KISS construction of LiFeP04 prismatic bank

[PNjunction]

Use the following as a BASE LEVEL construction of an LFP bank, 12v, 24, whatever.

Read, and delve into other threads if you want to go down rabbit-holes of engineering. BUT, still - be safe. That means wrapping/taping your construction tools from handle to head. No wrings, watches etc. Basic battery safety should be followed.

We are making the assumption that you are building a bank that will draw no more than about 0.1C, maybe 0.2C on peaks for long-lasting solar autonomy. IN other words, your application is that of solar storage, with plenty of capacity, and not a high-current EV.

Because you are using the typical solar charge controller (with NO temperature-compensation active!), this means top-balance as shown below. Bottom balance is valid too, BUT only if you have the specialized circuitry to actually use it properly. But for practical charge-controller reasons, we'll stick to top-balance.

1) You purchased good quality prismatic cells from a reputable dealer. No used, old-stock, gray-market, crap.

2) You have access to a programmable bench-supply (and know how to safely use it), or a "single-cell" 3.2v nominal charger (upper limit usually 3.6v max), or a quality hobby charger like an iCharger, Revolectrics PL6 / PL8 and know how to use it.

2A) Hobby charger users: note that I'm NOT talking about using individual bleeder/balancer leads, but just setting your charger for a "1S" configuration, and charging each one individually, one by one.

3) INITIAL CHARGE: Charge each cell individually to 3.6v, until the current naturally tapers off to 0.025C. (ie, a 100ah cell should be stopped when absorb slows to 2.5A). Be mindful of polarity if your bank is already link-connected. No need to exceed 0.33C, but also don't do the initial charge at less than 0.1C for efficiency and the ability to reach 3.6v *in time* so as not to take all day to do so! As we know, sitting at full charge, or being near it all day long is not good for a cell's health.

4) Clean your terminal links if necessary with a non-wire brush. A plastic scrubber followed by a *thin* coat of aluminox, penetrox and the like should be applied between it and the terminals themselves underneath.

5). For normal charging, you can charge at a bank-level of anywhere from 3.45v to 3.6v per cell (ex: 13.8 to 14.4v for a 12v batt) with a CC/CV algo. For best results, do not charge in CC mode faster than 0.33C. Your battery bank will reach full charge anywhere from a CV of 3.45v to 3.6v, the only difference is that this absorb to full will take much longer at 3.45v per cell (0.025C again) than it will at 3.6v.

Don't want to charge to full? You choice: stop at a specific voltage. OR, stop somewhere before a full-absorb has been reached. Up to you. Many threads on this. You'll find out soon enough.

6) Preventative Maintenance: During charge, now that you have done the so-called "top balance" by charging them individually (and are not CRAP), you should see no cell lower than 3.45v, and no higher than 3.6v during the absorb phase. Ideally, you'll be much closer than that, typically no larger than 0.1V difference between cells. *at the end* of a full-charge. (again. 0.025C absorb current).

7) Use a quality voltmeter, not a shirt-pocket cheapie.

8) Set your LVD conservatively at the start to 3.1v per cell. If you want to go lower than that, you can, but generally no need at first - do not let any cell go lower than 2.75v.

8A) Option: Because we are not designing for a high-current EV, but drawing a piddly 0.1C or less, a "pack level" LVD can be used, especially if it is set conservatively at 3.1v per cell. Basically, because you purchased quality cells close in both capacity and internal resistance, AND because you are not trying to drive a high-current EV home, this may suffice because of our gift of time with a low-current application.

9) Just like taking care of a lead-acid bank, this means a little preventative maintenance from time to time. Every once in awhile, watch the individual cell charge voltages and as long as they are not 0.1V or more different from each other, you are good.

10) Notice I have not mentioned any use of bleeder / balancer boards. Because you did step 3 properly, there is no need as long as you didn't fall prey to junk prismatic cells.

It's that simple. Before going down other rabbit-holes, keep *our application* in mind. Most often, you'll find it straying into the inevitable comparisons to EV, Radio Control, racing, or even lithium battery chemistries that are not LiFeP04 (typically the 3.7v nominal, not our 3.2v nominal LFP).

These are the BASICS. If you want to throw a lot of gadgetry on top, be my guest. Just ask yourself if you trust a $2 little bleeder/balancer mosfet on your $2800 bank! Or do you have the skills to do step 3 properly, and look at step 9 once in awhile?

11) PRACTICE FIRST: unless you have a huge budget, try it out on a small 40-60ah 12v bank to prove it to yourself like I did, that this can be done. This will serve nicely as your auxiliary / learner bank. it would be irresponsible to tell someone to just follow these instructions and build some monster 48v / 2kw bank with no previous experience. We don't do that with lead-acid, and we shouldn't do that here.

[Mike90250]

Should step 9 be PM (preventive maintenance) not PV ??
( I thought about editing it myself, but theres a chance I am wrong)

And, I'd like to see a Bottom Balance "step by step" too.

And the standard precautions of physical boxing of the cell/pouch so they don't flex, warp and short

[max2k]

...

7) Use a quality voltmeter, not a shirt-pocket cheapie.

...

'quality voltmeter' is relative to application. Here someone spent time evaluating cheap commonly available DMMs: http://goughlui.com/2014/11/17/inves...it-multimeter/

It seems in 20V range some of them could fit the bill as they show DC within 0.05V off the real value in the full 20V range. That page is also interesting to illustrate when someone posting here 12.6V could mean anything from 12.45V to 12.75V in reality if they happen to use $20 widely available model. Batteries present unexpectedly high precision requirements.

[Mike90250]

Here's a vendor with a pre-set precision reference standard to check your meter against
http://shop.voltagestandard.com/prod...1&categoryId=1 $30 not too bad.

[PNjunction]

And, I'd like to see a Bottom Balance "step by step" too.

I hesitate to go there since it has been covered a bunch of times elsewhere, and takes it beyond the KISS concept. While the balancing at the bottom isn't hard, the average solar controller off-the-shelf stuff isn't programmed to stop at an exact voltage, or have individual cell monitoring to trigger this abrupt stop at the top. Thus, when using an off the shelf controller, which is programmed primarily to top-balance lead acid batteries if you will, the user ends up trying to do both top and bottom balancing at the same time, which is a recipe for disaster.

Basically it comes down to asking yourself if you actually *need* to do a bottom balance, *given our low-current application*. And of course, bottom balance is not a substitute for the lack of an LVD! If one is headed towards the bottom, you have not sized your bank properly for autonomous days-worth of storage, or have wasted money by not doing a proper power-budget.

And the standard precautions of physical boxing of the cell/pouch so they don't flex, warp and short

The precaution here is not to play with trash and try to DIY this yourself. Much like building your own solar panels, the manufacturers can provide the proper banding and strapping of the prismatic cells. Basically one puts aluminum or steel plates on each end of the bank, and then elongated thin u-channels are fashioned to just keep things tight. One BIG mistake is to try and take some nylon strap banding tool, and accidentally compress the crap out of the prismatics. We just want things snug to prevent movement either sideways or up and down.

If one is playing with stand-alone pouches and the like, then that is like trying to build your own lead-acid battery out of individual plates found at the dump, and is more of a bench-exercise, like building your own solar panel - frequently with cast-off's, rejects, and used materials. If you were somehow to get hold of pristine quality flat-packs, when you price it out, it costs far more than having the manufacturer's own product already cased up with terminals and hardware.

Again, the analogy here is like trying to save money by building your own solar-panel. Good for a goof, but not for any serious use, both financially and from a performance aspect.

[zamboni]

I've been reading through all the LiFePO4-related posts to learn about using these cells in off-grid applications. First off, thanks PNJunction for writing this post, because it pulls together a lot of info that is otherwise difficult to glean, as it's spread through a lot of other posts. I'm surprised it's not a sticky?

I see PNJunction, Sunking, and a few others consistently citing charge guidelines based on certain VPCs, apparently regardless of manufacturer, size, type. I am one who takes manufacturer specs seriously, and with FLA i have always charged per the bulk/absorb/float/equalize charge voltages on the spec sheets -- which vary between manufacturer and also product-to-product. Now with LiFePO4, where 0.1V can mean the difference between a battery and a brick, suddenly every cell out there is identical? Yet i've seen some spec sheets stating voltages well outside those which are suggested here.

Is this a case of the "chi-coms" (as Sunking would call them) publishing disingenuous spec sheets to make their products seem magically better than competitors - while in reality they are basically identical? Or is it because solar/off-grid is not the target application of these cells, so the spec sheets are skewed towards applications with different priorities (presumably high-C)? Something else?

[sensij]

I've been reading through all the LiFePO4-related posts to learn about using these cells in off-grid applications. First off, thanks PNJunction for writing this post, because it pulls together a lot of info that is otherwise difficult to glean, as it's spread through a lot of other posts. I'm surprised it's not a sticky?

That reminds me, I had wanted to ask PNjunction about maybe combining this with his previous post that is already a sticky, since they cover some of the same ground.

[Sunking]

PN .1 volt difference is way to much with the exception of fully discharged on Top Balanced Systems. 3.3 to 3.4 volts is a 50% SOC difference,

[jflorey2]

I hesitate to go there since it has been covered a bunch of times elsewhere, and takes it beyond the KISS concept. While the balancing at the bottom isn't hard, the average solar controller off-the-shelf stuff isn't programmed to stop at an exact voltage, or have individual cell monitoring to trigger this abrupt stop at the top.

Right. But with top balancing you get exact balance at the top, and can be pretty assured of accuracy at the top end (and thus can accurately set charge voltage.) At the bottom end you have to allow for a lot more slop in your LVD setting, since you won't get good balance at the bottom. With bottom balancing you get exact balance at the bottom, and can be pretty assured of accuracy when you choose an LVD setting. At the top end you have to allow for a lot more slop in your charge voltage setting. Same problem, just at different ends of the voltage scale.

[karrak]

A good primer on using LFP batteries. Well done PNJunction.

We are making the assumption that you are building a bank that will draw no more than about 0.1C, maybe 0.2C on peaks for long-lasting solar autonomy. IN other words, your application is that of solar storage, with plenty of capacity, and not a high-current EV.

This is not taking advantage of one of the major advantages that LFP batteries have over LA batteries used in off-grid systems which allows one to run larger inverters . The lower internal impedance/resistance of LFP batteries means you can charge draw up to 2C from them. For longer life you should limit this to a peak draw of ~1.0C and an average draw less than 0.5C.

Because you are using the typical solar charge controller (with NO temperature-compensation active!), this means top-balance as shown below. Bottom balance is valid too, BUT only if you have the specialized circuitry to actually use it properly. But for practical charge-controller reasons, we'll stick to top-balance.

Another reason for using top balance and in my opinion a very important reason is that you can see if your battery is staying in balance every time the battery is fully charged. With bottom balancing you have to fully discharge the battery to see if it is still in balance.

2A) Hobby charger users: note that I'm NOT talking about using individual bleeder/balancer leads, but just setting your charger for a "1S" configuration, and charging each one individually, one by one.

You can speed up the initial balance charge by charging them in series using the bleeder/balancer leads then finishing off the charge with the cells in parallel.

9) Just like taking care of a lead-acid bank, this means a little preventative maintenance from time to time. Every once in awhile, watch the individual cell charge voltages and as long as they are not 0.1V or more different from each other, you are good.

Make sure that you make the measurement at the end of the absorb phase of the charge. All the cells can be within 0.010V of each other for most of the charging process and diverge dramatically to well over 0.100V in the last few minutes of the absorb phase.

10) Notice I have not mentioned any use of bleeder / balancer boards. Because you did step 3 properly, there is no need as long as you didn't fall prey to junk prismatic cells.

This statement says to me that LFP batteries do not go out of balance. This does not match my experience and the experience of many others. One of the cells within the battery can develop a fault which would result in the battery going out of balance over a short period of time or they can naturally go out of balance over a period of months or years.

11) PRACTICE FIRST: unless you have a huge budget, try it out on a small 40-60ah 12v bank to prove it to yourself like I did, that this can be done.

You can go even smaller and cheaper and use some of the small cylindrical LFP cells to experiment with.

I would add that I think you should always have some sort of individual cell voltage monitoring on any form of lithium-ion batteries which have more than one cell is series. I agree with PNJ that it is more than likely that if you take the steps he has outlined that you will never have a problem but there is always the chance that a fault or set of circumstances will occur during the long life of the battery that will result in an individual cell going outside its safe operating voltage range and becoming a fire risk.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[Sunking]

Is this a case of the "chi-coms" (as Sunking would call them) publishing disingenuous spec sheets to make their products seem magically better than competitors - while in reality they are basically identical? Or is it because solar/off-grid is not the target application of these cells, so the spec sheets are skewed towards applications with different priorities (presumably high-C)? Something else?

Let's see if I can shine some light on the subject.

First thing to understand is a Lithium Ion Battery does not mean squat, just like saying I have a car. Neither statement means anything because it does not tell you anything. There are many types of Lithium Ion Batteries just like Lead Acid. Each chemistry lends itself to specific applications. There dozens of Lithium Ion battery chemistry, and each has its own unique voltage signature. Cell voltages range from 2.6 to 4.3 volts. I will not go through all of them, just the major players.

LTO Lithium Titinate, voltage = 2.4 volts. Very high Specific Power, highest of all lithium Ion Batteries. Specific Power is a measure of how many watts the battery can deliver based on its weight. In other words very high charge and discharge rates or C-Rate. Can go as high as 50C.They are the only long life Lithium battery out there of 3000 to 10,000 cycles on paper Was suppose to save the world just 2 minor catches. Extremely expensive, and poor Energy Density or Watt-Hours/Kilogram. About the same as Lead Acid. No practical use EV or high drain electronics. Would be great for home solar, but you could never afford them or recover your investment.

LCO Lithium Cobalt Oxide. Nominal voltage = 3.6 volts. These are what your cell phone, laptop, and some electronics use. One of the highest Energy Density batteries out there or Watt-Hours / Kilogrram. Exactly what you want in an EV so you can get long range. It takes a 4000 pound lead acid to equal a 1000 pound LCO. Exactly why Tesla used them for their first Roadster. It was the best at the time. However there are down side. LCO is unstable and prone to thermal runaway, thus all the bad press. They also have low Specific Power and do not tolerate high discharge rates, and at best 300n to 600 cycles until dead. Only one EV manufacturer is foolish enough to use them.

LMO Lithium Manganese Oxide, NMC Nickel Manganese Cobalt, NCA Nickel Cobalt Aluminum I will lump together. Nominal Voltage 3.7 to 3.7 volts. This is what all EV manufactures use, even Tesla today. Even the commercial Solar Lithium Batteries use on eof these variants. Much more stable than LCO (safe), good Specific Power and Energy Density. Cost is high, decent cycle life up to 1000 cycles.

LFP Lithium Iron Phosphate. Nominal voltage 3.2 volts. OK the low voltage clues you in Energy Density is low, thus why EV manufactures will not use them as the battery would be to darn heavy. They are the safest of the lithium batteries, and have good Specific Power, good but not great. They can be charged at C/2, and discharge at 1C to 3C which makes them a good choice for power tools and power lawn equipment. Moderate Expense and fair cycle life of up to 1000 cycles.

CHI-Coms are LFP made to very low standards vs say A123. A123 was the first to come out with them and soon went bankrupt. LFP was the other miracle battery that was suppose to take the EV and Utility markets. Never happened. Soon after A123 Chi-Coms ripped off A123 and released Prismatic cells thinking Prismatic was a better approach. That company was called Thunder Sky aka Thunder Turds by users. They went bankrupt from warranty claims. Thundersky was divided into three Chi-Com called CALB, Sinopoly, and Winston. Each made a tweak to the design, CALB is the best of the bunch, but that is not saying much. CALB, Winston, and Sinopoly market was DIY EV, but not even DIY EV guys will use them anymore because they are such poor products. Instead we use used and salvage batteries from Nissan Leaf, and Chevy Volt, and some have even cracked Tesla. A used or salvage EV battery is less expensive and 10 times better quality and will last longer.

OK to address Top or Bottom Balance , from a consumer POV, Top Balance is the only way to go. It works better for manufactures, screw the consumer. A consumer is not smart enough to know what they are doing, or what the manufactures are doing to them. All parties are happy happy. You are getting screwed and you like it a lot. That is why you go to Las Vegas. Where else can you go, be over charged for everything, robbed at the tables, and when you get home cannot wait to go back and get screwed over again. Am I right? Of course I am so listen up.

Top Balance requires charging the battery fully to 100%. Do you know what happens to battery cycle life, and the danger that puts you in? Heck no you don't know, manufactures count on that. They have you trained from Lead Acid to keep that battery fully charged up. Am I right. Dang straight I am right. At this moment you are wondering why wouldn't you want your battery fully charged. Real simplle you just cut cycle life 50 to 75%. That means frequent battery replacement and you battery manufacture very happy happy.

Guess what? EV manufactures know this and thus why they would never ever allow any customer to fully charge the vehicle battery. If they did, They would go bankrupt from warranty claims. It is the only possible way for them to offer warranties they do. The Lithium battery types they use are only good for about 500 cycles if fully charged like your laptop or cell phone. Depending on the EV manufacturer only allow you to charge up to 80 to 90% SOC, and will never allow you to fully discharge and cut you off at 10 to 20%.

So by all means, Top Balance. It is what you have been programmed to do. Now obey the Karrak God and Top Balance. You are not smart enough to know any other way.

To answer your question, Solar and Utility are just benefactors of EV's, and a secondary market. I classify using lithium just the same as AGM Lead Acid. They have some niche applications. But Lithium is also very extremely temperature sensitive. You can take Lead Acid down to -40 degrees, and the cold weather king is AGM, not Lithium. In fact anything below freezing is dangerous. OK so what are they good at.

1. Energy Density. LFP is about twice as good as Lead Acid in that they only take up half the space and weight off lead acid. Great for a mobile application, worthless for a home or cabln because Lead Acid cost 1/3 as much as last twice as long.

2. High Charge and Discharge Rates. This can be useful especially if you live in a Hell hole sewer like Seattle Ws where you only got 1 Sun Hour on a winter day on a good day. Goth and Junkies do not care for day light. Now this goes contrary to what PNJunction said. If you size a lithium battery properly to equal lead acid, for each 100 AH of Pb you would use 75 to 80 AH of LFP. This means the battery is smaller and thus runs higher charge and discharge rates than his Pb cousin. If you are foolish enough to use a 12 volt 100 AH battery on a 1000 watt Inverter, you had better use a EV grade lithium battery or a very special Pb AGM battery. Typical charge rate on Lithium is C/5 to C/2 where Pb is C/10.

Until there are significant improvements in economics and technology, Lithium is not ready for Solar or RE. So if you are going to use lithium, you really need to justify its use. Lithium cost too much long term, and still not as safe. Let your neighbor take the chances and spend his money.

Last peic eof advice.

[Moderator note: deleted personal attack]

[zamboni]

Wow SK, that was a lot of info. Thanks for laying it all out like that.

I don't think any of it illuminated my question, though? I get that LFP are all the same nominal 3.2VPC, but FLA batteries are also all the same nominal voltage, yet the various manufacturers suggest different charge regimes. I'm wondering why it wouldn't be the same with LFPs. There's way more than just the "three daughters of Thundersky" out there making cells, and it strikes me as unusual that every one of those cells would respond the same to some universal charging regime. I mean, you could put forth a similar "universal" regime for FLA batteries, too - and it would work great, since (as you often point out) FLA batteries are really tolerant of imperfect treatment. But lots of people agonize over their bulk/absorb/float voltages anyway, and many of us try to stick with the manufacturer's specs whenever possible. Given that LFP are fragile, fickle prima donnas, why are we all OK with a general set of voltages regardless of cell maker, capacity, etc?

This is genuine curiosity, not trying to be a wiseass; i am grateful for the solid experience-based info available here...because it really doesn't exist anywhere else. I found this site after i had already built up an LFP pack (but before i put it into service), and it answered a LOT of questions, saved me a lot of money, and a lot of mistakes. I'm running my system according to your voltages today, and i trust those parameters more than the dubious-sounding voltages from the Chinglish spec sheet for my cells (cutoffs at 3.85 and 2.0 VPC...really). I'm just wondering if it's possible to trust anything from a LPF manufacturer's spec sheet, or if we should only believe what we can measure / observe for ourselves.

[karrak]

I don't think any of it illuminated my question, though? I get that LFP are all the same nominal 3.2VPC, but FLA batteries are also all the same nominal voltage, yet the various manufacturers suggest different charge regimes. I'm wondering why it wouldn't be the same with LFPs. There's way more than just the "three daughters of Thundersky" out there making cells, and it strikes me as unusual that every one of those cells would respond the same to some universal charging regime. I mean, you could put forth a similar "universal" regime for FLA batteries, too - and it would work great, since (as you often point out) FLA batteries are really tolerant of imperfect treatment. But lots of people agonize over their bulk/absorb/float voltages anyway, and many of us try to stick with the manufacturer's specs whenever possible. Given that LFP are fragile, fickle prima donnas, why are we all OK with a general set of voltages regardless of cell maker, capacity, etc?

This is genuine curiosity, not trying to be a wiseass; i am grateful for the solid experience-based info available here...because it really doesn't exist anywhere else. I found this site after i had already built up an LFP pack (but before i put it into service), and it answered a LOT of questions, saved me a lot of money, and a lot of mistakes. I'm running my system according to your voltages today, and i trust those parameters more than the dubious-sounding voltages from the Chinglish spec sheet for my cells (cutoffs at 3.85 and 2.0 VPC...really). I'm just wondering if it's possible to trust anything from a LPF manufacturer's spec sheet, or if we should only believe what we can measure / observe for ourselves.

There are many different varieties of LA batteries with various amounts of additives like Calcium and Selenium which change the charge and discharge characteristics, see Wikipedia for more information. NMC, NCA and other variants of lithium-ion batteries have different proportions of Lithium, Nickle, Aluminium and other elements in the cathodes and do have differing charge and discharge characteristics. As far as I am aware LFP batteries have much less variation in the composition of the cathode and all have similar charge/discharge voltage characteristics regardless of the manufacturer.

The most important voltages with LFP batteries are the voltages above or below which damage will occur to the battery in a short amount of time. These voltages are above ~4.3V at which the electrolyte starts to break down and below ~2.0V at which the copper current collector in the cathode starts to dissolve.

The next most important voltages/SOC are the voltages/SOC that will significantly reduce the lifespan of the battery. Holding the voltage above ~3.45V for any period of time will speed up the growth of Lithium dendrites growing on the electrolyte side of the SEI layer which will reduce the capacity of the battery. This process is slow and partially reversible as you discharge the battery. The higher the SOC the faster this occurs. This reduction in capacity is also dependent on whether the battery is being stored at a high SOC or is in continuous use as in an off-grid system. This is why myself and many others are seeing very little loss in capacity in our batteries even though we charge and float them at greater than 95% SOC when the sun is shining.

Unless you want to charge an LFP battery very quickly there is no reason to set the charge voltage to more than 3.45V/cell. This voltage will result in an SOC > 98% if you let the charge current taper off to below C/50. At the lower end you will only have a few percent of charge at a cell voltage of ~2.8V. Floating at 3.35V/cell will keep the battery at >97%.

As PNJunction has said it is very simple to set up and run an LFP battery successfully if you follow a simple set of guidelines. For a 12V battery charge to 13.8V-14.4V(3.45-3.6V/cell) and don't discharge below 12.4V(3.1V/cell). I would use individual cell monitoring and set the overall LVD to 12V(3.0V/cell) and as a safety measure an individual cell voltage of 2.8V. With a 12 volt battery this gives a voltage range of 14.4V-12.0V. To start doing serious damage to a 12V LFP battery you would have to take the voltage above ~17V or below ~8V.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[zamboni]

That clarifies things a bit better. I guess there's just not that many different ways to make an LFP battery so they pretty much all charge the same way ; ) Thanks!