Battery Sizing

Hi All,

I'm wiring a beetle for a friend (going great thanks to HPA) I need to size a battery.

The Beetle is a classic, with air-ride and modern studio, an amp and sub.

Air ride

25a

Amp

15a

Sub

12a

Those are max draw numbers.

The Starter tops out about 80a

Any tips on changing the battery out?

Are you going to operate the car without the engine running (i.e. blasting the stereo, hopping on the air ride), then you have to size the battery for the loads it will see. A 52 Ah battery would allow doing that for about an hour (then the battery would be completely discharged and wouldn't start the car. If that's in the cards, then a larger battery (like 150Ah deep discharge), would let you run the stereo for a few hours, and still start the car when done. Note you might need an external battery charger to restore that battery to full charge, if you aren't driving for a long time after discharging it.

Since the usual main purpose for the battery is to start the car (the alternator providing all the current to run the car and re-charge the battery for the next start). We typically size the battery for that, so perhaps a 70Ah, 625 CCA battery.

As David said. I'd also suggest sizing the alternator ( I trust it's not still running a dynamo) for the load and how the vehicle is driven. If it's used almost entirely for open road driving, the alt' will be close to it's max' output, but if it's spending most of it's time idling and at low rpm in traffic, a larger alternator, and/or maybe a smaller pulley, may be required to give the headroom for the draws on the electrical circuit AND charging the battery.

Don't forget to take the lights' draw, and other incidentals like rear screen demister (if fitted) heater blower, etc, into account because the winter is when you're going to be running into issues.

Thanks, both; that helps a lot.

We went for a happy medium in the end, and I'm going to fit a volt meter, so he can keep an eye on the levels.

He is thinking about a create motor next year so we will have an option to resize the altenator then.