USB output pulsing on 20A PWM Charge controller and the cause.


New Member
Jan 26, 2022
My plan was to use a cheap(I won it) PWM charge controller with a small PV panel charging a 33aH AGM battery in my greenhouse to power USB connector LED strips of grow lights. During testing, everything appeared to work ok, but then the two light strips started pulsing with a 75-85% duty cycle. I note that the LCD shows the battery voltage to oscillate between 12.5 and 12.7 volts in sequence with the light pulsing. The documentation for these PWM chargers is very poor(You probably have seen them with the blue backround faceplate and four graphics left of the LCD screen and three buttons:Menu, up, down/load control), so I do not think there is a diagnostics manual out there. Is this normal for the PWM outputs or is it caused by an overload? They spec 3 amps for the USB, but do not clarify if that is each port or total. Each three meter light strip is spec'd to draw 1A, so I thought I was safe either way. I unplugged one of the light sets and the pulsing stopped, although the LCD still showed the battery voltage oscillating between 12.5 and 12.7 volts. I swapped light sets and no pulsing. I then replaced the charge controller as the source for the USB LED light strips with a DBPower portable power supply. I plugged one of the light strips into the 5V1A USB socket and verified no pulsing, I plugged the second strip into the 5V2A USB port and verified no pulsing. Apparently the cheap PWM solar charge controllers that are so prevalent get overloaded with a 2 Amp total load at the USB connectors. Cell phones may be fine, but I would think twice about trying to charge a tablet on the USB port of a cheap PWM solar charge controller.


Staff member
Sep 21, 2019
Belmont, NC
Absolutely correct.

Any charge controller with a 3A usb rating, will NEVER have close to that.

I think the load from the usb helps balance the caps in the pwm circuit. They are always crap, and ya usually can’t turn it off, so it is always a drain on the battery.

It would be better to use a better controller, and a dedicated usb converter.
Or, heck… use battery voltage lights.