How long to leave the battery charger on?

[pimping]

Genius level IQ here, watch out.

Just hastily booked the car in for an not and the bloke said leave it with him and he might be able to fit me in.

So I go to start the car up and bzzzzzzzz then dead. Turned over once. Tried jumping it, no joy and now it has the charger on.
But how long should I leave it on, I don't want the car dying if the testers are testing the lights with motor off etc.
All my own stupidity but it was only ten days ago I started it and left it running 20 minutes or so.

[300bhp/ton]

pimping said:

Genius level IQ here, watch out.

Just hastily booked the car in for an not and the bloke said leave it with him and he might be able to fit me in.

So I go to start the car up and bzzzzzzzz then dead. Turned over once. Tried jumping it, no joy and now it has the charger on.
But how long should I leave it on, I don't want the car dying if the testers are testing the lights with motor off etc.
All my own stupidity but it was only ten days ago I started it and left it running 20 minutes or so.

erm... well how flat is the battery, what capacity is the battery and what is the charge rate of the charger.

Basically if you don't know these things, how on earth can you expect anyone else to be able to give you answer based on no figures??

MoT light check is likely to be with the engine running.

[pimping]

Don't know to all the above.

2 hours?

[pimping]

Once the wee Beastie starts is it quicker to charge the battery on charger or by letting it sit running?

Yes I know.

[fangio]

Fast tick-over or driving it charges far more quickly than your battery charger.

[monthefish]

pimping said:

Once the wee Beastie starts is it quicker to charge the battery on charger or by letting it sit running?

Yes I know.

Ignore the miserable unhelpful bugger 2 above.

5-6 hours should be enough for an average charger in average conditions to charge an average flat battery.

If you can take the car for a reasonable (20 minute?) run that should be good enough to charge your battery (unless the battery and/or your alternator are actually goosed)

[300bhp/ton]

monthefish said:

pimping said:

Once the wee Beastie starts is it quicker to charge the battery on charger or by letting it sit running?

Yes I know.

Ignore the miserable unhelpful bugger 2 above.

5-6 hours should be enough for an average charger in average conditions to charge an average flat battery.

If you can take the car for a reasonable (20 minute?) run that should be good enough to charge your battery (unless the battery and/or your alternator are actually goosed)

But what is average? A 1.2 Corsa will have a much smaller battery than say an S-Type R. Maybe 4 times smaller. The charge rate of chargers can vary dramatically also by a factor or 2 or 4.

This means a slow charger on a larger battery could take maybe 8 times longer to charge to the same percentage as a small battery on a fast charger. When you are talking handfuls of hours as your unit, then it makes quite a considerable difference in time.

e.g. the largest battery I have on the slowest charger would probably take 48-72hrs to charge. While the smallest battery on my fastest charger would probably take 1.5-2 hrs.

[andy-xr]

Whenever I've done it I've either

a) Taken the battery off, brought it inside and left it on for 24 hours til the red light turns green or
b) Charged it on the car long enough for it to start, then taken it for a drive. You need revs and load to recharge, idling wont do a lot for it

[sinbad666]

Until its full?
Seriously, i'd say as above, 4-6 hours should give its a decent charge. You really need to go for a long drive after, maybe do this just before you go for your MOT.
Any decent battery charger should have an in built trickle charger so you can leave the battery charger on as long as you want and it wont kill the battery.

[LuS1fer]

Let's go back to basics.
(1) how old is the battery?
(2) is it f*cked?

If (2) then no amount of charging is going to stop it going flat again and a new one is not expensive.

[DottyMR2]

sinbad666 said:

Until its full?
Seriously, i'd say as above, 4-6 hours should give its a decent charge. You really need to go for a long drive after, maybe do this just before you go for your MOT.
Any decent battery charger should have an in built trickle charger so you can leave the battery charger on as long as you want and it wont kill the battery.

Just to highlight, if the car sits for a while not being used as my understanding of your post (maybe I've mis-read) then I'd recommend taking the battery out the car and sitting it on the charger. It's what I do for the MR2 and no problems.

Doesn't solve the problem now but one for the future. Took around 4 hours to get enough juice in the battery to get the car started.

[monthefish]

300bhp/ton said:

monthefish said:

pimping said:

Once the wee Beastie starts is it quicker to charge the battery on charger or by letting it sit running?

Yes I know.

Ignore the miserable unhelpful bugger 2 above.

5-6 hours should be enough for an average charger in average conditions to charge an average flat battery.

If you can take the car for a reasonable (20 minute?) run that should be good enough to charge your battery (unless the battery and/or your alternator are actually goosed)

But what is average? A 1.2 Corsa will have a much smaller battery than say an S-Type R. Maybe 4 times smaller. The charge rate of chargers can vary dramatically also by a factor or 2 or 4.

This means a slow charger on a larger battery could take maybe 8 times longer to charge to the same percentage as a small battery on a fast charger. When you are talking handfuls of hours as your unit, then it makes quite a considerable difference in time.

e.g. the largest battery I have on the slowest charger would probably take 48-72hrs to charge. While the smallest battery on my fastest charger would probably take 1.5-2 hrs.

Do you understand the concept of average?



So you don't assume a "1.2 Corsa" nor an "S-Type R".
You don't assume "the largest battery I have on the slowest charger" nor "the smallest battery on my fastest charger".

Those are extremes.


Anyway, the point I was trying to make is that instead of the negative and basically unhelpful post you posted, why not say something like:


"Hello mate. It really depends on the car and charger you're using but around 6 hours should be enough to see it back to life (unless you have other problems, such as the battery being dead rather than just flat).
If you let me know the details of the car and the battery & charger you are using, I should be able to provide more specific details as I am indisputably the foremost internet expert on battery charging."

With responses like your initial one above, it's not hard to see why you have such a bad reputation on here.

[DottyMR2]

LuS1fer said:

Let's go back to basics.
(1) how old is the battery?
(2) is it f*cked?

If (2) then no amount of charging is going to stop it going flat again and a new one is not expensive.

Or possibly swing by a scappies on the way to MOT? If the battery is struggling to hold charge one of the cells may have collapsed. If that has happened, then you're as well getting a new battery.

A scrappy charged me £6 for a battery and it's done the job for a while. If you battery is toast, pick one up on the way to MOT and you should be fine.

[RicksAlfas]

As soon as you get it started, leave it running while you tidy your charger up, go for a whizz about and take it for it's MOT. It'll be fine. I've started mine with it still attached to the charger to see if it works. Probably not good practice but what the heck. Just make sure you've no cables near any whirly bits.

[Mr2Mike]

monthefish said:

Ignore the miserable unhelpful bugger 2 above.

5-6 hours should be enough for an average charger in average conditions to charge an average flat battery.

That would depend entirely on your definition of an "average" charger. A typical 4 Amp charger might put enough charge into it to be able to start the car, but it won't be anything like fully charged. A 10-15 Amp charger would be getting most smaller batteries toward full charge in the same time.

[swisstoni]

Is there no indication of charge on your charger?

If in a hurry, charge for a bit, get it going and drive around for a while ending up back at your place. Turn off. Restart. If it starts fairly happily drive around a bit more and end up at the MOT station. If it doesn't start, more charging and no MOT for you today.