Advice on using small batteries with our electric A/C

[classicretrofit]

Apart from cranking the engine, the role of the battery is to provide reserve power when the alternator does not provide enough output to cover the electrical demand. Typically, this happens at low RPM (e.g. idle), since alternator's output is dependent on engine speed.

Even the 'best' 110A 911 alternator (from the 964) can only put out 40A at idle. This means that when you are stopped with lights and defogger and wipers, the battery is providing some of the juice and it is not charging. If you have a voltmeter in your car, you can tell as the voltage will dip. As a rule of thumb, if the voltage is below 12.5 volts, the battery is not charging.

Typically though, you only need a small RPM increase for the alternator to provide enough amps to cover the full electrical load of all the accessories. Car makers rely on us driving the cars, not leaving them to idle for hours on end. It has always been so.

So, if you fit a smaller battery, you will find that the reserved power available is less. Depending on your usage of the car, this could result in battery drain. About the worse thing you can do, is start the car and leave it idling with heavy current accessories on, like AC.

ElectroCooler's ECU has logic inside to monitor the battery voltage and the current consumption. In conjunction with configurable threshold and timing parameters, the system can be 'tuned' to meet the requirements of the vehicle and protect the battery from drain. Setting the AC up to be more cautious results in the logic shutting down the AC sooner when idle is detected. Conversely, an 'optimistic' setting, will not shut the AC off at all, but may result in battery drain. It's all configurable.

Alternators. The 90A unit fitted to the C3.2 is the best of the bunch but it is pretty lame by today's standards. Many folks including ourselves have 'rewound' these alternators to produce more power but there is a problem here. Although it is possible to get more than 100A from this design, the case does not have very good cooling and there are no internal fans. In stress testing, we have seen very high temperatures so we took a new approach.

Our modern completely redesigned alternator is a brand new 6-phase unit with internal fans. The first test unit was a huge success: the 180A unit providing an idle output of 90A. We've had it running at 140A with no belt drag which is surprising. This opens up the door for electric heat that actually works.