Battery Charging

[Nemo2000]

More out of curiosity that anything, and I realise that any answer will be only approximate, but....

roughly how far would I have to drive my petrol RAV 4.2 in order to recharge the Battery enough to make up for the initial drain when starting the car?

[Gods_gift]

Im no expert on rav's but my understanding is that an alternator doesnt kickin until a higher than idle rev range and it trickle charges your Battery. So a motorway journey of a decent run is required.

[local hero]

I know this isn't the answer you want, but i replaced my Battery last week, so iput the old one on a trickle charger in my garage, and after 5 days continuous charge the little magic eye in the Battery is showing fully charged, Red ctre, blue outer, Stew

[crofter]

I always believed that alternators charged at low revs due to them running faster than the old dynamos

At the moment my car would think it it's birthday if it did a ten mile run!!

Due to circumestances at home my journeys are often less less than a mile and so far the Battery is holding up quite well,although it is from a diesel but by no means new.

So I would think you have no need to worry unless your Battery is about to give up on you.

Just saying it as I see it

Del

[Tech01]

. . .roughly how far would I have to drive my petrol RAV 4.2 in order to recharge the Battery enough to make up for the initial drain when starting the car?

After a typical cold start, with a Battery that is in a decent state of charge, the alternator will kick out a very large charge - anything up to 50A or 75A - into the Battery. Under normal circumstances (that is, unless you turn on every electrical load imaginable) this initial high charge will reduce after less than 5 minutes driving to 20A or even 10A. It will then stay at this low figure, and taper off to a trickle (5A or less) over the next half an hour of driving.

So the answer to your question is, probably, somewhere in the region of 15-20 minutes of driving. If your battery is holding up on journeys of less than 1 mile, you're doing really well, especially for a diesel, which takes far more out of the battery than a petrol on a cold start. Two or three minutes of Fast Idle will help after you start it, as will a top-up charge from a longer journey (or battery charger) now and again.

______

When does the alternator 'kick in' ?

Once an alternator starts turning, it produces an output. Alternators typically run at around 2-3 times the rpm of the engine (their pulleys are roughly one-third the diameter of the crankshaft pulley). So at 1000rpm tickover, the alternator's actually doing 2000-3000rpm, and is quite capable of delivering a fair output at this speed. The alternator's output rises with speed, but levels out somewhere around 5000rpm (2000 engine rpm). Maximum output can be anything between 80 and 120A for today's cars.

Large, slower-revving engines (V8's, diesels) tend to have quite small alternator pulleys, to keep alternator revs up at low road speeds (eg 30mph).

Dynamos (on old cars) do not like high revs. They have segmented commutators (like a typical mains electric drill), and not the smooth, continuous slip-rings of a modern alternator. Dynamos started to disappear from vehicles in the 1960s. The output from a dynamo at low engine revs was not good, and did indeed 'cut in' at something above idling speed. (It was the function of the Cut-out box to prevent current actually flowing 'backwards' into the dynamo windings at idle speed. The bank of diodes in an alternator (which also convert its AC to a DC output) also prevent reverse current drain.

Chris