• I have 2 batteries 12V 80Ah deep cycle lead in series for a 24V system.

  • I connected an invertor directly to the battery pack.

  • I have ~600W (~44V) of solar panels connected to the PWM charge controller.

As soon as I use like in this example 1000W on the system, the charge controller just stop transmitting Amps to the battery. (And voltage drops.)

Resulting in a voltage drop like that (captors on battery pack 24v) :

enter image description here

  • Black line is battery full + voltage added from solar panels. ~28.8V
  • blue line is battery full. (At night should be around there) ~26.8V
  • red line is maybe ~23.5V

It seems to restart the charge after few hours.

Is there something wrong with my Charge controller ? With my setup ? :/
Thank you.


You can't charge the battery while drawing power from it at the same time It isn't like a bucket of water where you can dump in on one side and suck out on the other. It is more like a T-intersection on a pipe where water can only flow in the middle pipe in one direction at a time. But you can draw power from the charger to power your load and charge the battery at the same time. Your charger can't supply 1000W to power your load so it shuts down to protect itself and the battery ends up powering your load instead.

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  • \$\begingroup\$ Thank you for the answer. So it's a normal behavior ? I will change this cheap PWM with a better MPPT. Will it turn off as well and wait for the load to be below power from the roof to turn back on ? Is it supposed to be instant for a good controller ? This one do nothing for several hours... \$\endgroup\$ – bob dylan Jan 30 at 15:44
  • \$\begingroup\$ @bobdylan I don't know if it is bad or not. The charger could be kicking repeatedly in to try and charge but going into protective shutdown which could be bad but it depends on the charger's design \$\endgroup\$ – DKNguyen Jan 30 at 15:46
  • \$\begingroup\$ So...if i understand correctly, If my load is 500w but solar panel are sending only 499w it will all turn off and switch to the battery ? (Aka just wasting 499w of power ?) Isn't there a smarter solution, like I don't know, another battery bank is getting charged instead ? That seems like a huge drawback... \$\endgroup\$ – bob dylan Jan 30 at 16:07
  • \$\begingroup\$ Yeah...I have not given it much thought before now but it is. There are ways around it to let the charger and battery both share the load. Either the battery would not charge and would make up for anything the panels could not cover, or the panels would entirely power the load and surplus would charge the battery. I would expect a system made for solar to accommodate this. But you did says yours is a cheap charger. Maybe it was just never designed for solar? An MPPT adjusting voltage output according to the battery voltage should take care of this \$\endgroup\$ – DKNguyen Jan 30 at 16:40
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    \$\begingroup\$ @bobdylan A battery only charges if the voltage being applied to it is equal to or greater than its own voltage. A simple battery charger will just apply the voltage it think is required to charge the battery to its outputs. But if something else is there alongside the battery to take power, it will take power from only one or the other unless the battery voltage and the charger voltage are kept very, very close to each other. As soon as either charger or battery voltage get out of balance, the one with the higher voltage supplies everything. \$\endgroup\$ – DKNguyen Jan 30 at 17:00

may i add my answer here. you can charging + use it the same time, the chemistry in your lead battery will balancing it self. that's why automotive tends to use alternator with secondary magnet from your battery it self.

yes it could be your charger design have "superior design" which cut off whenever certain point voltage drop. but unfortunately your charger seems to tend stay open circuit rather than stay charging with unregulated output.

so if i may suggest, you should change your PWM charger if you want to stay charging whenever you are using heavy load or normal load. or you can try to tweak the charger. inside the PWM charger almost 50:50 it have voltage sensing adjustment with trimpot / potensiometer. you can try to adjust that. but beware it could change your top voltage limit.

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