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)


On Thu, 17 Jul 2003 16:19:10 -0400, "Belgian"
wrote:

Hi , i was wondering if there is any danger to fry the magneto of an
outboard 40hp Suzuki engine if install 2 12 volts battery in parallel .
I do use lots of 12 volt equipments on the pontoon, so i figured that 2
batteries would be better and last longer .Last year a battery died on me in
the midle
of the lake. I just bought 2 brand new marine ones .
would that be too much strain on the magneto ?
thanks for your help !