please advice an EE idiot on using chargers

[Steve Simpkin]

Vaklaff,

For your HP-25 it may be easier to just use disposable AA alkaline batteries in the original battery pack than a lab power supply. I have been doing that for the last 36 years. I removed the thin center plastic post that was between the batteries in the original battery pack which then allowed me to then slip the batteries in and out when needed (see photo below). Be sure to observe the proper polarity! This is the easiest solution but beware that the raised part of the positive terminal on standard batteries may not make good contact with the terminal on the circuit board. The added length also makes it harder to remove the pack.

*Ideally* I would recommend using flat-top rechargeable AA batteries (NiCad or HiMH) similar to these: https://www.amazon.com/dp/B072KKCBVY

They can be recharged externally via a changer like this model: https://www.amazon.com/dp/B0721JP6FK
For rechargeable batteries, make sure the charger accommodates flat-top batteries like the above model does (many do not).