Meant to post this a while back but......life is busy. So my dead 4D battery prompted me to take a look at lithium batteries. I ended up running into this site and went down the rabbit hole:
After a lot of reading and such I decided to build up my own LIFEPO4 cell and ended up replacing my 4D agm with a smaller 31M battery to act as a backup for the starter. The lithium will be the main house load battery, but I can switch it over to the House AGM in case I drain it.
Many folks are familiar with the smaller LIFEPO4 cylindrical cells. With the EV car market like it is, there are tons of different larger amp hour "prismatic" cells out there. Most, if not all, originate in china. They can be bought on ALI, Ebay, Amazon, etc.
I chose smallish 120AH prismatic cells. I wanted a small form factor and lighter weight. I bought Cells from a company named NINTHCIT. They boast a USA warehouse, although I am not sure if that was true. I bought through Amazon, figured that gave me a safety margin with returns etc. I paid a bit more, but ordering direct from China can be dicey if your cells are not good grade or arrived damaged. There are less than reputable suppliers.
My cells came well matched (Voltage) and looked to be in great shape. They came with studs, nuts and bus bars. First thing you do with new cells that are in good balance is to TOP Balance them. You hook them up in Parallel and charge them up to 3.6 to 3.65 volts. You need a reasonable bench power supply to do this and it takes a long time if you are working with a 5 amp supply like I was. These took the better part of a week. There are very detailed instructions on how to do this on diysolar. I reviewed my findings on these cells in this thread:
I will say here that I most likely did not save any money over a packaged Li battery, but I learned a ton and that is worth it to me. I bought a decent Victron smart charger and a bench power supply, so that put me over. But I now have a true 120AH cell that will run a long long time.
Second part of the build is making a box to add compression to the cells. Some cells like a bit of compression, or to not be able to expand during charging. An overcharged Lithium cell will blow out of it's housing and is done and could be a fire risk. Many folks have made the mistake of upping the voltage over 3.65 on a Top Balance and wrecked their cells. I kept mine under 3.6 and quit. Weird thing about LI is it will sit in the 3.4-3.5 range for days while charging, then suddenly just take off to 3.6. Remember, these are the individual cell voltages. They are in parallel when being top balanced charged.
I made my box out of plywood and used a floating front to compress the cells will threaded rod. It worked great. My cells came a little bulged on the big flat sides. A little compression and cycling took that bulge out. Once in the housing it is time to add a BMS.
Most critical part of an LIFEPO4 setup is the Batter Management System. This is the small unit that tracks cell voltages, system voltages, charging and discharging amps, and should also have a thermocouple to shut down charging below freezing and perhaps at high temps. Good ones have bluettooth and custom programming and monitoring through an app. Mine is DALY, but there are other better ones out there. A highly rated one is the overkill solar. Mine is only rated to 100 AMPS which means 100A of charging or discharging. I have mine custom set bit lower. Another feature is to be able to shut down charging and discharging at certain voltages. You have to also look at the specifications on the cells. They will give charging and discharging numbers like 1C, that means 1 capacity, or for a 120AH cell, 120 AMP max discharge or charge. Some are 0.5C, which would mean half the amp hour capacity in amps charging etc.
Use caution when buying a BMS, it can get confusing. they go by the number of cells in series. In my case, I need a 4S for 12V. The BMS has individual sensor leads that go in order onto the terminals. The BMS then can read each cell's voltage as well as charging and discharging currents. This way you can also see if one or more of your cells is lagging or leading. It is important to have matched, balanced cells that do not lag or lead each other, this reduces capacity as the BMS will shut down charging or discharging depending on those leading and lagging cell voltages.