PWM charge controller sizing and selection

A PWM charge controller is rated to operate at a particular system voltage and maximum current. PV modules designed to work at the system voltage must be connected in parallel in order to achieve the minimum PV source size and the charge controller therefore must be sized to handle this amount of current. If the current rating of a PWM charge controller is exceeded, it can be damaged or destroyed.

Step 1: Determine PV module power rating

The chosen system voltage limits the choices of modules and configurations that are possible:

• 12 volt system = 1 × 36-cell module per string.
• 24 volt system = 1 × 72-cell module per string or 2 x 36-cell modules in series per string.
• 48 volt system = 2 × 72-cell modules in series per string or 4 x 36-cell modules in series per string.

36-cell modules are typically below 150W. 72-cell modules are typically 300W-400W.

Step 2: Determine proposed number of PV modules

This calculation will give a minimum number of PV modules. Different modules sizes and configurations can be explored to find the optimal design.

Minimum number of PV modules	= minimum PV source size ÷ PV module power rating (Step 1)

The final number of PV modules should always be larger than this value, thus if the the result of the calculation is a decimal, it should be rounded up.

Step 3: Verify the proposed design

Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range - typically between .05-.13 (5-13%) of their C/20 rating. It is best practice to consult the manual or manufacturer for recommended maximum and minimum charging currents.

Minimum required charge current	= final Ah capacity × .05

It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.

Available charging current	= maximum power current (Imp) × minimum number of PV modules × charge controller efficiency parameter × module degradation parameter × soiling loss parameter

If a system uses many loads during the day, this will limit the available charging current for the energy storage system and should be taken into account. If the number of PV modules does not meet the recommendations outlined below, increasing the PV source in size should be considered.

• A minimum charge current of .05 (5%) of the C/20 Ah rating is recommended for a system that is used infrequently or is primarily used at night.
• A charge current of .1 (10%) of the C/20 Ah rating is recommended for a system that is used regularly with significant load usage during the day.
• A maximum charge current of .13 (13%) of the C/20 Ah rating is recommended.

Percentage of C/20 rate	= available charging current ÷ final Ah capacity

Step 4: Final number of PV modules

A final number of modules should be chosen that can meet the requirements of Step 2 and Step 3.

Step 5: Determine minimum current rating of charge controller

This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The Isc rating of the PV module can be found on its specifications sheet.

Minimum current rating of charge controller	= final number of PV modules (Step 4) × Isc rating of chosen module (Step 1) × irradiance safety parameter

Step 6: Select a charge controller

The final chosen charge controller should function at the system voltage and have a current rating that is larger than the minimum current rating (Step 4).

Step 8: Determine final PV source power rating

The total power rating of the PV source can be calculated by multiplying the power rating of the chosen PV module by the final number of PV modules (Step 4).

PV source power rating	= PV module power rating (Step 1) × number of PV modules (Step 4)