Hi, Comments imbedded below:

On Mon, 14 Nov 2005 19:22:36 -0800, jaykchan wrote:

First, are you SURE you need a 24VDC trolling motor? As you can see, it
complicates things greatly. Minn Kota has a nice 55-lb-thrust motor that
runs on 12V: the Max55T. I think it would push your boat fine, and will
pretty much run all day on a fully-charged group 24 battery (at
trolling speeds).

Last time when I asked around here, I came to the conclusion that I am
probably better off having a 24V trolling motor.

That's a common problem with advice "around here". It seems it's always
buy Bigger, Stronger, Faster, and damn the consequences! You might get a
bit more performance from a 24V motor, but look at all the problems it's
causing! Maybe see if you can make some agreement with a local shop that
you try a 12V motor (measure speed with GPS), and if it's not
satisfactory, trade it up for a 24V model.

I'm using a 12V motor (Max55T) as the PRIME "engine" for my 10ft
inflatable, and I used a smaller Max40T just as you describe on my 12-ft
skiff.


Second, if you decide on a 24V motor, you CAN buy a third
battery and put it in series with your deep-cycle battery: just
make sure it's the same type, etc. (yours is basically
brand-new,right?).

Unfortunately, the existing deep-cycle battery is not new. It is
probably newer than the boat itself. But it is not exactly new.
Therefore, I don't intend to combine the existing deep cycle battery
with a new battery to make a 24V system.

Although it's IDEAL to have two nearly-identical batteries in series for
your 24V system, it's by no mean required. I might buy two new ones when
the older one wears out, but no reason to throw out a perfectly good
deep=cycle battery!

The other thing that makes me uncomfortable about your suggestion is:
If we combine the existing 12V deep-cycle battery with a new 12V
battery to make a 24V system for the trolling motor, we will only have
one battery left to do everything else (starting motor, powering fish
finder, VHF radio...etc), right? Unless I am misunderstanding
something, I will be very hesitated to do that. Please clarify.

Two things:
First, it wouldn't be a big problem running "everything else" on a single
battery. "House" batteries (the second one) is great if you have a cabin,
with cabin lights, DC fridge, stereo running all evening, etc. But for an
18ft open boat: I'd guess you have less accessory-power on it than your
car, and your car only has one battery, right?

Second: you can still use the one 12V battery of the 24V system as your
"house" battery. It will mean it will be drawn down SLIGHTLY more than the
other one, but realistically it just won't make a difference. I'm an
Electrical Engineer: I can run the boring numbers for you if you like, or
you can just trust me on this.

Use quick-connect connections, and when you want to charge
them, re-arrange them to parallel. ...

I always like the concept of "Quick Connect". That's why all the water
hoses in my garden have quick-connect. How does the "quick-connect"
look like in the context of battery connection?

You could probably use one of the heavy-duty connectors used in automotive
or RVs (most are made by Molex - try searching for that). But again the
simplest is the butterfly-connectors you probably have on the battery
already. Just unscrew them and pull out the spade connector. Then
reconnect the two batteries in parallel and charge them with a 12V
charger. It takes a few minutes, but I'm pretty sure that's what 99% of
the people using 24V trolling motors do.

Again, IN THEORY you shouldn't charge unlike batteries in parallel, but
I've been doing it with wildly-different sets (like 50%-charged golf-cart
battery (actually 2 in series to get 12V) in parallel with 90%-charged
Group 27) and have never had a problem. Again, I can run the numbers if
you like...

Lloyd