Lithium Iron Phosphate (LiFePO4) Batteries

[davidbrady]

Here's a study that talks about LFP power and cycling degradation when discharging at various temps and loads.

  LiFePO4 Cycling Degradation Battery.pdf (Size: 1.38 MB / Downloads: 1)

There's certainly a bunch of marketing hype out there regarding Li-Ion. This paper suggests keeping our LiFePO4's at or around 68 deg F, limiting depth of discharge, and keeping the discharge rate down at fractional C levels. Also, there are exciting anode and cathode technologies on the way. One of them is Lithium Titanate (Li2TiO3) which may take us to the next level. In any case, all of these technologies will likely have similar concerns for us; namely, cell balance and voltage reversal when put into series strings, strict low and high cutoff voltages, and special constant current and constant voltage charging regimens with limitations on float current.

I need to study the use of Vanner Equalizers in an LFP bank, but a quick review of the Vanner documentation causes some concern. Center-tapping a LFP bank can lead to cell charge imbalance and possibly cell voltage reversal. The Vanner Equalizer poses concern during alternator charging. If the Vanner is unable to keep up with the alternator, excessive charge can be delivered to half the battery bank. The Vanner is also a concern during battery disconnect where active 24V loads can drive the 12V center-tap into voltage reversal. These issues can possibly be caught and managed with sophisticated per cell battery management and balance technology, but that's a whole lot of complexity I'd prefer to avoid.

I think the simplest and most reliable approach is to treat the LFP bank as a monolithic block; i.e., no center-tapping, and derive 12VDC from a DC-to-DC converter. Also, cycle lifetimes of the LFP are most likely optimized if we use them like we use our AGMs - 50% DOD and fractional C levels of load. This means not reducing the battery bank size when going from AGM to LFP which means the generator capacity will need to stay the same. I think it's optimistic to claim that we can reduce generator capacity until we get some real world experience with LFP's in a modern day Prevost conversion. My current inverters are Hybrid but I rarely, if ever, use that feature. It's probably worthwhile to sit down and look at realistic use-cases to see under what circumstances supplementing grid power via the LFP battery bank power makes sense and for what duration???