Sizing PWM Charge Controllers

Charge Controller Sizing, Part 1: PWM & Shunt

All renewable energy (RE) systems with batteries should include a charge controller. Charge controllers prevent battery overcharging and also prevent the batteries from sending their charge back through the system to the charging source (i.e., the solar panels). Think of a charge controller as a battery nurse—its job is to monitor the battery bank, feeding it what it needs and checking its vital signs. Since a charge controller does its work in line between the solar array and the batteries, it would make sense that its selection and sizing would be influenced by those components. And that’s exactly the case.

Voltage and amperage (or current) are the parameters we use in charge controller sizing. The charge controller must be capable of accepting the voltage and current produced by the DC source and delivering the proper voltage and current to the batteries. This situation might make you think that the DC source, charge controller and batteries must all share a common voltage. While that is one system design strategy used in many installations, it’s not the only one. More on the alternatives later. For now, it’s one voltage for everyone!

Technically speaking, the DC source must always have a higher operating voltage than the battery bank in order for current to flow from one to the other. A handy way to remember this fact is the statement, “Voltage flows downhill.” For the purpose of this discussion, we’ll use nominal voltage which means common battery voltages. Nominal voltage in this sense is synonymous with system voltage. Since batteries (where they are used) are in many ways the heart of an RE system, we can call the bank’s voltage the system voltage. The system voltage selected for any given installation is usually, though not always, determined by the battery bank required by the application; the inverter, if one is used, will also influence the choice of system voltage.