12V 100-Ah LiFePo4 Battery – Final Review

In 2021, I participated in a “Builder’s Group” session with my local ham radio club, Williamsburg Amateur Radio Club. We had made portable 12V 100Ah Lithium Iron Phosphate (LiFePo4 or LFP) batteries from Fortune 3.2V 100Ah cells and a separate 120A Battery Management System (BMS). The end product was a portable 1280 watt-hour battery pack. I configured mine differently because I installed it in my car. See the original entry here. Its capacity tested very well at first, but declined with each passing year. I was able to find small “cheats” that would improve capacity a little, but the practices were not realistic to sustain because they only helped the load test and not real-world use. This year’s capacity test returned just 74Ah. My JBD BMS has just 100 milliamps of passive cell balancing, which is not nearly enough for a battery that’s charged only when the car is being driven. I decided to experiment with a different BMS in 2025. The new BMS is by JK and features 2-amperes of active balancing. The JK is much more aggressive and successful at balancing my cells. However, one cell still reaches “low voltage disconnect” (LVD) before the others. The same cell is usually first to reach “high voltage disconnect” (HVD) when the battery is charged from a relatively deep discharge, too. It’s safe to say that one cell has degraded to a point that’s unlikely to be recovered.

What happened? It took me years of casual observation to figure out that only one cell was responsible for the capacity loss of the entire battery. Cell #3 either has internal damage or simply has a lower internal resistance than the other cells. I honestly don’t know which is more accurate, not until I can disassemble the pack for some measurements. Looking at this older uncluttered photo, I’ll point out that Cell #3 is in the lower right side of the stacks. That position happens to be the closest cell to the center muffler in my exhaust system. I’ve never had any indication of excessive heat there, but I’ve also never placed a thermometer at that corner for more precise measurements. I will admit, though, that ALL of the cells have been subjected to heat in excess of 110°F since it does get hot in the car while parked during the summer months. Cell #3, if it was indeed a little warmer than the rest while driving, simply had a head start as the cabin temperatures rose when the car was parked. Another possibility is damage created from charging at high current from my relatively new 700W charger. But, if that’s the cause, then why only Cell #3?

What now? As I mentioned above, I can remove the battery and disassemble the cells for top balancing and resistance checks. That is not a speedy process in my application; plus, I’ve always been one to really dislike any down-time in my car or its systems. I probably should have done my testing last year when I had the fuel tank suction pump recall done. The LFP battery was out of the car for just over a week. That would have been the perfect time for testing, but I don’t think I was yet aware of how much my capacity had degraded.

Some sites suggest that it’s time to replace a LFP battery when it no longer holds more than 80% of its rated capacity. Most sites are more broad and say to replace “when your battery no longer holds charge like it used to” or “if the voltage drops more quickly than normal.” Those are not very objective standards. My subjective thought is that 100Ah is more than enough most of the time, but 75Ah is NOT enough some of the time. I think my current capacity would be sufficient for the winter months because my refrigerator doesn’t run as much when it’s cold outside. But I’ll quickly find myself wanting more capacity during the summer months, especially during heavy radio use. Until recently, I had a plan to run the pack for one more year… HAD a plan.

I’ve always been one to shop long before my actual needs. So, I’ve been watching LFP battery trends and pricing for quite a while. Today’s pre-built batteries are far more affordable than when I built my component setup. And by “more affordable,” I mean that I can buy a replacement 100Ah battery that would put me back to where I was for just 25% of what I paid four years ago! Put differently, I can replace the whole battery for what it would cost to replace one faulty cell! Some batteries are even available in a compact form that will fit where my current battery resides. As a result, I’ve been looking at where I could fit a pair of 100Ah batteries. That’s WAY more than I need, but also gives me plenty of room to spare in the future when the capacity inevitably begins to diminish.

My shopping came to abrupt end when the early “Black Friday” sales became too good to ignore! It is said that batteries and technology will only get cheaper with time, as has happened since 2021. But it’s also said that tariffs will drive up prices very soon. Which is true? I decided to make an impulse purchase, a purchase that will give me more capacity than I’ll ever truly need, a purchase that I hope will last long enough to eventually serve in my next vehicle (whenever that comes). This will not be a direct swap; so, it’ll take about 2-3 weeks for me to design the layout, round up the parts, and then get everything installed, tested, and ready to share. I’ll share my selection after I’ve installed and tested it. Believe me when I say that the new setup will be ridiculous, especially in a GTI! 😀

See a related video here.

Until Then,

Scott