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If you have a fridge on your boat you are
probably obsessive about batteries. If you are
not the fridge is turned off. If you do not
have a fridge but have a radio you need power
to run it and the power comes from your
batteries. You might find the following which
came from the Ample Power Primer (www.amplepower.com)
useful.
Best methods to kill your batteries:
Overcharge the battery by applying a voltage
above 13.8 Volts for extended periods.
Undercharge the battery by never charging it
beyond 13.8 Volts.
Discharge the battery and leave it that way
for a few days or weeks.
Let the battery sit unattended without
charging (refer below).
Repeatedly discharge the battery beyond the
optimum 50%.
Allow the electrolyte to slosh around when
the battery is deeply discharged.
Boil enough electrolyte to expose the
plates.
Charge or discharge at too high a rate.
Use a starter battery in a deep cycle
application.
Leaving Batteries unattended.
Non sealed batteries with a liquid electrolyte
are likely to have lead-antimony plates. While
antimony makes the plates stronger, it also
causes battery cells to self–discharge more
rapidly. Self–discharge is a deleterious
discharge because it creates a hard lead
sulfate that crystallizes and ultimately
destroys the battery. The only way to avoid
self–discharge is to keep lead–antimony
batteries on a charger when not in use.
Maintenance free batteries may have
lead-calcium plates which have a lower rate of
discharge, particularly in cold weather. They
may appreciate a charge a couple of times over
winter.
GEL and AGM batteries have low rates of
discharge.
Types of batteries:
Starting Battery
A
car battery. To produce the current necessary
to start your car it has thin plates the
combined surface of which allows high current
to flow. A new battery may fail if deeply
discharged. It is designed to start an engine
and then be recharged by your alternator,
again at a fairly high rate. Some articles
recommend not discharging more than 20%.
Deep Cycle Battery
To
enable deep discharges a deep cycle battery
has thicker, fewer and batter insulated
plates. The battery won’t sustain as high a
rate of current, but will permit deeper
discharges without imminent failure.
Gel Batteries
Gel batteries use the same lead–acid chemistry
of conventional liquid units but capture the
acid in a silica gel. The plates can be made
thinner and they therefore allow higher rates
of charge and discharge that a deep cycle
battery. Because of the manner in which they
are built
they will withstand deep charges. Any sealed
battery is potentially more sensitive to
charging voltage which may boil the
electrolyte. Some suggest a charging current
limited to 14.1 volts.
AGM Batteries
Other sealed batteries (absorbed electrolyte)
capture a small amount of acid in a fiberglass
matte separator. Because there is no liquid to
slosh around the plates of a sealed battery,
plates can be made thinner and still withstand
deep discharges. A gel battery is thus capable
of high rate charge and discharges, and offers
a great many deep discharge cycles. Some
suggest is not as sensitive to charging
voltage.
Charging batteries:
The Bulk Charge step.
When a charge source is first applied to a
well discharged battery current begins to flow
typically at the maximum rate of the charge
source. Because most of the charge is
delivered at the maximum charger rate, the
first step of the charge cycle is called the
bulk charge step. During the bulk step,
battery voltage will steadily rise.
The Absorption Step
At
the instant battery voltage has risen to the
maximum allowable voltage of the charge
source, which is typically around 14.4 volts,
current through the battery begins to decline.
It declines because the battery is absorbing
all it can at the set voltage. This
simultaneous event of reaching maximum voltage
and the start of current decline marks the
beginning of the absorption step. The
absorption step should continue until current
through the battery declines to about 2% of
battery capacity in Amp–hours. Good batteries
will reach a steady state current at less than
1% of Ah rating.
The Float Step
Once a battery is full, a lower voltage should
be applied that will maintain the full charge.
Depending on the type of battery, (liquid,
gel), and the age of the battery, 13.4 – 13.8
Volts is appropriate as a float voltage.
Hydrometers and Specific Gravity:
Hydrometer Measures Weight
A
hydrometer measures the specific gravity, SG,
of electrolyte, that is, how much the
electrolyte weighs compared to pure water. A
reading should be corrected for temperature
and the battery should have been rested for 24
hours prior to taking a reading. e or
discharge for 24 hours preceding the SG
sample.
Battery Health Determination
Despite problems obtaining valid state of
charge measurements, the hydrometer is easily
used to determine battery health. In a healthy
battery all cells will have about the same SG.
If there are small cell to cell variations,
then an equalization charge is needed. A
typical SG is 1.265. That is the electrolyte
weighs 1.265 times as much as water. Usually
the decimal point is dropped and the SG is
referred to as 1265 points. Cell to cell
differences of up to 30 points can be
corrected by equalization. A difference of 50
points or more from cell to cell indicate a
bad battery.
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