How to Select a Charge Controller

How to Select a Charge Controller

Charge controllers are one of the most important components in a renewable power system, next to the battery bank and power inverter. To know how to select a charge controller for one’s off-grid power system, one needs to know how charge controllers work, their features and settings (including if they have preset or adjustable settings), and the difference between PWM and MPPT charge controllers.

The first half of this post explains how charge controllers work; this includes explaining how charge controllers charge the battery bank as well as the many features and settings which will factor into choosing the charge controller. In the 2nd half, there will be a lengthy PWM vs MPPT charge controllers comparison that will go over how they function, their differences, and where each is best used.

Before you continue, here is a collapsible Table of Contents menu for easier navigation throughout this post (using anchor links). If you click ‘Top (⇑)’ next to any of the section and sub-section headers, it will take you back to this menu. For many of my future posts, I will add similar Table of Contents navigation menus.

Post Table of Contents:
Click to see Post Table of Contents

I.How do Charge Controllers Work (top⇑)

Because charge controllers are so important in renewable energy systems, it’s important to understand how charge controllers work. First, there are the charging stages for safe charging of the battery bank. Next, there are the numerous functions and features explained.

Then, there is a lengthy overview on adjustable/preset settings for the earlier mentioned functions and features. This is especially important since a charge controller’s available setting values will determine if it will fit into an off-grid power system. Finally, some physical features of charge controllers like input/output ports, display LEDs, and screens will be explained.

A.All Modern Charge Controllers Have 3 Official Charging Stages (top⇑)

Battery Charging Stages
When power comes in from the renewable power source, it gets distributed to the battery bank in a controlled manner; otherwise the DCLA (deep cycle lead acid) battery bank becomes damaged. The three official stages are:

  • Bulk Charging(Stage 1): If the DCLA battery capacity is under 80-90% full, the charge controller charges at high voltage and amperage to quickly fill up the battery until the battery bank fills up the program-specified capacity which is anywhere around 80-90% full. This is like letting a fire hose charge at a steady rate.
  • Absorption Charging(Stage 2): Once the bulk charging stage is passed which at around 80-90% full, the voltage is at its peak and the amperage is then tapered down as the charge controller’s internal resistance is increased. This is like switching from a fire hose to a garden hose which is operating at a strong pressure(same voltage is kept), but the amount of water is gradually lowering(lowering the amperage).
  • Float Charging(Stage 3): When this stage is reached, the battery bank is just about full. However, if charge trickles out of the battery bank by various means(ex: reverse-current) outside of normal battery usage, the charge controller send in small tickles of charge into the battery as maintenance charging. The feeding voltage is usually just above the battery bank’s voltage to prevent overcharging.

While charge controllers only have 3 stages; many are advertised as “four-stage” or even “five-stage”. These extra “stages” are basically features and services provided by the charge controller like battery equalization. While the 3 main stages are always used during charging; a service like battery equalization is done only few times per month or few times per year and is not part of the continuously active 3-stage charging process. The reason many charge controllers are advertised as “4-stage” or “5-stage” is because it sounds bigger and better than 3-stage charge controllers.

B. Functions Besides the 3 Charging Stages (top⇑)

Besides the 3 charging stages, there are several features and services that enable a battery bank to be charged and maintained safely. Some of these functions/features should be found in any charge controller; whereas others are found in some charge controllers. It’s important to be aware of these features so it can be determined if you have the needed features in your charge controller.

i.)Low Voltage Disconnect(top⇑): As a battery or battery bank depletes from full capacity, the voltage pressure slowly decreases. By reading this decreasing voltage, the charge controller can tell how much charge the battery bank has left. When voltage reaches a certain low voltage, the low-voltage disconnect will trip and stop anymore power from being taken from the battery bank until it has been charged. Check out this table:
State of Charge as Measured by Specific Gravity and Voltage
The above table roughly shows the state of charge measured by voltage (and specific gravity) for a flooded lead acid battery. As shown above, a fully charged “12 V” battery bank (with 100% state of charge) has a voltage of 12.7 volts. If a charge controller has set the low voltage disconnect to 12.2 volts, that means the lowest allowed state of charge of the battery bank is 50% (=50% depth of discharge).

Essentially, the low voltage disconnect feature is meant to protect a battery bank from over-depletion. A state of charge of 50% is considered by many to be the maximum a lead acid battery bank can be depleted (without severely shortening the battery life) before recharging.

If you wanted to enable the low voltage disconnect at a higher state of charge like 60% (=40% depth of discharge), the low voltage disconnect can then be set to a voltage slightly higher than 12.2 volts.

Because protecting the battery from damage is critically important, this feature should be found in all charge controllers in one way or another. However, you should check if the low voltage disconnect is preset or can be manually set.

ii.)Maximum Voltage Disconnect(top⇑): This feature is similar to the low voltage disconnect; except the power charging is cut-off when the voltage signifying a full battery bank is reached. Looking at the earlier figure, this would mean that a “12 V” battery bank would stop being charged when the charge controller detects a voltage pressure of 12.7 volts from the battery bank.

This feature is also critical to battery bank safety because unchecked battery bank charging will cause the battery bank to explode from the built-up electro-chemical gas. Hence, this feature should be also found in all charge controllers.

Lastly, because different battery banks can have different voltages signifying full capacity; it should be checked if the maximum voltage disconnect is preset or can be manually set if needed.

iii.)Equalization(and Desulfation)(top⇑): Equalization is the process of deliberately overcharging the battery bank in order to reverse negative chemical effects like stratification and sulfation (also spelled sulphation). This maintenance feature is done after the float stage when the battery capacity is full; which is why this is sometimes called the “4th stage” of charging. Equalization is not always done after the 3 charging stages; and is usually done once per month to few times per year. Also, equalization takes roughly 1-2 hours unless instructed otherwise by battery specifications or charge controller manual.

The below lead-acid battery diagram, along with commentary, will help explain Equalization better:
Lead Acid Battery Cell Diagram
To start with, all lead-acid batteries have a positive and negative electrode(cathode and anode, respectively). The positive PbO2(s) plate and the negative Pb(s) plate are dipped in the sulfuric acid solution H2SO4(l); which is also known as the battery electrolyte.

As the battery discharges, the sulfuric acid solution reacts with the positive and negative plates creating PbSO4(s) on both plates’ surfaces; this chemical reaction is called sulfation. When the battery recharges, the PbSO4(s) reacts with the H2O(l) and reforms the sulfuric acid. Then the cathode and anode respectively become PbO2(s) and Pb(s) again; this chemical process is called desulfation.

However, even after charging in accordance with the 3 charging states (with battery bank supposedly full), there may still be residual PbSO4(s) left. The problem is that if the residue is left to harden, the battery capacity will decrease. With equalization, the extra charge is to make sure the remaining PbSO4(s) residue that was missed during the 3-stage charging will dissipate after reacting to any remaining  H2O(l).

Related to reducing excess sulfation, Equalization also helps reduce stratification; which is a condition where acid concentration is greater at the bottom of the battery than at the top. As a result, the higher acid concentration at the bottom induces unwanted sulfation on the lower half of the plates. With the extra charge from Equalization, the electrolyte will be mixed via electrolysis to enable a more uniform acid concentration. Additionally, letting the battery to rest for a few days, light shaking, and tipping the battery on its side can also help undo stratification.

As far as I know, the charge controller itself can’t detect if the battery bank needs equalization. The charge controller will perform equalization (after float stage) when set to do so; which should be once per month to few times per year. Additionally, the charge controller’s Equalization function is only suitable for dealing with minor sulfation and stratification. If the sulfation is heavy, a desulphator machine is needed to reverse heavy sulfate build-up.

Battery Acid StratificationEven though Equalization can be set to automatically run at a set interval (like once per month); it’s recommended to manually determine when Equalization is really needed. This is done by measuring the specific gravity (SG) of each of the flooded lead acid (FLA) batteries in the battery bank (using a hydrometer); and then comparing them. If the SG difference between any of them is significant(i.e ≥ 0.030), then equalization needs to be performed. The benefit of doing this is to minimize the number of times Equalization needs to be performed. Also, because electrical loads have to be removed before Equalization charging, reducing the number of Equalizations performed will reduce the load removal/adding hassle.

Finally, as implied earlier, Equalization should only be done on FLA batteries. When H2(g) and O2(g) gasses are released from the battery bank’s electrolyte due to the high equalization charge (as high as 16 volts for a 12V battery); only FLA batteries can be opened in order to replenish the electrolyte with distilled water (since H2(g) and O2(g) come from water).

Because AGM and Gel batteries cannot be opened; Equalization will shorten the lifespan of these batteries since distilled water cannot be added to replenish the water released as H2(g) and O2(g) gasses. Hence, a charge controller with the Equalization feature is only necessary if the chosen battery bank uses FLA batteries.

iv.)Temperature Compensation(top⇑): This charging feature helps ensure that a battery is neither undercharged nor overcharged despite how temperature affects battery chemistry during charging.

Because battery charging is an electro-chemical process, it is affected by temperature like all other chemical reactions in nature. Specifically, because colder temperatures dampen electro-chemical reactions, a higher voltage is needed to push current into the battery plates and electrolyte. With warmer temperatures, a lower voltage is needed since the heat already helps catalyze the electro-chemical reactions; if too much charge is added on top of higher temperatures, the lead acid batteries experience unnecessary gassing as well as potential damage.

By default, most charge controllers are set for charging at room temperature (25°C, or 77°F). The temperature compensation can be fixed (e.g. ±5mV/°C/cell) or adjustable. The latter is preferable because different battery manufacturer specifications can recommend different temperature compensation values.

Some charge controllers have a built-in temperature sensor; and others have to be connected to an external temperature sensor. If the sensor is built-in, the charge controller needs to be placed close to the battery bank so it can detect the same temperature as the battery bank with regards to the ambient surroundings. In the latter case, only the external sensor needs to be next to the battery bank so it can relay the battery temperature to the charge controller.

Finally, to minimize temperature fluctuations around the batteries (or if you want to minimize use of this charge controller feature), the battery bank container should be kept at a stable (room) temperature along with ventilation for gasses. Here is how to calculate temperature compensation if you are interested.

v.)Charge Cut-Off or Diversion(top⇑): The Charge Cut-off feature is used when the battery bank is full and needs to be disconnected from the renewable power source. In the case of solar panels, because the DC output is relatively small, the connection between the panels and charge controller can be safely disconnected until there is a need to reconnect to the panels.

Dump Load Controller Diagram

For the AC power output produced by hydro-electric and wind generators (or if the cut-off feature is unavailable for a solar charge controller), diversion is needed. These hydro and wind generators cannot simply be disconnected because if the rotors in these generators continue to spin, the built-up charge won’t have anywhere to go and the generators will be damaged or destroyed.

Therefore, either the rotors have to stop spinning or the charge has to be diverted away from the battery. In the latter case, a separate device called a dump controller(with the dump load wired to it) has to be connected to the battery bank to take power away when it senses a charge overflow.

Lastly, you should check this post from SolarHomeStead which better explains dump controller circuitry as well as how to size a dump controller to the dump load.

vi.)Reverse Current Protection(top⇑): Even though the entire renewable power system is one circuit, the power from the renewable energy source is supposed to flow in one direction to the battery bank via charge controller. However, when the circuit is off and still connected, a bit of the battery power flows out to “fill” the rest of the circuit. One way to prevent this is with a blocking diode (which only allows the current to flow to the direction of the battery bank and not the other way).

Reverse current protection should be found in all charge controllers or in some other capacity or form.

C. Settings and Physical Device Characteristics (top⇑)

To find the right charge controller (after checking if it has the needed features and functions), one needs to determine if a charge controller’s preset/adjustable setting values match the needs of the off-grid system.

The first subsections cover how to account for preset/adjustable setting values in the final charge controller selection. The later subsections cover some minor details on the physical characteristics of charge controllers like input/outputs and information display.

i.)Preset vs Adjustable Settings(top⇑): So far, all the above mentioned features’ values are preset or can be manually adjusted depending on the charge controller. Below is a summary list of all the above settings/features that can be preset(fixed values) or manually adjusted; as well as their implications on voltage and battery type compatibility:

Absorption VoltageFloat voltageLow-Voltage Disconnect, and Maximum-Voltage Disconnect: signify at what voltages the charge controller will start using these charging stages/features. If these values are preset, that means the charge controller would only be suited for a battery bank of specific charge (Ex: if these settings are preset to voltage values suited for 12 V batteries, then only a 12 V battery bank would be compatible with that charge controller). If these voltage values are adjustable, then the charge controller can work with battery banks of different voltages.

Bulk Voltage: often has no setting as the voltage only increases. Generally, when the voltage for the Absorption Voltage stage is reached, the Bulk Voltage state ends. If a charge controller does have a preset/adjustable Bulk Voltage setting, it would only signify the voltage at which the Bulk Voltage charging state ends; which would only be slightly lower than the voltage where the Absorption Voltage stage starts.

Equalization: if its operation voltage is preset, then the charge controller is only compatible with flooded lead acid (FLA) batteries of a specific voltage. If the Equalization voltage is adjustable, then FLA batteries of different voltages can be used. Lastly, if Equalization is unavailable or can be disabled, then AGM and Gel batteries can be used.

Temperature Compensation: If the setting for this value is preset, then the charge controller will mainly be suited for batteries requiring similar temperature compensation values. If adjustable, the charge controller is then compatible to batteries of different compensation values.

Cut-off or Diversion: If the charge controller has a Cut-off feature, then the charge controller is most likely suited to be a solar charge controller (as long as the input voltage from the panels is low. For example: only 17.5 volts from “12V” panels). Otherwise, the charge controller would have to be used with a dump controller for solar, wind, and/or hydro power.

Reverse Current: Lastly, this feature should be considered in some way to prevent unneeded power loss.

Charge controller with preset setting values will be cheaper than those with adjustable setting values; this is due to lacking the extra electronics needed for adjusting setting values. The trade-off is that charge controllers with preset setting values will have limited flexibility and use in terms of system and battery bank specifications.

Given how common 12 volt battery banks are, charge controllers with preset values for 12V battery banks should be relatively common. Hence, charge controllers with matching preset values for a 12V battery banks are easier to find than for battery banks of different voltages(like 48 volts) or utilize non-standard preset values. On the other hand, it can be worthwhile to get a charge controller with adjustable setting values to have more freedom in making wanted adjustments to the system and battery bank.

For the many different charge controllers on the market, there are manuals available online. As charge controller manuals would definitely have information on the preset/adjustable setting values; you can check these online manuals to determine beforehand if a charge controller is suited for one’s system and battery bank design.

ii.)Examples of Preset and Adjustable Charge Controllers(top⇑): In this continuation of the previous sub-section, 3 adjustable and preset charge controllers are explained to demonstrate what kinds of charge controllers you can pick from depending on your personal needs:

1. OutBack Flexmax 80 Amp Solar Charge Controller: This charge controller can only be used for solar power and has MPPT(maximum point power tracking); which I’ll explain more later. Also, this charge controller only works for 12-60 Volt battery banks and can handle up to 150 Vdc (volts DC) from the solar array. The settings are highly adjustable (via digital screen) as several earlier-mentioned features can be digitally set including the bulk, absorb, float, and equalize voltages. Finally, this device can handle up to 80 amps of current from the solar array. Here is the online manual for your viewing.

2. TriStar TS-60 PWM Charge Controller: This charge controller also allows values to be set; however, the settings are mechanical. This uses PWM(pulse width modulation) instead of MPPT; which makes this charge controller much better suited for hydro and wind power generation. This works on battery banks of 12/24/48 Vdc. Lastly, this charge controller can handle up to 60 amps of current from the power generator. Here is the online manual for more information.

3. Renogy Wanderer 30 Amp PWM Charge Controller: Unlike the previous two charge controllers, this one uses preset voltage values. The preset values apply only to 12V battery banks. Also, there are three sets of preset voltage values depending on which deep-cycle battery bank type is used. The 3 battery bank settings to choose from are flooded, sealed(AGM), and gel. For example, if the flooded setting is selected, the respective voltages for boost(aka absorption), float, and equalization are set to 14.8 V, 13.2 V, and 15.5 V, respectively. One would then need a 12V deep-cycle flooded battery bank whose charge state specifications match these voltages. Finally, here is the online manual for your further viewing.

Two things that are brought up that I will explain more later are charge controller sizing and how the PWM and MPPT charging methods work. Knowing how to size a charge controller means picking a charge controller that can handle the maximum possible current from renewable power generation. I will also later explain the differences between PWM and MPPT charge controllers.

Lastly, like I said before, you should read the online manuals of charge controllers (when possible) to determine if the given  features and characteristics are suited for one’s needs.

iii.)Displays and Metering(top⇑): A charge controller displays data with a small digital display screen(s) and/or use blinking LEDs to show the currently in-use feature(s).

LEDs on charge controllers tend to be used to describe the on/off state of qualitative features. For example, there are LEDs that can determine which of the three states of charge the battery is currently being charged in. Also, there can also be an LED signifying whether equalization is on or not. The most common LED(s) tell if the device is on/off.

Digital Screens can show what LEDs already signify plus it can show more quantitative data than LEDs. For example the digital screens will show numbers as to monitor how full the battery is, how much volts/watts/amps is coming from the renewable power source, and how much volts/watts/amps is outputted.

Another benefit of digital screens is that these enable fine tuning as some device buttons and knobs enable to quantify its effect on the system on the display screen. If the charge controller doesn’t fully display the conditions of the system, a battery bank monitor can be attached to the system to display system conditions.

One last thing to point out is if a charge controller only has LEDs and buttons/knobs, that means it most likely uses preset voltage values. If the charge controller has a digital screen, it could possibly means values are adjustable since a digital screen can enable fine tuning.
Solar Charge Controller Wiring Diagram

iv.)Inputs and Outputs(top⇑): A charge controller should have at least 2 (+/-) wire terminal pairs. One is for taking input power from the renewable energy source, another pair is taking and adding charge to the battery bank.

Any other wiring/connections are for auxiliary purposes like grounding, DC loads for lighting (this is not the same as using a dump controller for diversion), or even sending data to a computer.

There can even be more terminal pairs if the charge controller is hybridized with an inverter.

v.)Finally, Charge Controllers Consumer Power too(top⇑): In most discussions about charge controllers, there is surprisingly little mention on the power consumption of charge controllers. As electronic devices, charge controllers will consume power.

For Example: the TriStar TS-60 Charge Controller from earlier has an operation voltage range of 9-68 volts; and a consumption current of ≤ 20 mA (= 0.02 Amps). These values came from pg.32/39 of the online pdf manual.

With Power = Voltage x Current, the power consumption range is about 0.2-1.4 Watts per hour. When using any charge controller, check for the power consumption from the manual or from the company as this value will play a part in battery bank sizing.

D.How to Size a Charge Controller for a Renewable Power System (top⇑)

Like how a battery bank is sized to ensure enough power is provided for all appliances (without excessively depleting itself), a charge controller has to be sized to be able to handle the maximum possible current produced by the in place solar panels or renewable power source. Here’s how it’s done:

First, it’ll be assumed that 1000 Watts per hour will be generated from a solar array.

Next is to take the voltage of the solar array. Although solar panels are labeled “12 V”; in actuality, the working voltage fluctuates around 14-20 volts (depending on temperature and light intensity) when pushing the charge into the 12 V battery bank.

If the solar array currently yields 14 volts, then the current is:
(1000 Watts)/(14 volts) ≈ 71.43 amp.

Otherwise, if the solar array yields 20 volts, then the current is:
(1000 Watts)/(20 volts) = 50 amp.

As shown, the lower the voltage, the higher the current. Because 71.43 amps is the highest possible current, the charge controller needs have an amp capacity higher than 71.43 amps. In this example, the OutBack Flexmax 80 Amp Charge Controller from earlier would suffice since it can handle up to 80 amps.

In actual charge controller use, it’s not advisable to let the working current repeatedly saturate the amp capacity. Doing so will overtax the charge controller and reduce it’s life span. There should be some margin like there was between 71.43 amps and the capacity limit of 80 amps. Lastly, remember that charge controller sizing is as important as inverter and battery bank sizing.

II.PWM vs MPPT Charge Controllers (top⇑)

PWM vs MPPT Charge Controllers
Finally, the PWM vs MPPT charge controller comparison will be explained. Since PWM and MPPT charging use different methods to charge the battery bank, the differences in charging can best be explained using math.

Then, the differences in when and where to use PWM and MPPT charge controllers will be explained based on individual features and real-world factors.

Before demonstrating the charging math, here are the assumptions:

  1. A “12 V” deep-cycle lead acid (DCLA) battery bank will be used
  2. A solar array of “12 V” and 400 Watts will be used. The working voltage generally ranges around 14-20 volts (which varies based on temperature and light intensity) so the higher voltage can push the charge into the 12 V battery bank.
  3. The charge controller’s current charging voltage will be 14 volts. This is from the 13.2-14.8 volt range commonly used during the charging stages (except equalization) for a 12 V battery bank.

A.How PWM (Pulse Width Modulation) Charging Works (top⇑)

PWM controllers allows battery charging to be done via converting the input charge from the solar panels and pulling the voltage down to a level suitable for charging. This will be demonstrated via example:

First, the solar array’s current (which goes into the charge controller) needs to be calculated. The solar array’s working voltage is 14 volts and the power rating is 400 Watts as assumed earlier.

Here are the calculations:
Currentsolar = (Powersolar) ÷ (Voltagesolar)
Currentsolar = 400 Watt ÷ 14 volts
Currentsolar = ≅ 28.57 amps

Now, here is the charge controller’s output power using it’s voltage and the solar array current:
Powercontroller = (Voltagecontroller) × (Currentsolar)
Powercontroller = 14 volts × 28.57 amps
Powercontroller ≈ 400 Watts

Because the solar array voltage (Voltagesolar) matches the charge controller voltage (Voltagecontroller), no voltage pull down takes place; and there’s no loss of power going into the battery bank. However, if the solar array is working at 20 volts, there will be a voltage pull down that will result in a power loss.

This can be measured by the following new calculations:
Currentsolar = (Powersolar) ÷ (Voltagesolar)
Currentsolar = 400 Watt  ÷  20 volts                                                                                                                                                                        Currentsolar= 20 amps

Powercontroller = (Voltagecontroller) × (Currentsolar)                                                                                                                                                          Powercontroller =  14 volts × 20 amps                                                                                                                                                                                        Powercontroller = 280 Watts

With Powercontroller reduced from 400 to 280 Watts, the battery charging power decreases by 30%. Ultimately, the power generation voltage must match the charge controller’ voltage to get the most efficient battery charging.

Disadvantages aside, the PWM charge controller charges the battery bank via rapid pulses. Pulses have been documented to have a much better effect on the plates of lead acid batteries than a steady high-voltage DC charge. PWM charge controllers utilize pulses to charge a battery bank in during the 3 stages (equalization included) depending on how much the battery bank has been charged.

Because of the three stage capability of the PWM charge controller, it has secured its place as a mainstay for many years as a charge controller of choice.

B.How MPPT (Maximum Power Point Tracking) Charging Works (top⇑)

Whilst PWM have been around for a while, MPPT charge controllers are relatively recent. MPPT already has most of the same features as PWM charge controllers. These include the three charging stages(as well as equlization) and an algorithm to determine which charging stage to utilize; MPPT also utilizes pulses like PWM controllers to control the charge rate into a battery bank. However, unlike PWM charging, MPPT charging works while trying to prevent power from being lost.

To explain how power is preserved, first check this following equation:
(Voltagesolar) × (Currentsolar) = (Voltagecontroller) × (Currentcontroller)

Like before:
Voltagesolar = voltage of solar panels
Currentsolar = current from solar panels
Voltagecontroller = voltage output of charge controller
Currentcontroller = current output from charge controller

If the solar array outputs 20 volts and all other assumptions are kept, then the following happens:
(Voltagesolar) x (Currentsolar) = ( Voltagecontroller) x (Currentcontroller)
20 volts x (400 Watts ÷ 20 volts) = 14 volts x (Currentcontroller)
Currentcontroller  ≅  16.67 amps.

As explained earlier about PWM, the current was kept constant while the voltage is brought down to match the battery bank’s voltage; and that causes power to be wasted. In the case of MPPT, the current was boosted to keep the power the same despite the solar array’s voltage being pulled down to match the charging stage voltage. While MPPT should theoretically save all power; in reality, only 94-98% of the original power is saved which is still a huge improvement over PWM’s charging efficiency.

Thus, MPPT is called Maximum Power Point Tracker because it tracks the maximum power at any point in time for the given circumstances and tries to output the same amount of power as the output into the battery bank.

C. PWM vs MPPT Charge Controller Usage Factors (top⇑)

After learning how PWM and MPPT charge controllers work, then comes determining which one is needed for one’s off-grid power system. This is decided by understanding the several situational factors that determine which one will perform most effectively for the user. Here is a summary of the main factors which affect the final decision-making:

i.)Solar Versus Wind and Hydro Power(top⇑): Since the beginning, MPPT technology was best suited for solar array systems since the slower voltage change made it easier to track and harness power.

As for wind and hydro generators, the rotation speed of their turbine rotors can change quickly and intermittently. This results in the DC voltage (first converted from AC voltage) changing too quickly to apply MPPT. On top of power loss from being unable to apply MPPT, there is also the slight power loss that comes from needing to convert from AC to DC power (for charge controller use) via inverter.

Recently, there have been attempts to create MPPT charge controllers for non-solar power. For example, this Ten-High 400W MPPT Wind Charge Controller claims to be able to apply MPPT to wind generators. However, until it is widely proven that this works, you should be cautious if this or any seller is telling the truth or not. Also, if you hear any advertisement like “MPPT charge controller for solar and wind”; it means that MPPT charging is active only during solar power collection and not for wind power collection (meaning that PWM charging is used for wind power). 

Until MPPT charge controllers can be truthfully applied to wind and hydro power, current wind and hydro systems will have to be built around PWM charge controllers. Dump controllers and loads will continue being used to shave excess wind and hydro power.

Only PWM charge controllers should be used with wind and hydro power regardless of scaling since MPPT is currently not suited for these forms of renewable power. Lastly, the PWM charge controller current capacity needs to be sized to handle wind and hydro generators.

ii.)Scaling(top⇑): If a solar array system is scaled from small to medium, it would be more cost-effective to use a PWM charge controller over an MPPT one. Although an MPPT charge controller can get more power, the power loss avoidance maybe marginal as a smaller array does not produce significant power to begin with.

On the other hand, if the solar array size was much bigger, the power loss avoidance would be much more noticeable if MPPT charging was used over PWM charging. I would guesstimate that if the solar array was scaled at 400-500 Watts or less, PWM charge controllers should be used. If the solar array was scaled larger than 500 Watts, MPPT charge controllers should begin to be considered.

iii.)Storability(top⇑): This maybe obvious, but it is strongly recommended to store the charge controller in a closed safe place with a stable temperature. As long as the charge controller is safe from extreme temperatures and other environmental hazards, it will run smoothly.

Besides making the storage place safe, it also needs space to hold the charge controller (and other devices) as well as allow for connection wires to go in and out of the storage. Lastly, as explained earlier, the charge controller can be stored together with the battery bank (as long as storage is safe and ventilated) to do temperature monitoring and compensation.

iv.)Cost(top⇑): Because MPPT requires sophisticated circuitry to both track power and to prevent power loss, additional cost is added onto the charge controller. Generally, MPPT charge controllers are at least 2-3 times the price of PWM charge controllers.

v.)Device Efficiency(top⇑): MPPT is most efficient; but only for large solar arrays. PWM is efficient only when the voltages of both the power source and charge controller closely match to avoid a voltage pull down. However, this is impossible to do at all times. If a PWM charge controller must be used, the renewable power system should be built around it to account for potential losses of power when they happen.

III.Summary & Ending (top⇑)

In the first-half of post, I heavily explained to you the functions and settings of charge controllers. The main functions I explained were:

  • 3 charging states (bulk, absorption, and float)
  • Low Voltage Disconnect
  • Maximum Voltage Disconnect
  • Equalization (and Desulphation)
  • Temperature Compensation
  • Charge Cut-Off and/or Diversion
  • Reverse Current Protection

I explained what each function does, where and when they’re used; and for some of them, I explained what battery types (flooded, AGM, or Gel) to use them with. When choosing your charge controller, you need to make sure it can provide the features relevant to your system’s needs.

Afterwards, I emphasized checking if the charge controller had preset or adjustable setting values for its features. The ones with preset values are only viable if the battery and system specification value match the preset values; otherwise, charge controllers with adjustable setting values should be preferred even though it’s more expensive.

In the latter part of this post , I first covered the difference between the charging methods of PWM and MPPT charge controllers. Finally, because of the difference in charging, I explained the situational factors that determined whether one should use a PWM or MPPT charge controller.

With all of the above factors combined and considered, you should now know how to select a charge controller. Together with the renewable power generator; the battery bank, charge controller, and inverter are the main components of an off-grid power system. It is important to know how all the components work as well as their compatibility with each other to avoid any haphazard configurations. After covering battery bank sizing and charge controllers, the next post will finally cover inverters in-depth.

If you liked this post, be sure to share this post (via social media bar) and subscribe for email updates(if you already haven’t). Leave a comment below regarding your thoughts, experiences, and questions regarding the use of PWM and MPPT charge controllers and their place in tiny house off-grid power systems.

Image Attributions (You may skip this):
Click to see all image & diagram attributions for this post
  1. Featured Header Image:  This composite image is made up of the following:
  2. Charges Stages Diagram: Derivation of the “Four-stage charging cycle” diagram from Strikhedonia Seawind. However, please check the “Content Reuse and Attribution Policy” page on the bottom to understand the content reuse policy for all images on this post.
  3. State of Charge as Measured by Specific Gravity and Voltage: table from “Specific Gravity Performance” pdf.
  4. Lead Acid Battery Cell Diagram : Derivation from this JeffBlessing blog page.
  5. Battery Acid Stratification Diagram : Derivation from this Battery University web page.
  6. Dump Load Controller (sample) Diagram: Derivation from this SolarHomeStead post.
  7. Charge Controller Wiring: Derivation from this EagleReady page.
  8. PWM and MPPT on Scale: This composite image uses:
    • the PWM and MPPT charge controller images from featured header image #1.
    • the weighing scale is a vector image from this all-free-download web page.
  9. Pinterest Hidden Image(only visible with Pinterest Browser Button): I reserve ALL RIGHTS to this image as I created this for my (and not anyone else’s) Pinterest sharing needs. This composite pin image consists of:
  10. Pinterest Hidden Infographic Image #1(Introduction to Charge Controllers):I reserve ALL RIGHTS to this image as I created this for my (and not anyone else’s) Pinterest sharing needs.
  11. Pinterest Hidden Infographic Image #2(PWM vs MPPT Charge Controllers):Same as #10.
  12. Facebook/Google+/etc. Post Header Image:  I also own all rights to this image as I made this for my other social media sharing needs. Just like in #8 and #9, this is also made up of the two PWM and MPPT charge controller images found in featured header image #1.
How to Select your Charge Controller. #Charge #controllers are needed to regulate how the #batterybank is charged since improper charging will damage the #batteries. To choose the best #chargecontroller (also called a #charger) for your #offgrid or #renewableenergy system, you need to know how #chargecontrollers work, their features and settings, and the differences between #PWM and #MPPT charging. Click through to learn everything you need to know for choosing your own #charge #controller in real life.
Introduction to Charge Controllers. #Charge #controllers are devices which limit the rate at which power is added or drawn from a #batterybank. This device is critical to any #offgrid power system; especially those that utilize #renewableenergy. This #infographic introduces charge controllers and how their charging capabilities work. Click through to learn more about charge controllers as well as how to select a #chargecontroller in real life.
PWM vs MPPT Charge Controllers. #PWM and #MPPT are the two main charging methods used by charge controllers in #offgrid and #renewableenergy systems. The above #infographic shows the differences between #PWM and #MPPT charge controllers and enables one to determine which charging method is best suited for ones needs. However, if you truly want to select a #chargecontroller in real life, click through to learn everything needed to do so.

2 Responses

  1. Kenny says:

    I recently learned a lot about electricity at my technical college I graduated from. You make the amperage analogy super simple when you said its like going from a fire hose to a garden hose from the bulk charging to the absorption charging stage. My dad has a charge controller for his solar panels and wind turbine and I have always been fascinated by them, after reading your article, I have even more knowledge to talk about with him. Do you think green energy will eventually take over the world?

    • Bobby Kundu says:

      Kenny, I am glad you found my explanation(s) understandable regarding the functions of PWM and MPPT charge controllers. In fact, I was a little worried that readers would have trouble understanding my explanations despite my efforts to simplify.

      Also, it does seem like your dad is serious about off-grid living if he has set up a renewable energy system using both solar and small wind; this is because solar and wind complement each other well since solar panels are good for sunny days and wind turbines can are better for cloudier days(which tend to be windier).

      Finally, regarding green energy eventually taking over the world; that might be a ways off. While green energy is now well-known, many still see it as a form of supplementary power or as something for ‘hippies’. However, with new wind farms, giant solar arrays, and hydro-turbines(in dams) being built into America’s infrastructure, utility companies can sell this generated electricity to the masses; this trend should be continuing slowly. Finally, there is the fact that that non-first world countries are still the slowest with getting into green energy.

Leave a Reply

Your email address will not be published. Required fields are marked *