I have a couple of old lead acid batteries, both have voltage around 10V which means one cell is dead. I want to use them for some other purposes like lighting up some LEDs before selling them for scrap. I can use a boost (step up) converter to get 12V out of them. But how do I charge them? Should I use my smart car battery charger or apply 11.5V (2.3V for each cell) from my PSU?
Or is there any method to bypass that dead cell, like draining out all acid from that cell?

 

All of the cells are in series, with the connections internal to the battery.

Emptying one cell would leave you with an open circuit.

if you can figure out a way to short the cell, after discharging with a resistor, you can have a battery with a little lower voltage.

 

But how do I charge them? Should I use my smart car battery charger or apply 11.5V (2.3V for each cell) from my PSU?

I have used Harbor Freight battery maintainers to restore 4 wet lead acid batteries and 2 SLA. It took weeks to months for each battery and there were half a dozen failed attempts on SLA's and one car battery.

I haven't capacity tested any of the batteries, but a couple of the batteries that are now in cars will hold a charge for more than a month and a riding lawnmower battery lasted all season. Several of the failed SLA attempts resulted in bulging batteries.

All of the wet lead acid batteries are years outside of their warranties.

 

Hi chris,
Please post a photo of the top face of your battery.
It is possible with some battery types to gain access to the inter cell lead linking bars.

E

 

Should I use my smart car battery charger or apply 11.5V (2.3V for each cell) from my PSU?

The smart charger isn't smart enough to know there's a cell shorted, thus will try to charge the battery to about 14V, so I would try charging it with the 11.5V or so (to get a few amps charging current) from your PSU and monitor the current every few hours, to see if it will charge the remaining good cells.
The current should drop off as the cells reach full charge.

If one cell is already shorted, then that likely can be ignored if it has a low enough resistance to power the load you want to connect after its charged, as it's likely a mechanical short in the cell from flaked off plate debris.

 

In the past, when rechargeable batteries weren't so easy to come by, I turned a bad 12V battery into first a 10V then an 8V battery by chewing into the top epoxy to reach the links around the dead cells, and soldered to them. I was powering a 9V device, so I used a rectifier in series to drop the 10V to 9.3 (probably unnecessary), and it ran fine from 8V. For charging I used a bench power supply set to 2.4V x # of cells and current limited to about C/5 (C being battery capacity in ampere-hours).

 

I have used the six volt position on my 6/12 volt charger to gently make an automotive wet-cell battery start to accept a charge. Not very scientific but eventually most would start to accept a charge.
A caution about bypassing a single cell is that the bypass added will need to be capable of carrying whatever current the battery is asked to deliver. And also, make sure that the shorted cell is really dead, because if not it could run a lot of current thru the jumper and generate a lot of heat some place.

 

I think I sawed right through a link to isolate a dead cell when necessary. I squirted in some hot glue if it looked like I'd breached the seal, in case the acid gel tried to escape. Current handling wasn't a problem since I was just running an early portable CD player that ran on 9V.