Redarc says the 25 model is for batteries between 75-200ah size.
You say you have a 100ah so the answer seems clear.

"For most applications, the BCDC1225D is suitable for battery bank sizes
of 75-200AH. For higher capacity needs of over 200AH, REDARC recommends
the BCDC1240D. "

Mark

Bill Berg wrote:
> Anyone on here familiar withe RedArc battery charger /solar controller ?
> Considering it to charge 100ah Lithium house battery and trying to decide
> between 25 or 40 amp models. Anyone have experience with these? Just want
> to make sure I don't go to high on amperage and overheat the alternator .
> Currently running a 90 amp alternator with no unusual loads over stock
> Vanagon.radio and interior lights and usb all on house battery and
> eventually a all electric fridge.
>
>