Battery Charging

Yes but they need to be in parallel & it'll take twice as long. Why?

Yes and no. Yes provided both batteries have no duff cells and your charger can cope with the current and you obviously put the batteries in parallel. No if you're unsure about the state of the batteries and whether your charger can cope and probably one of those newfangled electronic ones may get confused by having two batteries. My caving lamps have cells in parallel and I charge them using a simple fixed voltage charger with no problems.

I have taken two batteries off my horsebox ( 2x 12v Exide 700amp -

Positive to positive, negative to negative.

Thanks all! uk.rec.cars.misc I've been asking too!! Reassuring that all you guys said the same thing! If I'd have had a 50/50 response id still be screwed!

Thx again. Lorry moved (jumped) - problems solved - batteries on charge so that I dont have to ask again!

If they've been flat for a while - fear the worst as lead acid batteries rarely recover from prolonged deep discharge - apart from a few designed for it.

Hmm.

As long as they are both in reasonable condition and start from a similar state of charge it should work. The more-charged battery initially will dump power into the lower charged battery (assuming they are wired in parallel...) until the voltages equalise.

Then it should Just Work (TM) except obviously it will take twice as long to charge.

I wouldn't try it if one of the batteries was suspect though.

It may overheat or even burn out the charger- two batteries in parallel will, all things being equal, draw twice as much current from the charger, which it may or may not be able to handle. Especially given the huge size of the batteries.

If the gauge on the charger has a red zone or a max safe current reading, you can try your luck as long as the gauge stays in the safe zone over the charging period. Lead-acid chargers are generally constant voltage chargers, not constant current like NiCad/NiMH chargers, so the load can and will destroy them if it is excessive.

Morse

To which the brief answer is "bollocks" :-) Any car or van battery on it's own can pull more current than almost any charger can deliver, they're all current limited, rather constant current.

Well, you might want to actually research the technical meaning of 'constant current' and 'constant voltage' chargers. Lead acid chargers do not use constant current as in constant current charging, but they may use overcurrent protection, which is *not* the same thing. Watch the meter when the battery is charging, it's hardly a constant current process.

The average cheapie car charger is rarely designed with any current control/limiting per se, other than a failsafe fuse. The transformer is wound to produce a known voltage into a known load, ie a lead acid car battery. The battery's internal resistance will alter over the charge cycle which causes the current drawn to alter and drop as the charge cycle goes on.

The battery won't 'pull more current than almost any charge can deliver' because the charge voltage is never high enough to cause that to happen, not because the charger is current limited.

NiCad/NiMH chargers OTOH are designed to provide a constant current regardless of load (within reason) In a good charger, the voltage is effectively varied by the charger as the charge cycle progresses and the NiCads resistance alters to maintain a fairly constant current. This is constant current charging, and it is unsuitable for charging lead acid technology.

Dave

So is the lorry 12 or 24 Volt? If it's 12v, and you jump-started it from a

To charge both of the batteries, you just need to connect the positive lead from the charger to either of the positive terminals on the batteries, whichever is easiest to reach, and the negative lead from the charger to earth, either a neg terminal on the batterie(s) or the vehicle chassis. Almost certainly the best way will be to use the terminals you used to jump-start it.

HTH,

Steve

All right, never mind that, I've just seen in the other ng that it's 24V.

Steve

Every single battery charger I've seen limits its maximum current in one way or another. They have to given the very low impedance of a battery.

For a car battery you speak the truth

Try charging a flat battery & you'll discover that the battery can easily pull more current than the charger can handle. Given the variation in internal resistance of the battery with temperature relying on the voltage to limit the current doesn't work.

Actually if you voltage limit it it's how you charge sealed lead acid batterys. Car battery chargers have a cheap thermal switch that cuts in & out to prevent them overheating. You can't sensibly limit the current by the voltage of the transformer as the voltage difference between 1A & 40A at

Of course- every charger or PSU has a max current limit imposed by the laws of physics, I never implied they didn't. There is no such thing as a infinite current source, they have imposed practical limitations due to the components used. That is not the same thing as constant current charging, nor is it the same thing as having current control devices installed. OTOH some more modern chargers may well be designed with overcurrent protection circuits and will safely operate into a dead short without damage.

All the low end/older car chargers I've seen are little more than a transformer, rectifier, fuse and ammeter. Short the croc clips together and after a while, if the fuse doesn't blow, the transformer will become very hot and smelly then burn out. It may well be that the OP has a newer or more complex charger with overcurrent protection, but I don't know that so I prefer to err on the side of caution and not speculate that everything will be OK when other people's gear is at stake!.

Morse

Yes but the charger has its own internal impedance which is a factor in limiting the current. It doesn't mean it necessarily has fancy current control/limiting circuits. It may rely, as virtually all the older ones do, on the transformer's current output limitations, ie the way it is wound. The older ones often have 'current limiting' in the shape of a bitmetallic strip wound with a heating element in series with the + output, which breaks the circuit on prolonged overcurrent until it cools down, but that kicks in in extreme circumstances to protect the transformer from burnout, rather than strictly speaking being an active component in the charging control circuit.

Yes, I agree with that. They are far from infallible though and will fail if seriously overstressed long enough. I've certainly seen these designs burn out, usually by the element wound around the bimetallic strip going open circuit after charging a 'dodgy' battery overnight, and I've seen at least one where the bimetallic strip became warped and was keeping the contacts apart.

Yes, it is true that a lead acid battery, when supplied with a fixed voltage, will draw widely varying currents depending on other factors, but in the more crude and older designs the transformer will be chosen by the designer to prevent that very scenario happening. Certainly my (ageing) car battery chargers are- one is a 5 Amp and the other a 7 Amp. Those figures are arrived at by the capacity of the transfomer, rectifier etc and the bimetal strip is set to cutout at the transformer's upper safe limit. It's nasty and things get hot, but it does work I suppose :-)

It's possible that the OP's charger is a modern one with proper overcurrent protection which can take charging two 700A/H batteries in its stride, but I don't like to make assumptions and thought it worth pointing out that damage to the charger is not out of the question.

Morse

edit:

They also have crude cutouts (thermal based- bimetal strips etc) to protect against damage, those buggers slipped my mind :-) They only kick in in extreme circumstances where the limit of the transformer and other components is reached, ie the max rated current of the charger.

Morse

HOwever the internal resistance of any partially discharged battery is almost completely irrelevant, it's orders of magnitude less than that of the charger for a 20Ah battery, the size of the battery makes little difference.