Lithium Iron Phosphate (LiFePO4) Batteries

[davidbrady]

Seeking advice. I've long stayed away from Li+ batteries because of the cell balancing issues, the cost, and the risk of thermal runaway and fire. I'm now revisiting the idea and I see that a technology that's been out for a handful of years largely eliminates these concerns. Lithium Iron Phospate, LiFePO4, batteries are thermally stable, they don't require complex and dangerous balancing circuitry, and the price per AH per cycle of use is very attractive. They can safely and reliably handle 5C of current delivery or charging current, they don't require charge voltage temperature correction, voltages are very consistent from cell to cell, power densities are around 2X's an AGM, they offer 80% DOD versus AGM's 50%, virtually no Puekert loss (virtually no voltage sag at discharge), and they can be cycled multiple thousand times offering a longer effective lifetime. The list goes on. There's no question that Li+ is the new King on the block. The question is how to adapt my Vantare to using them.

Luckily the Trace SW4024 inverter is very configurable and the 3 stage charger can easily be configured to correctly charge Li+ batteries.

Issues which need to be worked out in this thread include:

1) What to do in place of the 100A Vanner Equalizer. With a 24V LiFePO4 battery bank there's no possibility of center tapping to obtain 12V, and if there were the idea of using an Equalizer to inject charging current mid-bank is contrary to Li+ conventions. That is, its creating a user induced un-balanced cell condition which goes against the holy grail of keeping all cells balanced,

2) How to handle generator auto start. What conditions should the auto start sense before deciding to start the generator. Li+ are sensitive to over charging and to under charging. These two scenarios can quickly destroy a battery bank. LiFePO's lack of voltage sag when discharging makes it difficult for the auto start circuitry to work properly,

3) How do I supply 12V power to the generator starter? Today it's taken from the 12V tap off the 24V battery bank.

4) What other 12V loads need power and is there a DC to DC converter available at a reasonable cost that can supply the amperage.