Battery charging voltage

AFAIK, cold temperatures don't affect the capacity of a battery, or affect it's charging requirements. All it does is reduce the maximum amps it can supply to the starter. By as much as 25% in freezing conditions. Mike.

Nope, cold weather needs higher charge voltage. Boschs graph

I sit corrected. I also thought the max charging voltage was set by the regulator, and not dependant upon ambient temperature. Mike.

It is, it's the regulator which compensates for temperature.

How can that work efficiently? With the alternator and regulator under the bonnet heated by the engine, and as with my car, the battery in the boot, where it will be much colder. Mike.

Well most cars nowadays vent the outside air to the alternator to keep it cool. For expensive batterys you actually do stick a temperature element to them, but that's not really worth it in a car.

Ford have used Smart Charge for about a decade. The charge rate is controlled by the ECU, and there is a fairly sophisticated charging strategy.

I'm guessing others do something similar?

Chris

whatever

It's charge current. A flat battery will show a different "charge voltage" than a charged one. The battery acts like a zener diode the voltage across it will not exceed the sum of the cell voltages and a small volt drop for its internal resistance. This is why a 6V battery will destroy a cheap 12V charger. But a current limiting charger in the same situation will still supply the charge current required.

The alternator doesn't regulate that though (unless you've got a Ford with smart charge & it's not the alternator then), it just delivers as much as it can up limited by the maximum current or the maximum voltage

Which isn't that small when charging it after the 1st couple of minutes, charge current is limited by the charge voltage from the alternator, it doesn't know whether the current's going to the battery, the heated front windscreen or the ICE, so it just limits the voltage.

">>> Recently I've been concerned by my car's charging voltage going up to

And if you look at that graph, even at -40 the maximum voltage should be less than 15.5 volts. I have never know any automotive charging system which increases the charging voltage depending on temperature.

No, the voltage shouldn't get much higher than about 14.8, don't forget that in the majority of cases the battery will be under the bonnet, and much warmer that the outside temp.

I'm going to check the 'sense' wire as I've been reading that any resistance in that will cause the alternator to 'see' a lower system voltage and hence overcharge.

I don't think I've ever seen a car with a battery sensed regulator, all the ones I've ever seen are machine (internal to the alternator) sensed, hence no sense wire.

You only ever need a battery sensed system if the alternator and the battery are a long way apart, and there is a possibility of a voltage drop in the power cable.

Perhaps the indicator light wire is what he meant? That excites the alternator in the first place, without it there is no charge unless you rev the alternator high enough to get it going on its own.

My Rover P6 had a battery sensing regulator. It was pathetic, though, - the alternator couldn't balance the maximum load. Not that that had anything to do with voltage sensing - just BL penny pinching.

There's voltage drop in any cable, so voltage sensing could make sense. My Tom-Tom power supply uses it. Or rather I assume it does - three wires to a two pole connector.