MPPT charge controller sizing
This is a step by step guide for MPPT charge controller sizing. 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
There are many types of lithium batteries but the most popular now is:
- Lithium iron phosphate (LiFePO4, LFP)
- Lithium ion (Li-ion)
Different types of lithium batteries charge at different voltages so you need to make sure the charge controller can charge at the rate recommend on the battery datasheet.
Charge controller datasheet
In the example above, it shows this charger is compatible with lithium batteries, for example LiFePO4.
STANDARD TEST CONDITIONS ADJUSTMENT - WHY A DESIGN SAFETY MARGIN IS 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).CONCLUSION
It is good practice to allow a 25% safety margin in all the following steps when sizing you solar panel charge controller.
Step 3 - Check the watts
The solar panel watts must be less than the charge controller specification.
In the example below, assuming a 12V system, the solar panel datasheet says nominal maximum power is “100W” which is 25% less than the charge controller datasheet nominal PV power at 12V of “260W”.
For more than one panel, add up the watts of all the solar panels. connected to the charge controller.
Solar panel datasheet
Charge controller datasheet
Step 4 - Check the voltage
This is very important since having too much voltage will damage many 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 less 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 5 - Check the amps under short circuit conditions
This is also important since having too much amps can damage some 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 (Isc) is “6.11A” which is 25% less than the charge controller datasheet maximum PV charging 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
Over-sizing your solar panels (also know as power shedding)
In the winter, the MPPT charge controller will often be working at a low rate. In the summer, the charge controller may be working at maximum capacity.
The idea is to oversize the solar panels by up to 20% (in the UK). This means, summer output will be similar or slightly higher, but in the winter, more energy will be produced.
IMPORTANT: This idea will damage all PWM solar charge controllers.
IMPORTANT: This idea only typically works on current. Over-sizing the voltage is likely to permanently damage the charge controller.
IMPORTANT: This idea will damage some MPPT controllers – check the datasheets/user manuals.
This is an example wording for a MPPT controller user manual:
“Limit charging power & current over the rated value”
“In any charging stage, as long as it is detected that the current is beyond the rated current, it shall enter into the current-limiting charge automatically and the charging current is limited to the rated current.”
If it doesn’t specifically say in the instructions over sizing current is permitted it is strongly recommended not to oversize.
You still need to consider the 25% safety margin noted above.