Alternator Information

Alternator Setup & Sizing Guide

To use an external regulator, your alternator must be modified. This can be done by:

Yourself (if you're confident)
A reputable auto electrical shop
Send your alternator to us and we will convert and test on our alternator test rig.

Depending on your alternator type, an external brush block is often available from most auto electrical suppliers.

We recommend a Bosch 80–90 amp alternator—this offers excellent performance without stepping into the high-cost range of larger units.

⚠️ No matter how smart your regulator is, if the alternator isn’t spinning fast enough, you won’t get a strong charge.

To ensure optimal output:

Calculate your lowest cruising RPM
Adjust your pulley size so the alternator spins at the correct speed
to calculate your pulley ratio, divide the outside diameter (OD) of the large pulley by the small pulley OD
2.5 or greater is the target.

Aim for at least 3,000 RPM, ideally 5,000–8,000 RPM at cruising speed.

💡 For every 25 amps, your alternator draws approximately 1 horsepower from the engine.

Here’s a rule of thumb for matching alternator size, battery bank capacity, and belt requirements:

We have many Yanmar engines turning a 120amp alternator on a standard A section belt