My 2011 Silverado truck has a 145 amp alternator controlled by a computer with a very sophisticated 7 stage charging algorithm to charge the OEM starter battery and to maintain the proper amps in the system as various loads are applied to the system by the head lights, brake lights, fans, radio, air conditioner, heater, wipers, etc. When I installed a deep cycle battery (with an isolator) and tried to use the existing charging system to charge the deep cycle battery, as soon as the OEM battery was fully charged (which did not take long) the alternator stopped generating amps. Consequently, the OEM system would not adequately charge the deep cycle battery. After much research, I installed the Ctek 250S charger which can provide up to 20 amps utilizing a five stage charging algorithm. The unit was pricey at $238, but it works great.
https://www.etrailer.com/Battery-Charge ... 56676.html My alternator, like most alternators, is designed to operate continuous duty at no more than about 65% of rated capacity (145amps x 65%= 94 amps maximum), otherwise the diodes in the alternator are at risk of will burning out. Before I bought the Ctek unit, I made sure that the extra 20 amps drawn by the unit would not exceed the 94 amp maximum.
I tested the alternator output with an amp meter by turning on the various loads one at a time. With every load turned on the alternator was outputting 76 amps. Consequently, I had about 18 amps to spare before installing the Ctek unit. I monitor my batteries with a Bogart Trimetric meter as I drive and am very happy with the Ctek units charging stages.