The battery’s a bit weak
QAfter a day’s travelling my leisure batteries are at 14.8v, but the voltage drops immediately I turn the engine off, even though I’m not using any power apart from the inventor. The individual batteries are 12.6v with one at 9.5v. I have noticed a ‘rotten eggs’ smell since plugging in the battery charger, and two batteries seem to be bubbling. Do I need to change all four leisure batteries?
ATONY REPLIES: The smell, 9.5v reading, bubbling, and any tarnished bright metal, higher than normal charging currents or a charger that refuses to step down to the float voltage all indicate internal shorting cells, so one or more batteries need replacing. They will be the ones with cells bubbling far more than the others.
What’s happening is that the charging current through a battery is proportional to the difference between the battery voltage and the charging voltage, so the battery showing 9.5v draws a far greater current than the others. This reduces the current available to the other batteries and probably reduces the charging voltage, which ‘fools’ the charger into thinking the batteries are not fully charged so it never steps down to its reduced float voltage.
Then when charging stops, the other batteries will try to charge the 9.5 volt battery so they discharge faster than normal – hence the drop to 12.6, which would continue dropping faster than normal until the bank was flat.
The deeper you discharge your batteries, the more life you use up. So your battery problem is making things worse for the other batteries, because you never fully charge the bank and they are discharging themselves to an unacceptable degree.
Best commercial practice is to say yes, replace all four; but if you understand what is going on and the potential for further problems, and are happy to test your batteries regularly, you can just replace batteries that are bubbling or show a low voltage after being disconnected from the bank overnight.
Once you replace the shorting batteries things should go back to normal. But we can be fairly sure the remaining two old batteries will fail sooner than the new ones. When one fails, it will start to discharge the new batteries leading to the same problem, so it should be removed immediately.