Measure a flooded lead acid battery voltage while it's charging?

Thread Starter

tracecom

Joined Apr 16, 2010
3,940
Logic tells me that it's impossible, but my logic has been wrong on occasion. I think that there's no way to measure a battery's voltage without pausing (however briefly) the charging voltage. Right or wrong? Thanks.
 

crutschow

Joined Mar 14, 2008
23,293
Even pausing the charging will not generally give a good indication of the battery state of charge, as it has to sit a few hours until the voltage settles sufficiently for an accurate reading.

The best you can likely do is roughly infer the state of charge from its charging voltage as shown here.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,940
By flooded, I mean that it's filled with liquid as opposed to gel.

So how does a "smart" charger know when to stop? Maybe measure the rate of battery voltage increase during brief pauses in charging?
 

#12

Joined Nov 30, 2010
18,076
I measure mine by the acceptance of current. When the charger I built can't push more than 3 amps into a car battery, the battery starts bubbling.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
So how does a "smart" charger know when to stop? Maybe measure the rate of battery voltage increase during brief pauses in charging?
I built mine many moons ago, it has to see a battery connected to output and charge current is dependent on battery state/level.
Max.
 

crutschow

Joined Mar 14, 2008
23,293
So how does a "smart" charger know when to stop?
Many will charge at a constant current (current limit) until a certain voltage is reached (typically about 14.4V for a 12V lead-acid battery) and then drop to a constant voltage of typically 13.6V to top off and maintain the charge.
Those voltage levels may change some depending upon the exact battery chemistry, which varies slightly whether it's flooded-cell or sealed or AGM.
Here's a circuit based on an LM317 regulator that does such a charge procedure.
 
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