Lithium Battery DIY

Didn’t see a dedicated electric forum and didn’t want to limit this to just fifth generation, so I’m gonna start this thread and flesh it out as time goes on.

I have had additional electric storage in my trucks for over a decade. Last one was a Silverado with 4x6v golf cart batteries for (nominally) 400 or so Ahr storage at 12v or just under a KWhr. Batteries were heavy, pricy and bulky and I rarely got >2 years of use from a set. They started at $65 apiece but got up to >$140 ea. I ran a 3KW inverter and 300 watts of solar on the bed so the truck powered an ARB 70 can capacity fridge continually for over a decade until some bastard stole the fridge during the COVID zombie apocalypse.

I sold the truck- fu€£ing 1st gen duramax known for bad injectors and was rebuilt multiple
Times by me at 45 hrs each time.

I bought a 4Runner. Not sure if I’m doing solar again unless I can find decent thin flexible panels. I found a source for brand new prismatic, LiFePO4 cells. These were originally slated for use in luxury ICE starting batteries. They are made in Japan brand new and extremely high-quality cells; capacity is 74 Amp hours. The nominal voltage is 3.2 V so for a four cell battery the nominal voltage is 14.8 V.

—warning: ⚠️ Mansplaining——
based on the voltage a forerunner typically generates i.e. the max when the alternator’s running and low-voltage cut offs on a fridge- approximately 80% of the battery’s capacity is usable within that voltage range. You’d get more if it were charged higher to 3.65 volts a cell and discharged to 2.5 but surprisingly, it’s not a huge amount. And it’s a way bigger investment to have a boost charger and fridges cut off voltage is typically high 11 v. One strange requirement is compression- you have to restrain the cells from swelling. Also, they cannot be charged when cold. That’s defined as 40F. If you’ll be operating in such an environment, a battery heater or clever placement is needed.

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so you’ll need 4 cells, busbars, nuts, a Battery BMS (battery management system board), wire, connectors, and a way to safely store and compress the cells. luckily, everything save for the case and hardware is available from one place and cheap.

I want you to go and price a commercial 74AH 12v lithium battery. They’re $400-$1000. Most will say stuff like 100Ahr but you can’t trust that. There may be a warranty and rarely a heater.

this can be done for around $200.
Battery: $35/cell=$140
Busbars: $6 for all
Compression all thread: $8
Wire/connectors/hardware: varies
Battery BMS: 4S LIFEPO4: $10
.50 cal ammo can: $20

74 Ahr is about the storage cap of a typical group 24 lead acid battery; though deep discharge will damage a lead acid; I typically drain a lead acid battery once and it’s compromised; two to three times and it’s toast. It only
Diminishes life in a lithium battery from 8,000 cycles to maybe 1500.
Best of all- NOT A FIRE RISK! Yea it will burn if you start it in fire with gasoline or whatever but it won’t set itself on fire anywhere nearly as frequently as other lithium chemistries. I’ve never had a lifepo battery go up and I’ve have tens of other lithium batts burn.

If there’s interest I’ll post a build thread; I just got the parts. The wholesaler has 7,000 cells left and they are the absolute nicest cells I have ever seen.

my advice- buy cells in sets of 4 or 2x6 and sit on them while you amass the rest of the stuff.

This will fit easily in the passenger firewall space.