Chuck Husick: Techno-Talk, January 2001 BoatUS Magazine - Updated December 2008
- What Type?
Boats eat batteries. Replacing disposable
batteries is simple, buy spares for all sizes, and the one you don't
have will be the one you need. In contrast, buying the right storage
battery can be confusing. A number of different types are offered, at
widely varying prices.
If your boat's electrical system powers
only a radio or a GPS when the engine is off, you won't go wrong by
replacing the worn out battery with a marine starting battery of similar
size. f your boat has equipment that consumes power when the engine
is off, for example, a refrigerator or a microwave, you need a marine
deep cycle battery. A battery undergoes a deep cycle whenever more than
20% of its stored energy is consumed before it is recharged. Used correctly,
deep cycle batteries can provide 200 to more than 3000 discharge/charge
cycles. Starting batteries are not designed for sustained discharge
and will last for only 50-60 use cycles.
You need to make three decisions when you
buy a deep cycle battery. How big should it be? How long do I want it
to last? What type of battery technology is best for my use. The "how
big" is easy. Buy the largest capacity battery that will fit in the
battery box. Deciding how long you want the battery to last is more
complicated. High cycle life batteries cost more. However, the cost
per cycle goes down as battery life goes up. You will want to find the
best trade-off between price and cost per cycle.
A boat used six months of the year will
experience about 60 battery cycles, about one for each day of use. A
light/medium duty battery good for 200-300 cycles will last about three
years. Use the boat almost every day of the year and you will accumulate
about 200-250 cycles. A medium/heavy duty grade battery will provide
500-750 cycles, or about three years use. A sailor off on a world ranging
voyage can put 700-1000 cycles on a battery in one year. The price of
a 3000 cycle life very heavy duty battery will be a bargain for his
boat but will give you a non electric shock. Remember, batteries wear-out
even when not being used. Buying one that whose cycle life won't be
consumed for 10 years is not a good idea.
Once you have selected the battery life
that's best for your boat you need to choose the type you want from
among the three generally available; flooded cell, gel cell or AGM (absorbed
glass mat). Flooded cell batteries have removable cell caps and need
monthly inspection of the fluid level in the cells (and addition of
distilled water if it is low). To ensure long life these batteries must
be recharged soon after each use. Like all batteries, they lose charge
over time and need to be recharged about once a month. The gel cell
and AGM batteries have no cell caps, only pressure relief valves, and
require no maintenance.
When not in use Gel Cell and AGM batteries loose charge more
slowly than the flooded cell type and can withstand being left in a
partially discharged condition far better than a flooded cell battery.
However, they cost about 90% more than a flooded cell battery; and neither the gel cell nor
the AGM can last as long as the highest grade flooded cell brutes. A number of "new" batteries based on the use of exceptionally pure lead are now available (at premium prices that exceed the price for a quality gel cell or AGM battery of equal capacity. The in-service worth of these batteries will become clear during the next couple of years.
Now that you have bought it, keep it healthy:
(1) keep the top of the battery clean and dry. A film of dirt will allow
current to flow, slowly but surely discharging the battery; (2) avoid
consuming more than about 50% of the stored energy from the battery
before recharging it. For most types, the battery will be 50% discharged
when its voltage drops to about 12.2 volts; (3) recharge promptly, no
battery benefits from sitting around in a semi discharged state.
Consider your answers to these three questions
when you buy your next battery: (1) are you willing to service it (2)
will you recharge the battery after each use cycle (3) your battery
budget. Regardless of the type of battery you choose, a battery that
will last about three to four years is a reasonable trade-off for most