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12/24V MPPT Solar Charge Controller Rev.1

By John Clarke

As presented in February 2011, the MPPT Solar Charge Controller was designed for use with 12V solar panels rated up to 120W or 24V solar panels up to 240W. A number of readers have requested modifications to allow it to be used with larger panels. Unfortunately, that’s not practical because the necessary component changes required for more power cannot be accommodated within the existing box size or on the PCB.

We may publish a higher power MPPT Charge Controller at a later date but for now, we are simply presenting some enhancements to the original design, to make it run cooler and more flexible to use. The increased efficiency will be a significant improvement where the Charge Controller is being used inside a vehicle and is subjected to high ambient temperatures.

The software in this revised design adds some options to the way the charger functions. As originally presented, the 3-stage charging feature includes bulk charging, an absorption phase and float charging. Plus there is the option to periodically run equalisation to make sure that all cells in the battery or battery bank have been equally charged.


When charging with the equalisation cycle, the battery will produce hydrogen gas which is explosive. For this reason, make sure that the battery is located in a well-ventilated area during charging.

Additionally, if equalisation is used, the battery voltage will rise above 15V and this could damage any equipment connected to it. If there is any risk of damage to such equipment, it should be disconnected during equalisation.

A test point (TP>15V & <11.5V) is provided on the PCB and this point goes to +5V when the battery goes above 15V during equalisation. This output could thus be used to automatically disconnect equipment when the voltage goes above 15V.

A suitable circuit for doing this is the DC Relay Switch published in SILICON CHIP, November 2006. However, a latching relay switch would be more effective for this application since the relay only draws power when switching. A suitable latching relay circuit was published in June 2011.

Note that the TP>15V & <11.5V output also goes to +5V if the battery voltage drops below 11.5V and only returns to 0V when the battery subsequently rises above 12V. As a result, this output can also be used to disconnect equipment when the battery voltage is low, to prevent over-discharge.

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