Metering Lead Acid Battery Voltage

Thread Starter

CarlPace

Joined May 3, 2022
17
Hello World;

I recently got a hold of a small Lead Acid battery that I extracted from one of those portable jump-start kits for cars. 12V/10Ah.

I've noticed that whenever I meter the voltage, the reading starts relatively low (8-10.5V depending on charge level), but then slowly ticks upward over the course of a minute or two, sometimes as much as 2 or 3 volts higher than the initial reading.

Can anyone explain what's happening there? Furthermore: Which reading should I accept? High, Low, or Average of both?

Thanks.
 

peterdeco

Joined Oct 8, 2019
481
Did you put the battery under load just before measuring it? The output voltage of a battery that needs charging will drop under load and under no load the voltage will creep up.
 

Dodgydave

Joined Jun 22, 2012
10,310
It should read between 11.5 to 13.5V under load, it sounds like it's gone flat and requires charging.
Otherwise it's useless.
 

MrSalts

Joined Apr 2, 2020
1,762
Hello World;

I recently got a hold of a small Lead Acid battery that I extracted from one of those portable jump-start kits for cars. 12V/10Ah.

I've noticed that whenever I meter the voltage, the reading starts relatively low (8-10.5V depending on charge level), but then slowly ticks upward over the course of a minute or two, sometimes as much as 2 or 3 volts higher than the initial reading.

Can anyone explain what's happening there? Furthermore: Which reading should I accept? High, Low, or Average of both?

Thanks.
A lead acid battery is a half-flow cell. Shaking or even just moving it can move lead sulfate and expose elemental lead. Or, the delicate concentration gradient of acid can be swirled when you move it. Each cause will cause a short-lived rise in voltage.
 

Thread Starter

CarlPace

Joined May 3, 2022
17
Ok. When I load a small USB charger module, then meter in parallel, the USB shuts off and the voltage reads 0.

I’ll try recharging. It HAS charged a phone and portable battery in the past, so it still has a little something left in it, but maybe just terrible capacity.

Is this a case for the ol’ distilled water refill trick?
 

dl324

Joined Mar 30, 2015
14,467
I’ll try recharging. It HAS charged a phone and portable battery in the past, so it still has a little something left in it, but maybe just terrible capacity.
Lead acid batteries don't like to be fully discharged. An open-circuit voltage of less than 12V (for a 12V battery) = completely discharged battery.
 
Top