Dracoro

Hi

Car - BMW 740 4.4v8
Battery - Merc 74Ah, 12V, (400A???)

Car normally starts OK. However, if you leave it 4/5 days it won't start. If you use the TV screen for 3/4 minutes whilst engine isn't running, this stops the car from being startable. Jump lead to another battery (66Ah, 12V) and it starts OK (combo of 2 batteries???). All the car lights, interior etc. all work ok, just not enough to get the car started.

Battery has 11.92 volts which is enough surely?

TV/Monitor, when everything is turned off, drains 1amp (car drains 1.85 amps altogther when turned off).

Any thoughts? Is it the battery despite being charged? I reckon it should have a 90Ah or 110Ah but don't wanna buy one if it's the car at fault.

gotmashed

the battery if ****ed just had the same thing would start first thing in the morning then 20-30 min later would die then nothing charged for 3 days did the same new battery fine

GC8

You should be seeing 13.8v not under 12v; Id say that it was f*cked.

Dracoro

Forgive my ignorance, but why 13.8? I thought, as the battery was a 12V one, that it should be close to 12?

When all turned off and locked, how many amps should a typical car be drawing? I don't wanna buy a new battery and then **** it cos of some other electrical gremlin.

davegtt

it should be 13 odd volts when the engine is running as its being charged by the alternator....

as a standing battery it should be reading about 12.5... problem is your seeing 11.92 but I dare bet when you go to start the car the battery being underload will drop to something like 6volts which wont be enough to turn the engine over...

Hope that makes sence? It sounds too me the battery in your motor isnt for your motor.... for example you have the battery out of a 1litre saxo in a 2.5litre V6... I had similar problems trying to get my Cougar to start when I bought it, found out I had the battery from a smaller car.

HTHs

Leslie

Do you have Tracker, they usually give the battery a pasting when all else is switched off.

Otherwise it sounds as though you will have to bite the bullet and buy a new battery.

Les

Dracoro

The battery is a 74Ah from a merc. I suspect it needs a 110Ah as 4.4 litres of V8 takes some turning. I get the impression that the current battery needs to be full charged. Anything less, even if just slightly, isn't enough to turn the engine. I wire another battery (newish 66Ah one) and it starts up ok.

farmer1

The capacity won't hugely affect its turning over power. The crucial bit is the max discharge rate/cold cranking which yours appears to be 400A as would a larger battery be.

Having a larger capacity more amp hours, just means that it can run devices or turn the engine over for longer.

The fact that you battery has dropped below 12V (is this with a current draw on it?) would suggest that it is fubared. Usually a new fully charged 12V battery will indicate around 13.8V

Brendan Hughes

I think the problem is that Merc and BMW are rivals, and specifically design their batteries not to work in each other's cars.

HTH

Dracoro

Got a new battery. So far so good, will wait & see. Interestingly the new battery has a cranking power of 760 amps which is almost double the 400 amps one that was in it! £70 from local motor factors (£100+ from bmw)

GC8

That should have read 12.8v; sorry (13.8v whilst charging.....).

Simon

ALi-B

iTrader: (1)

I was going to say a CCA of 400amps seemed a bit weedy for such a large engine/electrical load (IIRC the battery in my scoob has the same CCA rating).

Might be an idea to do some googling on what happens to batteries if left to discharge too much, as sulfation is the enemy - Which is one of the main causes responsible for killing most batteries, through being allowed to discharge too much and for too long. Something many people aren't really aware of (including me...well, until I had to maintain 6 bike batteries ).