Solar Charge Controllers – An Overview
Do you need a solar charge controller? If your solar PV system will require batteries, then YES! If not, you can skip right on by this page. Why? Because solar charge controllers control or regulate the charging of batteries in a battery-based solar PV system. And because most residential solar electric systems are “grid-tied” systems, they do not use batteries and thus have no need for solar panel controllers. But all the off-grid homes and remote equipment power by solar-charged battery systems (lights, sensors, instruments, etc.) need charge controllers to safely and efficiently charge their battery investments. To skip the rest of this overview, click to see our charge controller lines here: Large Charge Controllers, Small Charge Controllers and Lighting Controllers.
What do charge controllers do? Didn’t we just cover this? Kind of. But let’s get a little more specific. You may wonder why these devices aren’t just called battery chargers? Generally speaking, a battery charger refers to a device that you plug into an AC outlet and then connect to a battery to charge it. You may have one in your garage to charge the car battery when dead. A charge controller is used to regulate the voltage and current coming from a DC energy source like a solar PV array. It sometimes helps to think of these as solar charge controllers or DC charge controllers, especially if your system may also include an inverter/charger (mentioned on our Inverter page). In that case you would still need both the inverter/charger (which involves AC charging of the battery bank) and a DC solar charge controller to regulate the flow of solar DC energy into the batteries.
The analogy. The goal in charging a battery is to bring the battery’s voltage up to a factory-defined level that represents its full charge. A classic analogy will illustrate how solar charge controllers work. Imagine you want to fill a glass from a water faucet. Think of the faucet as the charge controller and the glass as the battery. The water level in the glass represents the battery’s voltage level. The water flow represents the current. As long as the water level is low enough, and the flow of water slow enough, then the glass will accept more water. Just like a low battery will accept DC current. But turning the faucet wide-open will quickly overflow the glass from the force of the water flow. Similarly, without a charge controller, excessive DC current flow can overcharge the battery. But adjusting the faucet, reduces the water flow to more steadily fill the glass without spillage. Likewise, a charge controller controls regulates the DC current flowing into the battery to steadily raise the battery’s voltage to effectively and efficiently re-charge the battery without causing damage.
Set points. More specifically, a charge controller regulates current into the battery by based on charging “set points” for that battery. Battery manufacturers specify certain voltage levels (“set points”) for each of the 3 stages of battery charging. There is a set point for the bulk, absorption and float charge stages that solar panel controllers keep track of. So a depleted battery is like that empty glass. The charge controller feeds the the highest current (like the fast water flow of an open faucet into an empty glass) during the bulk stage until the voltage level rises to the bulk stage set point at which point the battery is about 80% recharged. At that point, the charge controller reduces current (or flow) like a slow-running faucet until it reaches the absorption set point. After that the charge controller basically just drips enough current to keep the battery voltage at its float set point (filled to the brim).
What kind do I need? There are two types of charge controller technology used these days. The cheaper, effective but more limited PWM or pulse width modulation charge controllers are one type. While MPPT or (maximum power point tracking) charge controllers are the more efficient and flexible, but more expensive type. On our website, you’ll find PWM charge controllers under the Small Charge Controller page, but you’ll find MPPT charge controllers on both the Small Charge Controller and Large Charge Controller pages. We define Small Charge Controllers to generally be those up to 30 Amps, and Large Controllers above that.
PWM. In the typical scenario, we look to see if we can simply match the voltage of the controller to your battery bank. So a 12 volt controller matches with a 12 volt battery bank/system. This is the typical situation with an RV or boat. Similarly, if you have a 24 volt battery bank for your off-grid house, you would match it with a 24 volt charge controller. Same for 48 volt systems. In these scenarios, the cheaper and perfectly good choice would be a pulse width modulation (PWM) solar panel controller. Huh? We’ll explain in a minute. Contrast the PWM controller with a maximum power point tracking (MPPT) charge controller. The MPPT controllers cost more, but allow you to use a larger voltage solar panel/array with a smaller voltage battery system. So a 60 cell, 30V solar panel can charge a 12 volt battery bank when an MPPT controller is used.
MPPT. An MPPT charge controller feeds the maximum amount of energy produced by the solar array to the battery bank. Voltage may vary from the solar array depending on the sun’s intensity. But despite that, an MPPT charge controller can use all of the available energy to charge the batteries Why? Since watts = voltage x current, an MPPT controller can adjust the voltage to match the battery by adjusting the current. This also allows an MPPT controller to use a larger voltage solar array than the battery bank uses. So while the MPPT controllers cost more, they get more charging bang, for the solar array buck.
Conversely, a PWM charge controller can only use the voltage supplied by the solar array that matches the battery bank voltage to charge the battery bank . So when panels produce more voltage in better sun and/or cooler temps, the PWM controller can only use up to its rated voltage to charge the batteries–which may mean energy wasted. Similarly, a PWM controller cannot use a higher voltage solar array (say 30 volts) to charge a 12 volt battery bank. But a PWM does a very nice job charging batteries if its voltage is in sync with the battery bank. In the same way pulsing a faucet on and off in quick small bursts get a glass of water nicely full, a PWM controller charges a battery by pulsing quick small bursts of solar energy.