That's really a very small alternator. I would suspect that while running
you don't have much more that 8 to 10 amps available for charging. The
alternator has to supply the power to operate the boat as well as to charge.
I would not plan on being able to bring two trolling batteries up very far
at the end of the day with that configuration. I suggest you mount a good
electornic charger capable of charging your both your trollling batteries at
the same time you install them. Then simply plug it in when you get home.
If you are going to have 2 dedicate trolling batteries you really don't need
to have two additional batteries for the regular operation. One regular
battery will suffice to operate the boat. You can add a simple switch to
use the trolling batteries for starting in an emergency.

wrote in message
ups.com...
My boat has an alternator that has a 16-ampere stator (and voltage
regulator). Is there a formula to calculate the number of batteries
that the alternator can charge? Can it charge two or four batteries
all at the same time?

I am asking this because my boat already has two dual purpose group-24
batteries (for starting battery and house battery), and I will add two
deep-cycle batteries for trolling motor. I think this will be nice if
I can use the outboard motor to recharge all four batteries when I am
done with fishing and heading home. And all four batteries will be
ready for next day boating/fishing trip without worrying about
recharging them at night.

Can this alternator handle AGM batteries instead of flood batteries?
I am under the impression that AGM batteries may be accepting charge
too fast that they may over-stress the alternator and overheat it (or
something like that if I understand this correctly). I am wondering
if this is relevant to my alternator.

I am asking this because the batteries-storage area in my boat is very
difficult to reach (tiny room with a tiny access door). Seem like AGM
batteries will be suitable for me because they don't need as much
maintenance as flood batteries - at least I don't need to physically
pull them out to add water. But if they could overheat the alternator
in my outboard, I would have to pass on them and choose flood
batteries instead (and relocate the batteries to some place more
accessible).

Thanks in advance for any info on this issue.

Jay Chan