Quote:
Originally Posted by Brit In Norway
Thanks for your help. The battery was new last year, but I've checked it and got the following results (seems ok to me, but I'm no autoelectrcian).
Voltage with battery connected after standing for 24 hours without the engine running and key out.
11.79 volts. May seem low but I had been checking fuses the previous day without running the engine.
With the engine running it was giving 14.1volts. After 5 minutes this decreased to 13.98 volts with the engine running. When I turned the engine off after that 5 minutes, it had increased to 12.73. (from 11.79 volts)
With the battery disconnected it was 12.67 volts.
After leaving the battery disconnected for 1 hour it was 12.42 volts (down from 12.67 volts)
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Just my take on this (I'm not necessarily right). When the battery is tested with the engine off, if it's still connected, it's not going to be a useful reading because there may still be a computer running or even just a trunk light, and this drops the battery voltage significantly.
For the measures with the battery disconnected, the battery voltage will start dropping after the car was driven, until it stabilizes to its real charge. So the actual charge would be that measured a good moment after disconnecting it. At 12.40v this means the battery is only about 60-70% charged. A new battery would have about 12.7v, an almost fully discharged one, around 12.0v (these values increase and decrease a little with the temperature).
There are a few reasons the battery is not fully charged. Possible the car was driven too gently for several short distances - usually less probable. Your alternator puts out 14.1v which normally should be higher when the battery is this discharged. For a battery at 70-80%, the alternator should put out its maximum voltage, around 14.6v or so. It is also possible some power cables or grounds are not in good condition or have some cracks: near the alternator, near the fuse box inside the engine bay (right near the ABS unit). Another possibility is the battery has internal damage and is no longer accepting a full charge, which may force the alternator to give a lower charging voltage. Ideally the alternator charge is to be measured on a new battery. Finally, what usually damages a battery is two things: poor alternator charging, or overnight current drain (this causes the battery to have many charging-discharging cycles which can damage a new battery in a few months). I would first try to check if there's a current drain: it's fairly simple, but need a voltmeter with a 10Amp socket (usually it will have three sockets). Leave the car unlocked. Disconnect the - at the battery, put the voltmeter
on the 10Amp socket and set it on the 10A direct current (straight line above), connect the voltmeter between the 'neg' battery post and between the disconnected battery cable with a tape and wait exactly 10 minutes after connecting it - at sharp 10 min all car computers will go to sleep. At that moment see what the numbers are: if they are small, switch to a smaller scale (20mA) : if you see 0.01 or 0.02 it's fine, 0.04 or 0.05 there may be a current drain.