well the problem with GEL batteries is they can take a surface charge and fool the BDI into thinking they are good voltage wise but when you load test them that will tell the real story. AGM batteries are the same way.

Those type batteries are great because they hold a strong charge for longer than lead acid batteries, but thier falloff point even tho is longer than lead acid batteries are they fall off real quick when they do. The Pro's being longer stronger discharge rate before falloff but the Con's are when they go bad its not gradual, they usually just go suddenly.
At least from my experience anyway...