Voltage on a Jag battery

[Dave Plowman (News)]

Oh yes - but that's not the same as turning the charge off when the battery reaches a specific state of charge.

[John Henderson]

You're missing the point. The faster charge results in a "fully charged" voltage earlier than it should.

Reducing the charge rate at that stage (with an appreciable surface charge already established) means it will take a long time indeed to approach true 100% charge. And that's because you're continuing to listen to battery voltage as the indication of charge saturation.

While that's taken to be close enough, it does reduce battery longevity from what it otherwise could be.

John

[Andy Champ]

That's surprising.

Engine is maybe 100kW, and to pull 0.4 seconds off a typical 10 second time would imply the alternator ate 4% of the output during that time.

4kW is a lot of electricity in a car.

I suppose though with a lower powered engine the time goes up and the power goes down, so it's possible...

Andy

[Duncan Wood]

Only if you under or overcharge it. You don't need 100.0000% full charge to avoid sulphation.

[Chris Whelan]

A 1.6 Focus is 75 kW, a 1.0 Micra 47kW.

An average alternator output is about 1kW; not sure about efficiency though.

(The top spec Touareg alternator can produce 300A; that's about 4.4kW without any allowance for losses, but clearly not a mainstream situation!)

Chris

[Dave Plowman (News)]

The one on my car is IIRC 130 amps - and water cooled. ;-)

[Chris Whelan]

[...]

The Touareg one is driven by a two-speed electrically controlled gearbox.

Chris