Care and Feeding of your
Sealed Lead
Acid Battery
(aka - gel cells)
Lead Acid (Gel Cell) charging
Lead acid (gel cells) should be charged with a constant potential charger specifically designed for these batteries. These are sometimes referred to as a CVC charger. You can charge them with a constant current charger but you must terminate charge when the voltage reaches 14.7 volts. You should not exceed the C/10 charge rate. If you have a 7 Ah battery in your field box the maximum constant current charge rate should not exceed 700 mA. It will take about 14 hours to charge from a fully discharged state (voltage less than 12 volts).
A CVC (Constant
Voltage
Charger) is exactly what the name implies. It is clamped at a
certain
voltage and puts out all the current it can until the battery reaches
the clamp
voltage, usually something around 14.5 volts, then the current drops
off to
maintain it at this voltage. A constant voltage charger is
characterized as one
having a current capability of supplying a fixed voltage to whatever
load is
applied. A constant current charge on the other hand will provide
whatever
voltage is necessary to force a fixed
value of current
though a
load. Constant current charges have a much higher internal resistance
than the
load so that any variation on the load will not change the current
being
supplied. Constant voltage charges have a very low resistance as
compared to
the load and will supply whatever current necessary to maintain a given
voltage
at the load.
Many inexpensive chargers used for sealed lead batteries are what is called taper chargers, these are set up so the voltage tapers off as the full charge voltage is reached. True constant potential (CVC) chargers can be quite expensive so a compromise is made in the design to control costs.
We have used the
term sealed
lead battery in this discussion. These batteries are not truly
sealed as
cylindrical Ni-Cds are. They have a gelled electrolyte system where
there is a
modest recombination of the oxygen in overcharge in some designs. All
require
venting of the oxygen and hydrogen byproducts of charging and
discharging. This
is why you should never totally seal these in a field box where these
gasses
can accumulate. Mixtures of oxygen and hydrogen can cause spectacular
"events" if a spark is provided (from an electric fuel pump motor).
How much
charge is there
in the battery?
Unlike Ni-Cds you
can read the
remaining capacity quite easily with a voltmeter.
After the battery has been on rest for a few hours read the voltage (no load). 12.0 volts is essentially fully discharged while 13.0 is fully charged. This is a fairly linear relationship so a reading of 12.4 volts means you have 40% of the capacity remaining.
Never leave a
lead acid
battery in the discharged condition.
The lead acid
battery should
never be left to set in the discharged condition or sulfation will
result. The
sulfuric acid in the electrolyte reacts with the sponge lead active
material
and forms lead sulfate. It is a poor conductor. This coupled with the
H2O left
after you take all the S out of H2SO4 is also a poor conductor so
trying to
charge requires a lot of voltage to push the current through required
to
convert the active material back to the charged state. Sometimes they
just
cannot be brought back from the sulfated state.
The good news is that sealed lead batteries retain their charge much longer than Ni-Cd, At room temperature it's well over a year. So all you have to do is make an occasional open circuit voltage check to see if you need to charge it.
For a great deal of information on Flooded Lead Acid (Automotive and Deep Cycle) go to:
Automotive & Deep Discharge Information
This material is oversimplified I know, but more detailed explanations can be had at my commercial rate of $125/hr plus expenses.