What's likely to be wrong with this car?

Sounds like knackered alternator (and now, damaged battery, to boot).

I ran a crappy VW Polo for a few miserable weeks during the winter, many ye ars ago. The alternator was broken and the symptoms were the same as you de scribe. The battery needed daily charging and I was fearful of putting on t he lights, rear screen heater or any other non-essential electrics, since t hese would increase the rate of depletion of the battery (beyond necessary consumption by ignition electrics) and consequently, my range, while drivin g.

Terry.

No, but its succint and to the point.

Miss Prude.

Yes - both originally US Military slang

Disconnect battery and charge with battery charger. Next morning/day, (ie when the battery has been left for a while not connected to anything) reconnect and try to start car. If car starts, battery is OK and something in the car is draining the battery or the alternator is not charging the battery.

The object of the exercise being to see if a known to be fully charged battery will hold a charge when it's not connected to anything that could discharge it.

Allowing car batteries to go completely flat, e.g., in a car which is not often used, destroys them. Until I got a trickle charger, I'd be lucky to get 8 months out of a car battery.

Assuming it isn't the battery (and the battery can go bad from just a very few deep discharges, os it probably is):

Get a multimeter and measure the voltage between terminals, around 13ish on the battery ("battery is full"), 14-something when running and charging ("battery charges").

A multimeter also can be used to measure the voltage drop *along* a cable: if there is a significant voltage difference (tens to hundreds of millivolts) along a connection with the car running, then that connection is bad... So, with the engine running, the voltage between (say) the negative battery terminal and the chassis should be a few hundred millivolts at the very most. More, and this is a sign that the connection is bad.

A bad connection is a Good Thing, because it can be easily fixed... usually corrosion on the ends/terminals.

Thomas Prufer

Yes I agree. A high discharge rate when parked needs eliminating but wouldn't explain all the symptoms. Low output alternator with rapid deterioration of the new battery is the likeliest explanation. Cheap modern batteries often don't survive even one deep discharge event. Once the alternator is fixed I recommend a new battery, even if the old one seems to work.

Well apart from that what about the contacts to the battery and elsewhere?

Also could there be a rather high current leak across it. I remember back in them days of yore when cars were simple, a friends cortina was doing this, then one night in the street it caught firere due to a short in the loom. Brian

No if it were the starter the clock would not reset.

Really needs to check the drain when nothing should be going on. Care needs to be taken here as taking the battery off line for too long can bugger up the radio coding.

Brian

Batteries are very expensive items. And are easy enough to test with the correct equipment. Not something I'd fit a new one of on a whim.

Any auto electrician should be able to test the battery, charging system and check for high quiescent load in a matter of minutes.

Why let it go flat? If the car isn't much used, fit a battery disconnect device. A lead acid battery has very slow self discharge. Of course you may them have to enter the radio etc code each time you use it. Same as if you let the battery go flat. ;-)

You'd simply lash out up to about £100 or so without testing the old one?

That does suggest that the battery isn't charging after it has started the engine and during the subsequent running - either because the alternator/charging circuitry has failed or because something is drawing a large current so the alternator cannot supply sufficient current both to supply that load and at the same time replenish the battery charge that has been taken out during starting.

I'd suggest:

- If you can manage without the car for a day, charge the battery using a battery charger and then leave it for 24 hours without being connected to anything: this will eliminate self-discharge.

- If you have an ammeter that can measure large currents, connect it in series with the battery in the car with a) the ignition and everything else switched off, b) the ignition on but engine not running. DO NOT LEAVE IT IN PLACE WHEN YOU START THE ENGINE because the starter motor current will probably blow the ammeter or fry its leads. If there is a significant spark when you reconnect the charged battery (without the ammeter) then that suggests that a large current is being drawn, so maybe it's not wise to use the ammeter in that case.

- Measure the charging voltage (voltmeter across the battery) when the engine is idling and when it's revving at 2000 rpm. Certainly in the latter case you should see around 14V - higher than the voltage without the engine running.

- Measure the battery voltage with the ignition on but the engine not running - does this cause the battery voltage to fall below the no-load voltage of about 12V?

- Get the battery tested at a garage or at Halford's.

You mean one who sells new batteries?

If the current drawn by the starter motor causes the terminal voltage to drop too far (eg due to high internal resistance of battery or its contacts) then it will drop below the threshold that the clock needs to make it work. That will reset the clock.

If I knew it had been too flat to start the car a few times, yes. Life's too short.

No. I mean an auto electrician. Might be hard to find one, of course.

I bought myself an ACT battery tester ages ago. Not cheap, but does test a battery quickly and easily. I'd hope any place claiming to test batteries would have something similar.

If you have a charging fault, the new battery will be exactly the same.

But now comparatively cheap