[Tom Hudson]

I am no expert on Battery technology but as an Electrical/Electronic Engineer have, I hope, gained a fair amount of knowledge about Batteries and Alternators over the years. In anticipation of being corrected by someone with more experience than myself I will add some thoughts to the above posts.

Malcolm:

You mentioned that Racemettle set the regulator to 15v knowing that it would drop to 14.5v when the car was running. This doesn’t sound right. A modern Electronic Regulator should be able to control the Alternators output voltage to 14.5v under all loads from 0 to full load. Any Alternator Voltage above the absolute maximum of 14.5 will result in too much charging current flowing, which will seriously decrease the life of the Battery. This may have been the cause of your Battery problems, particularly as you mention you already cradle the Battery on a foam pad, I have found Batteries do not tolerate excessive shock loads for long.

In another post you mention that the Optima is not similar to the DMS/Varley type as the Optima is a Lead Acid Battery whilst the DMS/Varley type are Gel type.

A Varley Gel type is also a Lead Acid Battery, the Gel just means that the Electrolyte is not the normal Sulphuric Acid/Distilled water liquid solution but it is in a Gel form so the Battery can be mounted on its side if required.

Ross:

“12v or less (without the motor running) This is the Battery voltage only

14.4v or close (with the motor running) This is the Alternator output voltage applied to the battery.

Now with the alternator voltage held constant at 14.4v and this voltage being higher than the Battery voltage we get current flow from the alternator to the battery (a charging current).
A flat battery has a low internal resistance so the alternator will supply quite a high initial current (amps) to the battery.
As the battery charges and it's internal resistance rises, less current will flow to the battery and it will eventually reach a fully charged state where it will only require maintenance current from the Alternator.”

I agree with your first two lines but would disagree with some of the last paragraph. In my opinion the following more accurately describes the principles involved.

The internal resistance of a Lead acid battery in good condition is low, approx 0.01 ohms, whether it is in the fully charged or discharged state. It is not a rise in the internal resistance that causes the charging current to fall but an increase in the Batteries voltage from approx. 10.5v for a dead flat Battery to approx. 13v for a fully charged Battery. This voltage opposes the 14.4v Alternator output. It is this difference in voltages that dictates how much charging current will flow into the Battery.

Tom