The lithium iron phosphate battery (LiFePO4 battery) or LFP battery (lithium ferrophosphate), is a type of rechargeable battery, Because of low cost, low toxicity, well-defined performance, long-term stability, etc. LiFePO4 is finding a number of roles in vehicle use, utility scale stationary applications, and backup power. Lithium-ion batteries aren’t just powering your cell phone and laptop anymore, they’ve gotten bigger and more powerful and now they’re powering cars, homes, and even the electric grid. They’re becoming increasingly less expensive and the technology behind them is getting better leading to longer useful lifespans. In fact, some car batteries are finding a second life as energy storage systems for homes, businesses and for keeping the amount of energy on the grid stable.
Until the last few years, the majority of batteries used as energy storage or backups for a solar panel system were lead-acid batteries. Particularly deep-cycle batteries like those used in golf carts. But there were and are some drawbacks to using them, like the toxic materials in the batteries. Lithium-ion batteries are also toxic but, like lead, they can be recycled, it’s just less cost-effective than recycling lead at this point. Lead-acid batteries also had a short life-cycle, about 5 years. That’s also the length of a warranty for most lead batteries.
How long do LiFePO4 Batteries last?
LFP batteries can also last a very long time. Our Boye Energy LFP batteries are rated at 3000 cycles, at a full 100% charge/discharge cycle. If you did that every day it makes for over 8 years of cycling! They last even longer when used in less-than-100% cycles, in fact for simplicity you can use a linear relationship: 50% discharge cycles means twice the cycles, 33% discharge cycles and you can reasonably expect three times the cycles.
How Does a LiFePO4 Battery Work?
LiFePO4 Internal Structure
Lithium-ion batteries are referred to as a type of ‘rocking-chair’ battery: They move ions, in this case lithium ions, from the negative to the positive electrode when discharging, and back again when charging. The drawing on the right shows what is going on inside. The little red balls are the lithium ions, that move back and forth between the negative and positive electrodes.
On the left side is the positive electrode, constructed from lithium-iron-phosphate (LiFePO4). This should help explain the name of this type of battery! The iron and phosphate ions form a grid that loosely trap the lithium ions. When the cell is getting charged, those lithium ions get pulled through the membrane in the middle, to the negative electrode on the right. The membrane is made of a type of polymer (plastic), with lots of tiny little pores in it, making it easy for the lithium ions to pass through. On the negative side we find a lattice made of carbon atoms, that can trap and hold those lithium ions that cross over.
Discharging the battery does the same thing in reverse: As electrons flow away through the negative electrode, the lithium ions once again go on the move, through the membrane, back to the iron-phosphate lattice. They are once again stored on the positive side until the battery gets charged again.
If you have really been paying attention you now understand that the battery drawing on the right shows an LFP battery that is almost completely discharged. Nearly all the lithium ions are on the side of the positive electrode. A fully charged battery would have those lithium ions all stored inside the carbon of the negative electrode.
In the real world lithium-ion cells are built of very thin layers of alternating aluminum – polymer – copper foils, with the chemicals pasted on them. Often they are rolled up like a jelly-roll, and put in a steel canister, much like an AA battery. The 12 Volt lithium-ion batteries you buy are made of many of those cells, connected in series & parallel to increase the Voltage and Amp-hour capacity. Each cell is around 3.3 Volt, so 4 of them in series makes 13.2 Volt. That is just the right Voltage for replacing a 12 Volt lead-acid battery!
How to Charge a LFP Battery
Most regular solar charge controllers have no trouble charging lithium-ion batteries. The Voltages needed are very similar to those used for AGM batteries (a type of sealed lead-acid battery). The BMS helps too, in making sure the battery cells see the right Voltage, do not get overcharged, or overly-discharged, it balances the cells, and ensures the cell temperature is within reason while they are being charged.
The graph below shows a typical profile of a LiFePO4 battery getting charged. To make it easier to read the Voltages have been converted to what a 12 Volt LFP battery pack would see (4x the single-cell Voltage).
LiFePO4 Charge Voltage vs. SOC
Shown in the graph is a charge rate of 0.5C, or half of the Ah capacity, in other words for a 100Ah battery this would be a charge rate of 50 Amp. The charge Voltage (in red) will not really change much for higher or lower charge rates (in blue), LFP batteries have a very flat Voltage curve.
Lithium-ion batteries are charged in two stages: First the current is kept constant, or with solar PV that generally means that we try and send as much current into the batteries as available from the sun. The Voltage will slowly rise during this time, until it reaches the ‘absorb’ Voltage, 14.6V in the graph above. Once absorb is reached the battery is about 90% full, and to fill it the rest of the way the Voltage is kept constant while the current slowly tapers off. Once the current drops to around 5% – 10% of the Ah rating of the battery it is at 100% State-Of-Charge.
In many ways a lithium-ion battery is easier to charge than a lead-acid battery: As long as the charge Voltage is high enough to move ions it charges. Lithium-ion batteries do not care if they are not fully 100% charged, in fact they last longer if they are not. There is no sulphating, there is no equalizing, the absorb time does not really matter, you cannot really overcharge the battery, and the BMS takes care of keeping things within reasonable boundaries.
Charge Voltage Needed
So what Voltage is enough to get those ions moving? A little experimenting shows that 13.6 Volt (3.4V per cell) is the cut-off point; below that very little happens, while above that the battery will get at least 95% full given enough time. At 14.0 Volt (3.5V per cell) the battery easily charges up to 95+ percent with a few hours absorb time and for all intents and purposes there is little difference in charging between 14.0 or higher Voltages, things just happen a little faster at 14.2 Volt and above.
Lithium-ion cell structure
To summarize this, a bulk/absorb setting between 14.2 and 14.6 Volt will work great for LiFePO4! Lower is possible too, down to about 14.0 Volt, with the help of some absorb time. Slightly higher Voltages are possible, the BMS for most batteries will allow around 14.8 – 15.0 Volt before disconnecting the battery. There is no benefit to a higher Voltage though, and more risk of getting cut of by the BMS, and possibly damage.
LFP batteries do not need to be floated. Charge controllers have this because lead-acid batteries have such a high rate of self-discharge that it makes sense to keep trickling in more charge to keep them happy. For lithium-ion batteries it is not great if the battery constantly sits at a high State-Of-Charge, so if your charge controller cannot disable float, just set it to a low enough Voltage that no actual charging will happen. Any Voltage of 13.6 Volt or less will do.
With charge Voltages over 14.6 Volt actively discouraged, it should be clear that no equalize should be done to a lithium-ion battery! If equalize cannot be disabled, set it to 14.6V or less, so it becomes just a regular absorb charge cycle.
There is a lot to be said for simply setting the absorb Voltage to 14.4V or 14.6V, and then just stop charging once the battery reaches that Voltage! In short, zero (or a short) absorb time. At that point your battery will be around 90% full. LiFePO4 batteries will be happier in the long run when they do not sit at 100% SOC for too long, so this practice will extend battery life. If you absolutely have to have 100% SOC in your battery then absorb will do that! Officially this is reached when the charge current drops to 5% – 10% of the Ah rating of the battery, so 5 – 10 Amp for a 100Ah battery. If you cannot stop absorb based on current, then set absorb time to about 2 hours and call it a day.
LiFePO4 batteries do not need temperature compensation! Please switch this off in your charge controller, or your charge Voltage will be wildly off when it is very warm or cold.
Be sure to check your charge controller Voltage settings against those actually measured with a good quality digital multi-meter! Small changes in Voltage can have a big impact when charging a lithium-ion battery! Change the charge settings accordingly!
The very low self-discharge rate makes it easy to store LFP batteries, even for longer periods. It is no problem to put a lithium-ion battery away for a year, just make sure there is some charge in it before placing it in storage. Something between 50% – 70% is fine, that will give the battery a very long time before self-discharge brings the Voltage close to the danger point.
Storing batteries below freezing is fine, they do not freeze and do not care much about temperature. Try to avoid storing them at high temperatures (45 Centigrade and above), and try to avoid storing them completely full if possible (or nearly empty).
If you need to store batteries for longer periods, be sure to simply disconnect all wires from them. That way there can not be any stray loads that slowly discharge the batteries.
Post time: Sep-07-2019