Hi, a friend went away for a few months and his neighbour started the car every few weeks to recharge the battery. But it went flat anyway.
What would be the minmum interval time to restart the car again to charge it up; and how long should it be left idling in the drivaway each time to keep the battery in good charge, please? thanking you john west
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.
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!
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.
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.
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.
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.
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.
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.
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