I have 300 watts of solar panels , 12 volt lithium battery, mppt and pwm controllers of which some are installed on my caravan.
I have the mppt controller set to control the lithium battery. Charging at 14.6 voltage and drops to 13.8 volts when charged. I have installed an on off switch to turn off the solar panels when in storage so as to stop charging the battery 100% all the time.
I have a battery monitor so I can keep an eye on the battery voltage and turn on the solar panels when the battery drops to 40% and off when it reaches 80%.
I have an idea to use a cheap pwm controller wired in parallel with the mppt controller and have it adjusted to charge at 13.2 volts and maintain about a 50% charge, that's only when the solar panels are disconnected form the mppt controller. When the switch is in the on position both controllers are connected in parallel.
Is this doable??


Michael Goldberg commented 7 months 3 weeks ago

Thank you for the comment.


It sounds like your idea is to maintain a certain charge level using two different charge controllers in parallel, where the MPPT controller is set for a higher charge level, and the PWM controller is used to maintain a lower charge level. The general concept is innovative - we have certainly never seen this approach used - so there are some concerns and clarifications to address.

Voltage Conflicts: When both controllers are connected in parallel and actively trying to charge the battery, you might have conflicts. For instance, if the MPPT is trying to bulk charge the battery to reach its set point (14.6V) while the PWM is trying to float or maintain at 13.2V, the controllers will fight each other. This can lead to inefficiencies, potential damage to the controllers, or an unstable charging regime for the battery.

Different Charging Philosophies: MPPT controllers typically adjust their charging behavior based on the battery's state of charge and other parameters. In contrast, a simple PWM controller might just act like an on/off switch based on a certain voltage threshold. This difference in behavior could lead to unpredictable charging patterns when both are active.

Safety Concerns: Connecting two controllers to the same battery system could lead to unexpected voltage or current patterns that might not be safe for your battery, especially a lithium one. Lithium batteries are sensitive to overcharging, and even if you're careful with your settings, having two controllers might increase the risk.

Efficiency Concerns: PWM controllers are generally less efficient than MPPT controllers, especially in varying lighting conditions. If the goal is to maintain the battery at a certain charge level, using a less efficient method might not be the best approach.

Battery Health: Keeping a lithium battery at around 50% charge when in storage is good for longevity. But constantly toggling between 40% to 80% (with the MPPT controller) or maintaining around 50% (with the PWM) could lead to more cycles than necessary. Ideally, once you set the storage charge level, you'd want minimal cycling.

If your primary goal is to maintain the battery at around 50% when in storage, consider this approach:

Use a Single Controller: Stick with the MPPT controller, as it's more efficient and typically offers better charge management features. Adjust its settings for storage mode to keep the battery around the 50% level. Many modern MPPT controllers have custom settings or profiles that can be used for such purposes.

Switching Mechanism: Continue to use your on/off switch as a way to control charging. When in storage mode, turn off the solar panels. Monitor the battery voltage, and when it reaches a certain low threshold (indicating it's dropped below 50%), turn on the solar panels until it's charged back up slightly above 50%. Then, disconnect again.

Regular Check: Periodically check the battery state of charge or voltage during storage to ensure it's maintained at the desired level. Use a battery with a BMS that can report out the battery state of health and send warning messages, etc...

This simplified approach reduces the potential conflicts and inefficiencies of using two controllers while achieving the same goal. Always ensure that you're using equipment rated for your system's voltage and current levels, and only use qualified electricians when making modifications to electrical systems.

Colin Doughty commented 7 months 3 weeks ago

I received the below email but I can't see any response. Am I looking in the wrong place?

A new response has been posted to your question titled "Storage charging of lithium battery".

Hello. Thank you for the follow up. I just approved both comments you should be able to see them now. Thanks again.

Colin Doughty commented 7 months 2 weeks ago

Michael Goldberg,
Thank you for your reply.
I have decided to operate as described, that is, switching off and on the solar panels to maintain appropriate 50% charge.
Wouldn't it be good if the mppt controllers had a storage setting activated with a switch.

Hello. I have been working with our engineering team on these answers. In speaking with them they need to advise you to follow the manufacturer recommendations. Please contact the manufacturer for any for any recommendations.


Thanks again.

7 months 3 weeks ago
Asked by
Colin Doughty