This HP41C calculator battery case has corrosion on 3 of the four springs. These springs serves to hold the N-size batteries in place as well as provide electrical connection between the four batteries to provide the 6V DC needed. Two springs each is actually made from one single wire. So you actually have only two separate springs. Each pair of springs connect two N-size batteries together to create 3V. The battery contacts on the HP41C caculator itself connect these 2 3V supplies into create 6V.
Because the springs is very hard to find ( since most battery spings are single spring not paired like this ) I decided to clean off the corrosion and recoated the ends of the spring with solder. The springs are made of metal with a high copper content, most likely a cupro-nickel alloy so soldering is quite easy.
These pictures shows the before and after of recoating the springs with solder. A check with a ohmeter shows good electrical contact ( measuring 0.5 ohm - mostly like due to probe contact resistance and meter tolerances )
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HP41C Battery Case
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Note that the two thin flat springy metal clips that clips to the inner walls( one from the left outside and one from the right outside wall ) are missing. The intent of these thin flat strips of springs is supposed to prevent the N size battery from slipping out of the battery case when tilting the battery case to install into the HP41C calculator. However on closer introspection, these may not be needed since you do not need to tilt the battery case too much in order to install it properly into the calculator. Once installed the battery springs push against the battery contacts in the calculator and provide good electrical contact as well as hold the battery case securely in the HP41C calculator.
