main battery is warm - what does it mean?

[J Charlton]

I had an issue starting my '85 - no cranking from my main battery. A jump from another vehicle and the engine fired right up. It was a bit of a mystery as I had been charging the van for a day with an onboard charger. After I ran the van for a while I noticed that the main battery was quite warm - not quite to the hot stage.
Since my last usage of the van was on a Northern Ontario fishing trip - lots of km of pretty rough dirt road - and the battery is about 5 years old, I figure that I have lead precipitate short across a couple of cells in the battery causing both the lack of starting power and the heating under charge. That's my theory - I'd like to hear others' views.
Thanks
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[presslab]

If the cells shorted you will see that with a voltmeter with the engine off.

I have had two batteries short their cells. One was in a Baja Bug and it couldn't handle the abuse. The other shorted shortly after I put in a 200A alternator in my van - but it's warranty replacement has been fine.
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1986 Vanagon Westfalia EJ25
1988 Subaru GL-10 EJ20G --- 2000 Honda XR650L
2010 Titus El Guapo --- 2011 On-One 456 Ti

[tencentlife]

A battery is a chemical machine and like all machines it is not 100% efficient and the wasted energy ends up as heat. Any battery will warm when under charge or discharge, palpably if it goes on long enough or is deep enough. If your battery was deeply discharged (deeply for a starting battery, that is), then under charge it will get warmer than usual.

This doesn't rule out problems in the cells, an internal short is still a possibility, but the warmth of the battery is not an indication of a fault per se.

Under normal use you wouldn't notice the warmth because normally the battery supplies a short shot of energy to run the starter, that energy is put right back within a few minutes of running, and the rest of the time the battery is sidelined; there should be little to no current entering or leaving it as long as loads are in total less than the alternator's capacity.
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