Small Off Grid Solar System
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Charge Controllers
Charge controllers manage the power from your solar panels to deliver it into your batteries and inverter at the right rate and prevents your batteries from being over charged.
There are two main types of charge controllers:
- Maximum Power Point Tracking (MPPT) and
- Pulse Width Modular (PWM).
Both have their merits and work well in the right situation.
PWM Charge Controllers: These charge controllers effectively connect the solar panels directly to the batteries during the bulk charge phase and then switch the panels on and off rapidly during the constant voltage and float phase to maintain the correct voltage. Because of this direct connection, the PV panels will be operating at the battery voltage. There are two key things to note about PWM controllers:
- Because the panels operate at the battery voltage, it's important to select panels that operate best at the same voltage. So for a 12V system you would want to choose panels with a maximum power point voltage (Vmpp) of around 14 to 17V. If you have a 48V battery system then you would want four of those panels in series.
- Because they connect directly to the battery during the bulk charging phase there is no switching losses which gives them good efficiency and they perform very well in low light conditions.
PWM charge controllers are also quite simple which makes them cheap and reliable.
MPPT Charge Controllers: These controllers mostly consist of a step down switching power supply and they can efficiently change the voltage between the PV panels and the batteries. Because the PV panels can operate at a different voltage to the batteries, the controller can work out the optimum voltage for the panels to give the maximum power. However, because they are 'step down' switching power supplies, the incoming PV voltage must be greater than the battery voltage by some margin. If the maximum power point voltage of the string of panels is below the battery voltage then the controller won't be able to find the maximum power point and will probably continue to search up and down the voltage range in an inefficient way. MPPT controllers can usually manage higher power levels.
Bottom line is:
- If your PV panels match your battery system (this usually means 12V PV panels) then a PWM controller is best.
- If your PV panels don't match your battery voltage or you are after higher power levels then a MPPT controller is best (but make sure the PV string voltage is high enough).