12v lead acid sealed battery drops to 6v and recovers fully, why?

Thread Starter


Joined Sep 15, 2018
Hello, I am using a 7AH 12v lead acid sealed to run a USB/ 5v fan (normally runs on 2X 18650 batteries or USB), through a car USB adapter. Fully charged battery reads 13v. After running the fan 2.5 hours I notice that the fan has slowed down. So I disconnect and check the battery. It reads around 6.5v. Disconnected battery. After leaving it overnight without charging, it reads 12.6. I connect the fan again. Run it for 4 hours. Battery reads 6v. After just 30 minutes, it reads 9.8. I am waiting to see what the final recovery number will be. Is this behavior normal or is something wrong with battery? It is an old used battery.

If I were using a battery low voltage controller that shuts the system down at say 10v, the fan would have shut down long time ago. Or will it go off and on, off and on as it drops to 10v and then recovers etc. Should I use such a controller? Thanks.


Joined Nov 13, 2015
You have a number of worn out cells. Nothing shorted or open, but just very low in capacity. A weak cell sill recover to nominal voltage over time after a load has been removed. Time for a new battery.