For how long should I charge a car battery?

Belenus

BMW 116i Series1 F20 January 2015. Petrol engined, 6 years old on original battery. It has stop start. It has done about 24,000 miles with lots of short trips and fairly regular long trips.

Due to Covid, it has being doing little but short trips of 2 to 5 miles for many weeks. On Thursday my wife returned from the supermarket (1.5 miles each way) and as she arrived home a low battery warning appeared urging either a long trip or a charge. It was about 1 degree C so very cold. The car is parked in the open on our driveway as the garage is a bit too narrow for it.

Due to the fixing bracket covering part of the battery label I can't read the full details but I can see 80Ah and 800 something.

A long drive isn't a good idea at the moment so yesterday afternoon I dug out my very old basic charger which outputs 4 amps. I read the car manual which told me to use the positive terminal under the bonnet and a metal part of the chassis and not to use the battery terminals.

At first the charger read 4 amps being delivered. After about 6 hours this has dropped to about 2 amps so I disconnected it.

This morning I reconnected it and it read 4 amps again. I checked it after about 20 minutes and it is down to 1 amp.

I presume this is all normal and the battery is getting closer to full charge and offering more resistance to the charger.

It has had about 7 hours of charging at up to 4 amps so that presumably means no more than 28 Ah which is well under the full capacity of 80Ah.

I don't want to overcharge it so for how much longer should I leave the charger connected?

Does the car have any over charge protection with the ignition switched off?

Should I be considering a new battery?

Thanks

AdrianC

The full capacity of the battery is 80Ah, but it wasn't anywhere near 0% charge. It almost certainly wasn't even down to 50% charge - that sort of charge level simply wouldn't start the car, and is the threshold of causing damage to a wet lead-acid battery.


It certainly sounds like it's pretty well charged now.

But a car with stop-start will almost certainly have an AGM battery, not a normal old-school wet lead-acid, so your old-school charger won't be putting out the optimal charge voltage for it. It won't HURT it, but it just won't fully charge it.

Belenus

Thanks Adrian, that is reassuring.

I'll take it off charge now.

neilmcl

6 hours is unlikely to be anywhere enough to fully charge your battery. The instructions for the CTEK MXS 5.0 I got for my dad states 12h for an 80% charge for a 60AH battery, and that's with it's dedicated AGM mode. If your charge has a trickle charge feature then it won't overcharge your battery anyway.

BTW, you're not alone, I've been getting a low battery warning on my 4 year old 1 series too as it isn't getting anywhere near the sort of trips it should. Unfortunately mines parked on the road so I can't put it on charge overnight.

Scrapit

For a car thats done 6000 to as little as 4800 miles a year treat it to a new battery, it can't be costing you much else in service items.

thegentleway

neilmcl said:

Unfortunately mines parked on the road so I can't put it on charge overnight.

Take the battery out of the car so you can charge it; you will wreck it if the charge gets too low.

neilmcl

thegentleway said:

neilmcl said:

Unfortunately mines parked on the road so I can't put it on charge overnight.

Take the battery out of the car so you can charge it; you will wreck it if the charge gets too low.

Couple of issues there. Firstly as the AGM batteries are so interlinked with the BEM system it's not always advisable to remove them and I'd be concerned that I may have to re-register the battery once it's put back in. The second reason I wouldn't want to to do it is because the batteries in the BMW are in the boot which I may have difficulty getting into without the central locking being enabled etc.

thegentleway

neilmcl said:

thegentleway said:

neilmcl said:

Unfortunately mines parked on the road so I can't put it on charge overnight.

Take the battery out of the car so you can charge it; you will wreck it if the charge gets too low.

Couple of issues there. Firstly as the AGM batteries are so interlinked with the BEM system it's not always advisable to remove them and I'd be concerned that I may have to re-register the battery once it's put back in. The second reason I wouldn't want to to do it is because the batteries in the BMW are in the boot which I may have difficulty getting into without the central locking being enabled etc.

You only need to register a new battery and you can open boot from the inside so just need to fold the seats.

Homer_home

You can pick up a couple of times a year from aldi and Lidl a smart battery charger for about 13 quid , these are a sound investment to recharge batteries that are infrequently used and are ctek clones 

chrisw

You definitely need one of the smart chargers then you can leave it as long as you like, I use the Aldi one which works fine. 

Volvo recommend charging for 48 hours and then leaving the car overnight before using. They also recommend charging with the battery in the car as the charge in and out of the battery is monitored. This seems to remove any low battery or stop start issues I've had.