The V8 Owners Forum -

Assuming a good battery and the only load the engine electrics, how many amps is the alternator chucking into the battery at 14.4v? Trying to guess how long it would stay that high before reducing as the battery charges up.

DaveEFI wrote: ↑

Fri May 01, 2020 2:55 pm

Assuming a good battery and the only load the engine electrics, how many amps is the alternator chucking into the battery at 14.4v? Trying to guess how long it would stay that high before reducing as the battery charges up.

Stick a clamp meter and find out ?

But I doubt it would be very much ? But the alternator will run all the electrics once the alternator is up and running. Not the battery.

Not something I've ever gave much thought to...only ever tested to see total load on the alternator when running.

Was just trying to get a guestimate of how long the car has to run to fully charge the battery. In these days of it not getting used much. I do have a hefty mains charger which claims 15 amp output max, but the volts from that never go above about 14v when charging a low battery. Don't have a DC clamp meter.

Dave, may not be what you are after but I'm e-mailing a spreadsheet showing the amps for each circuit on my car. I used it to calculate the fuse required for each circuit as well as how I split baanced each circuit across different fuses.

If anyone else want it, jut PM me your e-mail address.

Thanks Richard, but it's more how much of the alternator output is used to charge the battery with little load on it from other things. The alternator has plenty capacity to run everything, but isn't going to use all its 100 amps or so to charge the battery.

I've never bothered with a clamp meter as they're not that accurate on DC - especially at low currents which I'd be more likely to want to measure, and a DVM is better for that. Unless DC clamp meters are more accurate these days.

Does not an alternatr put out a fair whack if the battery is low? I thought it monitored it and varied output to suit instant load and battery replenishment.

The yardstick I've heard is 20 mins of running is required to top up a battery.

Chris.

In the GT
Idling on a hot evening, lights on, radiator fans running, ecu, low pressure pump, high pressure pump, cockpit fresh air fan, and twin 4 amp fans blowing air through engine bay, volt meter dropped to less than 10v.
When driving again it took a while for the volt meter to get to its normal night time running 12.5, no fans but the rest of the list working.
Day time just fuel,pumps and ECU it runs just above 14v

So the alternator, 90 amp from memory is a bit marginal!

Rad fans draw 12 amps each, 24 amp
Fresh air and engine bay, 16 amp
Headlights 60 w each total10 amp
Tail lights, instruments lights 2 amp
LP fuel pump 5 amp
Hp fuel,pump 8 amp

70 amp hour battery so a fair reserve for night time idling!

Oh no sound system other than the engine and exhaust


Ian
Ecu 5 amp
That total 70 amp

When windscreen fogs up I have heated fans, 350w each, that draws another 60 amps. Turn them,on and the fan belt screams!
And two seldom used spot lights at 100 amp each 17 amp

Probably considered too much draw for the alternator! But it does work.

DaveEFI wrote: ↑

Fri May 01, 2020 4:06 pm

Was just trying to get a guestimate of how long the car has to run to fully charge the battery. In these days of it not getting used much. I do have a hefty mains charger which claims 15 amp output max, but the volts from that never go above about 14v when charging a low battery. Don't have a DC clamp meter.

All you need if it's sitting is a trickle charger, 1-2A.

And it's a 12v battery...you dont really need over 14v to charge it.

DC clamp meters are handy, can be had for £25 or so.

Ian Anderson wrote: ↑

Sat May 02, 2020 9:00 pm

In the GT
Idling on a hot evening, lights on, radiator fans running, ecu, low pressure pump, high pressure pump, cockpit fresh air fan, and twin 4 amp fans blowing air through engine bay, volt meter dropped to less than 10v.
When driving again it took a while for the volt meter to get to its normal night time running 12.5, no fans but the rest of the list working.
Day time just fuel,pumps and ECU it runs just above 14v

So the alternator, 90 amp from memory is a bit marginal!

Rad fans draw 12 amps each, 24 amp
Fresh air and engine bay, 16 amp
Headlights 60 w each total10 amp
Tail lights, instruments lights 2 amp
LP fuel pump 5 amp
Hp fuel,pump 8 amp

70 amp hour battery so a fair reserve for night time idling!

Oh no sound system other than the engine and exhaust


Ian
Ecu 5 amp
That total 70 amp

When windscreen fogs up I have heated fans, 350w each, that draws another 60 amps. Turn them,on and the fan belt screams!
And two seldom used spot lights at 100 amp each 17 amp

Probably considered too much draw for the alternator! But it does work.

Real world testing is a more reliable way to prove what load is actually being drawn.

On my car, turning everything on I could find whilst driving along I could only get it up to around 70A, and even at that, you'd never actually drive the car hard if you had all that stuff turned on.
Only thing I couldnt add was heated rear window as it doesnt work.

The simple rule is that a battery will take what it needs limited by the power available - the alternator does not force power into a battery, a battery sucks it in.

So if the alternator can supply 100amp and a 100ah battery is low say at 10% (flat but still oK) the battery will most likely take all the 100amp power that is available to recharge. If the battery is about 95% like most serviceable car batteries are then the battery will only suck in a couple of amps to get to full capacity.

So the simple answer is - it depends - how long is a piece of string but if the battery is low it will pull in more charge but if it is nearly full it will not take much charge irrespective of how much charge is available from the alternator.

Garry

Think we''ve all likely had a loose fan belt that screams when you start the car from cold, but soon stops. Meaning a very high output to the battery only (other than engine electrics) doesn't happen for long. And if you add a lot of load - headlights heated windows etc - can be made to scream again.

So just wondered if anyone had a curve or whatever showing the actual amps flowing, against battery volts, as the battery charges, assuming no extra loads.

I doubt increasing the size of the alternator would make any very much difference beyond adequate.

If none exists, perhaps a clamp meter would give an idea. But difficult to use with the car in use.

I'm curious as I recently replaced a failed regulator in a fairly new alternator, after the car had been in a garage for non related work. Could be coincidence. The new reg shows a consistently higher voltage at the battery than the original. Even after a half hour run, was still showing 14.4v, starting off with a fully charged battery via a mains charger, so only a cold start to drain it. The old reg (when working) would have dropped back to 13.8v in that time. So am concerned about cooking the battery.

The alternator output should be fixed, regardless of battery state / electrical load. 14.4V is at the upper end of what is acceptable, and 13.8V is at the bottom end.

I once had a Rover 200 which kept blowing bulbs, and I eventually discovered the alternator was peaking at about 16V. I replaced the built in regulator, and it stopped blowing bulbs!

The only time the voltage output from an alternator is lower than it's normal voltage is if the load is as much as it can deliver (flat battery, lots of electrical items turned on, low engine speed). But with a modern vehicle, this won't persist for long, if it exists at all.

Provided your alternator does not go about 14.4V, I wouldn't worry about it.

Your quest for a graph showing battery state (i.e. no load voltage), versus charge current versus charge voltage is probably obtainable from a reputable battery manufacturer, but might take a bit of tracking down!

Chris.

You could always fit one of these to find out
https://www.demon-tweeks.com/smiths-int ... lsrc=aw.ds

Ian

I can actually remember the days when 30 amps would cover the entire electrical load. Add a spotlight - as you did - and you were in trouble.