Battery testing

[DKL]

If I suspect a battery is dead or dying what's the best way to get a definitive answer with a multimeter?
Damn things only 15 months old but doesn't seem to want to hold charge for more than a couple of days whereas normally it is happy for 3 weeks or more.

[350Matt]

with engine off take a voltage reading
Start engine and re-read, if voltage is increasing at least your charging circuit is working.

As for holding a charge disconnect it from the car measure voltage
Leave overnight then measure again, it should be within 0.2-0.3V depending on temperature


Matt

[Cider Andy]

Check the voltage drop during cranking. If it's healthy it shouldn't drop below about 10V. Static voltage shouldn't be below 12V.

[HiRich]

Working from memory, so if someone wants to correct the voltage figures.

A good, fully charged battery should give about 12.75V. Good but discharged should still give >12.0V. Anything less probably indicates a damaged battery.
The alternator should deliver at least 13.0V, ideally closer to 14.0V

So step 1 is to check voltage across the terminals with the ignition key out. If it's a bit on the low side, there are two things to consider:

• A dodgy earth or bad connection may have been added or occurred. Consider disconnecting the battery. The voltage should increase only marginally (+0.1V). Another possible cause here is that you have been a pillock and left some lights on or something. To locate the cause, check all switches, then try removing fuses one-by-one, looking for the voltage to jump up noticeably. (Tip; if you disconnect the battery, make sure a door is open, just in case it decides to lock you out).
• Normal practice is to give the battery a good long charging session (12 hours, if there isn't a "fully charged" light on the charger). When you remove the charger, check the voltage (battery connected or not). It should now read much closer to the 12.75V. Leave it overnight and recheck - this will show you whether the battery is genuinely failing to retain the charge.

A battery can give good 'static' readings, but still be failing. So Step 2 is to watch the voltage as you start the car, <10.0V indicates a duff battery.

While you're at it, you should check your alternator. Start the engine and recheck the voltage across the terminals. A low figure suggests that your alternator is dodgy, belt slipping, or poor connections. This seems less likely in the warmer conditions and longer days, but is worth checking.