I have a battery that came out of service. It was in a UPS. The battery lost its capacity. As soon as power went out the battery was not able to supply the necessary current to keep the computer working. It would drop out in about 15 seconds. I replaced that battery with one just like it but instead of 12V @ 7Ah the SLA I put in its place is 12V @ 9Ah but it's physically the same dimensions and connections.

Here's the weird part: I put the old battery on a 13.8V source (radio shack 13.8V @ 19A power supply). Let it sit for a few days and last check with the PS connected the voltage was 13.6V. That was days ago. I connected an old computer fan to the battery and let it run. It ran over night and the next morning the voltage had dropped to 9.2V. I disconnected the battery and put it back on the PS. Two days ago I disconnected the battery from everything, just let it sit on the bench. For a few days I've been watching its voltage while resting with nothing connected whatsoever. Right now the battery voltage is 13.12. I've checked it with two other meters, and with a few hundredths of a volt have essentially read the same thing. So this battery isn't worth much other than powering small electronics for as yet some undetermined period.

The crux of my question is "Why does a 12V SLA (used) sit at or above 13V?" From all I've ever known about lead acid batteries is that they will rest (new) after charging and then leaving them alone for 12 hours the voltage drops to 12.6V which is normal for a fresh 12V battery. This battery is obviously sulfated on its plates which is why it can't provide any serious current. I'm just curious as heck as to why that voltage is present after two days of rest.

What'cha think?

 

I've seen something similar before.
Sometimes there is a shorted cell in the battery. Gel-cells can 'super-charge' the remaining bad (high resistance due to sulfated plates) but not shorted cells to the charging voltage but the extra voltage is mainly surface charge on the remaining 'good' cells that won't hold the stated voltage very long under load. The battery voltage will drop to the 9->10V range rather quickly under load.

 

I've seen something similar before.
Sometimes there is a shorted cell in the battery. Gel-cells can 'super-charge' the remaining bad (high resistance due to sulfated plates) but not shorted cells to the charging voltage but the extra voltage is mainly surface charge on the remaining 'good' cells that won't hold the stated voltage very long under load. The battery voltage will drop to the 9->10V range rather quickly under load.

A darn good explanation. Thanks.

First load I tried was an automotive LED marker light. After two days continuous running the BV was still in the 13V range. That's why I switched to a motor as a load. This case a couple of computer fans (one at a time). Think I'll grab a 12V secondary output transformer and use that as a load. I'd imagine it should drop quite quickly.

 

DURING choke test the voltage dropped to 1.13V. Upon disconnecting the choke there was a significant back EMF. I still think the battery was excessively sulfated. But the short inside doesn't readily suggest a plausibility in my mind. Each cell on its own should be 2.1V. Dividing five cells (excluding the shorted one) at 13.12 = 2.62V per cell when they should nominally be 2.1V.

While your answer seems like a good one, I'm not believing that the good cells can deliver 2.6V each. At present, after the choke short test the BV is at 12.86.

 

Think I'll grab a 12V secondary output transformer and use that as a load.

You do realize that a transformer acts as a short for DC (current limited only by the winding resistance)?

 

DURING choke test the voltage dropped to 1.13V. Upon disconnecting the choke there was a significant back EMF. I still think the battery was excessively sulfated. But the short inside doesn't readily suggest a plausibility in my mind. Each cell on its own should be 2.1V. Dividing five cells (excluding the shorted one) at 13.12 = 2.62V when they should nominally be 2.1V.

While your answer seems like a good one, I'm not believing that the good cells can deliver 2.6V each.

They are not still good cells, they have been hurt to deliver the higher voltage...

Sure, they can, happens all the time with car batteries with 5 good and one shorted cell. It can fool you if you don't understand what's happening as the acid is concentrated (causing more plate damage) due to overheating. Cell 'overcharging'' is very inefficient (water lost (and acid concentration increased)) past the normal REDOX voltage and likely to create a lot of heat but on a sulfated gel/sealed/AGM-cell there is some current/heating limiting due to the high plate resistance

 

It appears to have gone high impedance. If it is sulphated or an interconnect has failed then presumably the current path for self-discharge has also become high impedance. So once charged it will remain at a fully-charged voltage, because it can't be discharged.

 

"No User Serviceable Parts Inside"
If it doesn't perform as expected, recycle it.
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"No User Serviceable Parts Inside"
If it doesn't perform as expected, recycle it.

Lead-acid batteries are 99% recyclable (rather better than lithium!)