How long will battery last?

Post Fri Feb 22, 2013 8:44 am

Hi guys,

The altenator in my E30 is bust and I need to get the car home to my usual BMW mechanic who is a 3 hour drive away. If I had a garage charge the battery for me overnight, how long would the fully charged battery last on this journey? Bearing in mind I'll be driving it during the day and will NOT have lights, heaters, radio, wipers on etc..

Cheers..

Post Fri Feb 22, 2013 8:55 am

It needs a service too so might aswell get mechanic do the lot.. plus I like to keep the book stamped up

Post Fri Feb 22, 2013 8:58 am

There's no way it'll last 3 hours. Car batteries simply don't work like that. Once it's made the starter spin up, 90% of a battery's job is done, and it's up to the alternator to take over. If your battery is in tip-top condition, it might last 20 minutes before it's utterly dead.

If you drive it until the battery runs flat, the battery is now toast. You can recharge it once, and get it to maybe 75% of what it was, but run it flat and it'll never hold a charge again. At this point, you can charge it up as much as you like, but within 30 minutes UNUSED it'll be flat again. Now you've not only got to pay to replace the alternator, but the battery as well.

http://www.e30zone.net/e30zonewiki/index.php/Charging

Post Fri Feb 22, 2013 9:26 am

Thanks for the reply.

What would be draining the battery if I wasnt using any electricals in the car?

Post Fri Feb 22, 2013 9:29 am

Spark plugs, ECU, brake lights, indicators

Post Fri Feb 22, 2013 9:30 am

I disagree.

the drain on a battery is only the ECU and the coil assuming you turn off all electrical things such as lights, radio, fan, demister etc i reckon a fully charged battery will last a few hours and it wiull onyl be toast if you leave it discharged, if you charge it immediately once you arrive at your destination it will not suffer too much, normally batteries cannot hold charge because of suphation of the plates caused by long periods of time uncharged.

but anyway, go to ant local back street garage and have them fit a new alternator, or even better do it yourself.

and how do you knwo the alternator is not working? have you considered that poor wiring may be at fault?

Post Fri Feb 22, 2013 9:34 am

I seem to have kicked off a bit of a debate here

Is there any quick test I can do myself (with limited mechanics tools at my disposal) which could quickly tell me if it was either the altenator, the battery or in fact faulty wiring?

Post Fri Feb 22, 2013 9:36 am

If you deep discharge a lead acid battery it CAN buckle the plates, which has a serious impact on its ability to hold charge.

Fitting a new alternator only involves undoing 2 bolts and the electrical connection on the back. If you've got the right size spanners it's a 5 minute job.

Post Fri Feb 22, 2013 9:40 am

To test the alternator you need a volt meter. When the alternator is charging it should read between 13.8v to 14.4v. Lower than this and it does not charge the battery. Any higher and it fries the battery.
To test the battery you need a serious ammeter. Best get that tested at the local tyre/exhaust/battery centre unless you know what you're doing.
As for testing the wiring - where to start

Post Fri Feb 22, 2013 9:50 am

What mrLee has written is correct. The battery will last a few hours, and won't be harmed. The car will cease to work a long time before the battery is discharged enough to damage it.
I'd be happier making this journey if I had a spare battery on board as well, though!
The usual cause of E30 alternator failure is a worn out carbon brush on the regulator pack. If you remove the air filter box, you will see the regulator pack, looking like a TO3 transistor package, screwed to the rear face of the alternator by two small cross head screws. Remove these, withdraw the regulator, and check the brushes. If this is the problem, one will be very short and show signs of burning on the business end.
Gently pull the brush to stretch the wire braid that retains it straight, replace it in the alternator, and it'll all be good for a couple of months at least.

Post Fri Feb 22, 2013 2:28 pm

Thanks for all the info guys, much appreciated

Post Fri Feb 22, 2013 2:58 pm

Don't try driving the 3hour journey on just the charge from the battery. It won't make it, trust me ive been having constant alternator problems. Your best bet is to buy an alternator and either fit it yourself (not difficult if you have the right spanners) or just take it to a local garage and get them to do the work. You wont walk out with your trousers around your ankles as they shouldn't charge that much.

My cars currently getting a new alternator as we speak, refurbed alternator supplied and fitted £95