This is the underside of the HP 21 calculator. Notice the 2-cell AA size nickel cadmium battery pack. HP has cleverly design the battery pack as a cover to the battery compartment as well. It is expected that the user will have to buy a new battery pack from HP should the rechargeable battery becomes unchargeable. A more knowledgeable use will simply remove the old batteries and either put in normal AA size dry cells or another pair of AA size nickel cadmium cells.
The battery compartment or battery pack looks like the weakest link in this handsomely designed calculator. Nickel cadmium batteries has a tendency to leak badly when left in uncharged state. 
This particular calculator is extensively damaged by the corrosive battery fluid, as the following pictures will show.
The battery compartment or battery pack looks like the weakest link in this handsomely designed calculator. Nickel cadmium batteries has a tendency to leak badly when left in uncharged state. 
This particular calculator is extensively damaged by the corrosive battery fluid, as the following pictures will show.
The NiCd battery actually has a vent hole. This somehow can be a source of leakage of corrosive battery fluids especially when in the discharge state.
Given the high quality components and material use in the rest of the calculator, its seems a pity that not much though has gone into making the battery compartment more battery leak proof.
I rescued an HP-21 from a garage sale a few years ago. The batteries were dead of course but they had not leaked so I replaced them with two AA cells and it works fine.
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