New batteries are on the upcoming season’s want list. Lithium based batteries are showing up more and more in the world. Tesla brought the electrification of cars to the front burner. We’ve been using lithium batteries in all of our marine projects. I thought it may be time to move forward on Lithium for Risa. Energy density is more than tripled that of lead, at half the weight!
There are several good companies making lithium ferrous (iron) phosphate batteries as lead acid replacements. Based on the cell prices I can get, I decided to build my own. The system is pretty simple: Four cells in series make about 13.3 volts. A BMS to manage the cell balance and monitor charge and discharge rates. Some adjustments to the existing charging systems and I should be good to go.
Except for one thing, like Spielberg’s Gremlins, they have one major caveat: Never feed them after…below zero. It seems that while it may be possible to charge at very low rates, the value that won’t change the batteries (into crap) is not easy to confirm. It seems that .01C is permissible down to about -5V and it drops rapidly from there. Better to just not do it. With the low self discharge rate of 2% per month, we should have no problem getting through the winter with them in sleepy mode. I decided that I would use a traditional battery system for winter monitoring.
After three months, my cells arrived. I built a bank 3P4S of 280 AH cells. That’s three parallel and four in series. One BMS manages the pack. Over the winter I have experimented with the pack and am building a rack to hold them in place. I plan to install them in the spring.