Need to disconnect battery before charging?1

I've often charged a battery just by putting the charger on the terminals without disconnecting anything. Seems to work. Might not on all cars!

The message from Maerko contains these words:

Many will tell you that you're in grave danger of blowing up the ECU if you connect a charger to a battery when it's still in circuit. Personally I have my doubts, as electronics are generalyl fairly well protected against a bit of ripple and considerable overvoltage.

However, if you are going to disonnect a lead - then it makes sense to disconnect the earth lead (which in almost all applications is the negative) 'cos it's safer. If you accidentally scrape the spanner on the chassis it won't matter 'cos it's already at ground potential. If you scrape the spanner with the positive terminal it'll spark like crazy, which is scary and potentially dangerous and damaging. Of course, once you've taken the earth lead off - the positive hasn't got anything to be positive /to/ so you can bang away to your heart's content.

Same goes for reconnecting - positive first, then negative.

In message , Maerko writes

Yes, if you slip with the spanner on the positive terminal and catch the chassis you short out the battery, if you are on the -ve then no problem.

I've never found a need to disconnect the battery while charging, Theoretically I suppose it will take fractionally less time to charge because things like the alarm system will not be taking current but this will be very marginal.

Unless of course any one knows better, I'm an electronics/radio engineer not a mechanic.

In some cars when current is removed for a long while it will "forget" certain settings. Especially those with electronic stability programmes and the likes.

My father, in the final few years before he stopped motoring, only ever did very short local journeys, insufficient to replace the battery power that he used up starting the car. He used to put his car battery on charge at least once a week. If he disconnected his battery, as per his handbook advice, then he had to re-set his clock, he lost all his stored radio stations, and had to re-enter the security-code,, etc. He spoke with an auto-electrician who advised him that there was absolutely no need to disconnect the battery to charge it up. He stopped doing so and had no problems whatsoever.

Kev

The reason for this is slightly less chance of a short while working close to the battery - nothing whatsoever to do with charging requirements.

If you're careful not to short anything, no.

Not with a domestic charger - or indeed any well designed one.

I'll give you some 'ifs'

If the charger was an ancient high powered type, it *might* develop a pretty high open circuit voltage.

If the battery was totally and absolutely flat and therefore knackered, it

If the electronics on your car are badly designed, they *might* be damaged by severe overvoltage.

There is no need to disconnect the battery. The charger won't put out more current then the alternator would anyway. It also saves losing radio codes and ecu settings.

The only reason for disconnecting the negative is that you then remove the earthing on the car - With the negative connected, and the negative of the battery charger connected to it, then touching any metalwork of the car with the positive lead will cause a short.

Same reason why when removing a battery you always remove the negative lead first (and yes, I did once slam the ratchet against the engine whilst doing this, which drove home the wisdom of removing the negative first).

Regards

Mike Dodd.

I replaced the clips on my charger with a cigarette lighter plug so I just put the charger and extension lead into the footwell, close the door on the lead and put the lead through an open window to an internal socket (with a rcd adapter)

No disconnection's, a few mins to set up even quicker to disconnect and the whole system is waterproof.

Once you have got the polarity of the cigarette plug sorted no errors

This works fine on my trickle charger which only puts out a max of 5 amps.

Once either lead is disconnected from the car, the battery is no longer relevant to it.

The only difference is that when you put a spanner onto the nut holding the clamp to the battery terminal, when you turn the bolt, you can short the battery to the bodywork through the spanner. This can't happen with the negative terminal on a negative earth car.

I haven't encountered any problems charging in situ, to be honest, but the theory would be that a charger can create extra voltage and that can damage sensitive electronics that you might well find on a modern car.

My non-professional gut reaction, tells me that a battery (particularly a relatively low charge one) is dead good at absorbing spikes and voltage so this isn't particularly likely to be an issue if you turn the charger on after connecting it to the battery poles.

Having said that, if anyone ever welds a car with an arc welder, the sorts of spike that generates will most definitely eat the engine management computer, the fancy alarm circuits and lots of other stuff. Disconnecting the battery there will make no odds, possibly makes it worse. Isolating the sensitive bits is less easy. All a bit of an issue. Pulling out most of the fuses seems to be one approach. Anyone know?

The voltage the charger can generate is held down by the internal resistance of the battery in practice. And after ohm's law, the greater that voltage, the greater the current going into the battery. And since few domestic chargers are likely to put out as much current as the car's alternator, it sort of follows that the voltage from the charger can be no higher than that.

The exception could be if the battery was knackered and had gone high resistance - but most modern chargers are voltage regulated.

I've been there - the spanners stay too hot to touch for ages don't they? ;)

A decent battery can take a chunk out of them.

Don't wish to contradict you but I thought the internal resistanceof a car battery was particularly low (which is why it can deliver so much current).

Welders run on a few volts at very high current so I'm a bit undecided as to whether they can damage electronics, I doubt it myself unless someone can state actual damage. What kills electronics usually is high voltage spikes but the metal shell of a car is excellent protection against external voltages, did you see that Top Gear program where they zapped a car with 2 million volts with one of the (expendable) presenters inside, no damage at all to him or the electronics. Faraday cage and all that, but this effect doesn't give the same protection against voltages developed inside the car.

If you really wanted to protect the ECU you would have to unplug it while wearing an earth wrist strap connected to both the car body and a good ground (metal pipe in the ground perhaps or mains earth would do), and the same when plugging it back in. Otherwise your body is likely to have static charges on it that are more likely to kill it than anything else mentioned in this thread. For ultimate protection you would also need a plug with all its pins shorted out to plug into (while still wearing an earth strap) the ECU while it's not plugged into the car's wiring loom. I can't see anyone bothering with all that, it's nearly as safe to leave it alone and still connected. Taking fuses out won't help.

A battery charger gives out less than an alternator, even a knackered battery will absorb just about any spikes or ripple, I did measure it years ago with a scope, it's nothing, at least with domestic chargers, so I would favour not disconnecting the battery during charging.

The message from David R contains these words:

And that's why it's hard to get the voltage across one to rise much - because its internal resistance is so low.

Absolutely. And if you have a low resistance driven off a relatively higher resistance source, that will restrict the voltage - even assuming no other method of control. Resistors in series and all that.

There's another reason why this is important.

When you charge a battery, you generate hydrogen gas and this is extremely flammable. If the car is going along, the air flushes out any hydrogen that is being generated and it usually doesn't build up all that much. However, if you have a freshly charged battery, the last thing you want to do is cause sparks around it.

I'm fairly sure this is playing it safe rather than a common incident but it is quite relevant in some circumstances like badly ventilated garages, particularly as the gas explosion happens near a fuel tank, three rickety shelves of paint tins and turpentine and a gallon of two stroke for the lawnmower.

Welders run on rather more than 12V in general, the failure mode however is the ecu being the return path for lot's of current & the only way you can avoid that for sure is to unplug it. To be honest I've never bothered though, clamping the earth electrode next to what you're welding is fine.

They're normally fairly static proof nowadays