Re: Ample Power vs Balmar vs Xantrex
I agree with MaineSail 100%. As usual.
However, I believe you'd be better served if you could possibly squeeze out enough $$$ to replace that alternator, as well as add a good external charger.
You can find Balmars at decent discount prices, but one I've been very happy using with clients is the PowerMax alternators sold by Hamilton Ferris. I've been using the 125A model (about $458) and de-rating it to about 100A using the Balmar MC-612 or MC-614 regulators.
The PowerMax's are very well built and will easily put out 100A for as long as you need it. And, you don't need any fancy upgrade to the belt drive system. A single 1/2" or even 3/8" belt will do the trick at this output level.
There are good discounts available on the older MC-612 regulator, too, which I prefer to the newer MC-614.