The [Un]Redeeming Qualities of LiFePO4 Batteries

LiFePO4 batteries are relatively new to the scene, introduced commercially at the turn of the century. They have a set of properties which are desirable for the sub. But as with anything, they have disadvantages as well. 

Two sets of four batteries to operate the sub for over an hour

• LiFePO4 batteries have a high energy density, but not as high as LiPO.
• LiFePO4 batteries don't heat up and catch on fire when they're dinged.
• LiFePO4 batteries half trouble self-balancing during charging. Once one cell in a series is full, charge will not flow to cells beyond that one. They need a custom charge management board solution.
• LiFePO4 batteries can have a high discharge rate, making them ideal for the small thruster bursts needed to stabilize the sub in ocean currents.
• LiFePO4 batteries cost more than LiPO and batteries with similar energy density.
• LiFePO4 batteries have a long cycle life. As long as you're charging these babies correctly, you're going to see a very long lifetime out of them-- up to 2000 cycles.
• LiFePO4 batteries do not leak much charge. Charge them and leave them (set it and forget it?).
• Poor performance in low temperatures. Below 0 degrees Celsius, battery performance degrades specifically. Fortunately for us, the ocean does not reach that temperature at depth.

All in all, we like these qualities. Ocean temperatures reach ~4deg Celsius, so no problem there. We initially did not account for the inability of the batteries to self balance, but all that's needed to fix that is an additional charging board. The high discharge rate and energy density is ideal for controlling the Blue Robotics T200 thrusters (350 Watts max). They put a dent in my pocket, but hopefully they'll last a good long while.