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.

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 excess production

During periods of poor weather or low solar resource, an off-grid PV system is designed to discharge the battery to a certain depth of discharge which can leave the energy storage system depleted. It is important that the energy storage system is brought back up to a full state of charge in short period of time or the cycle life of the batteries will be reduced. The PV array therefore must be sized to generate sufficient excess energy, while continuing to meet all of the power needs from the Load evaluation. OSSP recommends that the array be sufficiently sized to reach a full state of charge within a week or that the system incorporate a generator to ensure adequate charging.

Proposed PV source low insolation production	= Minimum number of PV modules (Step 2) × Total PV source loss parameter × Design insolation × Charge controller efficiency parameter ÷ Energy storage efficiency parameter

Daily excess production in Ah	= (Proposed PV source low insolation production - Average daily Watt-hours required) ÷ System voltage parameter

Ah used at full depth of discharge	= Final Ah capacity × Depth of discharge parameter

Time to reach full state of charge	= Ah used at full depth of discharge ÷ Daily excess production in Ah

If the time to reach full state of charge is less than 7 days, then the size of the PV source should be increased until it is below 7 days.

Step 4: Verify charging current

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. The maximum charging current for most lead acid batteries is between .13 (13%) and .2 (20%) of the C/20 rate. It is necessary 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 (Step 2) × 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. A minimum charge current of .05 (5%) and a maximum charge current 13% of the C/20 Ah rating is recommended for proper battery charging . Most designs should have a charge rate between 5-10% - closer to 10% if the system is used heavily during the day.

Percentage of C/20 rate	= Available charging current ÷ Final Ah capacity

If the number of PV modules does not meet the recommendations outlined above, increasing the PV source in size should be considered.

Step 5: Final number of PV modules

A final number of modules should be chosen that can meet the requirements of Step 2, Step 3, and Step 4.

Step 6: 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 5) × Isc rating of chosen module (Step 1) × Irradiance safety parameter

Step 7: Select a charge controller

The final chosen charge controller should:

1. Function at the system voltage.
2. Have a current rating that is larger than the minimum current rating (Step 6).

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 5).

PV source power rating	= PV module power rating (Step 1) × Number of PV modules (Step 5)