From above:Originally Posted by TruckinMP3
"The ABSORPTION stage is where the charger voltage, depending on the battery type, is constant between 14.1 VDC and 14.8 VDC at 80° F (26.7° C) and the current decreases until the battery is fully charged, which is typically the last 20% of the recharge."
I guess you just have to trust me on the point that with a constant voltage, the battery will draw less and less current until it's internal voltage matches the constant voltage and no current is flowing. They don't mean that the 14.1 and 14.8 is varying, you choose a voltage between that and keep it constant.
Batteries have what is called an "internal resistance." If you model a battery as a true voltage source and a resistor, you can model how much current is going to be flowing. If the batteries 80% full and it's internal voltage is 13 volts, and you hook up 14 volts to it, it is going to draw an ammount of current based on it's internal resistance. If that resistance is 0.1 ohms, then 10 amps are going to be flowing in to charge the battery. V/R=I. (14v-13v)/0.1 = 10A Once the battery is fully charged it's internal voltage will be 14 volts and the internal resistance will still be 0.1 ohms and V/R=I tells us that (14v-14v)/0.1 = 0 amps. Thus the battery stopped drawing current. You can force more current into the battery by raising the voltage, but that would be bad for a lead-acid battery, hence the allowable range ~14.1 to 14.8
Ni-cad chargers work by putting in a constant current until they measure a certian voltage or temperature, or putting in a trickel charge current that you can leave on forever. If you keep Lead Acid battery on trickel charge you will start to kill it if you don't limit the voltage.