I see a lot of threads about how to recharge the Ni-CD or Ni-MH "9V" batteries. I designed a simple one many years back, schematic attached. It is a shunt regulator that charges with approximately 10 mA and will not overcharge because it locks into a constant voltage mode when the battery is fully charged.
The LED is an indicator of charge because as the charging current through the battery diminishes, the LED gets brighter indicating closer to full charge.
NOTE: Ni-Cd and Ni-MH "9V" batteries are typically six-cell which means they are actually 7.5V (nominal) and 8.4V when fully charged. This circuit is designed for these 8.4V batteries. It is not a fast charger. For a typical 9V battery (between 100 and 150 mA-hr) it will take 10 - 20 hours to recharge fully.
There are some "9V" rechargeables with seven cells which have 8.75V nominal and 9.8V full charge. This circuit would have to be tweaked up to 9.8V to work with those.
To calibrate the circuit: disconnect battery and adjust VSET pot to make the voltage across the resistor string (where it connects to the battery terminals) read 8.4V with the battery disconnected.
The LED is an indicator of charge because as the charging current through the battery diminishes, the LED gets brighter indicating closer to full charge.
NOTE: Ni-Cd and Ni-MH "9V" batteries are typically six-cell which means they are actually 7.5V (nominal) and 8.4V when fully charged. This circuit is designed for these 8.4V batteries. It is not a fast charger. For a typical 9V battery (between 100 and 150 mA-hr) it will take 10 - 20 hours to recharge fully.
There are some "9V" rechargeables with seven cells which have 8.75V nominal and 9.8V full charge. This circuit would have to be tweaked up to 9.8V to work with those.
To calibrate the circuit: disconnect battery and adjust VSET pot to make the voltage across the resistor string (where it connects to the battery terminals) read 8.4V with the battery disconnected.
Attachments
-
268 KB Views: 183