Charging car battery by leaving running idle

GabbaGabbaHey

facade wrote: »

Just sit in the car and play the radio/read a book for an hour with your foot tickling the throttle so it runs at a fast idle

If you are doing this, make sure that the car is out of doors or in a very well ventilated garage. Carbon Monoxide poisoning can often be fatal.

facade

GabbaGabbaHey wrote: »

If you are doing this, make sure that the car is out of doors or in a very well ventilated garage. Carbon Monoxide poisoning can often be fatal.

Good point, I assumed that he was on the drive from the posts.

A modern car that has passed its MOT emissions is producing very low levels of CO anyway, mine tested at 0.00 last time. (the relevance of the MOT pass is that the catalyst is known to be working)

bigjl

arcon5 wrote: »

Just wondering how long you guys would recommend leaving the engine running to charge a battery that has run flat? Bearing in mind I cant take it for a long hard drive to charge it as it's SORNed.

Need to jump it, then leave it running to build up enough charge to fire up when it's collected for an MOT next week

Leaving a car idling for a period of time won't charge the bettery up.

Hence why when the M25 was snowed in a few years ago people were advised against it. Not just due to the tailpipe being blocked or suchlike. But because the battery will be being discharged rather than charged.

Another reason that runlock systems on vehicles raise the idle speed, so the vehicles batteries are charged.

Only way to sort your problem is to remove the battery and trickle charge it.

Even taking the car for a run will not fully charge the battery. Hence why people needlessly change a battery after repeated flat batteries, they never trickle charge it, just jumped it and carried on.

bigjl

arcon5 wrote: »

I've taken the charger off the battery and my voltmeter is showing 12.8. Still is pretty much what you'd expect a battery to be right? The charger itself still shows as charging - but it can't be far from charged now, right?

Leave it on charge overnight.

If the charger you have bought is decent enough it will be no problem.

I flattened the battery on a car after leaving the interior light on. Was so flat neither the central locking or alarm worked.

Overnight charge with a cheap charger from Halfords sorted it out. Never had an issue with it again.

TrickyWicky

arcon5 wrote: »

The charger itself still shows as charging - but it can't be far from charged now, right?

Depends if your charger has a full indicator. Some chargers just charge constantly while others will sense a full battery and turn on a green LED before switching to trickle charge etc.

If you have a sensing charger the general rule of thumb is leave it charging for as long as it takes to indicate the battery is full. The longer it takes, the healthier the battery actually is. The deader the battery is the quicker it takes to show a full charge (usually anything from 5 seconds to 5 minutes and then it'll go green showing it's full - as soon as you connect it to the car it won't even unlock the doors!).

I have two batterys that take around 4 hours to fully recharge and thats a good sign - I'd actually prefer them to take longer because then I'd be happy knowing that they still have plenty of capacity left in them. I have one on the door step that shows it's full in 30 seconds yet won't power the interior lights without flickering.

mttylad

Just bear in mind that if you want to leave the car idling to charge the battery you need to be sat in the drivers seat all that time.

ISTR that it is against the law to leave the vehicle running without a driver in the driving seat.

Myself, I whip my battery out and use a charger, saves fuel, saves emissions, saves me spending time doing nowt.

TrickyWicky

mttylad wrote: »

ISTR that it is against the law to leave the vehicle running without a driver in the driving seat. .

Nothing in the highway code about it and on private land the rules of the road do not apply.

GabbaGabbaHey

TrickyWicky wrote: »

Nothing in the highway code about it

Apart, of course, for Section 123 of the Highway Code, which says:

You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road. Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine to reduce emissions and noise pollution. However it is permissible to leave the engine running if the vehicle is stationary in traffic or for diagnosing faults.
Law CUR regs 98 & 107

Available on-line at: https://www.gov.uk/general-rules-all-drivers-riders-103-to-158/control-of-the-vehicle-117-to-126

TrickyWicky

I stand corrected!

Joe_Horner

arcon5 wrote: »

[...] i've today bought a charger from Wilkinsons, called "12v 4A(RMS) Battery Charger"... on charge now - in fact has been on charge for over 2 hours now!!

How long you think this should take?

The charger will supply (up to) 4 amps charging current. In practice with a basic charger that usually means it'll supply maybe 5 amps when the battery's fully discharged but steadily less as it charges and the voltage rises.

If you treat that 4 amps as the maximum rate it can charge at then working out the absolute minimu time to charge a fully flat battery is easy:

The battery will have an "amp hour" (ah) - typically bewteen about 35 and 50 for petrol engines and maybe 70 to 90 for diesels. Dividing this figure by the rating of the charger will give you the minimum charging time.

So, say your battery is rated at 40 ah, with a 4 amp charger it can't physically charge from fully flat to fully charged in less that 10 hours.

That's assuming that everything works with 100% efficiency which it never does. But it's also assuming that your battery is 100% flat which it almost certainly won't be.

In practice, that calculation will give you something pretty close to how long it'll take for an "almost flat" battery to charge allowing for losses. It's by no means exact, but it gives you a ballpark to play in.