I think there is a lot of misconception about battery chargers and charging lead acid batteries (perhaps applicable to other types of batteries as well!). Most of my work is in other areas of electronics/microcomputers. Most of what I know about battery chargers came from maintaining a 400AH flooded cell lead acid battery stack on a cruising sailboat as well as understanding power supply design and along with some conversations with battery engineers. I also have extended experience with running an elevator system that powered the elevator motor with a large (220V) lead acid battery stack. I may not have all this exactly correct but it is pretty close.
In the following comments "battery" will mean a lead acid battery.
A charger is a power supply tailored to the charging of the battery. It is a source of current with some control of the output voltage. Any well designed charger will act as a power supply with current limiting, voltage regulation and reverse voltage protection (ie; it will not allow current to flow into the charger). If you place a high impedance resistive load on the output of the charger it will supply a current based on the load and will regulate the voltage to the load. A typical lead acid battery three stage charger set for flooded cell batteries will immediately adjust the output voltage into resistive load to the adsorption voltage (typically 14.8V) and output current that the load draws. ( If the load was low enough impedance the charger voltage would be less and the charger would go into current limit.) After some period, usually based on time, the voltage will be adjusted downward by the charger to the "float voltage setting" (typically 13.8V). At this time the current into the load will decrease as the load is resistive.
Batteries are not resistive loads. The amount of current a battery will draw (accept is a better definition) from the charger depends on the battery state of charge. The state of charge also determines the terminal voltage of the battery. Due to the internal characteristics of the battery the terminal voltage cannot be pulled up to the charger output voltage but will rise as current is accepted. This means that with a discharged battery when the charger begins charging the battery the charger voltage will follow the battery terminal voltage. The battery will determine how much current is drawn from the charger and the charger output voltage will rise as the battery terminal voltage rises. This is called the "bulk" phase of charging. Voltage is rising and current is a combination of charger current limit and battery current acceptance ability.
When the battery terminal voltage rises to the charger set point the voltage will become constant. This is called the "acceptance" phase of charging and the current will decline as the battery accepts charge. Again, it is usual to keep the voltage at the acceptance value for some chosen time, although the charger could be designed to go to float voltage based on the current into the battery. ( My 400AH stack will accept about 2A forever at 14.8V when fully charged.)
As I cited in an earlier post the Amplepower web site has a very good description of this process as well as some graphs showing same.
Hope this helps.
John Acord
Flatwater Electronics
www.flatwaterfarm.com
"Neurosurgery for computer looms."
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