Micra battery at... 1 volt!

Yes you need to disconnect all cables.

that equals a 5 watt load. glove box light? boot light?

The battery is shagged. probably O/C

That's what I thought - but no. Just put a 10A lamp load on it and it shining away fine!

Re the 5 watt load - yes - but no int lamp or boot lamp on - not the radio either. (sigh) here we go.

The usual alternator fault that causes it to draw current is a failed rectifier diode - and that is part of the alternator output so you need to disconnect the heavy cable(s). But you usually get a current draw of about

You've possibly got two faults, 0.5A will take an undercharged battery that low. What's the chrging voltage with the engine running?

Intermittent O/C then. It's shagged whatever way you look at it.

Assuming a 38AH battery a .5A load would take 76 hours to run it down.

It is, however, quite enough to sulphate it and make it look as if it's discharged. Been there, done that, night before last. Left the V70 passenger door ajar overnight, interior light stayed on. Ten hours later, not a hint of a start. Everything dead. No dashboard lights. Nothing.

Two hours on a 3 amp charger and we were hot to trot and good to go. Quick charge knocks the coating off the plates, revealing the nice fizzy electrolyte.

OP, your experience is entirely typical of a small drain on a car battery. Find what's staying on (probably an interior, glove box or boot light), fix it and you should have no more problems. The battery won't have suffered at all.

Ian

In message , The Real Doctor writes

With the caveat that car battery life is shortened by allowing it to remain in a low state of charge for too long. They last longer if kept fully charged.

Eh?

You're implying a gentle longish discharge does something to a fully charged battery?

So what is the magic figure? Most modern cars have a quiescent load - immobiliser, clock, radio etc. Usually of the order of about 20 mA. And extra 500mA shouldn't hurt an otherwise ok battery overnight. Sulphation occurs when you deeply discharge a battery. And 3 amps for a couple of hours ain't going to sort that.

Only if it's fully charged.

That's usually the case with most modern cars, used normally.

Err yes, but if it's a consistent fault you'll never notice it until your usage pattern has left the battery part charged.

In message , George writes

Temperature dependant and also assuming a full charge.

At risk of appearing a pedant, that's not strictly true; for vehicle applications the discharge current/time ratio is only accurate at the 10A discharge rate.

Agree entirely that a 0.5A load for 10 or 12 hours *shouldn't* leave a healthy battery unable to start the car, but for reasons I can't explain, it often seems to.

Chris

The capacity would be slightly greater at the lower discharge rate.

It's an interesting point. My brother and a neighbour both have BMW 3 Series Tourers of about the same age - roughly '02. And both refuse to engage the starter with a slightly low battery - ie nothing near flat. Perhaps they sense the voltage before allowing that relay to engage. In both cases a very short charge with a small charger restored things to normal. Until the next time. A new battery sorted things in both cases - but my brother put the old one into another car where it seems to work just fine.

Would he have been happy crossing a desert and relying on it?

Why would you try a cross a desert in a 3 series tourer?