Fwiw, I'd be tempted to move the items that I will use when trolling over to
the trolling batteries along with the trolling motor. The fishfinder and
livewell. That way your primary run battery is not in use much while the
outboard is not running. You can pull 12v off the first of the two trolling
batteries for accessories even though you have them wired in series for a
24v trollling motor. An a-b switch correctly added would also give you the
ability to start from the first trolling battery in case your primary run
battery goes down. There are some really nice electronic chargers that can
be mounted with your agm batteries. You'll be able to simply plug them in
when you get home. Pricey though. Unless there is a big price difference
I'd go for the 70lb motor. The battery draw is more a function of how the
actual load on the motor than it is on the motor's maximum thrust.

wrote in message
oups.com...
Sorry, I missed one point that you had mentioned in your post. You
are saying that I really don't need the extra two batteries. I would
like to learn more. If I really don't need them, I will be able to
cut down at least one hundred pounds from the load, and this would be
a very good thing.

In order not to get the extra two batteries, I will need to use the
current two dual purpose batteries to do the following things:

o First dual purpose battery: I will need to use it to crank the
outboard and use it to power fishfinder, GPS, VHF radio,
livewell for baits, and _may_be_ a pair of trim tabs. I have
a feeling that one battery cannot handle that many tasks,
and I may need to split the tasks into two batteries.

o Second dual purpose battery: I will need to use it strictly for
trolling motor. This also means that I will need to pair it with
a 12-volt trolling motor, and this is limited to no more than
55-lb thrust.

Based on West Marine recommendation, 55-lb thrust should be enough for
a 18-ft 2000-lb fiberglass boat. But based on tech support in a
trolling motor company, they recommend 70-lb thrust for that size of
boat, and that will require a 24-volt system and two 12-volt
batteries. This makes me a bit confused. If I interpret this
correctly, this means more is better, and less is OK if I am not using
it in strong current or high wind situation. If I understand this
correctly, 55-lb thrust probably is OK for me, and this also means
that I probably only need one battery for the trolling motor.

In summary, I may only need totally 3 batteries instead of 4 batteries
as what I originally thought. This is good because I can cut down at
least 50-lb to 60-lb from the boat load. Thanks for getting me
thinking along that line.

Jay Chan


On Mar 22, 10:44 am, "Tim" wrote:
Jay, If I remember correctly, your boat has a Mercury outboard[?]

16a. is about all you're going to get out of it total. and as far as I
know,t here isn't any performance stator to give you more power.

In a nutshell, the outboards charging system will charge all your
batteries, but it will take quite a while. You can't expect automotive
performance with that small of output.

A basic rule of thumb is that if you take a 650 CCA battery that is
flat dead (or almost), it will take a 10 a battery charger a good 6
hr.s to recharge it. That rule is a bit inefficient, but gives you an
idea on how battery charge rates work.

Of course you're not taling charging 4 dead batteries on 16a either.
Another thing to consider, is that the stator builds up a lot of heat
under the enclosed flywheel, and doesn't have any fan cooling. It
will usually handle what it's intended for, but under extended hard
loads, the Stator gets really hot, and burns off the enamel insulation
on the stator wires, and shorts them out, then it doesn't charge at
all untill you get the Stator replaced.

Not handy, and not cheap.

But if you really feel you need to go this rout, instead of going with
a battery isolator I'd go with a battery switch where you can pick and
choose which set of batteries you need to charge . the problem with
battery isolators, is that because they use heavy diodes, is that you
have a voltage drop due to the power absorbtion of the diodes
themselves. and with only 16a (possibly 20a peak) you need all the
help you can get.

Honestly, I think you are going a bit of over kill by adding the extra
batteries, but then again, it's your boat.

Good Luck.



wrote:
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- Hide quoted text -

- Show quoted text -