SLA Battery Charging?

Quick question about charging SLA Batteries, I haven't really dealt with battery charging too much in the past.  I have two 12v 5Ah SLA batteries, in series, and I have built an under/over voltage protection circuit for each of them, but while charging I noticed that testing voltages across each battery, they are different, however, as soon as I remove the charger and test them as is, they have the exact same voltage (+/-0.05 volts, they are very close).  Is this just because the charger is hooked up, or should I be worried at all about that?

I am charging them to 14.75 volts each, or would like to, they say they should be charged to 14.5 to 15v right on the front of them.  While charging they both climb, but one climbs faster and stops once the voltmeter reads 15.2v across it.  0.2v over the rated value really doesn't seem to be of major concern, I just find it odd the different voltmeter readings across the batteries are different while the charger is connected and identical when its not.  Does it have something to do with charging them in series?  I did read that you really can't measure batteries while charging, you have to disconnect the charger first, but any insight would be appreciated.

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rickharris2 months ago

No 2 batteries will be identical even with the same chemistry, age, chemical changes etc will make slight differences.

Check voltage off charge or your measuring the charge voltage. You could measure the charge current which will fall to a very low point when fully charged.

If your charging them in series then you a) have a 24 volt battery and your charge voltage isn't high enough b) shouldn't because the slight cell difference will allow one to potentially over charge.


lot of info here

Wesley666 (author)  rickharris2 months ago

Oh the voltage is high enough to charge them, 29.5 volts. Some good reading there, great answers from everybody, so I did a little tinkering and built something like those little over voltage protector straps that go across large lithium cells like these;


So my under/over voltage circuit now shuts off all power from the batteries, minus the protection circuit if they drop too low, as it did before, but for over voltage on either cell it turns on or off a shunt resistor in the neighborhood of 330 ohms across either battery, and with a fabricobbled together board it seems to be working great, if either battery goes over 14.75v while charging, it turns on, and it drops to about 14.25v, the voltage rises on the other as it charges until it goes over that threshold and it turns on. Seems to be keeping them under 14.75 volts while they charge, which is right in the middle of the 14.5v to 15v range that it states on the front of them they should be charged at. Looking at doing a little more tweaking, possibly different resistor values, the LiFePO4 ones say 250mA through them when they are balancing which is more then what is going through mine. Once I am happy I will order the right resistors off Digikey and solder them to a proper board I have being made up.

Makes sense, Sorry I miss read you question as if your charging the 2 batteries at 14.75. :-)

Thanks for the best answer.

seandogue2 months ago

As a general rule, I would suggest that you charge the two cells independently to optimize their maintenance. Based on your observation, one is showing higher internal resistance.

A loaded battery shows evidence of its characteristic impedance (and state of the electrolyte/electrodes) via the temrinal voltage while discharging (or charging) a known current (of macroscopic value, not micro/millamp uA/mA) . A loaded battery that shows droop generally indicates some level of sulfation on the electrodes (or depleted electrolyte)

An unloaded battery, tested with a DMM, does not have to push any substantive current, (ie, do serious work) since the meter has a very high input impedance of many megaohms and draws teensy amounts of current, (Only nA or uA of current are required for the measurement) so it doesn't readily display the deleterious effects of sulfate crystal deposition on the electrodes. (electrode sulfation can be thought of like a fine deposit of insulation...thicker it gets over time and abuse (alwyas keep SLA fully charged when not in use) , the more it resists flow out of (or into) a battery. The meter just doesn't load the battery enough to see how it acts when being called on to do its job. (remembering of course that we're not talking about a coin cell powering a wrist watch, but something that can deliver current large enough to turn a motor or heat water or power a telescope dew heater, etc., ie "macroscopic")

Wesley666 (author)  seandogue2 months ago

well I would charge them separately, but I don't fancy taking the project they are going to be in apart regularly to do so. Laziness takes this round! That and I figure they are already 6 cell batteries, stick 2 in series and its 12 cells now.

2nd paragraph correction: A loaded battery that shows more droop when tested than a companion used in the same battery pack (like your two-battery pack) generally indicates some level of sulfation on the electrodes...

Downunder35m2 months ago

I would not go that high in voltage with SLA batteries.
As the name suggests they are SEALED, so you want to prevent gassing when charging.
13.8V would be the upper maximum, with a proper charger that regulates the amps (trickle charger) you can go as high as 14.2V but 15 IMHO means you produce hydrogen....

2 x 12 volt SLA in series!

At room temperature, 14.5VDC is not unreasonable in fast charge mode for a series of 6 lead acid cells (ie a "12VDC" SLA). It would not be suitable for slower modes, and definitely not for trickle mode, but as long as it's only used during the fast charge region it's fine.

I would suggest that the OP check the manufacturers tech guides online to determine the correct range of allowable charging voltages for their device just to be sure. As I recall, Hawker batteries allowed up to ~2.5VDC per cell in fast charge mode (just shy of that, actually, but roughly speaking), and as I recall, that's the guideline we used for the 14-cell (28VDC nom) batteries we used to provide power for a myriad of experiments at NASA ground based facilities during the decade I was responsible and afaik, for decades prior to my arrival.

Even when we moved to automatic charging stations, the voltage when in fast charge mode was ~2.4VDC/cell

Note though, that this max per-cell voltage is temp dependent. The maximum per-cell voltage in fast charge mode decreases with increasing temperature (and visa versa). For instance, during the summer in some parts of Australia, where outdoor temps are in the +30-40C range, one would not want to exceed ~2.3 VDC per cell. If it were instead -20C, you could use a value near to 2.7VDC per cell.

Granted, the difference vs temp is small, but there are tables to show the relationship available from various quality battery sources like Hawker, etc. if one wants to be or needs to be thorough.

In any case, if hydrogen production is of concern, one *should already be practicing good ventilation wherever sizable batteries are being changed for safety reasons.

OP says there are 2 12 volt batteries in series.