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Battery chargers are voltage transformers. They take 120 volt AC power and
transform it to lower voltage DC power for battery charging. The voltage of the
battery charger is set higher than the battery voltage. This is what causes
current to flow.
If the charger voltage is set too low it will take a long time to charge your
battery. Many battery chargers are intentionally set at a low voltage which
allows slow charging without over charging the battery. If the voltage is set
too high, your battery will be charged and it will begin gassing as it is over
charged. This is done intentionally during an equalization charge, but should
be avoided at all other times.
A constant voltage battery charger really has two stages. The first stage, or
bulk charging stage, occurs when the battery voltage is low and the charger
delivers its maximum rated current to the battery. The current remains constant
as the voltage rises. Once the voltage rises to the voltage setting of the
charger the current starts decreasing. This is the second stage or absorption
stage.
Some battery chargers have a third stage that occurs once the current has
decreased to a trickle charge and the battery is full. The charger reduces its
voltage setting to a float voltage and a small current flows into the battery
in order to maintain it at a full charge.
Maximum Charge Rate
A rule of thumb is to divide the capacity of your battery by 10 (this is called
the C/10 rate) to arrive at an appropriate charge rate. The initial charge rate
may be as high as C/3, but this rate will only be maintained for a short time
and it may not be economical to purchase such a large battery charger.
Inverter/Chargers
A generator powered battery charger is most efficient when the initial rate of
charge is between C/10 and C/3 of the rated capacity of the battery bank. This
is a consideration when purchasing an inverter with a built in battery charger.
There is an advantage to purchasing an inverter with a greater battery charging
capacity.
Charger Efficiency
Charger efficiency is important since the size of your generator may limit the
maximum charge rate of your battery charger. Cheap automotive type chargers
achieve only 50% to 60% efficiency when running on a generator. Better quality
chargers will achieve as high as 86% efficiency. Your generator must be large
enough to operate your battery charger and the other loads you operate while
battery charging.
Trickle Charge
The final or trickle charge rate of your battery bank is calculated by dividing
the battery capacity by 100 (the C/100 rate). Once the current from your
battery charger tapers to this rate, your battery is essentially full. Another
two hours of trickle charging is recommended, but this slow charge rate may not
be cost effective when charging from a generator. If you anticipate a sunny
afternoon, charge your batteries with the generator in the morning and let the
sun supply the trickle charge.
Charge Time
It will take longer to charge your batteries than you expect. A battery voltage
of 14.8 volts does not mean the batteries are charged and the generator can be
turned off. Do not be fooled by your voltmeter. Your batteries have not
completed charging until the charge rate has tapered to the C/100 rate.
Battery Life and Fuel Costs
Some people choose to charge their battery only to the 80% or 90% level, and
equalize more often. This results in a trade off of battery life for fuel costs
and generator maintenance. The best choice is to coordinate heavy appliance use
(vacuuming and washing) with your battery charging. Operate those loads while
the battery is trickle charging.
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