100-Ah Lithium Iron Phosphate Secondary Battery

This year is the first time in many years that I spent a lot of time using my ham radios. As much as I like to drive, operating on the HF bands often requires log work, especially during contests or “Worked All States” network sessions. Logging while driving is unsafe, especially at night when most HF nets occur. Spending an hour or two on the radio while parked is taxing on the starter battery; plus, I don’t like to idle the engine for extended periods to maintain the battery charge. What’s worse is that I had wired-in a bypass switch to activate my electronics panel while parked. I’ve had a few occasions where I accidentally left that switch in its “bypass” position and drained the starter battery overnight. I needed a better solution!

I had considered adding a secondary battery to my setup for quite some time. My initial design was inspired by Van Life setups, but I took a slightly different path, one which was offered by my local ham radio club, Williamsburg Area Amateur Radio Club (WAARC). Lithium Iron Phosphate (LiFePo4) batteries can can be easily found in ready-to-go form through Amazon and other sources. I use ready-to-go LiFePo4 batteries in my cargo trailer. They’re affordable and easy to install. However, those slick batteries are just a set of four 3.2-volt battery cells wired in series, totaling 12.8 volts, with a cheap battery management system (BMS), all invisibly contained within a sealed pack. They’re completely unserviceable, should they suffer from an imbalance between the cells or a cell failure.

WAARC has had great success with building complete packages from separate Fortune 3.2V battery cells, a high-quality 120-amp programmable BMS, and a plastic ammo box, shown with the previous paragraph. Some benefits of this setup are that the cells can be rewired to balance their charge (if needed) and failed cells can be replaced individually, which can save a lot of money if something eventually goes wrong. The parts used in this project are all high-quality items that are more than sufficient for my needs and highly regarded in solar energy circles.

The ammo box case is too large to fit behind my front seats; and I didn’t want it taking space in the trunk. I opted to build my battery in a flatter box so that it would fit inside my “rear seat delete.” The photo in the previous paragraph shows the wooden enclosure in its early design stage. This photo shows the box after paint and wiring connections. While the battery was out of the box for paint, I decided to buy a DC power supply, rewire the cells in series, and then top balance the cells.

The BMS is excellent and offers remote monitoring and management via Bluetooth app, called Xiaoxiang. There are versions for both IOS and Android. Google’s Play Store has a basic version of the app. Visit the link in the BMS documentation to get the full version with admin functions. Many of the readouts in the app are too small to be useful while driving… and that’s okay. The main dial, which shows charging current, is large, easy to read while driving, and what I find most valuable at a glance. The display also gives me a quick indication of whether I have a problem with my charger or individual cells. The temperature readout is easy to see, too. If it’s below freezing, it would easily explain a lack of charging current. I have a small RV tank heater under the battery pack that activates any time the engine is running and it’s below 45°F. When parked, I can also view the health of each battery cell and manage a few specifics with the battery.

What do I power with this battery pack besides the ham radios? My most recent accidental starter battery depletion was a result of me forgetting to turn off the Raspberry Pi3 D-STAR hotspot. It’s plugged into a factory 12V outlet, as are a phone charger, WeBoost cellular booster, and GMRS radio. Leaving any of those energized for extended periods can result in unwanted surprises. I pulled the 12V outlet fuse and then fed the circuit from the LiFePo4 battery pack. I also connected my Blackvue dash camera into the same circuit. I mentioned in this video that I now carry my Dometic refrigerator full-time. So, the battery pack powers that, too. Finally, I’ve routed my 400-watt pure sine wave inverter to run from the LiFePo4 pack, though I prefer to minimize how much I use the inverter when the engine is off! Click this image to see a larger view of the signal flow.

WHY did I do this in a small car? If you follow my projects, then you know that I’m a bit of a tech-addict. So, I continually pack more and more tech into my car. I *think* I’m just about done… maybe! There’s no doubt that a larger vehicle, perhaps an adventure van or SUV, would be better suited for my wants. Since I cannot currently justify the purchase of another vehicle and I thoroughly enjoy driving my GTI nearly every day, I decided to just install my projects into the fun little car that serves me well today. Everything I’ve done is scalable and would not be difficult to move into whatever I buy next. In fact, this very battery project is exactly what works well in travel vans and small RVs; it’s only 25% smaller than the system that’s found in the 2022 Winnebago Revel! Sure, I cannot camp or off-road in my GTI. But most campers and off-road vehicles cannot accelerate or handle like a GTI, especially for the price!

You may have figured out that I can be a bit of a “mad scientist” with my approach to building and testing my ideas. So, I simply HAD to test the capacity of the new battery pack. I bought a load tester and commenced to draining my new battery! Yes, the purchase and the test were totally unnecessary, but it’s what I do! The capacity measured 105 Ah and 1267 watt-hours, which was quite pleasing! That’s over twice as much juice as what my Jackery Explorer 500 provides! Best of all, having this new system in the car should help to prevent me from finding myself with a dead starter battery one morning. 😉 Speaking of that, this SuperCap 2 is fantastic for jump-starting without a second vehicle. Check it out! It will slowly charge from a “dead” battery, then discharge through the starter to get your dead car started. Watch this video to see my first use of this product. Another cool thing about having the LiFePo4 battery: It provides a source to quickly charge the SuperCap!

I’m going to limit this article to just the LiFePo4 batteries; I’ll explain my choice of charging system in my next entry. See my related video here and more photos in the album below.

Power to Spare!

Scott

About Scott

I grew up near Houston, TX and served in the U. S. Coast Guard for over 30 years. I have an electronics background and continue to work in the electronics engineering field. I taught myself the basics about automotive systems as well as how to perform some of my own maintenance (cars and bicycles). I became involved with Amateur Radio and computers in 1995. The explosion of technology has made my job and several of my hobbies quite interesting. My hobbies include Volkswagens, bicycling, photography, electronics, amateur radio, web management, and reptiles. Visit my websites to learn more.
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2 Responses to 100-Ah Lithium Iron Phosphate Secondary Battery

  1. Dave says:

    Thanks Scott; excellent project execution and writeup.
    -Dave aj4xv

    • Scott says:

      Thanks, Dave! I’m hoping to get the DC-DC charger write-up and video ready before Thursday. Sweeping changes, parts delays, and even the weather are slowing me down. But I think I’ll make it! HAHA! Take Care!

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