Restore ni-cd battery pack

How to repair and enhance the Texas Instruments BP-7 battery pack* (see disclaimer below).

Ni-cd battery lose the ability to take charge after 500/1000 charge/discharge cycles, but the battery pack life is usually shorter due to common misuse: total discharge (pack is damaged if not charged for months or years) or overcharging. With the original Texas Instruments BP-7 battery pack dead the calculator became unusable, also when the power adapter is connected. The reason is simple: dead battery short the charger or act as open circuit making the adapter current unstable and potentially dangerous for the electronics. The solutions is simple: you have to rebuild the pack with new ni-cd elements.

Disassemble the battery using small screw. Open the pack pushing tool into the plastic holes. Remove the old battery and clean the internal circuit. Usually you will find severe corrosion due to battery leckages. If you found the pack unreparable you have the opportunity to replace the BP-7 with a clip for any standard 9V battery. Remember to dispose properly the battery: nickel pollute the environment.

Restore the BP-7 battery with two new ni-cd cell (use cell with contact for soldering iron). Change and reconnect with the soldering iron the battery cables. Sometime you have to restore corroded copper strip onto the circuit board. I also suggest to change the old 470 uF capacitor (remember to respect the polarity!). Close the pack and recharge overnight.  

The BP-7 could be replaced with standard 9V battery. In this case connect a standard clip to the original calculator connector ad use the BP-7 back plastic as battery cover. Remember: some calculators (as my TI-51-III) reverse the expected cable polarity, using red for negative (-), and black for positive (+). If your calculator refuse to work or show a simple dot bar effect (i think not casual) reverse the polarity.
If you modify the calculator for standard 9V battery, you can also transform the original BP-7 adding a 9V clip connected to the 9V output pins. You will find holes and place for 'nude clip' installation in any BP-7 pack.

(*) Disclaimer: We offer this repair experience as it is, without any warranty about contents or results (results depends by your personal ability, electronic knowledge and also lucky). Use this information at your risk: we are not responsable for injury or damages eventually caused.

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