LIFEPO4 SOC and everything else you need to know!

LiFePo4 SOC

Want to know how to take care of your valuable new purchase: the best way to charge lithium batteries, how to discharge them, and learn how to maximize their life? This article explains the pros and cons.

Lithium-ion battery prices are slowly changing from obscenely expensive to just almost on par with traditional Lead Acid, and we at LiFePo4 Australia tend to find most users using them in Caravans, fifth wheels, RVs, and the like Vehicles while some are jumping into stationary off-grid systems.

The specific category of lithium-ion batteries; Lithium Ferro (iron) Phosphate or LiFePO4 in its chemical formula, also abbreviated as LFP batteries. This is a little different from what you have in your cell phone and laptop. These are (mostly) lithium cobalt batteries. The advantage of LFP is that it is much more stable and not prone to self-immolation. This does not mean that if the battery is damaged, it will not burn: there is a fair amount of energy stored in a charged battery, and in the event of an unplanned discharge, the results can be obtained Very interesting, very quick! Compared to lithium cobalt, LFP also lasts longer and is more temperature stable. Of all the different lithium battery technologies, this makes LFP best suited for deep cycle applications! no. BMS takes care of battery protection; disconnects the battery when it is discharged or overcharged. BMS also cares about limiting charge and discharge currents, monitors the cell temperature (and limits charge/discharge if necessary), and most of them balance each cell with a full charge time (think of balancing as bringing all cells inside the battery to the same state of charge, similar to the equalization for a lead acid battery) Unless you like living on the edge, DO NOT BUY a battery without a BMS! Below is the knowledge gained from reading a large number of web articles, blog pages, scientific publications, and discussions with LFP manufacturers. which we are writing about here is by no means intended as the ultimate guide to LFP batteries, we hope this article will bypass cattle excrement and give you solid tips on how to get the most out of lithium-ion batteries.

Why lithium-ion? We explained in our lead-acid battery article how that chemistry’s Achilles heel stays at a partial charge for too long It’s too easy to damage an expensive bank of lead-acid batteries in a few months by letting it sit at partial charge This is very different for LFP! You can let lithium-ion batteries stay partially charged forever without damage, LFP prefers to stay partially charged rather than full or empty, and for longevity, it is best to cycle the battery or let it sit partially charged. Lithium-ion batteries are currently what I would call the holy grail of batteries – with the right charging parameters you can almost forget there’s a battery. Maintenance. The BMS will take care of it and you can safely go cycling!

LFP batteries can also last very long. The once famous USA-made Battle Born LFP batteries are rated for 3000 cycles, with a full 100% charge/discharge cycle. If you do this every day, it gives you over 8 years of cycling! even longer when used in cycles less than 100%, in fact, for simplicity, a linear relationship can be used: 50% discharge cycles mean twice as many cycles, 33% discharge cycles, and you can reasonably expect three times the number of cycles. A LiFePO4 battery also weighs less than half that of a lead acid battery of similar capacity. It can withstand large charging currents (100 Ah rating is no problem, try it with lead acid! ) to allow for fast charging, it’s sealed to prevent fumes and it has a very low self-discharge rate (3% per month or less).

Lithium-Ion vs. Lead Acid

As of 2020 a Battle Born 12V 100Ah battery costs $950 US dollars ($1400-1500AUD). The full 100Ah is usable, so 12.8 x 100 = 1,280-watt-hours of energy storage or just under $1.50 AUD per Wh of usable energy storage. One of the best – bang-for-buck-deep-cycle lead-acid batteries is currently on sale at approx $300 for 12v 120Ah. With lead-acid only 50% is really usable. so we have 12 x 120 x 0.5 = 770wh  energy storage. That makes 400/60 = $ 0.67 AUD per Wh of usable energy storage.

This is where you say, “Wait a minute, those dam $ lithium batteries cost nearly twice the price of lead acid!”! Immediately followed by “but I still want one!” And you’re right: We haven’t determined the difference in battery life yet. A very good quality Lead Acid is suitable for around 1000 cycles at 50 ° discharge (DOD) during the battle Born does 6000 cycles with the same 50% DOD. This means that the lithium-ion battery will last about 6 times as long! Over the life of a single set of LFP batteries, the cost per usable Wh for lead acid is now 6 x 0.50 = $ 3.00, making lithium ions half the price! But wait, who even buys a Battleborn is Australia? No one, But it’s still a good example. If we were talking about SolarKing or iTechworld the LiFePO4 would be twice as good value. However, any of the much more expensive brands like Victron and Enerdrive would be more in line with BattleBorn. There is so much more: in real life, very few people will get the full life cycle of lead-acid batteries

Apply this to the Chinese-made 100ah drop-ins. The cost is cheaper for LiFePo4! It’s a no-brainer!

It is too easy for them to take offense at your treatment and leave prematurely for the Big Battery in the Sky, if you make them go far, there is water, specific gravity measurement, and taking care to recharge them regularly to prevent them from sulfating, none of this is necessary for lithium ions! We bet at this point you are starting to consider these Lithium Ions!

Battery Bank Sizing

Battery bank sizing for LFP I mentioned above: LFP batteries have a best practice usable capacity of 90%, while lead-acid really ends up at 50%. This means you can size an LFP battery bank smaller than a lead-acid bank and functionally it’s the same. The numbers suggest that LFP may be 55-60% of the amp-hour size of the lead acid. However, there is more. For longevity, lead acid battery banks should not be sized where they regularly see discharges below 50% SOC. With LFP this is not a problem! Round-trip energy efficiency for LFP is also a lot better than lead-acid, which means less energy is needed to fill the tank after a certain level of discharge. faster recovery up to 100%, while we already had a smaller battery bank, further reinforcing this effect. The bottom line is that it would be handy to size a lithium-ion battery bank to 55% – 60% the size of an equivalent lead-acid bank and expect the same (actually better!) performance. Even on those dark winter days when the sun is scarce.
This also means you can basically use half the solar panel size array, just because of this difference, you now need a smaller system overall. Making LFP the clear winner in every possible scenario.

Beware of batteries connected in series!

There is a potential problem when multiple Li-ion batteries are connected in series. For example, two 12 Volt 100 Ah batteries, each with their own built-in BMS, are connected in series to produce 24 Volt 100 Ah. Now assume that one of these two batteries is almost empty, the other quite full, and put a load on the batteries to discharge them. The low battery will reach the point where the BMS first decides “enough is enough” and shut down that battery, effectively disconnecting the entire battery bank, even if the other battery is still full. The same potential for trouble exists when charging both batteries simultaneously with a 24 Volt charging source. two batteries will fill first, increasing the charge voltage on that battery, until it reaches the point where the BMS once again steps in to protect the battery and shut down the full battery. When the BMS shuts down, the entire bank of batteries “goes. If both started erratically, the other battery may not be fully charged yet, and this will not resolve over time or even with multiple charge cycles. . Moral of this story is that you should understand the dynamics of connecting multiple lithium-ion batteries in series. They don’t behave quite like lead-acid batteries! Lead-acid batteries self-balance when charged, all reaching a similar state of charge in the end. Lithium-ion batteries because each has its own independent BMS. From experience, we know that connected batteries in series, each with its own BMS, can function properly. It would be a good idea to make sure the two are ‘in sync’ every now and then, charging them individually with a 12-volt charger, until both are fully charged, so they start with the same state of charge.

Temperature of LFP

But wait a minute! Is lithium-ion really the solution to all our battery woos? Well, not quite… LFP batteries also have their limits. One big problem is temperature: you cannot charge a lithium-ion battery below zero Celcius. Lead acid does not care. You can still discharge the battery (with temporary loss of capacity), but the charging will not occur. The BMS should take care to block the charge in freezing temperatures, avoiding accidental damage. This is a possible problem in our Australian climate! Temperature is also an issue at the high end. The main cause of aging batteries is use or even just storage at high temperatures. Up to about 30 degrees centigrade, there is no problem, even 45 degrees centigrade does not incur an excessive penalty, anything higher really accelerates aging and eventually the end of the battery, however, this includes storing the battery in a shaded possibly cooled environment.

There is a sneaky problem that can arise when using charging sources that potentially provide high voltage: when the battery is full, the voltage will rise, unless the charging source stops If it gets up high enough, the BMS will protect the battery and disconnect it, letting the charging source go up even more! This can be a problem with the car (bad) alternator voltage regulators, which must always see a load or the voltage will spike and the diodes will release their magic smoke. This can also be a problem with small wind turbines. which rely on the battery to keep them in check. They could possibly destroy an LFP battery. Then there’s that high initial purchase price! But we bet you still want one!

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 during discharge and vice versa always during charging. The drawing on the right shows what is happening inside. The red balls are the lithium ions, which move back and forth between the negative and positive electrodes. On the left side is the positive electrode, constructed from iron phosphate (LiFePO4). This should help explain the name of this type of battery! The iron and phosphate ions form a grid that freely traps the lithium ions. When the cell charges, those lithium ions are drawn through the membrane into the center, to the negative electrode on the right. The membrane is made from a type of polymer (plastic), with many tiny pores inside it, which facilitate the passage of lithium ions. On the side we find a lattice made of carbon atoms, which can trap and hold those lithium ions that cross. The battery is discharged in reverse order: When electrons flow away through the negative electrode, the lithium ions move again through the membrane back to the iron-phosphate grid. They will again be saved on the positive side until the battery is recharged. Almost all lithium ions are on the positive electrode side. In a charged battery, these lithium ions would all be stored in the carbon of the negative electrode. In the real world, lithium-ion cells consist of very thin layers of alternating aluminum-polymer-copper foils, The chemicals are stuck to it. Rolled up like a jelly roll and put in a steel container, similar to an AA battery. The 12 volt lithium ion batteries you buy are made up of many of these cells connected in series and in parallel in order to increase the voltage and ampere-hour capacity. Each cell is roughly 3.3 volts, so 4 of them in series makes 13. 2 volts This is the correct voltage to replace a 12-volt lead acid battery!

Charging an LFP Battery from solar

Most regular solar charge controllers have no problem charging lithium-ion batteries

The voltages required are very similar to those for AGM batteries (a type of sealed lead-acid battery). The BMS also helps to secure the battery cells to see the correct voltage, do not get overcharged or over-discharged, balance the cells and ensure that the cell temperature is within the reasonable range while charging. The following graph shows a typical Profile of a LiFePO4 battery being charged Make it easy to see the voltages that have been converted to what a 12-volt LFP battery would see (4x the single cell voltage). LiFePO4 charge voltage vs.SOC LiFePO4 charge voltage vs or half the Ah capacity, in other words for a 100 Ah battery this would be a charge rate of 50 amps. The charge voltage (in red) doesn’t change that much for LFP batteries with higher or lower charge rates (in blue) they have a very flat voltage curve. Lithium-ion batteries are charged in two stages: first, the current is kept constant, or with solar PV, which usually means we try to send as much current into the batteries as is available from the sun. the voltage will slowly increase during this time, until it reaches the “ absorption ” voltage, 14.6V in the graph above.

Once the absorption is reached, the battery is full to about 90 %, and to fill it the rest of the time, the voltage is kept constant while the current slowly decreases. Once the current drops to about 5% – 10% of the battery’s Ah rating, it is at 100% of l ‘state of charge. In many ways, a lithium-ion battery is easier to charge than a lead-acid battery: as long as the charging voltage is high enough to displace the ions it is charging. ion does not care if they are not fully 100% charged, in fact, they last a long time if they are not. There is no sulfation, there is no equalization, absorption time does not really matter, you cannot really overcharge the battery, and the BMS takes care to keep things within reasonable limits. Required charging voltage So what voltage is enough to move these ions?6 Volts (3.4 V per cell) is the breakpoint; below this very little happens, while above this the battery will fill up to at least 95% given enough time At 14.0 Volts (3.5 V per cell) the battery easily charges up to 95+ percent with a few hours of time absorption and for all intents and purposes and for purposes there is little difference in charging between voltages 14.0 or higher, things just happen a little faster at 14.2 Volts and above. it will work great for LiFePO4! With the help of a certain absorption time, a decrease to around 14.0 volts is also possible. Slightly higher voltages are possible. The BMS for most batteries allows about 14.8 to 15.0 volts before the battery is disconnected. There is no benefit to higher voltage but there is a higher risk of being cut off by the BMS and possible damage. Float Voltage LFP batteries do not have to be floatable. Charge controllers do this because lead-acid batteries have such a high rate of self-discharge that it makes sense to keep adding more and more charges to keep them happy. It is not good for lithium-ion batteries if the battery is constantly in a high state of charge. If your charge controller cannot deactivate the float, simply set it to a low enough voltage that actual charging will not occur. Any voltage from 13. 6 Volts or less. Equalize Voltage With charging voltages above 14.6 Volts actively discouraged, it should be understood that no equalization should be performed on a Li-ion battery! If the equalization cannot be disabled, set it to 14.6V or less, then it becomes just a normal absorption charge cycle.

Absorption time

There is a lot to be said for just setting the absorption voltage to 14.4V or 14.6V, and then stop charging once the battery reaches that voltage! In short, zero (or short) absorb time. At that point, the battery will be about 90% full. LiFePO4 batteries will be happier in the long run when they don’t stay at 100% SOC for too long, so this practice will extend your battery life. If you absolutely must have 100% SOC in your battery, absorb it will do! Officially, this is achieved when the charging current drops to 5% – 10% of the battery Ah value, i.e. 5-10 Amp for a 100Ah battery If you cannot stop absorbing the current, set the absorption time to about 2 hours and call Temperature compensation LiFePO4 Batteries do not require temperature compensation! Turn this off in the charge controller, otherwise, the charge voltage will be wildly turned off when it is very hot or cold. Check the voltage settings of the charge controller against actual voltage measured with a good quality DMM! Small changes in voltage can make a big difference during charging a lithium-ion battery! Change the charge settings accordingly!

Discharging an LFP Battery

Unlike lead acid batteries, the voltage of a lithium-ion battery remains very constant during discharge, making it difficult to guess the state of charge from the voltage alone. For a battery with a moderate load, the discharge curve seems LiFePO4 Discharge voltage vs. discharge voltage SOC LiFePO4 vs. SOC Most of the time during discharge, the battery voltage will be just around 13.2 volts. it was a really bad idea ™ to go below 20% SOC for a LiFePO4 battery. This has changed and the current LFP battery harvest will quite happily discharge down to 0% for many cycles. However, there is an advantage in pedaling less deep. It’s not just that going to 30% SOC will get you 1 / 3 cycles more than 0%, the battery will likely last more cycles than that. The hard numbers are, well, hard to find, but the cycle up to 50% SOC seems to show about 3 times the cycle life compared to .cycling 100%. Below is a table showing the battery voltage for a 12 Volt battery pack with respect to depth of discharge. Take these voltage values with a pinch of salt, the discharge curve is so flat that it is really difficult to determine SOC from voltage alone: small variations in load and accuracy of the voltmeter will negate the measurement.

Storing lithium-ion batteries

The very low self-discharge rate makes it easier to store LFP batteries, even for longer periods of time. It is not a problem to put a lithium-ion battery away for a year, but make sure it is charged beforehand. Storing between 50% and 60% is ideal as the battery takes a very long time before self-discharge near voltage Storing batteries below freezing is fine even at very low temperatures than -40 degrees Celsius (which is the same in Fahrenheit) or even less! The electrolyte in LiFePO4 cells does not contain any water. Even if it freezes (which happens at around -40 degrees Celsius depending on the formulation), this is not the case. Allow the battery to warm up a little before discharging it again. This is fine at -20 degrees Celsius and above. When discharging at temperatures below freezing point, an obvious loss of capacity occurs, reversed when the battery rises above freezing point and has a slightly accelerated effect on aging. Storage at low temperatures is certainly much better than storage at high temperatures: the aging of the calendar slows down dramatically at low temperatures. Avoid storing them at 45 degrees Celsius and above, and try to avoid keeping them as full (or almost empty) as possible. If you need to store batteries for a long time, just unplug all cables from them. In this way, there can be no parasitic charges which slowly discharge the batteries.

The end of your lithium-ion batteries We can hear you gasp in horror; the idea that your precious LFP battery bank is no longer thrills you! Alas, all good things have to come to an end eventually. What we want to avoid is the premature (and perhaps spectacular) end of the genre, which makes us have to understand how lithium-ion batteries die. Many batteries consider a battery to be “dead” when its capacity drops to 80% of what it should be. Thus, for a 100Ah battery, its end comes when its capacity is reduced to 80Ah There are two mechanisms at work for the disappearance of your battery: cycling and aging. Each time you discharge and recharge the battery, it does a bit of damage and you lose a bit of capacity. But even if you put your precious battery in a beautiful, glass-enclosed sanctuary, never to be rolled, it will always come to an end.

LiFePo4 Lifespan

The latter is called lifespan. Calendar It is difficult to find reliable data on the calendar life of LiFePO4 batteries, very little is available. Some scientific studies have been conducted on the effect of extremes (temperature and SOC) on calendar life and these help set limits. What we gather is that if you don’t abuse the battery bank, avoid extremes and generally only use the batteries within reasonable limits, there is a maximum limit of about 20 years on calendar life. In addition to the cells inside the battery, there is also the BMS, which is made up of electronic parts. When the BMS fails, the battery will be too. ion batteries with a built-in BMS are still too new, and we’ll have to see, but ultimately the battery management system has to survive for as long as lithium-ion cells do. The processes within the battery conspire over time to coat the boundary layer between the electrodes and the electrolyte with chemical compounds that prevent lithium ions from entering and exiting the electrodes. The processes also bind lithium ions into new compounds chemicals, so they are no longer available to switch from electricity ode to electrode. These processes will happen regardless of what we do, but are very temperature dependent! Keep the batteries below 30 degrees centigrade and they are very slow. Go beyond 45 degrees centigrade and things speed up considerably! The # 1 public enemy for lithium-ion batteries is heat by far! The calendar life has more to offer and how quickly a LiFePO4 battery will age: the state of charge also has something to do with it. Bad at high temperatures, these batteries really, really don’t like to sit at 0% SOC and very high temperatures!

Also bad, although not quite as bad as 0% SOC, is sitting at 100% SOC and high temperatures for them. Very Low Temperatures As we discussed earlier, you cannot charge LFP batteries below freezing (and the BMS will not allow you to). It turns out that while it can be discharged below freezing, it also has an accelerated effect on aging. Nowhere near as bad as leaving your battery at a high temperature. However, if you are exposing your battery to freezing temperatures, it is better to do so while it is neither charging nor discharging and there is some gas in the tank (though not a full tank). In general, it is better to put these batteries away. a t around 50% – 60% SOC if they need to be stored longer. Molten Battery If you really want to know, what happens when a lithium-ion battery is charged below zero is that metallic lithium is deposited on the negative (carbon) electrode. , which end up puncturing the membrane and shorting out the battery (leading to a spectacular, unscheduled rapid disassembly event as NASA calls it, involving smoke, extreme heat, and most likely flames). Lucky for us this is something the BMS prevents from We move on to cycle life It has become common to get thousands of cycles, even at a full 100% charge-discharge cycle, with lithium-ion batteries There are some things you can do to maximize cycle life.

We talked about how LiFePO4 batteries work – they move lithium ions between electrodes It is important to understand that these are real physical particles, which have a size for them, are torn off one electrode and inserted into the other, each time the battery is charged-discharged This causes damage, particularly to the carbon of the negative electrode. Each time the battery is charged the electrode swells a little and thins again with each discharge. Over time this causes microscopic cracks. it is for this reason that loading a little below 100% will give you more cycles, as will discharging a little above 0%. Also, think of those ions as exerting “pressure,” and the extreme numbers of state of charge exert more pressure, causing chemical reactions that aren’t good for the battery. This is why LFP batteries don’t like being stored away. at 100% SOC, or float charge at (near) 100%. How quickly lithium ions are dragged here and there also has an effect on cycle life. Given the above, this shouldn’t come as a surprise. While LFP batteries are routinely charged and discharged at 1 ° C (i.e. 100 amps for a 100 Ah battery), you will see more cycles from your battery if you limit this to more reasonable values for lead-acid batteries Limit of about 20% of the Ah rating, and adhering to this limit for lithium-ion also has benefits for longer battery life. The last factor to mention is voltage, although this is really the BMS Lithium-ion batteries have a narrow voltage window for loading and unloading. Going outside this window very quickly will result in permanent damage and a possible RUD event (NASA talk, as mentioned before) in the upper area. For LiFePO4 this window is about 8.0V (2.0V per cell) up to 16. 8 Volts (4.2V per cell). The integrated BMS should be careful to keep the battery within these limits.

LFP Take-Home Lessons

Now that we know how lithium-ion batteries work, what they like and dislike, and how they ultimately fail, there are some tips to take away. We have made a small list below. If you’re not going to do anything else, please take note of the first two, they have by far the most effect on the overall time you have to enjoy your Li-ion battery! Paying attention to others will also help, to make the battery last even longer. above, for long and happy LFP battery life, in order of importance, you should pay attention to the following: Keep the battery temperature below 45 degrees centigrade (below 30 degrees if possible) – This is by far the most important! ! Keep charge and discharge currents below 0.5 ° C (preferably 0.2 ° C) Keep battery temperature above 0 degrees Celsius when discharging if possible – This is all located below is nowhere near as important as the first two Do not cycle below 10% – 15% SOC unless Do not float the battery at 100% SOC if possible Do not charge 100% SOC if you have none need That’s it! Now you too can find happiness and a full life with your LiFePO4 batteries!