Lithium Iron Phosphate (LiFePO4) Batteries

[davidbrady]

Hi Greg,

Either one. I can have the generator start as a function of battery voltage along with the time held at that voltage, the same with cutting the inverters off from the batteries.

The Trace SW4024 inverters are "hybrid" and have been long before "hybrid" became the new marketing term. The SW series supports grid tie-in and supplementing grid voltage with inverter power drawn from the batteries; i.e., wave form synchronized. Here's the control for the SW4024 low battery cutoff, charging, and auto generator start:

FEATURE: PROGRAMMABLE RANGE
Low Battery:
Low Battery cut out VDC: 16.0 - 35.5
Low Battery delay (minutes): 0 - 255

Battery Charging:
Bulk Volts: 20 - 32
Absorption time, hour, minutes: 00:00 - 23:50
Float Volts: 20 - 32
Max Charge Amps (AC): 1 - 35

Gen Auto Start:
Load Start amps (AC): 0 - 63
Load Start delay min: 0 - 25.5
24 hr Start Volts (DC): 10 - 35.5
2 hr Start Volts (DC): 10 - 35.5
15 min Start Volts (DC): 10 - 35.5

The idea with the Gen Auto Start is to handle the voltage sag under high loads so as to not start the generator needlessly. For instance, I may want to tolerate a short-term high load voltage drop to start a motor. Float voltage on a 24V system is 26.8, so I'll set: "15 min start volts" to 22.6. This will accommodate the voltage sag on high current draw. This sag is related to concentration gradients and diffusion rates of battery charge carriers and inflicts all battery technologies; systems built around LFPs should also allow a degree of voltage sag before declaring an energy depletion condition. This is why its better to gauge DOD by adding up charge and discharge current rather than looking solely at voltage level. Simply cutting off the batteries at the first site of 2.8V per cell via a contactor will almost certainly leave usable energy still in the cell because low voltage cutoff is dependent upon discharge rate. 2.8V might be the spec at 1C, but if we draw 2C we're going to dip below 2.8V and cutoff the batteries needlessly.

I still think the trick to getting lifetime out of LFP's in a motorhome, with existing motorhome technology and abiding by the KISS principle, is to charge them to 90% and discharge them to 40%. This way we're sure not to destroy them thru over voltage or over energy depletion. We still get the benefit of more cylces over AGMs, but we aren't getting all of the energy density advantage, which brings up a good point. I understand in the GMC a few hundred pounds makes a difference, but 500 lbs doesn't make any difference in my 50,000 lb H3. In fact, I'd gladly go for heavier batteries if the cost were halved! For my motorhome, I'd buy the same AH capacity as I have now and I'd treat them like AGMs, 50% DOD. IOWs for the investment involved I'd baby them; my payoff would be over 8000 cycles of use. On the other hand, if I were building a lightweight vehicle I'd scour around for an optimal charging scheme and an optimal Fuel Gauge.

My H3 has an additional complication - it's a split 12V/24V system. The battery bank has a center tap for 12V and there's a Vanner Equalizer which draws power from the 24V side and feeds it into the 12V side. At first inspection, I don't see any way that an LFP bank will handle the user induced imbalances caused by this setup. Plus, who knows what the charging characteristics of a Vanner Equalizer are. I think the solution here is to have two separate banks and two separate charging systems, yet another complexity for minimal gain.