Boat Forum / Cruising / March 2004

Lead acid batteries in parallel are hard to manage for charge.  A better
solution is a starter battery for the engine and a deep cycle (house)
battery for everything else.  Switch between the two for charging.  Just
remember that most outboards that size usually put out only 10 amps or
less which is fine for topping off the starter battery but you are not
going to be able to charge the house battery at much of a rate once it
is down.  You will have to shed load down to what the OB charging
circuit will support in order to maintain voltage.  Best to charge the
house battery to full charge with shore power to before you go out.  If
you need more, add another fully charge house battery.  I have 3
batteries on my toon boat.

Ron

Not familiar with this particular engine. If it has a
generator/alternator like the comparable Evinrude - this only
puts out a few amps -  six tops, if I recall.

So if you feed two batteries in parallel instead of one, what's the
effect?    I don't think it alters the 'strain' or load on the
generator.    You won't be able to take more amps out of TWO
batteries than you can out of ONE if you rely purely on the OBs
generator to charge them.

So the real benefit of having two batteries is this:
If you charge them at home - you have twice the ampere hours for
use afloat, with the discharge rate slowed down a little by the OBs
generator.

That's probably a safe way not to get stranded without electric
power....
Some users with troll motors use seperate batteries for trolling - so
they retain electric start when the troll motor flattens its battery -
as it will just at the most awkward moment.

Brian Whatcott    Altus OK

Typically, outboard alternators are pretty weak. Yours probably isn't
giving much of a boost while running. Our 55 amp alternator can hardly
give us a night's usage in an hour because of it's (basically)
automotive regulator.

You (and we) might do better by mounting a small solar panel to charge
the batteries during the week. A half-amp for 70 hours or so a week will
probably be more than you get from the engine.

You *probably* won't cook the alternator with the extra battery.

I'm a degreed electrical engineer, which means my answer will work on
paper but may not in your boat.  Go easy the first couple times and
see.  That's the engineer's final test!  I'll give a little intuition
on how it will work and then give you the tools to crunch the numbers.

Think of electricity as a big water tank.  The water in the tank is
the electric current, or load.  The water pressure is the voltage.
Your magneto is a special device that has unlimited water to pump into
the tank, but has a fixed rate pump that shuts off at a given
pressure.  Thus it fills your water tank when the pressure drops until
the pressure is restored.

Now if you have two water tanks, you've got twice the water but the
same amount of pressure.  So if you run both tanks dry, it will take
twice as long, relative to a single tank, to fill the tanks back up.
But the pressure has not changed so the strain (pressure) on the
magneto is no different.

Anyway, it's fairly simple but a little tedious to calculate how
multiple batteries are going to change your situation.  I'm only going
to cover batteries in parallel.

There are a few numbers you should find for your batteries.  The
first, you've already got, is the rated voltage.  The second, is the
number of amp-hours the battery is rated for.  Third, either assume or
find out, how the manufacturer of your battery measures amp-hours.  

A brief digression on amp-hours.  The standard is to measure amp-hours
for a twenty (20) hour period.  Amp-hours are calculated as an average
load.  This average load is 5% of the rated capacity.  So for a
battery rated at 7Ah (Amp-hours), the manufacturer made the amp-hour
rating assuming a constant load of 0.350 amps.  If your load exceeds
this, the performance of the battery will not match manufacturer's
specifications and will discharge faster than expected.

Two batteries in parallel means double the amp-hour capacity (two
water tanks).  So if you have two 7Ah batteries in parallel, you
effectively have 14Ah of capacity.  Which, as your gut intuition told
you, means longer lasting.  Is this better?  Well it probably is for
what you're doing.  But notice, that you still shouldn't exceed the
0.350 Amps of draw if you want to meet manufacturer's specifications.

The following equation will let you do a "back-of-the napkin"
calculation.  It errors on giving you a liberal answer, meaning you
probably won't get the complete expected life.  In my line of
engineering we use a 50% rule meaning that we engineer to 1/2 of
manufacturer's specifications.  Expensive?  Yes.  Reliable?  Yes.  So
if you're willing to handle the expense, halve the answer you get
below.  I also realize you probably don't know the current draw so the
equation is probably useless.

(Expected life) = (Amp-Hour Rating) * (Number of Batteries) * (20) /
(Current Draw)