Lithium batteries have taken over as the primary battery chemistry from applications ranging from consumer electronics to electric vehicles and all kinds of other things in between. But the standard lithium ion battery has a few downsides, namely issues operating at temperature extremes. Lead acid solves some of these problems but has much lower energy density, and if you want to split the difference with your own battery you’ll need to build your own lithium iron phosphate (LiFePO4) pack.
[Well Done Tips] is building this specific type of battery because the lead acid battery in his electric ATV is on the decline. He’s using cylindrical cells that resemble an 18650 battery but are much larger. Beyond the size, though, many of the design principles from building 18650 battery packs are similar, with the exception that these have screw terminals so that bus bars can be easily attached and don’t require spot welding.
With the pack assembled using 3D printed parts, a battery management system is installed with the balance wires cleverly routed through the prints and attached to the bus bars. The only problem [Well Done Tips] had was not realizing that LiFePO4 batteries’ voltages settle a bit after being fully charged, which meant that he didn’t properly calculate the final voltage of his pack and had to add a cell, bringing his original 15S1P battery up to 16S1P and the correct 54V at full charge.
LiFePO4 has a few other upsides compared to lithium ion as well, including that it delivers almost full power until it’s at about 20% charge. It’s not quite as energy dense but compared to the lead-acid battery he was using is a huge improvement, and is one of the reasons we’ve seen them taking over various other EV conversions as well.
One of the big disadvantages that prevents using LiFePO4 in automotive applications to replace lead starter batteries is that it pretty much refuses to charge below 0 C. You can do some serious damage if you try, so the commercial batteries have nickel strip heaters between the cells to bring them up to temp after starting the vehicle.
Even at 0 C you can lose couple percentage points of capacity per cycle if you try to charge fast, so charging below 0.05C is recommendable. In practical terms, that means about 10-20 minutes to charge up what you lost cranking the engine up if the BMS allows charging at all.
That in turn presents another problem: with the mass of batteries to heat up, it takes time before it will take charge. When it’s really cold out, it takes 10-20 minutes to get the temperature up, but then you don’t have enough time left in your commute and you end up running the battery flat if the cold spell keeps on.
Lead acids charge and discharge as long as the electrolyte stays liquid, which is down to -54 C when the battery is full, and down to -25 C when half discharged, so they pretty much just work even in the arctic. Lithium-ion batteries struggle discharging at -20 C or below, and LiFePO4 generally likes it warmer.
Regarding BMS, many projects only require one LiFePO4 cell and if you do not want 100% capacity, you may find that 90-95% can be achieved by float-charging:
http://www.tycorun.com/blogs/news/lifepo4-float-voltage-guide