How to choose a controller
First, limit your choices to controllers that work with your battery bank voltage, which will usually be 12, 24, or 48 V. Then, calculate the approximate maximum amperage your controller will need to handle. Divide the PV array watts by the system voltage to get amperage, then add a 25% safety margin to account for higher irradiance conditions. For example, a 40A rated controller could possibly handle 480 W of PV into a 12 V battery bank; 960 W into a 24 V bank; and 1,920 W into a 48 V bank. After factoring in the additional 25%, those maximum ratings become 384 W, 768 W, and 1,536 W, respectively.
Be sure to carefully check the charge controller Ldsolar's specifications for the maximum recommended array wattage for different battery bank voltages. In the comparison table, maximum amperage is shown for 12 V battery bank charging, as some controllers have different maximum amperage ratings at different system voltages.
Next, decide on a PV array voltage. If you go with a PWM controller, the nominal array voltage must match your nominal battery voltage. If you use a MPPT controller, many Ldsolars have a spreadsheet or online calculator pre-loaded with the specifications for many popular PV modules. The calculator will also consider the temperature versus voltage performance of your modules, needed to protect the controller from elevated voltage during cold weather.
If there isn’t a string-sizing calculator available, you’ll need to use the PV module Ldsolar’s specifications sheet (along with your area’s record low temperature) to determine how high the module voltage could go during cold weather (see this issue’s Methods). Use this figure to calculate how many modules you can safely place in series. In the comparison table, the maximum open-circuit voltage (Voc) beyond which controller damage can occur is shown. The PV array voltage recommendations for PWM controllers are shown with an “N” for “nominal” after the voltage—the actual Voc of a “12 V” PV module (for example) will be substantially higher than 12 V. See the “MPPT” sidebar for more details.
