how long idling to charge the battery?

Also sprach "j west +++" :-

Don't. Few things worse for an engine than leaving it idling from cold.

Drive it ten miles and park it - it'll be much happier.

If this is a regular thing - get a battery charger.

I read some time ago that it takes twenty minutes *driving* to put back into the battery what a five second start took out...

Buy a battery charger!

Darren

IIRC, it is not advisable (or possible) to charge up a totally dead battery from the alternator. A dead car battery should be recharged from a battery charger, which will of course come in handy in the future.

--Nick.

And Lidl have them for £6.99,-6amp, very robust, all metal casing. Shop closed today. DaveK.

You can burn the alternator out by charging a flat battery at idle, the alternator will be working at full current, but it will get very little ventilation due to its fan only turning slowly. High current, low ventilation = overheat and burn out!

Also sprach "davek" :-

Minus 6 amp. How very appropriate for a battery charger.

It'll never charge up on idle. The alternator simply cannot generate enough current. You could leave it sitting there a week and all the alternator will do is barely meet the current requirements needed to keep the ignition system/fuel pump/ECU up.

Far better doing a 30 minute run once a week.

You What? I've never had a car that can't charge the battery at idle! Most alternator setups can't quite reach their rated output at idle, but they can get bloody close. The last car I stuck a meter on could quite happily maintain over 14V with the rear window heater, both seat heaters and the headlights all on at idle.

Of course, using the engine to charge the battery is hideously inefficient though.

Nope. If all you want to do is charge the battery, then a battery charger is the obvious way to go.

Cheers,

Colin.

How does that work? (Not saying it's wrong, but I'm a novice). Are there any facts & figures to prove this? How much current does a starter motor need - are the petrol and diesel versions different?

Cranking current depends on the size of the starter motor, size of engine, compression ratio, oil viscosity and temperature. A large diesel engine with cold, thick oil would need much more power to spin over than a motorbike engine with thin oil that was already hot - hence why bigger engines have bigger batteries and starter motors.

Darren

Also sprach "Darren Jarvis" :-

And generally bigger alternators.

And? Tried measuring the current its generating? After all, the starter motor draws current and alot of it.

Clue...

The voltage means nothing. Its the current thats the important factor.

What about one of those solar panel jobbies??, a mate has one for his m'bike as he's pretty much a fair weather rider only, solar panel on the garage roof and it seems to keep the battery charged enough over the alarm drain and whatever else is draining the battery during the winter months.

OK, back home by10.30pm. Any advance on +6amp. Lidl kit vg. DaveK.

Also sprach "Chris" :-

There's a couple of prombles. Apart from needing a really large panel for use in the UK - after all, it's winter when you most need it - when there's least sun, the other problem is that car glazing cuts UV - which is the most energetic end of the spectrum - so you lose a huge about of power just by putting it inside the car.

An alternator won't produce full output at idle.

If the alternator is producing volts above quiescent battery volts then it's charging. And most are designed not to try and chuck huge amounts of current into a 'flat' battery. They are limited to a safe amount - far less than their rated output. That maximum output is there to sustain the electrical load when on the move.

...And if you have 14ish V across a working 12V car battery, then it's charging at roughly it's ideal rate. End of story.

Nope. No need. You charge lead-acid batteries by putting a constant voltage across them. 14ish V is plenty for a 12V battery to charge about as fast as is reliably possible in a car. The current the battery draws will vary from tens of amps down to tens of milliamps depending on how flat it is. Put 14ish V across it and it'll take whatever current it can handle.

It does indeed. It's not running whilst the battery is charging though.

Hopefully, you have one now ;o)

If the voltage is above the battery's open-terminal voltage, then it will be charging. If it's below, then it will be discharging - simple as that. Measuring the charging current can tell you some interesting stuff about how flat or knackered the battery is, but if you want to know if the charging system on the car is working, then the voltage is all you need to worry about.

Cheers,

Colin.

The car first needs to go for a good run to get the oil hot if it's been left idling - that sort of treatment is very bad for the engine. At least

Then buy a battery charger....