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.

[davidbrady]

For instance, 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.

[davidbrady]

Like most Prevost's, my H3 is blessed with two Vanner Equalizers. Most Prevost converters are very careful not to intrude or cut into any factory Prevost wiring harnesses; the converter house stands on its own in most cases. The Prevost chassis uses one equalizer to split 12V out of its 24V chassis battery bank. The converter followed suit with the house batteries; i.e., 24V battery bank to supply the inverters and the slide motors and a Vanner Equalizer (center tapped into the house battery bank) to supply the 12VDC loads including the generator start. My focus is on the house; I'd leave the 24V chassis bank and equalizer untouched.

Here's a schematic of my house system with split 24/12VDC battery bank and twin SW-4024 inverters:



My largest 12V draw is the generator start motor. A simple solution here is to break out this draw with a dedicated 12V battery for the genny. That leaves the remaining 12V loads which include just about everything DC powered in the house: mostly lights, electronics, a handful of motors (shades, sofas, pocket doors, dump valves), Aquahot, Glendinning, tank levels, etc.

I'm thinking a good way to handle these loads is with a DC-to-DC converter, 24VDC in and 12VDC out. My 12VDC distribution panel is protected with a 100A circuit breaker so I need a 100A DC to DC converter, 24VDC in 12VDC out. I haven't yet found a high quality off-the-shelf converter with anything close to these specs; if anyone finds anything please let me know. I may have to build it.

Stay tuned!

[davidbrady]

Out of the attached study comes the suggestion to keep our LiFePO4's at or around 68 deg F, discharge them moderately, and keep 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. It's an exciting time to be an electrochemist, a battery material scientist, or a motorhome owner!

  LiFePO4 Cycling Degradation Battery.pdf (Size: 1.38 MB / Downloads: 2)

[travelite]

Here's an outfit offering an LFP battery conversion for the Prevost crowd: Volta Power Systems. I think they have a connection with Pantera Coach. Their solution is a complete swap out of the alternator, voltage regulator, inverters, battery bank, and battery equalizer, with the addition of a 48V to 12V DC-to-DC converter. To me it looks like a bunch of off-the-shelf parts. My guess is the DC-to-DC converter pushed them into the 48VDC world. I have yet to find a 24V to 12V DC converter able to supply a continuous 100 amps of current, and apparently they couldn't either. The 48V solution is okay for new builds, but it's a tough $30K sale for the Prevost owner who simply wants an LFP "upgrade"!

[CC_Guy]

The Volta system is based on a 48V battery pack, which makes a lot of sense. The DC to DC converter can support 150A of service. The alternator has more output than the 50DN most of us have and only pulls 7HP.

Changing to the Volta system can make sense for some that want more run time, want to use air conditioning at night during generator quiet time or select other cases. The battery pack can supply 30,000 watts and more than one can be installed.

The real value comes with new conversions. A single 48V bus air system is just one example of a change that really makes sense. The 2015 emission standards also drive the need for this kind of system. Generators with more than 10-12KW output will have to meet very similar requirements that 2015 drive engines have to meet.

[davidbrady]

Hey CC_Guy,

Great to hear from you and Happy New Year! I was about to round up a search and rescue posse!

I agree with the use of 48VDC. It's synergizes well with air conditioning units, DC refrigerators, and solar panels and control systems, etc. I just don't want to rip out my twin 24VDC Trace SW4024's inverters and 50DN alternator, plus I'd need an extra DC controller to generate 24V for my various 24VDC loads. That's in addition to the one needed for my 12VDC loads.

I may be reading the website wrong, but it looks like the Volta alternator consumes quite a bit more than 7hp at full tilt. Remember the First Law of Thermodyanmics. Here's a graph from their website which shows a horsepower consumption of almost 19hp at an output of 190A. (Not that it matters much).


I'm not quite sure how the Volta system reduces the 120VAC generator power requirement, nor am I sure what the 30kW battery power tells us. The Volta website I'm looking at specs a battery capacity of 15.7kWH which at the usual 20hr rate is a C rating of 327 amps. They can probably safely be drawn down at 3C or so for a max sustained power of 47KW. Ignoring Puekert 3C can be held for about 20 minutes. They can probably withstand bursts of a few seconds at 10C for a whopping 157kW, but get your wallet ready because your cells will be irretrievably damaged if over discharged. Replenishing 15.7kWH of LiFePO4 energy is the same as replenishing the 15.7kWH of my current AGM battery bank and my 17.5KW Powertech performs this task quite nicely along with supplying other house loads. I wouldn't want to reduce my generator capacity in either case. In both the LiFePO4 and the AGM case I'm charger amperage capacity limited, and the generator is limited by other loads which need to be supplied.

What I want is a simple solution. Give me a 24V LFP battery bank and a 200A 24V to 12V DC Controller and I'm set. Like you said, the Volta solution is really for new builds.

Since I can't find a 24/12V 200A DC Controller I'm thinking about building my own. PM if your interested.

[CC_Guy]

David,

Great catch on the HP needed. I hate it when I rely on marketing data :-(

There are numerous safety features with these batteries. One is that they can't be depleted beyond their design criteria. This is controlled by the battery control module. It also has fail-safe shutdowns. The most dramatic is creating a physical barrier between the plates to stop the flow of ions. Of course, this would render the battery useless. The battery bank is made up of several batteries, each of which can be replaced as needed. The entire bank doesn't have to replaced at the same time.

The size of the generator can be reduced as it would no longer need to be sized for peak loads. Above average loads could be supplemented by the battery bank. This feature is similar to the feature you have with your Trace inverters.

Why not install two 150A equalizers? They are simple, proven, and reliable.

You'll love this one. I was recently helping someone with a high end newer coach that has a 24V battery bank. The coach has one 24V alternator and uses a single Delco regulator that regulates the alternator based on the chassis batteries needs. The Lifeline house batteries are given whatever the chassis batteries want through a battery isolator. Simply a case of taking the cheap route; I can't imagine another reason.

FWIW, the Volta battery uses nickel, not iron.

[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???