Chemlec,
Do SLA batteries prefer a constant current (140ma on mine, for example) or constant voltage (7.25) with a varied current (higher, 140ma at the start and lower, 10ma or less, towards the end)?
Hi,
I am not sure if you have looked at the entire process of charging a LA battery because that entails BOTH constant current AND constant voltage, at least basically. It also depends on time which the battery takes to start to charge more and more.
"constant current" in this case however does not really mean constant current, that's just a way to describe the starting conditions. It should really be called, "current limit" because that's really what it is. The current is limited during the start of charge, and gradually as the voltage gets higher the circuit switches to constant voltage, but constant voltage here does mean constant voltage (or 'regulated' voltage).
For a 12v battery, if we start with say 14v constant voltage, we might see a very large current like 10 amps. To avoid that, we limit the current to the safe level. Lets say that it is 140ma as is your battery. That means we have the circuit set up so that it can put out a constant 14v BUT the current in that circuit is limited to 140ma also. So when the battery starts to charge and it wants to draw 10 amps, the circuit does not let it. The circuit cuts the current back to 140ma and that means the voltage of the battery is automatically lower, like maybe 13.6v or something.
As the battery charges, the current remains at 140ma but the voltage starts to creep up slowly, 13.60, 13.61, 13.62, 13.63, etc., until it gets close to the set point of 14.00v. At that time the current will NATURALLY be lower, even without the current limit, because of the internal resistances of the battery and the charger. That means that at 13.98v the battery might only draw 100ma (not 140ma anymore and certainly not 10 amps) so the constant current is not needed anymore and so the charger starts to limit the voltage and as the battery gets up to 14.00v it only draws a very small current which may only be 20ma or something.
So you see the process starts out with the max current, then when the current tapers off naturally the charger starts to limit the voltage so the battery does not overcharge. That is the basic charge method for LA and for Li-ion although the set point voltage for Li-ion is more critical and has to be more accurate. The set point for LA is also based on temperature, but often a temperature of 20 degrees C is assumed so if the temperature is significantly different than that, the set point voltage has to be changed a little. Some batteries actually have this stamped on them.
The simple characterization of the LA charger is that it is basically just a voltage source (power supply that puts out voltage) with an included current limit. It can put out the full voltage of say 14v but only if the load does not draw more than 140ma (in your case).