In article ,
Percival P. Cassidy wrote:

But are you saying that the "Automatic - Deep Cycle" setting on an
"ordinary" battery charger (it's one I bought originally for the car
batteries, but the "Deep Cycle" setting seemed like a bonus) is not a
reliable way of keeping the batery in good shape?

It's quite possible that this charger is not well suited for long-term
float charging of a battery, "automatic" or not.

Many standard chargers are designed mostly for "refueling" a depleted
battery. They often use a two-step charging algorithm, to perform the
"bulk" charge (high initial current level which brings the battery up
to about 80% of full capacity), and then a "topping" charge (lower
terminal voltage, resulting in a lower current) to bring them the rest
of the way up to full capacity. The switchover between bulk and
topping charge happens automatically based on the battery's terminal
voltage and/or the current level... it's done at a point which trades
off the speed of recharge, and the possible loss of electrolyte.

The terminal voltage during bulk and topping charge can often exceed
14.5 volts, and might be over 15 volts depending on the charger design,
battery type, and temperature. It's high enough to result in some
loss of electrolyte, if the battery gasses rapidly enough that its
recombination catalyst can't turn the gasses back into water.

Many of these chargers do *not* incorporate circuitry which will
detect the "full charge" state, and switch over to a proper
float-charging regime. Proper floating voltage is a good deal lower
than recharging voltage, and depends on the temperature... I've seen
figures ranging from 14.1 volts (freezing) down to as low as 13.4
volts (40 degrees C). It also depends to some extent on the specific
battery type and chemistry.

A two-phase bulk/topping charger is likely to keep the voltage on the
battery rather too high for proper floating. Gassing and loss of
electrolyte can occur as a result.

For best charging performance, you really want to have a sophisticated
three-phase charger, with temperature compensation for all phases of
the charging cycle.

As an alternative (possibly cheaper), use your car-battery charger to
recharge the battery after use, and buy/build a well-regulated
low-current voltage supply to use as a float charger. If your battery
will be stored in a location which has a relatively constant
near-room temperature, you could build a simple LM317-based voltage
regulator, trim its output for 13.6 volts, and be pretty confident
that you could float your battery on this without risking excessive
outgassing.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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