With a Balmar 90 amp dual output I have a slipping problem. I'm running a Gates Power rated 6839 belt (1/2"x39") Because the alternator is a third of the engine pulley and waterpump, it's not ideal. But I want to keep it that way.

The belt looks a little small in fit. Should I try a 6939 belt (21/32" x 39")?

Or, should I put a larger pulley on the Balmar. That would increase the belt to pulley surface area. I don't think the rpms would be effected, we don't idle to charge. If so, where's the best place to get a larger pulley to fit it?

I've used belt dressing. I've cut back the charging rate on the Xantrex but I'm not sure that has much effect.
I've also checked and adjusted alignment, even tried adjusting it in small increments and testing. That doesn't seem to be the problem.

The problem is when the batteries are down enough the regulator calls for the highest charging rate.