How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries
By: Rob Beckers
You have just sold your first-born into slavery, remortgaged the house, and bought yourself a lithium-ion battery! Now you want to know how to take care of your precious new purchase: How to best charge lithium-iron batteries, how to discharge them, and how to get the maximum life out of your lithium-ion batteries. This article will explain the do’s and don’ts.
Pricing of lithium-ion batteries is slowly changing from obscenely expensive to only moderately unaffordable, and we at Solacity are seeing a steady increase in sales of this type of battery. Most users seem to put them to work in RVs, fifth-wheels, campers and similar vehicles, while some are going into actual stationary off-grid systems.
This article will talk about one specific category of lithium-ion batteries; Lithium-Iron-Phosphate or LiFePO4 in its chemical formula, also abbreviated as LFP batteries. These are a little different from what you have in your cell phone and laptop, those are (mostly) lithium-cobalt batteries. The advantage of LFP is that it is much more stable, and not prone to self-combustion. That does not mean the battery cannot combust in case of damage: There is a whole lot of energy stored in a charged battery and in case of an unplanned discharge the results can get very interesting very quickly! LFP also lasts longer in comparison to lithium-cobalt, and is more temperature-stable. Of all the various lithium battery technologies out there this makes LFP best suited for deep-cycle applications!
Since this seems to be a cause of confusion for some: All Lithium-Iron-Phosphate batteries are in fact lithium-ion batteries, but not all lithium-ion batteries are Lithium-Iron-Phosphate batteries. The term “lithium-ion battery” is a generic description of any battery technology that uses lithium ions to move and store electrical charge. Many chemistries use lithium ions to do this, and LiFePO4 is one such specific lithium-ion chemistry.
We will assume the battery has a BMS or Battery Management System, as almost all LFP batteries that are sold as a 12/24/48 Volt pack do. The BMS takes care of protecting the battery; it disconnects the battery when it is discharged, or threatens to be over-charged. The BMS also takes care of limiting the charge and discharge currents, monitors cell temperature (and curtails charge/discharge if needed), and most will balance the cells each time a full charge is done (think of balancing as bringing all the cells inside the battery pack to the same state-of-charge, similar to equalizing for a lead-acid battery). Unless you like living on the edge, DO NOT BUY a battery without BMS!
While this is a very good general rule to live by, there are exceptions worth noting: Some brands, Victron being one of them, make batteries that do not contain a build-in BMS. Instead, they provide an external BMS with all the hardware and instructions needed to wire the battery bank up safely. There is advantage to an external BMS (it can be replaced easily if it breaks), and this too provides a safe and useful solution.
What follows below is the knowledge gleamed from reading a large number of Web articles, blog pages, scientific publications, and discussion with LFP manufacturers. Be careful what you believe, there is a lot of disinformation out there! While what we write here is by no means meant as the ultimate guide to LFP batteries, our hope is that this article cuts through the bovine excrement and gives solid guidelines to get the most out of your lithium-ion batteries.
The Weak Link - The BMS
We now have a few years of experience with lithium-ion batteries, and what is becoming clear is that while the LiFePO4 cells hold up very well, that is not the case with the Battery Management System (BMS). Overall the number of prematurely failed batteries is small, but with 10,000+ batteries sold it is clear that in 99% of cases it is the BMS that fails, turning the battery into an expensive piece of gender-neutral-cave decoration!
While we very much advocate for using batteries with a build-in BMS (without one the battery would be unsafe and likely fail very quickly!), manufacturers struggle to make their BMS as bullet-proof as it should and needs to be. Surge currents due to the input capacitors of large inverters, motors, and air-conditioners can and at times will kill the BMS, rendering the battery useless.
At least one well-known battery manufacturer is now enforcing their warranty conditions to the letter, and that requires the use of an external current limiter when their batteries are used with large inverters (“large” being defined as 3,500 Watt and up). This leads to the ironical situation where the BMS is there to protect the battery, and now a current limiter gets connected to protect the BMS. What will be next to protect the current limiter…
Seeing how the BMS has become the weak link, manufacturers should really work (hard) on hardening that. Nothing is 100% bomb-proof, but there certainly is room for improvement! Another solution could be to acknowledge that the BMS is the weak link and make it so it can be replaced without too much effort, for example a gasketed lid on the battery that is removable with a few screws, and a BMS that has connectors and bolted lugs, so a repair shop can swap the board. It makes no sense to throw away a battery where 90% of the cost is in the cells, and 10% in the BMS, just because the BMS failed.
Why Lithium-Ion?
We explained in our lead-acid battery article how the Achilles heel of that chemistry is sitting at partial charge for too long. It is too easy to pooch an expensive lead-acid battery bank in mere months by letting it sit at partial charge. That is very different for LFP! You can let lithium-ion batteries sit at partial charge forever without damage. In fact, LFP prefers to sit at partial charge rather than being completely full or empty, and for longevity it is better to cycle the battery or to let it sit at partial charge.
But wait! There is more!
Lithium-ion batteries are very nearly the holy grail of batteries: With the right charge parameters you can almost forget there is a battery. There is no maintenance. The BMS will take care of it, and you can happily cycle away!
But wait! There is still more! (Any resemblance with certain infomercials is purely coincidental, and, frankly, we resent the suggestion!)…
LFP batteries can also last a very long time. Our Volthium LFP batteries are rated at 2500+ cycles, at a full 100% charge/discharge cycle. If you did that every day it makes for 7 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. In actuallity you actually get more cycles than that, and very few real-life scenarios would see batteries drained a full 100% on a regular basis. For that reason we reasonably expect that most people will see their lithium batteries realistically last 20 years before needing replacements.
But wait! There is more yet!…
A LiFePO4 battery also weighs less than 1/2 of a lead-acid battery of similar capacity. It can handle large charge currents (100% of Ah rating is no problem, try that with lead-acid!), allowing for rapid charging, it is sealed so there are no fumes, and it has a very low self-discharge rate (3% a month or less).
Battery Co$t of Lithium-Ion vs. Lead-Acid
As I am writing this, our Enerwatt WPL31 100Ah 12 Volt battery costs $727 Canadian dollars. The full 100Ah is certainly usable, so that makes 12 x 100 = 1,200 Watt-hour in energy storage, or $0.61 per Wh in usable energy storage.
One of our best-bang-for-the-deep-cycle-buck lead-acid batteries is the Rolls/Surrette S6 L16-HC (formerly S-550), currently going for $493 for 6V 445Ah in storage. With lead-acid only 80% is really reasonably useful, going into the bottom 20% of energy storage is a recipe for permanent battery damage, so we have 6 x 445 x 0.8 = 2,136 Wh in energy storage. That makes for 493 / 2136 = $0.23 per Wh in usable energy storage.
This is where you say, “wait a minute, those d@m$ lithium batteries are three times the price of lead-acid!!”. Immediately followed by “but I still really want one!”, and right you are: We did not figure the difference in battery life in yet.
The Surrette S-550 is good for around 1900 cycles at 50% Depth-Of-Discharge (DOD), while the Enerwatt will do 6000 cycles at that same 50% DOD. That means the lithium-ion battery is going to last about 3.2x as long! Over the life of a single set of LFP batteries the cost per usable Wh for lead-acid now works out to 3.2 x 0.23 = $0.73, exceeding the cost of lithium-ion! There is more to it than that: In real life very few people will get the full cycle life out of lead-acid batteries. It is too easy to have them take offence to your treatment and prematurely depart for the the Big Battery in the Sky. If you do make them go the distance, there is watering, measuring specific gravity, and taking care to regularly recharge them lest they sulfphate. None of that is needed for lithium-ion!
We bet at this point you are willing to throw in that no-good spouse of yours (m/f, we don’t discriminate) with your first-born just so you can replace your lead-acid with lithium-ion batteries!
Battery Bank Sizing for LFP
We hinted at this above: Lithium-ion batteries have 100% usable capacity, while lead-acid really ends at 80%. That means you can size an LFP battery bank smaller than a lead-acid bank, and still have it be functionally the same. The numbers suggest that LFP can be 80% the Amp-hour size of lead-acid. There is more to this though.
For longevity lead-acid battery banks should not be sized where they regularly see discharging below 50% SOC. With LFP that is no problem! Round-trip energy efficiency for LFP is quite a bit better than lead-acid as well, meaning that less energy is needed to fill up the tank after a certain level of discharge. That results in faster recovery back to 100%, while we already had a smaller battery bank, reinforcing this effect even more.
The bottom line is that we would be comfortable to size a lithium-ion battery bank at 55% – 70% of the size of an equivalent lead-acid bank, and expect the same (or better!) performance. Including on those dark winter days when sun is in short supply.
Beware of Series Connected Batteries!
There is a potential issue when multiple lithium-ion batteries are connected in series. For example, two 12 Volt 100 Ah batteries, each with their own build-in BMS, connected in series to make 24 Volt 100 Ah. Now assume one of those two batteries is near-empty, the other pretty full, and you put a load on the batteries, to discharge them. The near-empty battery will reach the point where the BMS decides “enough is enough” first and it will switch off that battery, in effect disconnecting your entire battery bank, even though the other battery is still full.
The same potential for trouble exists when charging both batteries at the same time with a 24 Volt charging source. The fuller of the two batteries will fill up first, raising the charging Voltage over that battery, until reaching the point where the BMS once again intervenes to protect the battery and switches the full battery off. When the BMS switches off, your entire battery bank “goes away”. If both started off uneven, then the other battery may well be nowhere near full yet, and this will not resolve over time or multiple charge cycles either.
The moral of this story is that you should understand the dynamics of connecting multiple lithium-ion batteries in series. They do not quite behave like lead-acid batteries! Lead-acid batteries will self-balance when they are charged, all attaining a similar state-of-charge in the end. Lithium-ion batteries due to each having their own independent BMS do not! From experience we know that by-and-large series connected batteries, each with their own BMS, can work fine. It would be a good idea though to make sure both are “in sync” every now and then, by charging them individually with a 12 Volt charger, until both are known to be fully charged, so they start off with the same state-of-charge.
Because it is important to understand this, we will come back to it and other BMS-related peculiarities in more detail further below.
But Wait a Minute!
Is lithium-ion really the solution to all our battery woos? Well, not quite…
LFP batteries too have their limitations. A big one is temperature: You cannot charge a lithium-ion battery below freezing, or zero Centigrade. Lead-acid could not care less about this. You can still discharge the battery (at a temporary capacity loss), but charging is not going to happen. The BMS should take care to block charging at freezing temperatures, avoiding accidental damage. This is a Big Deal in our Canadian climate!
Temperature is also an issue at the high end. The biggest single cause of aging of the batteries is use or even just storage at high temperatures. Up to around 30 Centigrade there is no problem. Even 45 Centigrade does not incur too much of a penalty. Anything higher really accelerates aging and ultimately the end of the battery though. This includes storing the battery when it is not being cycled. We will talk about this in more detail later, when discussing how LFP batteries fail.
There is a sneaky issue that can crop up when using charging sources that potentially provide a high Voltage: When the battery is full the Voltage will rise, unless the charging source stops charging. If it rises enough the BMS will protect the battery and disconnect it, leaving that charging source to rise even more! This can be an issue with (bad) car alternator Voltage regulators, that need to always see a load or the Voltage will spike and the diodes will release their magic smoke. This can also be an issue with small wind turbines that rely on the battery to keep them under control. They can run away when the battery disappears.
Then there is that steep, steep, initial purchase price!
But we bet you still want one!..
Other BMS-Induced Peculiarities & Problems
The BMS (Battery Management System) that is build into the batteries functions as a simple on-off switch, switching the batteries off when Voltage, current, or temperature parameters get to the edge of what is safe. Contrary to what many think, the BMS does not change the charge or discharge current, it really is just an on-off switch; if you have a charging source that can push hundreds of Amps into the battery the BMS will not prevent you from doing this, but it will sense the large current, and switch the battery off when the upper safe limit is reached!
Lithium Batteries with Build-In Heater
The issue of not being able to charge LiFePO4 batteries below freezing is now being overcome by versions that have a built-in heater and a thermostat that senses when the battery is being charged at a low temperature. It will kick in a heater to get the cells above freezing before actually charging the battery (incidentally this is also how an electrical car works in winter).
That may look like a good solution to the no-charging-below-freezing problem, but it too comes with its own potential issues: When the heater is running the battery cells are not charging (and vice-versa). This could cause problems when your charging sources can deliver more current than the heater needs, which in turn causes the ‘charge Voltage’ (though note it’s not actually charging, just heating) to rise rapidly. Generally the charge controller will limit the Voltage, and keep it from rising too high. However, this may erroneously cause the charge controller to believe it’s done bulk-absorb charging and go to its float stage before it is done heating, at which point the batteries will not actually be charged at all! The work-around is to create a custom charging profile for the charge controller, with a suggested value of 14.2 or 14.4 Volt for bulk-absorb (for a 12V battery bank, multiply to fit your situation for 24/48V batteries), and 14.0 Volt for float. Absorb time should still be kept low, 0.5 or 1 hour is suggested, and temperature compensation should be set to zero (or off). This way the batteries will get charged to at least 95% State-Of-Charge even when the heating cycle leaves the charge controller at float by the time it is done.
Because the battery cells are switched off while the battery heater is running, there is in essence no battery. That leaves the system with very little Voltage-buffer, in other words a small current changes cause large Voltage changes, something that may not work with (for example) wind turbines that expect to see a large load to keep them under control.
If your system has a battery monitor that uses a shunt to measure the Amp-hours going in, and coming out of the battery, this will lose track of the State-Of-Charge when a heated battery is involved. Any Amp-hours going towards heating are not charging the battery, but they are counted by the battery monitor! There is no work-around for this, you will just not get good SOC readings from a shunt-based battery monitor when battery heating is involved.
Heated lithium-ion batteries should NEVER be connected in series! Unless the BMS’s in both batteries communicate, there is no way the heaters will be switched on/off at the exact same times, and that will cause all kinds of issues. So no series connection for heated LiFePO4 batteries.
The long and short of this is that you should think long and hard about heated LiFePO4 batteries. If at all possible it may be a better idea to stick with regular (unheated) ones, and use a well-insulated battery box to keep the temperatures above freezing.
Jump-Starting The BMS
For most conditions (over-current, over-Voltage, under-temperature, or over-temperature) the BMS will automatically switch back on again, either after a set amount of time has passed, or once the conditions are safe. However, there is one case where the BMS will NOT switch on by itself, the battery will stay off: When any cell within a LFP battery falls below the lower safe Voltage limit the BMS will switch off to protect the cells from over-discharge. It does this with still a little charge left in the cells, so the battery can sit for a while and self-discharge before damage to the cells occurs. The important part is that the BMS will not switch the battery back on by itself! When this happens the battery simply “goes away” and produces 0 Volt.
To make the BMS switch on again after a low-Voltage disconnect event the battery needs to see a charging Voltage. How much exactly varies from brand-to-brand, but generally this means 14.0 Volt or up (for a 12V battery). Keep in mind that inverter-chargers won’t work without a battery, nor will solar charge controllers. They need to see regular battery Voltage to function. That means you cannot switch the battery BMS back on by charging from a generator (via your inverter-charger) or your solar panels. To make the BMS switch on again you either need a 120V AC charger that can do “dead battery charging” as it is usually called in the brochure, meaning it puts out a charging Voltage even if it does not sense a battery. Alternatively you can “jump start” the switched-off battery by taking another battery of the same nominal Voltage, even a lead-acid battery, and connect it in parallel with the dead battery, and then charge via solar or your inverter-charger. As soon as the Voltage reaches high enough the BMS will sense it and switch the battery back on again. At that point you can disconnect the extra battery, but please keep charging so the empty battery does not immediately switch off again with the slightest load.
Balance & BMS
Another source of confusion, and potential problems, is when multiple single lithium-ion batteries, each with their own BMS, get connected in series to create a battery bank with a higher Voltage. For example, by connecting four 12 Volt batteries in series to create a 48V battery bank. Lithium-ion batteries do not at all self-balance! This is very different from lead-acid, where charging gets progressively less efficient as the battery gets more fully charged; this makes it so when multiple batteries are connected in series the ones that have less charge in them will automatically “catch up” to the fuller battery. Not so for lithium-ion batteries! Any difference in charge between series-connected batteries will persist from charge-cycle to charge-cycle and wreak havoc: Say a half-full 12V battery is connected to a fully charged 12V battery, in series, to create a 24V battery bank. When discharging the half-full battery will reach empty first, and at some point the BMS will intervene and switch that battery off, causing the entire battery bank to “go away”. Moreover, the empty battery will not switch on again until it sees a charging Voltage. The second battery will at this point be about half full, and if the entire bank is charged in this state the situation will be exactly the same as before; one battery will reach full while the other is only just half-full. Worse still, the fully charged battery will continue to rise in Voltage until the BMS intervenes and switches that battery off, causing the entire bank once again to just disappear (though it will switch back on again by itself, not needing a “jump start”). This situation will persist forever, unless manually corrected.