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I just purchased an HP 22 with the batteries replaced with rechargable. I have read lots of posts saying to replace them and how, but how do I charge them once replaced? Do I pry them out of the cover, charge them and put them back? Thanks.

You can do that, or you can charge them in the calculator. Just plug in the AC adapter.

Warning: do NOT ever plug in the AC adapter without a good battery pack in the calculator, or if the battery contacts are corroded or damaged. This can damage the calculator electronics, and the only source of replacement chips is other calculators.

If possible, recharge the batteries outside the calculator, avoiding risks of damage to the delicate and difficult-to-replace ICs.

If you feel so inclined, look in the MoHPC archives for postings from Norm (a.k.a. Captain Zener), who proposed the technique of soldering a certain Zener diode inside the calculator. Such diode will provide quite a degree of protection against possible overvoltages while charging the batteries inside the calculator.

The overvoltages will appear and will damage the ICs if the regulating behaviour of the batteries is not present, even briefly. That will happen if contact between the batteries and the calculator is not perfect (movement, oxide, corrosion, dirt are possible causes for imperfect contact). Also overvoltage may occur if batteries become bad over their lifespan.