By way of example, the PowerResponderTM 350 Farad product has a series resistance of 80 milliOhms. If we
charge the device at 5 Amperes, and use a cutoff voltage of 4.0 Volts – the charger will terminate charge at 3.6
Volts owing to the 0.4 Volts developed across the cell’s internal resistance. Ohm’s law states that V=IR, so 5
Amperes * 0.08 Ohms yields 0.4V of potential. Upon removal of the charging current, the cell voltage will drop to
a lower voltage which is dependent on the amount of charge that was provided to the device.
This short constant current charge is then followed by a constant voltage interval which assures that the device
stores the maximum amount of usable energy. In applications where the capacity of the device being charged is
unknown, the fast charge board adds additional charge and monitors cell voltage until the desired top-off value is
achieved. For lithium batteries, this constant voltage charge stage can take several hours, but for the
PowerResponder™ product, a few more seconds of constant current are adequate to attain full charge.
Specific implementation:
The Paper Battery Company’s fast charger board is a buck converter with an external current sensing loop
imposed onto the controller such that the output voltage collapses to maintain constant current.
The fast charge board is provided with a 36 Watt AC adaptor, but it can also be powered by any 9-20VDC source
with sufficient current rating for the desired charging power.
The image below illustrates the actual charging loop implementation: The specific buck converter used here has a
5 amp internal switch. It should be noted that any buck converter or other type of regulator can easily be used.
Linear regulators are not recommended as the power dissipation in the device may become prohibitively high.
The microprocessor is strictly optional in any implementation. The main purpose of the microprocessor in this
system is to turn off charging after the desired voltage has been achieved. In typical applications,
PowerResponder™ cells will be charged up to between 3.8V and 4.0V.
