The original golf cart and deep cycle marine lead acid batteries didn’t seem to have the oomph they once had. They are running down too quickly and are getting harder to recharge even though they are only 2 years old. Batteries Plus tested a couple of the golf cart batteries and said they had about a year of life left. They tested by drawing 125 Amps for a minute on each battery. Those results were an inspiration to look around for new batteries. Upgrading to AGM batteries would cost $1600 (from a dealer quote in Denver). Then a YouTube video was discovered about LifePO4 lithium iron phosphate chemistry. See Will Prowse the DIY Solar master at https://www.youtube.com/watch?v=zStEtH9Le-w&t=241s
Four LifePO4 100AH cells were purchased on eBay along with a BMS (Battery Management System) in an attempt to move up the learning curve and find out if LifePO4 would be a good upgrade. In ensuing months it was learned that there is a lot to learn. Lithium chemistry is far superior to lead acid for a solar application but there is a learning curve. For example, LifePO4 batteries can not be charged if the temperature is below freezing. That’s a significant hurdle for an outdoor storage shed on the Colorado plains. After a month of use and several paradigm changes the Lithium batteries are proving to be a great upgrade. One advantage is the space saved with Lithium. Compare the Lithium sitting on the shelf (green) to all the lead acid batteries on the floor it replaces.
The solar controller was replaced with a model that was designed for Lithium charging, which is a Morningstar ProStar. In addition a second order of 4 batteries was added for a total of 200AH rated capacity. Heater pads (12 volt, 25 watt) were place between the cells with a thermostat set to 5 degrees Celsius. Two autumn frosts have occurred and the heaters have kept the batteries above freezing. As of this writing the ambient temperature is 1C and the battery temp is 6C. Will they hold up over the cold winter ahead?
Followup after a snow storm and 108 hours below freezing at the end of October: Yes, the heaters kept the batteries warm. However, the Lithium did not have enough power budget to supply the heaters, the radio equipment, and the computers along with snow on the solar panels. A low voltage disconnect occurred causing no damage but the station was off the air. The heaters and the computers were moved back over to the lead acid batteries to get back on the air during the storm. To avoid spending any more money this hybrid arrangement will be used for a while. Long term solution is more Lithium batteries.