External battery charging while still on car

Probably, depending on the rating of the charger.

Why would you want to, OOI?

Chris

Thanks Chris. The car belongs to a 69-year old neighbour who's had her arm in plaster for a few weeks, so the car hasn't been used and she can't/doesn't want to risk lifting the battery out to charge it on the bench. She asked me to do it but my back injury is playing up and I don't want to/can't risk lifting the battery out either, so just thought about charging it in situ. It's a Ring 6A RCB206 charger by the way. I suppose I could leave the battery in place and just disconnect it from the car but was worried about losing the radio code (if it has one) and any other settings it may have.

Ah, OK.

It is obviously safest to disconnect the battery; it might be worth checking if the code is available. If not, a 6A charger is unlikely to damage anything.

It might be worth bearing in mind that a battery that has been left discharged for any length of time will have started to sulphate, and hence will have lost some capacity. If the battery was near end-of-life, a replacement may be needed now or in the near future.

Kudos for helping her out, BTW.

Chris

Not seen the original so replying to this.

If the battery is flat, the charger might not connect to it. Most modern ones have a relay to prevent reverse connection, and this needs some volts from the battery to operate.

Absolutely no need to disconnect the battery on any car when charging it.

Thanks again Chris, all noted.

Cheers :D It costs nothing to be nice to folks and I'm lucky to live in a street where we all tend to look after each other.

Dave Plowman (News) submitted this idea :

Oh right, thanks - but what does one do in that situation? The whole reason for a battery charger to exist is to charge a flat battery.

It would be logical to assume that; after all, the alternator provides a charge whilst the battery is connected to the car, but I always worry that the charge coming from an external charger is somehow 'different' because it's plugged into the mains :o)

Has anybody tried to start the car?

There's a big difference between a battery that won't turn the engine, versus one that won't take charge from a sophisticated charger. A test would be whether the electric door lock mechanism will unlock, or whether the radio works.

The issue with charging batteries occurs when the charging device provides a very large current - and it is still connected when you try to crank the engine. If/when the engine starts large currents may flow from the alternator around unexpected earth pathways and cause damage to the ECU. This may happen if the charging device is another car or the sort of high power device used in garages; and the failure might then occur in the car providing the charging power.

I've never understood the exact mechanism of such failures, and I suspect some failures arise from incompetence; but I have seen dire warnings.

In this instance, if the car does not start I suggest you connect the charger and leave it connected for a day. Then disconnect it, and turn on the headlights - do they glow with the correct brightness, and not go dim after a few seconds? If so the battery has taken some charge. Turn them off, then try cranking the engine.

A battery left for only a few weeks ought to retain its charge (think of the people who leave their cars at the airport while on holiday) but more than that might be a problem.

Never ever disconnected a flat battery. Always charged them in situ. Why would you need to disconnect?

Tim

A high powered old type charger might increase the voltage in the system to a dangerous level (dangerous to the ECU) particularly if the battery is knackered. Modern chargers should be absolutely fine.

If the battery had no output, it might be that a cell to cell connection has failed. It could also be the case that the connection between the battery post and the connector had gone high-resistance.

Connecting a cheap charger in those circumstances would mean you are connecting a 16 or 17 volt poorly regulated supply, with lots of ripple, to the car's electronics rather than the battery.

Chris

If it's that flat any radio code will probably also be gone

Some car manuals advise against charging whilst connected.

Perhaps they are aware that some trickle charges can give out a lot of volts?

Steve formulated the question :

No, a charger is designed to bring a low battery up to full charge, at least modern ones are. You can get around the problem, by temporarily connecting a good battery in parallel with the flat one and the charger, just to fool it into charging.

Steve has brought this to us :

Its a backside covering exercise by manufacturers - some really big or old fashioned chargers could damage the car's electronics.

I use a variable power supply any time I want to charge any battery. That way I can set both current and voltage limits. You can probably get one for the price of a cheapo diodes and transformer charger these days.

A modern smart charger is only 13 quid upwards. That is all any average consumer wants or needs, requires no attention and can be left permanently connected.

Christ I am seriously out of date. £3.35 on ebay. eBay item number:

Harry Bloomfield wrote on 03/10/2016 :

I understand what you're saying Harry but remember these things are meant for the general public. Most folks will have a battery charger in the garage for when the car refuses to start because of a flat battery

- and who has a spare good battery lying around? If you had, you wouldn't need the charger lol

As in heading. Claims to go into pulse mode automatically for very flat batteries.