Car advice - flat battery?

gilbert_and_sullivan

If the car battery is stone dead, after jump starting,leave the car on a fast tickover (something wedged on the throttle pedal) with the side lights on for half an hour, it will fully charge up.

Sounds silly i know leaving something turned on, but if the battery is completely flat the alternator will not kick in and start charging unless its got something to 'work on', hence leave sidelights (or heater fan or similar) on.

I would have been amazed had it started on the key after 10 weeks anyway, if you're going to leave it that long, either buy a special 'top up' charger for the purpose (classic car enthusiasts use them) or simply disconnect the battery, but 10 weeks is a fair time without topping up.

Oblivion

gilbert_and_sullivan wrote: »

If the car battery is stone dead, after jump starting,leave the car on a fast tickover (something wedged on the throttle pedal) with the side lights on for half an hour, it will fully charge up.

Sounds silly i know leaving something turned on, but if the battery is completely flat the alternator will not kick in and start charging unless its got something to 'work on', hence leave sidelights (or heater fan or similar) on.

Where on earth did you dig up that urban myth from? Modern car electronics will detect the need for battery charging regardless of whether anything else is switched on, and will apply alternator current appropriately.

Dave.

gilbert_and_sullivan

Oblivion wrote: »

Where on earth did you dig up that urban myth from? Modern car electronics will detect the need for battery charging regardless of whether anything else is switched on, and will apply alternator current appropriately.

Dave.

Its no urban myth, i've seen it happen times.

Having spent many years collecting cars from compounds where many would be completely flat, and seen some cars left ticking over for hours only to be completely flat afterwards.
But if you turn something on and give the throttle a blip, you will notice the speed of the engine has changed slightly where the alternator has cut in.

Most compunds still do this, but they tend to use hazard flashers, as otherwise a petrol car could be left ticking over for days until the fuel ran out.

I'm not saying that all cars need this little trick, but for the sake of turning something on, is it worth the risk?

Dynamo's on the other hand would charge, albeit slowly from completely flat without this.

Oblivion

gilbert_and_sullivan wrote: »

Its no urban myth, i've seen it happen times.

Having spent many years collecting cars from compounds where many would be completely flat, and seen some cars left ticking over for hours only to be completely flat afterwards.
But if you turn something on and give the throttle a blip, you will notice the speed of the engine has changed slightly where the alternator has cut in.

Most compunds still do this, but they tend to use hazard flashers, as otherwise a petrol car could be left ticking over for days until the fuel ran out.

I'm not saying that all cars need this little trick, but for the sake of turning something on, is it worth the risk?

Dynamo's on the other hand would charge, albeit slowly from completely flat without this.

I think what you are describing is the active engine management system's reaction to an increased current load requirement, so I do now understand your original point even though I do not condone it. All that is doing is increasing the revs above a point where the engine would be consuming all of the alternator's output and leaving nothing over for charging the battery. Switching something on is still going to leave the added difference in available current marginal, and makes a mountain out of a molehill in terms of battery charging.

Leaving a car on tickover is not an adviseable way of recharging a battery, even with the revs slightly elevated by something wedged on the accelerator pedal.

This does not mimic the way a car should be run and will only induce long term damage to the engine because in these circumstances, oil is not under sufficient pressure to keep the camshaft(s) and upper engine parts sufficiently lubricated. I have witnessed the damage this can cause.

I always have at least four cars in my collection, often more, so I am no stranger to battery depletion since some of them get run infrequently. In my experience, there is no real substitute for a good quality battery charger when a battery has gone flat.

Dave.

k4no_p

if it clicks then the battery is not totally dead and will charge once jump started.. set of jump leads is best bet and cheapest.. take your car for a nice run for about 5 miles and this should charge your battery up to optimum performance again.

Keith

k4no_p wrote: »

if it clicks then the battery is not totally dead and will charge once jump started.. set of jump leads is best bet and cheapest.. take your car for a nice run for about 5 miles and this should charge your battery up to optimum performance again.

The only way to tell if the battery is ok is to use a multimeter, with a charged batter if when attempting to start the car the meter shows less an 12.5v then the battery is knackered and needs replacing.

My car was clicking last week, jump started it, drove home, checked the battery voltage and it was fine, upon restarting it clicked, checked with multimeter and the battery read 8v whilst attempting to start

goldspanners

Keith wrote: »

The only way to tell if the battery is ok is to use a multimeter, with a charged batter if when attempting to start the car the meter shows less an 12.5v then the battery is knackered and needs replacing.

My car was clicking last week, jump started it, drove home, checked the battery voltage and it was fine, upon restarting it clicked, checked with multimeter and the battery read 8v whilst attempting to start

sorry keith, the only sure way to test a battery is with a battery load tester. this will tell you a lot more about the state of your battery than a multi meter.

hewhoisnotintheknow

ask a neighbour to jump start it and drive it, no biggy

Keith

goldspanners wrote: »

sorry keith, the only sure way to test a battery is with a battery load tester. this will tell you a lot more about the state of your battery than a multi meter.

I was under the impression that a charged battery, which under starting load, drops to under 10v is knackered?

I've little knowledge on this, I only know what I've read with google.

Ephemera

As the weather's been cold, it will depress the charge in your battery.

Try to charge it, however as it's been run completely flat there is a chance it's been damaged beyond full recovery, and as it's winter a new battery might be on the cards.

All you can do is suck it and see....get your car started, run it for an hour as suggested elsewhere, and charge the battery overnight off the mains.

I guess you only use your car for short trips? In that case, you will likely not be getting enough charge into the battery whilst driving to cover the charge needed to start it again next time, slowly draining the battery. If you use it regularly for long trips and the battery was new and in good condition, fully charged before you left your car, then you could have left something on or you have a possible electrical fault, draining the battery.

Go for the simplest fix...charge it up and hope that does the trick!