Each Smith-Root backpack electrofisher battery (for either the LR-24 or LR-20B) supplies 24 volts and has a capacity of either 7 amp-hours or 12 amp-hours (for the extended life version). With the exception of the new lithium iron phosphate (LiFePO4) technology, these batteries use sealed lead-acid (SLA) cells that require no maintenance beyond regular charging. Now 24V is a little unusual in the general lead-acid battery market (most are 12V) but, as you may recall from basic physics, wiring two batteries together in series adds their respective voltages. If we do this to two 12V cells we’ll end up with a 24V and, as you can see from the picture below, that’s exactly what Smith-Root has done.
In fact, the Smith-Root batteries are simply two commercially available SLA batteries glued to a plastic handle and wired in series. This handle is nothing more than a 0.25″ thick sheet of ABS plastic with a hole routed in it for a handle and a spot for the power connectors. Since the battery cases are also made of ABS plastic, it’s possible to create very strong glue joints between the three components. The only remaining pieces are the fancy orange connector between the center (-) and (+) terminals which turns out to be a 40A automotive fuse (MAXI size), and the connectors that attach the battery to the electrofisher. These connectors are known as Anderson Powerpoles and have a rather rather cool stackable, genderless design. They can be bought for about a dollar apiece from a number of different hardware supply websites.
First you need to source two 12V SLA batteries in either the 7A or 12A size (you can go with other sizes too, all you’re changing is the fishing time). There are actually a few competing brands for these, all with pretty much the exact same range of sizes and quality. The only real difference between brands is the color of the casing. Smith-Root appears to use AJC batteries but Sigmastek (which I’ve used here) or Powersonic will also work. AJC simply refers to the batteries by voltage and amperage (so 12V 7A or 12V 12A) while Sigmastek calls them SP12-7 and SP12-12 and Powersonic labels them the PS1270 and PS12120. All are equivalent so I recommend shopping around for the best deal.
Once you’ve done that you need to collect the other pieces. I’ve had good success getting 12″x48″ sheets of 0.25″ ABS plastic from Grainer (which will make a lot of batteries) and the other components can be obtained from McMaster-Carr. The Powerpole connectors should be the 45A size and I recommend getting the combined housing and connector kits. By mixing a five-part red kit with a similar black one you get enough connectors to make five batteries. Finally you’ll need the aforementioned 40A MAXI fuse, some 8AWG wire in red and black, and some heat-shrink tubing to protect the terminals.
The next step is to cut down your ABS sheet until you’ve got a chunk about 6″ by 5.5″. I did this on a bandsaw but a jigsaw, scroll saw, or even a hacksaw would work just as well. If you’d like to be fancy you can add the rounded corned at this time, although it’d certainly not necessary for normal operation. I’ve found that a coarse file works well for cleaning up any rough edges and doing final shaping.
After that’s done it’s time to cut out the rounded handle slot. In all honesty, you could skip this step completely (and the handle itself, for that matter) but it’s nice to have when you’re carrying the battery around and provides a place to run the strap when securing the battery to the electrofisher. I tried a few different ways of making this, including a dremel, but finally settled on a combination of a hole saw (for the rounded ends) and a keyhole saw (for the straight areas between them) with a bit of filing to finish up. A jigsaw or scroll saw would probably work well too.
At this point I recommend testing the fit to make sure that everything lines up, especially the fuse in its little slot. Then, if you’re happy with how it all goes together it’s time to get out the glue and set about making it permanent. I have no idea what Smith-Root uses but I went with Weld-On 16, namely because I had some lying around. Any glue that’s suitable for ABS plastic should work fine as we’re not doing anything exotic like joining dissimilar materials. Once you’re got that all set up it’s time to clamp it and step away for 24 hours or so.
After the glue sets, all that’s left is to solder the fuse across the middle two tabs (linking the batteries in series and adding their voltages to make a 24V unit) and then solder the wires to the two remaining outside tabs. Connect the matching color Powerpole connectors to the end of each wire and you have yourself a battery unit that’s 100% compatible with both LR-24/LR-20B units and the battery chargers associated with them.
Feel free to email with any questions or comments. I’m especially interested in hearing from anyone who’s built one of these and would love to see pictures of the finished product.
Update Sept. 2019: I’ve heard from a few people interested in whether this technique can be used to make replacements for Smith-Root 24V 9.6AH lithium batteries and the answer is yes, this definitely works for those too. You’ll just need to buy LiFePO4 SLA replacements which are available from many of the same suppliers selling regular SLA batteries. These tend to run in the $100-150 range for ~9.6AH so two will run you around $300, a far cry from the $997 Smith-Root currently charges. The only additional wrinkle is that Smith-Root’s lithium models use a 6-pin powerpole connector instead of the normal two and come with a 6-to-2 pin adapter that allows you to connect the battery to their electrofishers and chargers. This is entirely due to shipping regulations: if the two batteries were permanently connected they would have to be re-certified as a “new” battery. Instead, Smith-Root runs all four battery terminals to the connector (the extra two are just for show) and then uses the adapter to connect them in series. This allows them to rely on the original certification for the batteries and avoid regulatory hassles. If you’re not going to ship your homemade batteries you can simply follow the 2-pin instructions above or you can use a multimeter to figure out the terminal order that Smith-Root uses and go with 6-pin connectors (although you’ll need to acquire or make 6-to-2 pin adapters if you go this route).
Disclaimer: I’m not a professional engineer nor am I in any way associated with Smith-Root. This worked for me but I can’t guarantee that it will for you.