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MG MGB Technical - PaulH Solihull

Paul --- I would like to hear your comment on the hazards of wiring two 12v batteries in parallel. I hope all is well.
Dan Robinson

Dan - I am not Paul, but I can't see any particular hazard in paralleling 2 12 volt batteries. My question would be why? If you are looking to provide for a heavy current draw, you should think about a high current alternator in. Cheers - Dave
Daved DuBois

There is no problem with 2 or more batteries in parallel if you are looking for storage capacity. I have run a Landy for 10 years with 2 alternators, one supplying 3 batteries and one supplying 4 batteries. One set to run fridge and freezer in the bush in Southern Africa and the other set for starting and winching. There was no issues with this.

Ronnie
RA Potter

I thought there was a problem if the two batteries are not matched. When they are left alone and still connected, the stronger of the two spends its time trying to charge the weaker. After a while they have both worn each-other out and both of them are flat.

So I think it's OK if you start with two brand new batteries and don't leave them too long between charges. But of course that was your basic idea in having two batteries in parallel in the first place.

It's probably best to isolate them from one-another when the car is out of use.

Mike
Mike Standring

Hi Dan - very well, although would like some rain to wash the salt off to drive my cars!

Mike is right, one can discharge into the other, and if one eventually goes faulty i.e short-circuit it could be pretty spectacular.

If you are planning a very large load then you should fit a larger alternator as David says, otherwise your load is going to flatten the batteries anyway even when the engine is running.

I know some people fit two 12v in parallel and claim faster cranking, but it just isn't necessary - unless there is a fault somewhere.
PaulH Solihull

Dan

I'd be interested to know why you're looking to fit 2 batteries, unless you've got a large or high compression motor you're struggling to start I'd question the need for an extra battery.

If however you're still determined to fit the extra battery I'd suggest 2 of the same type and age, preferably new.

Regarding the theoretical possibility of a catastrophic failure, the theory says there's the possibility it could happen the same as I could win the lottery jackpot but at 14.7 million to 1 I won't base my retirement plans on it.

If we look at what could happen ie a battery going short circuit, yes that could be quite spectacular but all the batteries I've seen fail either die of old age and fail to hold a charge or a cell goes open circuit, either of these situations would fail safe. If however a battery went short circuit it would only be 1 cell, (the chance of more than 1 or all cells failing simultaneously is almost impossible), that 1 cell would have been charged at the time of failure and likely self destruct anyway, but even if it didn't you'd effectively have a 10v battery in parallel with a 12v battery and although the current could be quite considerable it wouldn’t be the same as complete short circuit across a 12v battery.

Putting batteries in parallel is accepted practice where greater capacity is required. I worked for some time with FFR Land Rovers, these had 4 x 100ah batteries, 2 lots of 2 in series in parallel with each other giving 24v at 200AH, these were charged with a 90A alternator. To the best of my knowledge we never had any failures of a catastrophic nature.

I fully appreciate the dangers of lead acid batteries having had 2 blowup on me, both down to my own stupidity and seen the results of a friends battery explode after the spanner he was using to undo the clamps go across both terminals.

I think we need to keep the dangers in proportion and I'd be the last person to advocate doing something that was inherently dangerous or unsafe. If you really want something to worry about I'd suggest that pumping volatile fuel into 2 containers that have a habit of overflowing and placing them directly above a red hot piece of cast iron would be of more concern.

Bob
R.A Davis

Hi Bob

I agree with all you have said and can tell you all of us who ventured into the wild game reserves in Southern Africa had at least dual batteries preferably identical deep cycle and I never heard of anyone having any problems. Most of us had a controller like the one made by National luna which only connected the 2nd battery when the primary one reached 13.8 volts. When the ignition was switched off the batteries were disconnected by a 150 amp relay. I would have no hesitation in putting 2 batteries in series though I don't really think it is necessary in a car.

Ronnie
RA Potter

Oops, think you mean parallel otherwise you would be converting the vehicle to 24v operation and all electrical items would have to be changed
Iain MacKintosh

Big trucks do it. Mine had 4x 6V in a parallel/series 12 V configuration. Money being a problem, I never replaced all 4 together, did it in pairs until I changed to 2x 12V.

FRM
FR Millmore

Hi, Paul H!
TheMiz

Hi Iain

Sorry about the slip up I meant parallel.

Ronnie
RA Potter

This thread was discussed between 28/02/2013 and 02/03/2013

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