An ordinary, low cost automotive battery charger is very bad news fo relatively small sealed lead-acid batteries. They are usually just transformer, rectifier, and ammeter with a switch to select nominal or 12 volt batteries. The charge rate is potentially too high fo small sealed batteries and, in the absence of active voltag detection and control, the final voltage is controlled only by th (fluctuating) line voltage and the turns ratio on the transformer.
The manufacturers design the transformer to provide adequate chargin
of an automotive starting battery in a reasonable time, whic requires a voltage that is high enough to force 8 amps (for your uni and mine) through the combined voltage drops of the low battery, th rectifier, the ammeter resistance, and the clamp and cabl resistance
The no-load voltage of the transformer-rectifier has been designed fo
this condition, but in the absence of active control, that voltage i too high for the final charging of a sealed battery. The result i that excessive current is forced through the battery during and afte the final "topping-off" stage, boiling off water that is no replaceable in a sealed cell. The 2 amps or so continuous termina charging current after full charge is acceptable in a flooded cel car battery because the rate of loss of water and heating of th battery are tolerable. The water can be easily replaced in non-sealed battery and the bulk of the battery limits the temperatur rise
Proper voltage detection and control is fairly critical and shoul
ideally be temperature compensated because the desirable voltag limits are a function of battery temperature
Actually, a discharged lead-acid battery can tolerate a moderatel
high charge rate below a certain temperature-dependent voltag because most of the energy is going into the chemical reaction, no in boiling off the water. It is important to reduce the chargin current at the boiling threshold voltage because that is the point a which water loss and excessive heating begins. Note that th Panasonic document cited above recommends a charging current limit o
40% of the amp-hour capacity of the battery, or 4 amps for your 1 amp-hour batteries. They also recommend limiting the voltage to 2.4 V/cell.
It is always a bad idea to charge batteries in parallel unless the
are known to be identical because any imbalance will result i unequal division of charging current between the batteries. If th current output of the charger exceeds the safe charging current fo either battery individually, damaging current can be driven throug the weaker battery. Even charging presumably identical batteries i parallel is risky because, at some point in their useful lives, on will become weaker than the other and hog current
Unless you have the ability, knowledge and patience to monitor an
control the voltage and current with external instruments, I thin you will save money by investing in a smart charger optimized fo your batteries and charge them separately.
Good luc
awrigh