9 Volt batteries extensively used in various electronics measurement equipment. Powering these devices with rechargeable batteries can be a cost effective solution. In this post a simple Lithium-ion 9 Volt Battery Charger design with overcharging protection will be presented. Charger is based on PIC16F876A microcontroller and uses its A/D module to monitor current flow into the battery.Number of required component was kept to a minimum as circuit simplicity was a main goal. The charger concept is relatively simple.Connect 9 Volt source to an empty rechargeable battery. Monitor the current during the charging period and stop the process at predefined current level. As battery recharges the current will decrease till there is no potential difference between the charger power supply and the battery.Max current was about 270-300mA while the battery was fully discharged and about 30mA after 4 hours of charging. Thus 30mA was set as a trigger level that will stop the charging. The meaning of this limit is that the circuit will not charge a battery that is fully charged therefore avoiding overcharging and protecting the battery from possible damage.
Charger software is a modified A/D Converter code published a few weeks ago in Code Examples category. Charger prototype was tested with Litelong 780mAh 9 Volt rechargeable battery and all calculation are based on this battery type. You’ll need to experiment with other similar batteries to determine their charging characteristic. 10 Segment LED bar was used to verify correct A/D reading however only one segment actually used to signal the end of the charging process. It can be replaced by a single LED connected to PORTB bit0. Charger assembly code is available here. I’m planning to improve this design by adding a smart charging algorithm in order to reduce the charging time and support other types of batteries.