I have a diesel truck with 2 batteries. There is also on this farm numerous diesel tractors that do not have a high rpm speed. All vehicles work fine. Most times the tractors aren't used at the highest rpms.

While optimal charging speed may be 1200-2500 rpm I still think that battery charging will happen at idle. Also, if it didn't, the alternator light on the dash would be lit until the rpm of the engine was raised.

If you use an axle with a differential, you could drive the alternator off the drive shaft U joint. It would be a higher RPM without a bunch of belts and pulleys.

Hook the alternator you want to use by the shaft into your drill press. Hook up the wires to a battery such that the alternator will charge it. Try the press at various RPM settings. When the battery shows a voltage rise with the alternator turning, you've achieved minimum charging RPM's.

I'd bet it'll be about 1500-2000rpm for the alternator.

I'd take the power off a differential (per Boz) and then use bicycle chain/cogs for final gear-up. They're more efficient than belts and pulleys.

I would think a few solar cells might be a useful alternative/augmentation too...

J

I wasn't thinking automotive rear axle... But interesting idea.

I like the differential idea but would a chain hold up to the high RPM required?

For some reason... I feel like DD's trying to re-invent the wheel.

Shirley! This has been done somewhere/somehow already!

I know of a guy who was on a cross country trek and read that he had an alternator connected to a rear wheel but that's all i know. As to the issue of mud and water, think in terms of three miles an hour.