Portions of this newsletter are reprinted from the AMA national newsletter © Academy of Model Aeronautics.
CARE AND FEED OF SEALED LEAD ACID BATTERIES
Lead acid gel cells should be charged with a constant potential charger specifically designed for these batteries. These chargers can be referred to as Constant Voltage Chargers (CVC). You can charge them with a constant current charger, but you must terminate the 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 takes about 14 hours to charge from a fully discharged state (voltage less than 12 volts.).
A CVC 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 around 14.5 volts. Then, the current drops off to maintain this voltage. A CVC is characterized as one having a current capable 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 through 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 are called taper chargers; these are set up so the voltage tapers off as the full-charge voltage is reached. True constant potential 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 Nickel Cadmium (Ni-Cd) batteries 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. You should never totally seal these in a field box where these gasses can accumulate. Mixtures of oxygen and hydrogen can cause a spectacular "event" 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). A reading of 12 volts is essentially fully discharged while 13 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 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 H20 left after you take all the S out of H2S04, is also a poor conductor, so trying to charge requires a lot of voltage to push the current through 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. All you have to do is make an occasional open voltage check to see if you need to charge it.