Car Battery Problems

27 Jan 2011
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United Kingdom
There are so many similar items in these forums, with regards to battery problems, that, as a retired battery laboratory testing technician, I thought I should give some guidance on car batteries.

A 12 Volt battery on a car, is 12 Volts ad infinitum (Unless completely shot!). This will be 12 Volts, until such time as a load is applied (turn on the lights etc) What happens when a load is applied, will indicate whether it is fully charged or flat. If it falls to less than 10.5 volts on load, you are in trouble. If it's less than 10.5 volts OFF load, it's scrap. (About a 2% recovery rate, but at MUCH reduced AH capacity.)
A good battery, is charged at 14.7 volts, and this will fall to around 13 volts when the charger is removed.

If the battery is allowed to discharge to less than 10.5 volts, it will decay rapidly, and needs charging IMMEDIATELY. If left at 10.5 volts or less for any time, you will probably NEVER recover the battery, even if recharged.
It MAY come back to 12 volts, but half the plates will probably be shorted out by paste falling off the plates, and shorting themselves out in the bottom of the battery. SOME plates may not get shorted out, which will still give the battery 12 volts when charged, but the Ampere hour capacity of the battery will be massively reduced. 8 positive plates, and 7 negative plates per cell, may be reduced to ONE of each still working. It will still give 12 volts after recharge, but the Ah capacity of the battery, could have gone from 40Ah to 5Ah for instance. May not even start the car!

Never judge a car battery by its voltage OFF LOAD!

If off load voltage of the battery is less than 10.5 volts, then chances are, that one cell already has a short (positive to negative plate), and charging it, will be like putting air into a punctured tyre.
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An old thread, but this is quite true.
I did think that some years ago, however I now have the tools to record what my smart charger is doing, and realise I was wrong in what I thought about batteries.

The main point is it takes time to discharge, and time to recharge, and no about of voltage will charge it any faster without damage. I found with an abandoned battery which was in good order when last used, I could not recharge it with a smart charger as not enough voltage for it to see the battery as 12 volt.

Only way to charge with a smart charger was to cheat, and put it in parallel with a good battery, the first battery I tried this with was a valve regulated lead acid (VRLA) also know as absorbed glass matt (AGM) from a stair lift, the charger had been disconnected, the batteries were used until flat, and then it was an emergency so new batteries fitted, and old ones simply left until my father-in-laws death, when I found them.

Putting the first one in parallel with a much larger battery and then putting the smart charger to charge the pair, I connected the smart charger though an energy meter, so I could see progress on my PC, then once one was charged repeated with other battery, this time with the now good 6 Ah battery.

With both got the same result, nothing for around 10 days, then standard charge pattern, and battery seemed as good as new. I only wanted to batteries for my 2 way radio, so no great current draw, but they worked well.

On moving found some more abandoned batteries, and again recharged using same method, one was a failure, but two recovered.

This revised my thoughts with part discharged batteries, and I found putting batteries on charge for an extended time with a smart charger could revive many a battery, in essence they had never been fully recharged as never left on charge for long enough, 10 days is a long time, and I would have dumped a battery well before that time on charge.

Done with both flooded and AGM, time it seems is a great healer with batteries.
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