The HP 41C uses a N size battery. Earlier version use the N size NiCd rechargeable battery 
while later versions used the N size alkaline battery
.
The N size battery is almost the same in external diameter as a normal AA size battery and about 20mm shorter.
The normal operating current of the HP 41C is about 20mA without the other peripherals like printer or card reader. The standy current is very minimal. Thus a normal alkaline AA size battery should last quite a while.
Even then the reason for the choice of N size battery may be know to the people in HP who design the HP 41C calculator. Would not the use of a normal standard AA size battery makes the HP 41C more a joy and cheaper to own? Would a extra 20mm in length of the calculator do that much damage to its aethetics?
Another unique feature of the HP 41C is the use of special springs for the battery contacts in the battery holder. A normal batery holder uses a spring that has a flat coil linked to another conical coil. The HP 41C battery holder uses a spring with 2 conical coils. This is arises as a result of using the springs to hold the battery holder secure in the casing. ( These springy battery contacts press against the printed circuit foil battery contacts on the IO connector ). It is a clever design, aimed more at manufacturing simplicity I believe. However the special springs are very hard to replace if they became corroded from leaking batteries.
( A less elegant replacement solution is to use four standard spring battery contacts and overlap one conical end with the flat coil end of another )
In restrospect, HP could well have ensure the long term survial of the HP41 if somehow they could have designed it to be use with AA size batteries then. ( Set aside the problems cause by the crack screw posts and the resulting poor contacts in the IO connectors and the CPU PCB to keypad connector )
However N size battery
are still available today, either as alkaline or rechargeable battery.
A good option is to use the newer rechargeable NiMH type rather than the obsoleted NiCd type. A NiMH battery provides 1.2V. Four will give 4.8V which is well with the minimum operating voltage of 4.5V of the HP 41C. NiMH are less prone to leaks. Use an external charger to charge the battery.
With this and some luck you may still be able to use your HP 41C for another 30 years.
while later versions used the N size alkaline battery.The N size battery is almost the same in external diameter as a normal AA size battery and about 20mm shorter.
The normal operating current of the HP 41C is about 20mA without the other peripherals like printer or card reader. The standy current is very minimal. Thus a normal alkaline AA size battery should last quite a while.
Even then the reason for the choice of N size battery
may be know to the people in HP who design the HP 41C calculator. Would not the use of a normal standard AA size battery makes the HP 41C more a joy and cheaper to own? Would a extra 20mm in length of the calculator do that much damage to its aethetics?Another unique feature of the HP 41C is the use of special springs for the battery contacts in the battery holder. A normal batery holder uses a spring that has a flat coil linked to another conical coil. The HP 41C battery holder uses a spring with 2 conical coils. This is arises as a result of using the springs to hold the battery holder secure in the casing. ( These springy battery contacts press against the printed circuit foil battery contacts on the IO connector ). It is a clever design, aimed more at manufacturing simplicity I believe. However the special springs are very hard to replace if they became corroded from leaking batteries.
( A less elegant replacement solution is to use four standard spring battery contacts and overlap one conical end with the flat coil end of another )
In restrospect, HP could well have ensure the long term survial of the HP41 if somehow they could have designed it to be use with AA size batteries then. ( Set aside the problems cause by the crack screw posts and the resulting poor contacts in the IO connectors and the CPU PCB to keypad connector )
However N size battery
are still available today, either as alkaline or rechargeable battery.A good option is to use the newer rechargeable NiMH type rather than the obsoleted NiCd type. A NiMH battery provides 1.2V. Four will give 4.8V which is well with the minimum operating voltage of 4.5V of the HP 41C. NiMH are less prone to leaks. Use an external charger to charge the battery.
With this and some luck you may still be able to use your HP 41C for another 30 years.
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