Gemini 105Mc Owner’s Manual
17
Copyright © 2004 Performance Cruising Inc.
Batteries
The Gemini comes with (3) three batteries #24 Deka
Deep Cycle Marine Batteries. There is room for (1)
one additional battery.
The batteries are number 24, deep cycle with a
capacity of 85 amps each. On the battery switch,
“Battery One” refers to two batteries connected
together while “Battery Two” refers to the singular
battery.
A Note on Deep Cycle Batteries: Unlike conven-
tional car batteries, deep cycle batteries hold a
charge better and retain an almost constant voltage
until just before the battery dies. However, in re-
taining an almost constant voltage, it is difficult to
tell how much charge is left in the battery. The
only accurate way is with the use of a hydrometer
to check the acidity of the battery fluid. Battery
managers that measure the flow of current into and
out of the battery are good for continuous use but
do not work well if the boat is left for long periods
of time without use. Batteries lose charge by inter-
nal degradation at the rate of as much as 1/8 am-
pere an hour. As batteries are used, the distilled
water changes to sulfuric acid and eventually the
formation of sulfate crystals coats the plates in the
battery and the capacity of the battery is reduced.
For example, if the battery is left at 60% charge for
long periods of time, then the battery will not ac-
cept a charge of more than 60% while charging. It
is possible to tell when the battery is near dead
when it can be fully charged in a short time.
Solar power is an efficient way to keep batteries in
good condition as solar power puts out low current
at high voltage. The solar panel, available as an
option, charge at up to 10 amps per hour.
A battery can be charged to 80% easily but the last
20% of charge requires a lot of time so that the bat-
tery is not overheated. That is why running the
engine to charge the batteries is not efficient no
matter what capacity alternator is in use.
The new smart chargers require a thermometer on
the battery.
Gel cell batteries are expensive and can be dam-
aged by over heating and theoretically can only be
charged and discharged one quarter as many times
as deep cycle batteries.
Checking Battery Levels
Deka recommends checking the electrolyte fluid in
your batteries weekly and/or before any prolonged
battery charging, such as extensive motoring.
Checking the batter level simply means ensuring
the electrolyte is filled to the indicator line on the
battery case (which covers the plates). If the bat-
12 Volt Power
tery needs fluid, use only distilled water. To go a
step further, you can use a hydrometer to test for a
specific gravity of 1265 following a charge. Start
with the cell closest to the (+) terminal. The spe-
cific gravity reading should be between 1.100 and
1.300 (Specifically 1.265).
Be sure to keep the battery connections clean as this
will help ensure longer battery life.
Battery Monitors
Battery monitors measure everything that goes in
and out of a battery through the negative side. This
is a highly effective way of keeping track of available
power. A voltmeter is not accurate enough as it will
not tell you what state of charge your battery is in.
Battery Switch
The battery switch is located above the batteries.
Caution: Never switch from position “1” to posi-
tion “2” through “OFF” – especially when the en-
gine is running as damage to the alternator could
occur. Always switch through the “BOTH” posi-
tion.
With Battery “1” selected on the battery switch, all
12 volt power is coming from the first set of batter-
ies and only the first set of batteries are receiving a
charge from the alternator with the engine running.
Battery “2” is not in use. When battery “2” is se-
lected all 12 volt power is coming from the single
battery and the single battery is receiving a charge
from the alternator with the engine running.
If battery “1” is used and left fully charged before
switching to battery “2”, then there will always be a
full spare battery and vice versa.
The Gemini does not make use of a designated en-
gine starter battery as the smaller Westerbeke en-
gine does not require a large amount of current to
start. In addition, a designated starter battery re-
quires the use of an automatic switch. With an auto-
matic switch, once the engine is started, first the
engine battery is fully charged and once it reaches
13.8 volts then the other ship’s batteries are
charged. The problem with this system, as discussed
earlier, is that it is easier to charge a flat battery
