This is probably a really stupid question but how do you know what battery to fit?

What affect does the Ah and A have on the performance or operation of the car?

The battery I have fitted at the moment seems to be "boiling" and is blowing liquid out of the vents and I was wondering if a change might help or the battery is just knackered. BTW its 12v 45Ah 440A.

Sounds like it is over charging --- alternator regulator problem

AH ampere hours is basically the charge capacity of the battery ie the reserve capacity you have.

A is the maxiumum rated starting current in amperes

[Edited on 12/8/07 by britishtrident]

batterys tend to boil/vent when over chargeing takes place. try checking the alternator output of your car, if its over 14.6 ish volts at 2000+rpm you have a problem.
i would also say if the vehicle starts ok the the capacity should be fine.

If the battery spins the motor over in most conditions then it's big enough for the car.

If you're boiling the battery then (a) you have a dangerous situation! and (b) your voltage regulation is faulty. Possibly there's (c) one of your cells has died, and the remaining cells are taking a beating.

Put a voltmeter on the battery terminals when disconnected from the car - you should get somewhere around 12 - 13 volts. If it's significantly less then the battery may be knackered.

Reconnect the battery, then run the engine at a fast idle - you should see around 13.8 volts across the battery. Get someone to rev the engine a bit and look again - it should stay roughly the same voltage. Much more than 14.5 volts and you need to look at the regulator circuit.

If you're unsure about the battery, take it to a decent local motor factors; many will have a tester that puts a load on the battery while checking the voltage. If they think that they may sell you a battery then they probably won't take any money for the test!

HTH
David

[Edited on 12/8/07 by David Jenkins]

quote:

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Originally posted by neilj37


What affect does the Ah and A have on the performance or operation of the car?

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quote:

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Originally posted by David Jenkins


Reconnect the battery, then run the engine at a fast idle - you should see around 13.8 volts across the battery. Get someone to rev the engine a bit and look again - it should stay roughly the same voltage. Much more than 14.5 volts and you need to look at the regulator circuit.



HTH
David

[Edited on 12/8/07 by David Jenkins]

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are you sure its not under bonnet temps getting to high ?

quote:

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Originally posted by Dangle_kt
I'd make sure I was fully protected though before going near a spitting battery... you only have one face

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quote:

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Originally posted by bigrich
batterys tend to boil/vent when over chargeing takes place. try checking the alternator output of your car, if its over 14.6 ish volts at 2000+rpm you have a problem.
i would also say if the vehicle starts ok the the capacity should be fine.

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Did you put the meter across the battery when the engine was stopped (or even when disconnected)?

That would give you a rough guide to its condition - but I still recommend a proper check.

DJ

Thanks for all the feedback guys.

I didn't disconnect the battery as I did not build the car I don't have any electrical test equipment. I just used the reading shown on the digidash.

Okay its looks like something is amiss.

Took the car out for a drive and everything appeared to be okay, battery showing 14.1v then all of a sudden its up to 18.1v for a few seconds then back to 14.1v. This continued for a while and appears to be worse when I am driving hard.

As I have no electrical knowledge what so ever is it going to be easier to find somebody local that knows what they are doing than trying to sort it myself.

[Edited on 13/8/07 by neilj37]

Get the auto-electrician to check the battery at the same time - it's very likely that it is 'unwell' after being boiled a few times.

Afterthought:

Don't go to a main dealer - find a friendly local garage who can do the job. This is NOT likely to be a big problem: in the worst case you might have to replace the alternator, but maybe it will just be a new regulator pack for the back of the alternator. A small garage won't throw a wobbler if you suggest replacing the alternator with a second-hand unit. In fact, they may have one on the shelf!



[Edited on 13/8/07 by David Jenkins]

Sounds like you have an arcing conection somewhere in the alternator charging circuit.

Go through the whole circuit looking for loose connection.

If the alternator has the Lucas/Bosch 3 spades connector block -- clean the blades connectors on the alternator and check the spades in the connector block are making a firm connection.

If the connections are OK then the alternator regulator pack is goosed.

I agree with the regulator diagnosis. Most alternators have regulators that are easy to change. Older alternators have interchangeable regulators so long as the alternator is the same capacity but more recently, the regulators have a communication function with the car's ECU and use different communication standards. These need like for like replacement.