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SG and voltage mismatch


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Tonight I saw something odd. Background: I have a 24V bank of 4 Trojan T125 batteries (that's the larger brother of the T105) that's very likely nearing the end of it's life, so there's quite a bit of variation in voltage. Interestingly the SG across cells is just about exactly the same (except for one cell that's clearly on the way out).

 

So tonight I took the bank down a bit low (by accident really) and it's sitting at 23V right now (load is off, except for quiescent currents). What is surprising is that the battery I considered to be the best in the batch sits at 4.5V at the moment, while the others (including the one with the bad cell) all sit above 6V, between 6.1V and 6.2V. But even more surprising, that battery that measures 4.5V.. it has a nice consistent SG of 1250 across all three cells.

 

So now, according to the Trojan spec sheet, at 1250 I'm barely at 20% DoD... but at 4.5V it's at >90% DoD. So I know one should not pay too much attention to voltages... but usually it is the other way round: High terminal voltages accompanied by a low SG. Here it is the other way round: a battery with consistent 1250 SGs only measures 4.5V.

 

So I'm just curious how such a thing happens.

 

The bad cell... well that one is at 1125... barely lifts the float in the hydrometer :-)

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Thanks Carl, I'll keep that in mind. These are 4-year old T125s that started life in a golf cart, so it's old and it's had a good life. I doubt it is worth the effort of schlepping it into the city (I'm in Somerset West). Time to step up the search for a replacement.

 

(Oh for the days when telecoms batteries were being thrown out and you could pick them up for a song...)

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Ja its probably not worth it but would be interesting to find out what is going on with that battery.

 

I also have 3 year old golf cart batteries (US 125) that have been hammered. So when the budget allows will be looking to upgrade.

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