Possible battery or charging problem

[TDIvolvo]

Originally Posted by oragex

You don't need to lock/alarm the car, but I guess the idea is to have current through the alarm siren to check if it's draining current. But if you know the siren is fine, you may leave the car unlocked and actually may leave doors or hood open as well. The way I test for drain is I remove the key from the ignition, put the voltmeter on the 10A plug and turn it on on , undo a battery post and connect the voltmeter in serial, then from the moment it's connected I wait exactly 10 minutes without touching anything. At 10 min sharp, by the second, all car computers will shut down and any interior light should be already off - on my S60 the trunk lights don't turn off so I disconnect them in advance. After 10 min sharp the voltmeter should show 0.01 or 0.02 if there's no current drain.

As for the battery voltage, 12.5 is usually not a full charge, I would expect to see 12.7v at room temperature for 100% charge. it may mean the charger needs more time, or it may mean the battery has wear to it and no longer accepts a full charge (which in the long term puts stress on the alternator regulator pack)

That is a good table and shows nicely to those not too battery savvy how it works. They really ought to be called 12.7V batteries! I have some Bosch batteries here that I keep fully charged for booster batteries and they will sit at 12.9-13V even after being sat for 2-3 weeks!

[oragex]

One thing the table doesn't show is the battery voltage will also vary with the temperature. A battery that shows 12.7v at 'room temperature', can rise to 12.9v or more inside the trunk of a car sitting in full summer heat in a hot summer day. Same in winter time, the voltage will slightly drop with the ambient temperature. It's actually a good thing to have the battery inside the trunk and away from the direct heat from the engine, it surely extends the life span as high heat is the biggest enemy (along with frequent discharges such as from parasitic drain)

A few years ago on another car, I had a rather weak battery. Knowing a very cold day will follow, I would bring the battery inside home the night before, the car would start easier the next morning

[damageandy]

Please correct me if not accurate...

I was always told a simple battery test was (with a fully charged battery) to connect the multimeter and then start the car and see what voltage it drops to while cranking over - if it drops below 10v(i think) the battery is bad.

Also a bad battery can show good voltage when just sitting and testing with a multi meter, but cant supply the power when its under load.

[cheshired5]

Originally Posted by VroomVroomClonk

Is it fair for me to say that if the voltage drops to 11.5V again over the next couple of days, but without being connected to anything, then the battery must be faulty?

I would say so.
My other video here shows the difference in behaviour between a dead and decent battery.