Clay -
If the alternator was designed for 90A and output was jacked up to 160A via new stator/rotor without replacing the rectifier/diode set/voltage regulator with stouter ones - and IF the battery and accessories were capable of drawing more than 90A - it would seem to me that there is a good chance of blowing one of the parts that was not designed for the higher load.
If the load never got above 50A (as you suggest) there would be no problem, but if the load is high and the alternator either runs at/near full output for an extended time, there is a danger of burning it up. Like engine life, alternator life isn't very long at maximum output.
I had my alternator built to partially take advantage of the AGM battery's ability to absorb a very fast high-amperage charge:
Any time your charge system is significantly smaller in ampere capacity than the charge acceptance of your battery banks, you need to ensure that the charging system can handle extended periods of maximum output. For example, the alternator that was easily capable of dealing with 400 amp-hours of flooded cell capacity could burn up when you switch to AGMs since AGMs can bulk absorb a lot more charge. However, installing a 4x bigger flooded cell bank could have the same effect.
Given the charge acceptance rate of Concorde AGM batteries, I could conceivably draw 4x AH capacity (95AH) = 380A while charging - IF the alternator was capable of that output. Since the battery is capable of the higher acceptance rate, the alternator goes to full (or near full depending on heat) output until the battery demand tapers off. If the alternator winding is capable of higher output than original, heat is greater and parts not designed for the job suffer.
There's a big difference in AGM batteries compared to flooded cells. Flooded cells have a charge acceptance rate of about 25% of AH capacity - so if you have a standard Group 24 battery at ~70AH you might be capable of only charging that battery at a rate of 18A - a huge difference from the 180A in the above example of my battery. The lower charge acceptance rate of a standard battery serves as a limiter to alternator output if no accessories are turned on. When accessories and/or higher charge acceptance rates are added, the alternator starts to work harder.
I guess my point is that he could have made a change to his electrical system that is calling for more juice and burning up alternators that are capable of higher than 90A output, but not without burning up due to all of the parts not being up to the task.
Frank
