- What a solar charge controller is doing
- What a solar charge controller is not
- How to choose a solar charge controller without a catalog slogan
- Frequently asked questions
- Do I need a solar charge controller if I have an inverter?
- Is MPPT always the right solar charge controller?
- Can one solar charge controller handle any battery?
- Sources and references
A solar charge controller sits between a photovoltaic array and a battery. Its job is to let the array charge the battery and to stop, or to taper, before the battery is held at a voltage the manufacturer forbids. It does not make alternating current for the house. That is the inverter. A cabin diagram that shows only “the solar box” is hiding which box does which job. If the battery voltage is wrong, the controller is the first place to look. If the outlets are dead and the battery is fine, the inverter is the first place to look. Mixing the two names is how the wrong part gets replaced.
What a solar charge controller is doing
The array produces current whenever light hits it. A battery will accept that current until it is damaged. The controller is the device that applies the battery’s charge voltages: a bulk stage while the battery is low, an absorption stage if that chemistry uses one, and a float or a cutoff when it is full. Lead-acid sheets often print all three, plus an equalization that is allowed only for some flooded types. Lithium iron phosphate sheets often print a charge voltage and a stop, and they often forbid equalization. The numbers are on the battery sheet. The controller only stores them. What each setting means, and how a wrong one shows up, is in solar charge controller parameters and solar charge controller problems.
Two families do that job differently. A pulse-width modulation controller is, in Victron’s description, essentially a switch. It pulls the array voltage down near the battery voltage, so a module whose maximum-power voltage is far above the battery cannot deliver its nameplate into that battery. An MPPT controller separates the two voltages. It runs the array near its own maximum-power point and converts that power to the battery’s voltage and current. In Victron’s worked example at 25°C, the PWM path delivered 81 watts and the MPPT path 100 watts, about 19 percent less for PWM in that example, not as a law for every roof. The advantage is clearest when the array is cold and the battery voltage is far below the module’s power point. In a hot climate the gap shrinks, because the module voltage has already fallen. Morningstar’s guidance is the practical version: PWM wants an array whose voltage is matched to the battery. A module near 30 volts at its power point, typical of many 60-cell layouts, is often a poor PWM match for a 12-volt battery. Match the method to the module, or buy the method that can accept the module you already have.
What a solar charge controller is not
It is not an inverter, and it is not a hybrid. It does not synchronize to a utility and it does not form a 230-volt waveform. Some inverter-chargers include a solar charger in the same chassis. The function is still a charge controller. The name on the box can be “inverter.” Read the terminals. A PV input with a stated maximum voltage is the controller function. An AC output is the inverter function. If the PV input is missing, you still need a controller, or a different inverter that has one, before the array lands on the battery. The map of inverter jobs is in types of solar inverters.
It is not a license to ignore cold open-circuit voltage. The array’s voltage rises as the cells get colder. The controller’s maximum input is a hard limit. A string that is safe at 25°C can exceed that limit on a bright winter morning, before the battery has pulled the voltage down. The check uses the module’s temperature coefficient of voltage, which is on the datasheet, and the coldest temperature the designer is willing to treat as real. There is no universal string length. How heat changes power is the other half of the same sheet: how heat affects a solar module.
It is not a battery. A controller can report a voltage. For a flat lithium curve, voltage is a poor state of charge unless the manufacturer published the table you are using. Do not invent the table from a lead-acid rule of thumb. Maintenance of the battery itself is in solar battery maintenance.
How to choose a solar charge controller without a catalog slogan
| Question | Where the answer lives |
|---|---|
| Battery voltage and chemistry | The battery sheet. The controller must have a profile that can be set to those voltages. |
| Array voltage, cold | Module Voc plus the cold coefficient, under the controller’s maximum input. |
| Array current | The controller’s charge-current rating, against the array, with the margin the manual requires. |
| PWM or MPPT | PWM if the module voltage is matched to the battery. MPPT if it is not, or if the cold-climate gap in Victron’s note matters on this site. |
A single published comparison is not a shopping percentage. A 2019 test in Cúcuta, Colombia, on one 30-watt module and a 12-volt, 18-amp-hour battery, measured an MPPT controller whose average efficiency was 14.9 percent higher than the PWM controller in that test. That is one setup. It does not mean every MPPT harvests 15 percent more, and it does not overturn Victron’s point that the gap shrinks in the heat. Use the table above. Leave the percentages in the papers they came from.
Wiring stays with a qualified person. The array is live in daylight. The controller’s manual states the order for connecting the battery and the array. This page will not replace that order. Pacific Northwest National Laboratory’s field rule still applies: only trained people connect and disconnect the wiring.
Frequently asked questions
Do I need a solar charge controller if I have an inverter?
You need the charge function. Some inverters include it. A plain grid-tied inverter that has no battery does not, because there is no battery to charge. An off-grid inverter that has no PV input does not either. Read the terminals before you decide the controller is already inside the box.
Is MPPT always the right solar charge controller?
It is the right controller when the array voltage does not match the battery, which is common with modern modules on a 12-volt or 24-volt bank. A voltage-matched array can use PWM. The 19 percent gap in Victron’s example is that example, at 25°C, not a promise for a hot roof.
Can one solar charge controller handle any battery?
Only if its settings can be made to match that battery’s sheet, including temperature limits and whether equalization is allowed. A controller locked to a flooded lead-acid profile is the wrong controller for a lithium pack that forbids that profile.
Sources and references
PWM as a switch, the 81-watt and 100-watt example, and the climate note: Victron, PWM or MPPT. One 30-watt comparison in Cúcuta, average MPPT efficiency 14.9 percent higher in that test: Colombia, 2019. Live field wiring: Pacific Northwest National Laboratory. Morningstar’s voltage-match point for PWM is used here as that manufacturer’s guidance; confirm it on the controller sheet you buy.
