I decided to post my experience with my car battery that I have replaced with a LiFe batery. It's been an adventure so far, so I figured I would share so others can learn.
Car = 1999 BMW M3. 3.2L inline 6 with a stock rating of 240 hp. I tuned it and did a some intake improvements, so hopefully it's around 270-290 hp with a 7k rpm limiter.
Original Battery = large 12v lead acid. I believe it was 55 lbs
Battery #1 - Shorai 8ah 4s LiFe motorcycle battery, used for 2-3 years previously in a Scion FRS with no problems but it was never balanced once in its life. Weighs about 3 pounds.
Battery #2 - Shorai 12ah 4s LiFe motorcycle battery, in use from July 2019 through January 2020 (today). Had a 100 mA balancer on it. No charge controller to limit under or over voltage. Weighs about 4 pounds.
Usage: My car is driven about 6 days a week. Every other day, it drives 5 miles to and 5 mile back to work. On alternating days, it drives either 15 miles or 50 miles each way to work (shared commuting car). On the weekends, the car is usually driven once. The car is also raced in the local autocross, so the lighter weight was a big motivator, but also for the slight fuel savings. The car did almost 20k miles in my whole year of ownership.
So when I switched to battery #1, I made myself some really nice battery posts that bolted on. Initial performance was not great. I found out that the lack of balanced damaged some of the cells and now only holds about 5 ah. This is a sketchy situation, especially in a car that likes to have a certain amount of power drain. This lasted not very long until I decided it was time that a fresh 12ah battery would probably be a good investment.
Battery #2 was purchased on eBay for about $220. It was brand new from someone who sold his bike that it was going to go into... roughly a $100 savings for me. I then bought cell balancer that I had used 12 years earlier in my battlebots in high school. It was an Astroflight Blinky A123 for about $35.
After initial installation, I wasn't too excited in how some cells would go over 3.6v. I was hoping since the balancer was working 100% of the time, that this would work itself out over time. I soon forgot about it and drove the car for 5 months. Unfortunate for me, the same day I put the new battery in, my alternator shorted out on me driving up the busy road in town and made my engine die in traffic... I rolled off into a parking lot and found my brand new battery had been effectively shorted out to ~5 volts. A new alternator and I was on my way. Later that summer, I had drained the battery twice while playing the radio in my driveway.
By the time December came, the battery was less and less convincing on starting the car. After dying for the second time, I took it out and found out that the cells were 100% unbalanced... some were 3.65v and other's were 2.8v. This made me very disappointed in the balancing board. I spent 3 days getting it back to balanced and I found it had about 8 ah of capacity left in it. In my dozens of LiFe packs I had used in my life leading up to this, I had never lost that much capacity, even after some extremely abusive scenarios.
So I decided that my battery was being charged at too high of a voltage. At 14.2 volts coming out of the alternator, the cells had to be impossibly well balanced to not go over 3.6v when averaging 3.55 volts over all 4 cells. So my brother and I build a 150 A diode out of (5) 30A diodes that we laying around. A full afternoon went into building this. We put it in the car and by day three, we had noticed that things were not charging enough. We found the diodes had dropped the voltage by a full 1.05 volts and in reality, the battery was sitting at about 12.95v (~25% SOC) after 4 hours of driving. So as of yesterday, we took it back out to have the battery over charged instead of under charged.
I'm trying to decide what to do next. I'm tempted to build a 15 ah LiFe pack out of 2.5 ah A123 cells, but I need to fix the charging situation so the pack sees 13.5-13.7v instead of 14+. I have yet to find info on how the voltage sensing wire works on the alternator. It would be nice to just wire a resistor in line to adjust the voltage that the alternator produces.
Note: the alternator pumps out 14.2v at idle and keeps up with the head lights on. If the heated seats, high beams or rear defroster go on, it dips a few tenths. It is a 140A alternator (the big one for the chassis, but small compared to the 180 A liquid cooled one in our 2001 740iL).
I will post some pictures in the next few days.
