When I last posted, I had shared that my four year old component-built 100Ah Lithium Iron Phosphate (LiFePo4 or LFP) battery had degraded to a capacity of only 74Ah. Given the cost and shipping charges for a replacement cell and the chance that it might only present a different problem, it seemed faster and more economical to just replace my setup with a new 100Ah battery. WattCycle’s 100Ah Mini LiFePo4, just $160 in December 2025, could probably fit perfectly in place of my existing battery for less than 25% of what I paid for 100Ah in 2021. It would also restore my lost 25Ah of capacity… for a while, anyway. Remember, all batteries degrade over time. How long would it take for another 100Ah battery to wind up right where I was in November 2025?
I had a nutty idea: What if I could fit TWO WattCycle 100Ah Minis in the car? I brain-stormed a few mounting locations that were impractical and decided that I could easily fit a pair inside one of my Build-Out tool boxes. The thought of 200Ah in this car is amazing, as is the 200 amperes that a parallel pair would support! In fact, it’s total overkill, which is sort of what I do! The batteries would even continue to be “too much” as they age and begin to degrade. Still, I went to bed one night visualizing how I could frame and wire a pair of batteries into a Build-Out box and then awoke the next morning with a thought: “Why worry about how to brace two batteries and parallel wire them inside the box if one larger battery will fit better without additional complexity?”
Sure enough, WattCycle was having a sale on their 314-Ah “dumb” battery, that is a battery without Bluetooth monitoring. It also features a 200-amp BMS. I can monitor most charging functions with my Victron Smart Shunt. I added the shunt’s optional battery temperature sensor for this new setup, too. The only information missing from the shunt’s report is the individual cell balance. I should be able to blindly manage cell balance through routine trickle-charging at home. My first capacity test at 10 amps returned 324Ah. Recharging at 10A in my garage took 36 hours! 😀 See a tear-down and review in this 7-minute video.
This battery has WAY more capacity than I ever imagined putting into my GTI! My new battery effectively quadruples the capacity of the 74Ah diminished battery that it will replace. To add some perspective, I’ll be carrying more battery capacity than most compact RVs had just ten years ago! It’s crazy! The Build-Out box has room for a heating pad at the bottom, a small fuse block, a thermostat, a cut-out switch for safety (not shown here), and pass-thrus to route the cables out of the box. I used strategically placed 2×4 pieces to seize battery movement. See more photos in the album below.
The box is removable for security, troubleshooting, or to power other equipment outside the car. I considered using a box with wheels for my application. In fact, Husky’s “Build-Out” series has a roller toolbox, shown here. However, I want the battery box to be a PITA for anyone who might consider stealing it from within my car! 😀 I’ll use a luggage cart or compact dolly if *I* need to transport it. The 82-lb package is fastened to at least one anchor point for added retention. The truth is anything can be stolen with enough tools, time, and privacy. I hope to compel potential thieves to consider finding an easier target!
What are the drawbacks of having so much battery capacity in my small car? The most obvious is that of space. I my previous battery could fit inside one of my “rear seat delete“ compartments. I had to put the larger Husky box into my cargo space. I can mount it either in the trunk or with the other tool boxes shown above. I still need to rearrange the refrigerator and tool boxes so that heavier items are on the passenger side of the car for improved weight distribution. Regardless of where the battery rides, it takes space that could be used by an overnight bag or something else… ANYTHING ELSE!
Another drawback to the large battery is that I usually don’t drive the car enough to keep it fully charged. Parking the car with only 50-80% state of charge is not the end of the world; some might argue that it’s actually good for the battery. Regardless, I trickle-charge at home on the weekends to keep the internal cells balanced. That makes the recent addition of an AC-DC charger a great decision! In case you’re wondering, I don’t consider solar panels to be a viable option because my roof space is limited and its loads are too diverse (see photo below). I think hood-mounted panels are too small and expensive to consider, too.
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eah, I “need a larger vehicle” for sure! But I enjoy driving this one far too much to give it up; plus, I don’t want to manage a separate vehicle. In addition to being “my everything car,” I’ll also consider it to be my “training platform” so that installing my mobile ham radio setup into something larger in the future will be a snap! One thing is certain, though: Having the battery and initial circuits housed in the Husky box makes the power package very flexible, albeit very heavy, too! I can remove it and use it to power almost anything! I’ve shared photos of the completed system in my next post.
Time for a Flux Capacitor!
Scott
