Car - battery/charging issues - any ideas?

[SV650S_DUB]

Good morning org, I trust all are well and good.
Over that past few months my car has often failed to start as the battery appeared to be entirely drained. First I'd put this down to the boot not being shut and as result the boot light draining the battery.
After being very conscientious in ensuring the boot was properly closed the problem seemed to go for a while. But it returned and I then replaced the battery, the the old one, 5 years old, wouldn't hold a strong charge
so I figured that was the root problem.

But now after say 4 or 5 starts to complete short trips of around 3 to 4 miles the car was dead this morning and simply wouldn't start.
Is it likely the alternator/charging system isn't working properly?
What would be the most likely culprit for draining the battery?
I should add the interior lights are set not to come on. Also if it helps any the car is a Merc A150.

All help much appreciated. A car that fails to start can drive a poor person mental!
Thanks.

[Alpinestarhero]

Have you replaced the battery? This would be the first thing to do. If you HAVE replaced the battery, get it tested first - sometimes you just get a duff one even though its brand new.

[davepreston]

its the reg rec lol
do as ash say's then have a test of the alternator cos that is the most likely problem, time to get the volt meter out hth

[keith_d]

These voltages are from memory, hopefully someone will correct me if I'm wildly out.

1) Check the acid levels then charge the battery on a charger. Disconnect the battery, measure the voltage. If it's less than 13 volts you've probably got a shorted cell.

2) Leave it twenty minutes, measure the voltage again. It should be steady be around 12.7 volts. If not you've probably got a duff cell.

3) Put it back in the car. Make sure everything is switched off. Connect the -ve clamp then use a 10A ammeter to measure the current between battery and +ve connector. More than 100mA (0.1 amp) and you've either got an electrical fault or a rectifier problem.

4) Connect the +ve clamp. Switch on the headlights, measure the battery voltage. It should drop to around 12V then stay steady. Switch off the headlights, then cover the terminals in Vaseline otherwise it will get forgotten.

5) Start the engine. Measure the voltage across the battery. It should be at least 13.5 volts. If it isn't you've got a charging problem. Suspects include alternator, regulator and fan belt.

6) Turn the headlights on and get someone to bring the revs up to around 3500. Check the voltage again. It should still be above 13.5 volts, if not I'd suspect shorted turns in the alternator.

That's my preferred set of tests, but I'm sure you'll get better recommendations from the serious mechanics on the forum.

Keith.