Lithium Iron Phosphate (LiFePO4) Batteries

[davidbrady]

Astute observation Greg, demand for AGM's tapering off and prices dropping could be the best outcome of incrementally better battery technology.

My Trace SW4024's can be programmed with new setpoints to charge LFP's (LiFePO4's). The problem is balancing the cells to make sure each cell in a string is at a voltage level within a few 100ths of the others in the string. This helps to be certain that no single cell is discharged below the low cut off voltage threshold where destruction of the cell takes place via cell voltage reversal. LFP cells are very sensitive to failure if over discharged; do it once and the cell is gone. This requires a very good battery "Fuel Gauge" to determine the remaining SOC. My Trace inverters use a simple voltage reading along with a time duration at that voltage to determine DOD. This doesn't work very well for LFP's because their voltage slope is very slight on discharge, also cell voltages are dependent upon discharge rate, temperature, and internal resistance. A better approach is to monitor the actual charge and discharge current into and out of the battery, but again with variable battery loading this becomes difficult to do. The best we're likely to end up with (unless we invest in specialized LFP chargers and battery management devices) is a charge cutoff voltage well below optimal and a discharge cutoff voltage well above optimal. In the real world this means well less than 80% DOD capacity and probably closer to AGM's 50%, so reducing your battery banks AH rating from AGM's 1000 to LFP's 625 and saving weight and dollars probably won't happen with current motorhome chargers and inverters.