P38A What can kill a battery?

Grrrrrr said:

13.8V is the old alternator set point. Not enough for a calcium battery. You can replace the alternator regulator to give 14.3 or even 14.5V but it will only have the same power so takes longer to charge a big 1k CCA battery. Replacing the regulator is about £20 and will need to be done anyway. After that you're looking at parasitic drain or RF interference.

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Datatek said:

Sorry that is just not true, the higher voltage will increase the current flow into the battery and so charge it more quickly. The charge current will never be at the alternator output current maximum for long or with 150 amps available, it would overheat and potentially explode.
The Mobiltron regulator will allow up to 15 volts in some circumstances, normally 14.8 just after starting gradually dropping as the battery charges to 13.8 or even lower on a long run.

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Grrrrrr said:

The max power is set by the number of turns on the coils. Only rewinding the alternator will change that. Volts go up, amps go down.

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Mark Piercy said:

I think the 150a was for the v8 and 105a for the diesel which is strange as the diesel I would have thought required more grunt to start

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kermit_rr said:

I would say the diesel needs more grunt, being a much higher compression engine

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Datatek said:

But only for starting which is catered for by a higher CCA battery. Full alternator power, as I said, is not really used to charge the battery for long or the battery would overheat.

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Mark Piercy said:

I think the 150a was for the v8 and 105a for the diesel which is strange as the diesel I would have thought required more grunt to start

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Datatek said:

It's got nothing to do with the turns on the coils, the higher the voltage across the battery, the greater the charge current which rarely reaches the alternator maximum output. V over R = I The P38 diesel alternator is a 150 amp unit I think, more than enough to cook a battery.

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Grrrrrr said:

The power cones from the number of coils - that's roughly what is causing R[esistance] in your formula, albeit with a bit of feedback so it varies on demand. Volts times amps is watts (power). If one goes up the other goes down but the power remains the same. The only way to change the power is to change the number of coils unless the thing isn't at its limit in which case that changes. The early ones, especially the models with lots of toys, clearly were near their limits because they put the bigger 150 amp alternators on later models. Short journeys probably caused some issues until they put bigger alternators on them.

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Datatek said:

You forgot the internal resistance of the battery which is where V over R comes in and also the PD between the alternator output and the battery voltage. the number of windings on the field oils and the gauge of the wire determines the maximum out put of the alternator but in practice has bugger all to do with the charging current going into the battery.

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Grrrrrr said:

No, I didn't. That's just another part of the overall system but it cannot affect the max output from the alternator. It will of course affect how much is drawn down from that max. What you're suggesting is that if you put a bigger load on an alternator it will always mysteriously come up with more power to service that load. It cannot adjust the load past its max output, hence why they realised they needed a bigger alternator and put it on later cars. Below that max output the alternator will adjust to the load put on it, one of the reasons they are preferred over a dynamo.

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