LiFePO4 batteries (or where 260 is greater than 440)

Lithium Ferric Phosphate (aka LiFePO4 or LFP) batteries have a lot of promise for use as house batteries in RVs. Compared to lead-acid, they are small, light, long lasting, and efficient. Prices are about the same as the AGM variant of lead-acid for the same amount of usable power, and they have the same advantages as AGM over flooded lead-acid.

LiFePO4 cells are mainly marketed for use in electric vehicles (EV). All of them available in small quantities are made in China, the plants in other countries have large contracts only do not have retail distributers. Being rated for EV use, they have specs for fast charge and discharge, and tend to perform even better when used at the relatively slow charge and discharge as house battery banks. (Where relatively slow is the two hour rate, rather than the 20 hour rate of lead-acid.) For house battery use, a rough rule-of-thumb is you will need half as many amp-hours of LiFePO4 as lead-acid. For large loads, (such as microwave ovens) the advantage is even further for LiFePO4.

Treating LiFePO4 batteries like lead-acid batteries want to be treated will shorten their life, and visa-versa. Lead-acid like being left fully charged, and trickle-charging a full battery will extend its life. (The low self-discharge of LiFePO4 means they don't need trickle-charged even for long term storage.) LiFePO4 should never be overcharged, and that includes trying to add power to a full battery even at a slow rate. LiFePO4 prefer being 20%-80% full, and never fully charging them will extend their life. Since they will accept charging faster than lead-acid, and don't have the rate reduced as they near full charge, less generator runtime is needed to charge them. You also do not need to oversize solar arrays in order to get the last 20% charge like you do for lead-acid. Some battery chargers, converters, and solar controllers designed for lead-acid can be set reasonably for LiFePO4, but not all. Few have LiFePO4 presets, and exactly how LiFePO4 should be treated for maximum life as house batteries is still being worked out. (They may only last twice as long as AGMs with the current best guesses, rather than up to ten times as long as is theoretically possible.)

Other than when fully charged or discharged, the voltage on LiFePO4 batteries is almost constant, with some almost random fluctuation. This means that the battery voltage cannot be used as a state of charge indicator. The 12.6 volts nominal on a six cell lead-acid battery is at full charge, the 12.8 nominal of a four cell LiFePO4 is near full discharge. LiFePO4 stay between 13.4 and 13.0 volts for most of their discharge. This higher voltage (which does not drop much under load) can cause problems with some devices. The sustained high charge current can also cause problems, some alternators will burn out if asked to put out their rated current for more than a short period of time.

While I have only been using a LiFePO4 house battery for a few weeks, Technomadia has had them two and a half years. If you are considering them, I recommend you read their blog series on their batteries. I have four Winston (marketed in the US by Balqon) 260 Ah cells in series, and am using my Morningstar Tristar 60 MPPT solar controler as my main charging source. I have removed the IQ4 from my Iota DLS-55 converter, and plan on using the high voltage (14.2 volt) jumper when on generator, and taking it out (13.6 volt) for the rare cases when I am on shore power, and not leaving it turned on for days at a time. (The Iota is not what I would have selected for LiFePO4, but for my occasional use seems adequate.)

For use as house batteries, current recommendations are bulk charging to 14.0 or 14.2 volts, leaving at that voltage until the current drops down, (not long) then using a float voltage of 13.2 to 13.4 volts just to supply the loads without discharging the battery significantly is the current best practice. The cells should be balanced before installing, but active balancing does not seem to be needed on four-cell batteries used at less than 1C both charging and discharging. (1C for a 260 Ah battery is 260 Amps.) Monitoring to see if the cells are getting out of balance seems like a good idea. LiFePO4 batteries should never be "equalized" by overcharging like lead-acid batteries are.

In general, prepackaged "12 volt" LiFePO4 batteries are not set up for use as house batteries and will not do well in that application. (Some are four "matched" cells, others include a battery monitor of some kind.) Claims of such as drop-in no thought required replacements for lead-acid batteries have turned out not to work well in practice. (The problem with the matched cell version is they should not be put in parallel, cells should be put in parallel then series.)

The battery tray in my motorhome was 14 inches wide and 12 inches deep. Two golf-cart batteries fit side-by-side because they are only 7 inches wide at the bottom, and a bit wider at the top. The 14.1 inch wide LiFePO4 cells I am using would not fit, and I built a wooden platform that fits in the tray to support them and hold the LiFePO4 cells in place and an open-top box around the cells. Even with the platform, they are not too tall for the space, but the taller 300 Ah cells would not work there. The box is thin plywood on the sides, and 2x4 on the front and back so it takes most of the 12 inches.

The wiring in my motorhome was undersized and overlong, and I am in the process of redoing parts of that, as well as installing a 2000 Watt inverter and adding a 12v outlet.

See Part 2 for a continuation.