A battery is a power supply that delivers nearly constant voltage. It maintains a fixed potential difference between its terminals. This stability occurs even when the circuit elements change.
A battery is a time-varying constant voltage source. In order to understand this a little bit better, you have to understand why an AC-DC power supply is not constant voltage. The source of the electrons across an AC-DC converter comes from free electrons on a conductor.
Constant current is a simple form of charging batteries, with the current level set at approximately 10% of the maximum battery rating. Constant current/constant voltage is a combination of the above two methods. The charger limits the amount of current to a pre-set level until the battery reaches a pre-set voltage level.
Constant current is a simple form of charging batteries, with the current level set at approximately 10% of the maximum battery rating. Charge times are relatively long with the disadvantage that the battery may overheat if it is over-charged, leading to premature battery replacement. This method is suitable for Ni-MH type of batteries.
Constant voltage allows the full current of the charger to flow into the battery until the power supply reaches its pre-set voltage. The current will then taper down to a minimum value once that voltage level is reached.
In a battery, the number of protons and electrons in the system are fixed, causing a constant voltage that varies with the charge of the battery.As the electrons flow from one terminal to the other, the voltage drops because there are less free protons.
The current will remain constant until the voltage rises to 28V. At this point the power supply will transition to constant voltage mode and the current will decay to zero when the battery is fully charged. The charge current is controlled to avoid overheating and the float voltage limited to avoid over-charging.