Intermittent flat battery ... ?@&$%£**

[Cool Mart]

Tuesday morning the battery in my S430 was flat .... managed to jump start it using a Mini (!) battery I had charged in another car and went to work.
I measured the current drain across the battery with the doors closed and everything locked and it was <300mA - nothing really to concern a 100A/hour battery. The car has been fine all week until this morning when it was flat again.
Electrics problems I can cope with (locate and fix) but intermittent electric problems drive me mad !
Anyone experienced this kind of fault, anyone have any ideas where to start looking ?

Thanks

Mart

 

[pammy]

How old is the battery? Is it due for replacement?

 

[DieselE]

How far is your journey to work?

Don't forget that when you jump start a car, the battery is still flat. The only difference is that the engine is now running and, hopefully, the alternator is charging the battery. If your current motoring is a series of short journeys then the fact that your battery has a 100Ah capacity is irrelevant as it will never be fully charged.

 

[Cool Mart]

The battery age is unknown but it's probably due for a replacement.

Journey to work is 8 miles but takes 35 mins. On the day it first happened the charge it got on the way to work was enough for me to start the car 5-6 times throughout the day just to check it wasn't flat. In the past if I've done a weeks worth of short journeys with all heating systems at full blast (seat, mirrors, back screen) then the battery has been a bit slow by Friday, but a good run has sorted that.

This problem is a sudden one, to discharge a battery of this size in 12 hours I'd guess there'd have to be a drain of over 5A somewhere - and that's worrying - something could be getting hot !

I'm tempted to try a new battery just because of its age, but I'm not convinced that's the problem.

 

[BTB 500]

To drain a fully charged 100 Ah battery in 12 hours would require over 8 amps ... that's over 100 watts - like leaving the headlights on.

Starting the car 5-6 times throughout the day would flatten the battery very quickly, it would probably need 10-20 mins of driving to fully recharge after each start!

I think it's a simple combination of the age of the battery and your usage pattern. I've had several cars in the past that had 'centre zero' ammeters on the instrument panel, rather than just the voltage reading you get nowadays. I still routinely reduce the electrical load if the car is idling in traffic, because I know the battery is being discharged otherwise!

Also remember that regularly allowing the battery to run down throughout the week and giving it a 'good run' on Friday to top it up will result in battery damage. Car batteries are designed to be fully recharged immediately after use, the plates start to suffer damage below 12.4V (which represents 75% charge - fully charged is 12.7V).

 

[DieselE]

If you have access to a trickle charger and that battery you used to start the car is available to you for more than 24hrs I'd suggest trying the following before changing the battery.

Connect the additional battery up to your car's leads using jump leads.

Disconnect the car's battery (The spare battery maintains supply to the car whilst the original battery is disconnected. This avoids the risk of having to reset various parameters).

Remove the car battery and trickle charge for 24 hours (or until the charge current drops to virtually zero if this takes longer).

Put battery back in car and re-connect, finally disconnecting the additional battery.

This procedure stands a good chance of saving the battery. Mind you, everything BTB500 says about the possible cause of the discharge is spot on. Like him, I think the decision years ago to discontinue fitting ammeters to cars was wrong.

 

[Cool Mart]

Bought and fitted a new battery yesterday, and just out of interest I put the old one on trickle charge .... it's been on charge for 24 hours now and won't go above 50% charged .... guess it was definately a battery problem then !

Thanks for all the advice.

Martin