Chuck Husick: Techno-Talk,
March 2001 BoatUS Magazine - Updated September 2008
storage batteries on your boat must be properly recharged after use.
So long as you use the boat fairly frequently and don't run the battery
down too far by excessive use of lights or radio when the engine is
not running, the engine's automotive-style alternator will do an adequate
job of keeping the battery charged. With reasonable care, the battery
will probably last about three years. However on some boats, especially
those where the batteries are located at a distance from the engine,
the standard charging system will fail to fully recharge the batteries.
Batteries that are not fully and promptly recharged will gradually lose
energy storage capacity and will have a shorter than normal life.
power for the boat and for battery charging is provided by an engine-driven,
automobile-type alternator, controlled by a voltage regulator built
into the alternator's case. The voltage required to properly recharge
a storage battery varies with battery temperature. Cold batteries
require a higher voltage, warm batteries a lower voltage. The built-in
regulator's temperature sensor measures alternator temperature,
not battery temperature. This charging voltage control system design
assumes that the battery is close to the engine/alternator and its
temperature is directly related to the "under the hood" temperature.
system works well on boats where the battery is in the engine compartment
and its temperature is influenced by engine temperature, as it is
in a car. However, when batteries are located away from the engine's
heat, as is frequently the case for house batteries and especially
on sailboats, the batteries will be much cooler than the voltage
regulator's temperature sensor assumes them to be. The charging
voltage delivered from the alternator will then be too low, slowing
the recharge process and often preventing a full recharge even after
a long period of engine operation. Repeated failure to fully recharge
batteries can result in a loss of 20% or more of energy storage
capacity and shorten the useful life of the battery.
charging problems created by inaccurate sensing of battery temperature
can be eliminated by replacing the alternator temperature sensing
voltage regulator with one that measures the actual temperature
of the battery being charged. The external voltage regulators sold
for marine use include a temperature sensor that is clipped onto
the terminal post of a battery. Using this actual battery temperature
information, the regulator will ensure that alternator voltage matches
the battery's charge voltage requirements. By sensing battery voltage
at the positive terminal of one of the batteries in the boat's battery
bank the remote voltage regulator will eliminate any charging voltage
error that may arise due to the resistance of the wires between
the alternator and the battery.
"smart" microprocessor-controlled external voltage regulator will
also sense the charging voltage at the terminal of the battery being
charged. Direct sensing of battery voltage eliminates the low battery
charge level problems that result from the use of diode type battery
isolators. In addition, a smart regulator can periodically "equalize"
flooded cell (wet) deep cycle batteries by applying a controlled,
higher than normal charging voltage that brings all cells to an
equal and optimum state of charge, thereby assuring maximum performance
and battery life.
external voltage regulators can be adjusted to match the charging
voltage requirements of the type of battery being used. This especially
important with gel cell and AGM batteries that require a lower charging
voltage than flooded cell batteries. Gel and AGM batteries can be
seriously damaged or destroyed if charged at the voltages appropriate
for flooded cell batteries. Each type of battery must be provided
with an appropriate charging system.
regulators equipped with an alternator temperature sensor will automatically
control the alternator's power output as necessary to prevent damage
from overheating. The temperature of an alternator depends largely
on how much power it is being asked to produce. Large DC/AC inverters
can impose very large power demands on the alternator, as can the
high charge currents that AGM batteries will accept.
multi-step battery charging program managed by a smart, external
regulator charges batteries as rapidly as possible while protecting
them from being damaged by excess current or overheating. Programmed
charging also ensures the most complete recharge possible, maintaining
maximum battery energy storage capacity for the longest possible
time. The $150 cost of an external regulator will be recovered by
improved battery performance and life in a few boating seasons.