PWM charge controller sizing
This is a step by step guide to sizing your PWM solar charge controller. It can be quite overwhealming making the correct choice, so we are only a phone call away to help you make the best choice for your needs.
Step 1 - Check the battery voltage
The majority of campervans, caravans and boats in the UK use 12V batteries. Charge controllers only typically work with certain battery voltages. So if you decide to have a 12V battery system, check the charge controller matches.
Charge controller datasheet
Step 2 - Check the battery type
Lead acid
There are three main types of lead acid batteries:
- Flooded lead acid (FLA)
- Absorbant glass mat (AGM)
- Gel
Flooded lead acid are also known as “wet” or “flooded” batteries.
AGM and gel are both sealed lead acid (SLA) and are both are types of Valve Regulated Lead Acid (VRLA).
Lithium
- Lithium iron phosphate (LiFePO4, LFP)
- Lithium ion (Li-ion)
Charge controller datasheet
In the example above, it shows this charger is compatible with lithium batteries, for example LiFePO4.
STANDARD TEST CONDITIONS ADJUSTMENT - SAFETY MARGIN REQUIRED!
Solar panel datasheets show information at Standard Test Conditions (STC) of irradiance (energy from the sun) of 1000 W/m²
In reality, some parts of the UK receive around 1,250 W/m2 or irradiance in perfect conditions, with the sun at right angles to the solar panels. Therefore, in perfect conditions in certain areas of the country, we need a 25% larger charge controller than we think.
Solar panel datasheets show information at Standard Test Conditions (STC) with cell temperature of 25°C
Contrary to expectations, solar panels increase their power as the temperature drops. Therefore, in the example on the right, accounting for -5°C minimum temperature this increases the Voc to 83V (a 11% increase).
Step 3 - Check the voltage in short circuit conditions
This is very important since having too much voltage will damage all PWM charge controllers.
The solar panel voltage must be less than the charge controller specification.
In the example below, the solar panel datasheet says Open circuit voltage (Voc) is “21.24V” which is 25% less than the charge controller datasheet maximum PV open circuit voltage (Voc) of “50V”.
If there are more than one solar panel wired in series – add up the voltage of all the panels and compare to the charge controller datasheet.
If there are more than one solar panel wired in parallel – take the voltage of a single solar panel.
Solar panel datasheet
Charge controller datasheet
Step 4 - Check the volts in normal operating conditions
12V systems
Solar panels which have 36 solar cells wires in series are typically ideal for charging a 12v battery.
Solar panels with maximum power voltages (Vmpp) in the range 16-19V are typically suitable.
24V systems
Solar panels which have 72 solar cells wires in series are typically ideal for charging a 24v battery.
Solar panels with maximum power voltages (Vmpp) in the range 32-36V are typically suitable.
If there are more than one solar panel wired in series – add up the voltage of all the panels and compare to the charge controller datasheet.
If there are more than one solar panel wired in parallel – take the voltage of a single solar panel.
Step 5 - Check the amps under short circuit conditions
This is also important since having too much amps can damage most PWM solar charge controllers.
The solar panel amps must be less than the charge controller specification in short circuit conditions. In the example below the solar panel datasheet says the short circuit current (Isc) is “6.11A” which is 25% less than the charge controller datasheet maximum PV short circuit current of “20A”.
If there are more than one solar panel wired in series – take the amps of a single solar panel.
If there are more than one solar panel wired in parallel – add up the amps of all the panels and compare to the charge controller datasheet.
Solar panel datasheet
Charge controller datasheet
