Recharge time

Tricky one, this.

Most alternators won't start charging the battery until there's a couple of thousand rpm on the clocks.

However, I've managed to get a car to re-start in such a way after 5-10 mins of idling, but I guess it all depends on how dead the battery is.

They will charge at idle most of the time, but not very much. Probably not enough. It's usually enough to raise the voltage on a charged battery, but a dead flat one will be much harder work.

That's usually OK, but you'd do better by driving somewhere. Best of all would be to put a charger on it overnight.

Cheers.

I used an old halfords charger last month for 2 hours and after that it started up ok. Its been going more than 20 minutes now.

Not insured to take it out. Got a guy coming around to view it for a sale.

Get the battery in the house/garage, and charge it overnight. If you've got a good one that guesstimates the current charge-state, you'll probably be shocked to see how low it is. Some batteries are unrecoverable, I had one that was nearly dead, charged overnight, appeared fine, but just couldn't start the car for whatever reason.

It'll also be doing the future-buyer a favour, instead of flogging a car with a known nearly-dead battery.

The message from snipped-for-privacy@italiancar.co.uk (SteveH) contains these words:

All the cars I've ever had wtih alternators have happily got to 13ish volts at idle. More as the revs come up, generally, but still charging.

Alternators should be able to supply all continual battery loads at idle (ie all lights on, heater on, radio on, wipers on). It may not be managing to maintain full voltage with that load on, but it shouldn't be low enough to allow the battery to discharge. By 1500rpm, it should be maintaining at least 14volts.

Going for a brief drive is better, as it means the alternator will be spinning faster and being kept cooler (it is possible to overheat alternators by leaving them idling with a flat battery)

True, but the charging effect from an alternator at idle is negligible. Depends on the application, but, for example, my Honda VFR800 doesn't charge at all at idle - as I found out a few weeks ago.

Yup. Agreed.

How long is a piece of string? You could start a car with no battery at all with jump leads so of course it would never recharge. And it's a bit the same when a battery is present - it depends on just how flat it was, and what caused it to go flat.

If you geared a bike alternator to charge at idle, the thing would explode once it got anywhere near full revs!

Not so - all charge at idle, unless you've got a very heavy load switched on.

"Moray Cuthill" wrote in news:4501e1be$0$19602$ snipped-for-privacy@free.teranews.com:

Bollocks, the battery fizzes a bit, that's all ;-)

I know bike electrics are crude, but surely a regulator isn't that expensive? Or are you saying it's a mechanical thing due to a wide rev band? If so quote some figures - ie idle speed and maximum revs. There's usually not a huge difference in this ratio regardless of peak rpm - indeed it's sometimes less with high revving engines.

Typical figures for a bike in the 600-1000cc range would be a tickover of

1500rpm, max revs 13,000rpm. IOW, a range of roughly double what would be normal in the car world.

In fact, low alternator output at idle is becoming a problem for some cars also. That's why the VW Touareg drives its alternator via a two-speed, electronically controlled gearbox.

Chris

Really? That's a ratio of 8.7 - 1. My Rover V-8 idles at 700 rpm and can run to over 6000, so pretty close. If you were talking about a peak engine speed of 6000 - just for argument - on a car, half the ratio of that bike would mean an idle speed of 1400 rpm - which would be ridiculous.

It becomes a problem if you want near maximum output at low rpm, perhaps, with a vast alternator, usually because of cooling. But that shouldn't be needed on a motorbike.

The regulation isn't the problem, it's the mechanical limits that's the problem.

On a bike (especially a sports bike), weight/size is the main issue, so the alternator needs to be as compact as possible, but is also limited by the fact it's a direct drive of the engine.

A smaller alternator means it's got to spin faster to charge, so by sacrifising the charging capacity at idle, you can have a smaller lighter alternator. But also, as it's direct drive, the physical size is limited, because a large rotor wouldn't take too kindly to being spun at 13000rpm.

Off course none of this applies to alternators that don't use a conventional alternator rotor/stator set-up, but instead use a permanent magnet rotor with field coils interwound with the stator coils, which I can't actually remember the name of just now. But I'm sure the magnets will also have a factor in maximum speed with this design.

The message from "M Cuthill" contains these words:

Or like my brother's VW - two-speed water-cooled alternator.

Chris Whelan wrote in news:K2uMg.14535$ snipped-for-privacy@newsfe5-win.ntli.net:

I thought it would be more like 900-15000 rpm.