Charging car batteries

I have to charge 2 12v batteries for my wife's invalid buggy, but the 24v charger that came with it has died. I have charged the two batteries separately using a 12v car battery charger, and they are both fully charged now, but I have to keep swapping the charger from one to the other to keep them floating. Can I wire them in parallel and leave them on the charger, or will one of them end up discharging and the other overcharging?
J.
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John Rouse

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If the batteries are not car batteries you could be damaging the batteries using a car battery charger!
If they are get SLA (Sealed Lead Acid) or GelCell type, then disconnect car charger and get a proper charger before you have to buy new batteries (a lot more expensive!!)
Sparks...
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How do mean the charger "has died" ? Have you checked that the fuse in the plug or the flex to the charger are all OK ? Did you get maintenance instructions with it ? How old is it ? Chargers are very simple things, even when they are called intelligent, so are very easily repaired.
Do the lights all come on as they should ? Does it make any humming sounds when it's switched on ?
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This one isn't, its riveted together!

It appears to be doing all the things it should - it just doesn't charge the batteries. It has a Cannon connector to plug into the scooter, and there is 24v on that, but when I leave the batteries in the scooter they don't charge as they should.
One battery was dead when I rescued the scooter from mother-in-laws garage, where it had supposedly been left on charge, but the other has recovered. I now have one new battery and one older one. The scooter runs happily when I charge the batteries one at a time, but during periods of inactivity I have to keep swapping them over to top them up. I'm wondering whether a 24v charger might be better and charge them in series?
J.
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writes

Drilling the rivets out is no problem.

Series charging is not a good idea, believe me. Does the charger have automatic cut-out on it ? Have you checked that all the connections are clean and clear, and made off properly ? If you have any loose connections or broken cables then the system is not going to work. A bit obvious, but I'm thinking out loud. :-))
My first question should have been, how old are the batteries, because these types of batteries only have a life span of around five years, so if they are any older than that, then it might be time to go for new ones.
If the charger has automatic cut-out, then it could be possible that it is cutting out to quickly because it was left on to long, so isn't giving time to charge the batteries properly. Most chargers did have thermostatic cut-out on them, so this might be the problem. If you can get a test meter on the batteries when they are in the scooter and then put them on charge and leave it for a while, the meter might tell you if the cut-out is activating to quickly and needs replaced.
If you find that the charger isn't doing the job right, then drill the rivets out and you can get in to make repairs on it. Seal it again with self-tapper screws or nuts and bolts.
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Given that a battery is made up of series connected cells, why not? Milk floats have been doing this for many many years...
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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wrote:

The idea of having damaged cells in the middle of a battery preventing the other cells from taking their full charge. In parallel the two batteries have a chance for at least one to get fully up to speed, and could show that the other battery needs replaced. But having them in series while charging often shows as both batteries being faulty. Maybe that's just me though.
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Well, a thus damaged cell will prevent the battery from giving the correct output too. ;-)
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On Sat, 31 Jan 2004 09:19:54 +0000 (GMT), Dave Plowman

Oh go and read _any_ battery textbook FFS ! When you ask a question that makes Walloper look smart, then it really isn't worth replying further.
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Oh, so a car battery isn't made up of series connected cells then? So each cell is in fact 12 volts and they're parallel connected? And I've never seen a truck with two 12 volt batteries to provide the 24? Or does it use some clever system to charge them separately? And John is wrong that this is *precisely* how the wheelchair was charged as supplied?

I'd say you need to take that 'textbook' out of your arse and try understanding it. Or get one which gives real world applications.
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On Sat, 31 Jan 2004 23:57:01 +0000 (GMT), Dave Plowman

Idiot.
Series-connected cells voltage-hogging problem.
Plonked fuckwit
Goodnight
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That's what IMM calls everyone. When he can't flannel his way anymore.

So you *can't* explain how this doesn't apply within a battery, or is of no importance? Thought as much...

Why do some prats like you think the contents of their killfile interests others?

Hope your hangover isn't too bad today.
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If the batteries are both the same age, type and have had the same use, then yes.
However if you have an old battery, and a new one, then NO!
You may end up overcharging one of the batteries because they are not matched properly.
This will then cause damage to that battery - possibly the risk of it overheating, and maybe exploding in an extreme case!
Sparks...
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Yup - this will work ok.
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On Fri, 30 Jan 2004 00:40:16 +0000 (GMT), Dave Plowman

Assuming they are car batteries that is, I would still say they are SLA batteries though. A car battery would more than likely be one, usually to get the required Ah 2 SLA's are required.
SJW A.C.S. Ltd.
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I would agree with the above, may father and mothers, both use SLA.
Dave
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You can charge SLAs in parallel too.

I use 12v 70 a/h SLAs at work.
I'd also say you'd get away with charging a large SLA with a modern car charger of the type most would have - say up to 10 amps (genuine) or so - provided you didn't leave it on charge for long periods and kept an eye on the charging voltage. My few years old Halfords 11 amp (bit generous that since I've never seen more than 7 amps out of it) never exceeds 14.4 volts as most car batteries are now of the low maintenance type and don't like being allowed to 'gas' like we once did.
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Dave Plowman wrote:

I have a cheapo that does at best 3-4A. Halfords finest (cheapest) I would think tis is entirely suitable for small lead acid, sealed or otherwise, down to 20A/h or so.
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John Rouse wrote:

Yes, you can. Aklthough this seems at fairt sight a dodgy thing to do, teh charge/disharge voltage/current relationships mean that it actually works wuite well.
Campers with motor and leisure batteries will charge them in paralell - so it's done on a regular basis.

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Many thanks for all the helpful comments on this. Luckily my wife doesn't need the buggy at the moment, so I have time to play around with the charger.
The car-battery charger I have is a fairly intelligent one, it charges in three stages, the final stage being a trickle charge. The two batteries[1] that were on the scooter were sealed Yeasu <sp?) ones, and one recovered when I charged it, but the other didn't. I also suspect the latter is losing electrolyte as there is a damp patch on the floor underneath it. So I bought a new battery and the buggy ran fine on those two, but it wouldn't re-charge using the charger that came with it. So I took the batteries out and charged them with the car-battery charger, and both are now at the trickle charge level.
From what I have read here, I'm going to connect them in parallel and leave them on trickle charge till I can hack my way into the original charger and find out what's wrong with it. However there is always the thought that it may interact with the electronics on the buggy, but I suspect they are not very sophisticated.
J.
[1]"accumulators" to the pedants.
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