A PWM charge controller is rated to operate at a particular DC system voltage and maximum current. PV modules designed to work at the DC system voltage must be connected in parallel PV source circuits 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 and series configuration
The chosen DC system voltage limits the choices of modules and configurations that are possible with a PWM charge controller. Below is a table of the number of modules that can be connected in series for each PV source circuit.
| DC system voltage
|
36 cell module
|
60 cell module
|
72 cell module
|
| 12 V
|
1
|
—
|
—
|
| 24 V
|
2
|
—
|
1
|
| 48 V
|
4
|
—
|
2
|
- 12 volt system = 1 × 36-cell module per PV source circuit.
- 24 volt system = 1 × 72-cell module per PV source circuit or 2 x 36-cell modules in series per PV source circuit.
- 48 volt system = 2 × 72-cell modules in series per PV source circuit or 4 x 36-cell modules in series per PV source circuit.
| Number of modules in series
|
=
|
Step 2: Determine proposed module configuration
This calculation will give a minimum number of PV modules - the result should always be rounded up. Different modules sizes and configurations can be explored to find the optimal design.
The final number of PV modules should always be larger than this value - the result should always be rounded up.
| Minimum number of PV source circuits
|
= Minimum number of PV modules ÷ Number of modules in series (Step 1)
|
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.
This step is only necessary for systems that will be used heavily for consecutive days (most systems). If a system is used lightly or infrequently then it will have ample time to recharge during days when it is inactive and this step is not necessary.
For a system that is used heavily for consecutive days, it is recommended that the array be sufficiently sized to reach a full state of charge within 7 days or that the system incorporate a generator to ensure adequate charging.
| Time to reach full state of charge
|
= Ah used at full depth of discharge ÷ Daily excess production in Ah
|
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 that depends upon battery type. Flooded lead acid and gel batteries should be charged with current that is between .05-.13 (5-13%) of their C/20 rating.[1] AGM batteries can should be charged with a current that is between .05-.2 (5-20%) of their C/20 rating.[1] 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 by increasing the PV source size. Most designs should have a charge rate between 5-10% - closer to 10% if the system is used heavily during the day. It is necessary to consult the manual or manufacturer for recommended maximum and minimum charging currents.
These calculations are performed with the Ah rating of the total energy storage system.
It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.
If the number of PV modules does not meet the recommendations outlined above, increasing the PV source in size should be considered.
Step 5: Determine final number of PV modules
Determine a final number of modules and a PV source configuration that can meet the requirements of Step 1, Step 2, Step 3, and Step 4.
| Final number of PV modules
|
=
|
| Final number of PV modules in series
|
=
|
| Final number of PV source circuits
|
=
|
Step 6: Total PV source current
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.
Step 7: Select a charge controller
A single charge controller is the simplest and cheapest option, but for larger systems multiple charge controllers often are used in parallel. The final chosen charge controller should:
- Function at the DC system voltage.
- The charge controller(s) should have a total current rating that is larger than the minimum current rating (Step 6).
The result of the following equation should always be rounded up.
| Number of charge controllers
|
= Total PV source current (Step 6) ÷ Chosen charge controller current rating
|
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) × Final number of PV modules(Step 5)
|
Step 9: Verify PV source and charge controller compatability
PWM charge controllers often have a maximum PV source power rating in watts that limits the size of the PV source. Verify that the maximum PV source power rating is greater than the final PV source power rating. If it is not, the charge controller size needs to be increased.
| PV source and charge controller compatability
|
= Final PV source power rating must be less than the maximum PV source power rating of the charge controller
|
Notes/references
