Voltage on a Jag battery

[Duncan Wood]

Same alternators normally. 100% charge just takes longer if you float charge.

[John Henderson]

We're going to have to disagree on this. I can't find my references at the moment.

For the time being, I'll simply quote this:

"Firstly a standard vehicle charging system will not (and cannot) charge a conventional lead acid battery (either starter or deep-cycle) beyond 70%. With big battery banks, 65% is more common. For this reason alone, that 100 amp/hr battery is good for only 70 amp/hr - and that¢s when it¢s new!"

It's a longish URL and it might line-wrap:

[Duncan Wood]

Fortunately that's also irrelevant to car starter batterys, if you =

discharge your car battery by 75% then you won't top it to full in a =

couple of hours, but you'll still knacker a starter battery however you = =

recharge it if you do that. You could try a battery manufacturer.

Simple

charging's slow, hence Ford fast charge, but if you're not doing =

multiple short trips it's not very relevant.

[John Henderson]

We continue to disagree. The Yuasa manual is an oversimplification. The dip in charge from starting takes a long time to undo completely, as does the discharge from just sitting in the car for days with standby electrics connected.

is a better source of information.

John

[Dave Plowman (News)]

That's just bollocks.

[Duncan Wood]

Again it's showing you a bateery charging from flat. Car batterys & standby batterys are not charged or discharged in similar fashions. Starting a car uses a fraction of an amp hour. Leaving it to go flat for a month won't do it any good & it won't be fully charged in 1/2 an hour, but that's not a normal usage cycle.

[John Henderson]

As someone who occasionally takes a "fully charged" battery from a car (meaning various cars) and monitors the voltage and current as I give it a slow topup charge (one amp, voltage regulated and temperature compensated), I disagree. It takes many hours for the voltage to peak and stabilize and the current to regulate down to a trickle.

John

[Dave Plowman (News)]

Try driving for a few hours before doing this measurement.

And use a higher voltage if you wish to reach that peak quicker.

[Duncan Wood]

You can just measure the sg. You can always flow current through a battery, it's not necessarily useful though.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember John Henderson saying something like:

[Duncan Wood]

Well they are Belgian.

[John Henderson]

What makes you think I haven't?

How do you get the voltage higher at a 12v battery when you limit the charge current to 1 amp? Other than waiting for the battery to achieve full charge, that is?

Using a lot more current will lead to a premature charge termination - "surface charge".

John

[John Henderson]

And not very accurate.

John

[Duncan Wood]

Well not as accurate as measuring the SG, but you can get pretty close with charge/load/measure the voltage.

[Dave Plowman (News)]

Because all cars continue charging the battery while the engine is running.

Is 'a lot' more than the magic one amp?

[John Henderson]

Agreed. And when I go for a drive which empties my fuel tank from full, I'm confident that my battery is at or very close to

100% charge by the end.

There's nothing magic about 1 amp of course. It's simply that reaching float charge voltage with a small current gives a more complete charge than reaching it with a large charge current.

It follows that a battery charged to cut-off voltage with an automotive alternator is not really fully charged (other things being equal).

For most of my working life, I thought that the charge current didn't matter. When informed by sources who should know that it does, I experimented and satisfied myself that they were right.

John

[Dave Plowman (News)]

Pretty well every charger reduces the current as the battery voltage increases. You'd have to deliberately design one which didn't.

But there's no cut off voltage on any alternator I've ever seen. They just reduce the charge rate. Of course I'm not denying batteries on some cars used for short journeys only will probably never be fully charged.

What would be the point in charging a car battery to this ultimate state if the car charging system doesn't do it?

[Duncan Wood]

Well most car alternators taper charge, so the cut offs the regulated voltage, modern Fords cut off when the fast charge peaks.

[Dave Plowman (News)]

Indeed - but they don't stop charging until you stop the engine. Newest car I have says 13.8v any time I look at it.

I have an accurate LCD voltmeter in the old Rover sitting across the battery. That varies quite a bit - but the lowest it ever shows with the engine running is 13.5v.

[Chris Whelan]

[...]

Some smart charge vehicles do. Ford's system, for example, prevents any alternator output during cranking in order to reduce starter loading. Some models also turn alternator output off under conditions of hard acceleration in order to improve performance; the gain can be as much as a 0.4 reduction in 0-60mph.

Chris