Hi All & happy new year

I got a call from a pal who finished his built a while back, but the battery just drains after a few days. The test show that the alternator is charging when the engine is running at thicker over. He has check over all the earth contacts on the chasis and they are all fine. Any idea anyone why the battery is losing charge so fast?

Cheers

To rule out a dodgy battery I would check the current draw at the battery with a multimeter. If there is a draw then do the same with each fuse one by one util you find the circuit at fault.

The most likely candidate is the alternator diodes but it could be a dead (aka gassing or shorted) cell in the battery.
Be aware most multimeters generally has a 10 amp internal fuse which if it blows will not be that obvious as the meter will read 0.0 amps.
The easy way to check for an alternator diode failure is simply to disconnect the alternator when the car is left parked for 24hrs or longer.
Note is essential the alternator is reconnected before the engine is run.
To check the battery you can try a cranking voltage test, fully charge the battery leave over night, voltage with everything off should be 12.5 To 12.8v depending on ambient temperature. When cranking the engine in cold start conditions if the voltage across the battery drops way below 9 volts there is a dead cell in the battery.

These are the most likely causes if the car passes both tests then post again and give methods of tracking down faults on individual circuits.

[Edited on 1/1/14 by britishtrident]

To rule out the battery make sure it is fully charged then disconnect it. After a few days reconnect and try to start the engine. If it's flat then the battery is duff, if it turns the engine easily then some thing is draining the power when it is left connected.

sounds like alternator diode packed up to me

[Edited on 1/1/14 by baz-R]

as a mechanic we used to check battery's by shorting the +ve and -ve terminals with a jack handle (or good jump leads, light weight jump leads get VERY hot) with the battery cell covers removed....dont do it for longer than a few seconds, look at the cells as you do it if any fizz the cell is knackered and so is the battery. if none fizz the battery is fine and will be good for use on the car.

It is worth noting that some alternators are designed to be controlled by the cars ECU and internally they can lack the regulators and diodes of older alternators, these are often activated via a relay (or electronically) so that they are electrically isolated when the engine is not running.

Thank u all, this is tremendous help.

Cheers again!

Shorting the battery is unbelievably dangerous . The bar doing the short can weld itself to the posts and then you get a faceful of acid when the battery explodes Then you're , qualify for a white stick, cheap TV licence and the audiobook version of PPC.

I had a similar story at the end of my build - the battery would lose charge over a few days. The new one ( cheapest from Halfords and now 2 years old ) starts the engine on a cold day after 2 months standing idle.

[Edited on 2/1/14 by Macbeast]

Dead shorting a good battery will result in a current of about 1000 amps, dead shorting a battery with a dud cell will release a lot of hydrogen gas either way I wouldn't care to be anywhere near it when it happened.

Battery load testers draw about 100 amps to 200 amps for the one intended for testing large batteries., the starters on most petrol car engines draw between 60 and 100 amps.

i worked for lotus and BMW and Lucas auto centres, it was common practice....so long as you dont do it for more than a few seconds (its all you need, 1 second is often enough) definitely quicker than looking for the battery tester.

Words fail me…..…..................

I hope you like Labradors

Apart from the danger of explosion, crowbarring a good battery will probably buckle the plates, thereby turning a good battery into a duff one.

When you were at Lucas, did you sell a lot of replacement batteries ?

The last battery I bouht was second hand from a breakers its how I checked it before taking it home ..been on my pinto powered car for three years and never given me any problems. The car is a daily driver 365 days of the year. It was a battry from a fist punto ....quite small....I bought it for the size to fit the spacd I had. Cost all of a tenner.bargin.

I also know of people who check batteries this way, but it is VERY dangerous.

I used to do a lot of work for a big CAT construction equipment dealer. I saw a young apprentice lad lose his eyesight from a battery blowing up in his face. He (accidently) dropped a 1/2" breaker bar onto the battery. It went with a very loud bang. I'm even wary of using drop testers on them, thats scary enough!!