PNJunction Battery Test Challenge

[Sunking]

Hey PN you like to test and play with batteries don't you? Want to try a test I did about a year ago and post your results so we can compare results. I did this just over a year ago when I decided to switch from Top Balance to Bottom Balance LiFePo4 cells in my cart. You will have to saccrifise at least one 18650 cell maybe two because it is a destructive test. I have two 1450 mah 18650 LFP cells if you need them and the Power Resistor required. Just hop on DIY Electric EV and drop me a PM with your address. Otherwise I think you have everything else.

Material needed:

1. 2- 3.2 volt LiFePo4 18650 cells. Any manufacture or capacity will do. They just need to be the same.
2. A Hobby Charger capable of Charging/Discharging, and Capacity test. I think you have an Icharger? I used a Cellpro Power Lab 8. Both work
3. 1 10-watt 2.2 Ohm power resistor. Value is not critical, all depends on what you have and how much time you want to take on discharge. C-Rate does not matter as long as you do not exceed the maximum discharge rate of the cell. IMO the faster the better.
4. DMM
5. Jumpers to connect components

Method:

1. Run each of the two cells individually through a couple of cycles on on your hobby charger. Label them and record full capacity and Internal Resistance at 50% SOC for future reference. Do so in a room where temps are stable and cells have plenty of air circulation to keep temps consistant.
2. Choose either battery, makes no difference which one, or if it is even charged up. We are going to try to kill it. Terminate it with power resistor and walk away. I did this just before going to bed for the night.
3. Come back after some time and measure and record voltage with resistor still terminated. Should be ZERO volts with Resistor still terminated. Remove resistor, and take a break for a few minutes.
4. Come back after break and measure voltage of dead cell. If like me you should see something just over 2 volts. That should make you scratch your head.
5. Try to cycle the battery with 1 charge to 3.65 volts, 1 discharge to 2.5 volts, and then charge to 3.65 volts until current tapers to C/33 if you can. I think you can reprogram the Icharger from default C/20. If not C/20 is just fine. Record capacity and leave it charged up. What i recorded is a battery with only minor capacity loss of less than 1%. Not enough to really get an accurate measurable difference.
6. Now with both batteries fully charged, discharge either one and bleed off about 10% capacity to 90% SOC. You want one battery with 10% higher AH capacity than the other
7. Connect both batteries in series for 2S configuration or 6.4 volts.
8. Terminate with resistor and walk away again. Let it die.
9. Measure voltage with resistor still terminated and you should see ZERO volts again.
10. Remove resistor and separate the two batteries, and take a break.
11. Come back from beak and measure the voltage of both batteries again. One should be 0 volts (the weaker one you partially discharged before the test), and the other up around 2 volts. You should be scratching your head again and lights going off in your head by now.
12. Try to cycle both individually in your hobby charger and report results.

Bet you get the same thing I did. 1 dead battery and 1 good working battery. Result: The stronger battery drove the weaker battery into reverse polarity and killed it. It will just bring a tear into your eyes knowing the weaker cell gave its life so the stronger cell(s) could live to fight another day. What you did is simulate a Top Balanced system where capacity tolerance was a 10% delta spread.

[inetdog]

A revealing experiment, but I will keep it as a thought experiment only, thank you.
One procedure which you did not follow that PNJunction strongly recommends is limiting the charge rate of a heavily discharged (into the lower voltage knee) cell until it is back up to a normal operating SOC value. That may or may not have made a difference with your reverse biased battery.

[Sunking]

He can charge/discharge at any rate he wants. I pulled the idea or repeated an experiment by Dr Jay Whitacre @ Carnege Mellon University and Dr. Jeff Dahn @ Dalhousie University. Neither of them is a Top Balance fan either.

[PNjunction]

That test is great, and I think the Elithium guy has a nice chart of various manufacturer's cells for much the same thing. Careful, the data may be stale and showing the typical generation-1 versions of LFP from 2008 or so. Improvements have been made since then, but still I know what you are trying to show to people.

I'd participate if I were doing an EV with high currents. But I'm not. I'm a low-current solar energy storage application, running no more than 0.2 to maybe 0.3C MAX, where a power-cell would be a total waste of money, unless one can get them at the right price. Sort of like buying an Enersys / Odyssey pure-lead agm to run a string of holiday led lights. Total overkill, but if the price is right ....

That stuff would be important to me if I were the owner / driver of Plasma Boy's White Zombie (Running Optimas early on, and now Dow / Kokam for propulsion and yep Winston's for 12v).

http://www.plasmaboyracing.com/history.php

It is also for the same reason that Burgerman who custom builds his own non-commercial LFP powered powerchairs uses "power cells" like Headway's, and NOT "energy cells" like GBS, Winston etc. No, he won't do a build for anyone. He has to sit on his own project every single day. Again, waaaay too overkill for our uses:

http://www.wheelchairdriver.com/BM-M...powerchair.htm

Additionally - and you'll like this:

http://www.wheelchairdriver.com/BMS.htm

The test also shows that the GBS cells are in fact, quite close to my LFP "KingSolar" 26650's, which are basically a slightly larger variation of cells that power solar garden lights! Not the highest energy-density, that's for sure - but is it necessary for a non-ev application?

So .. yep, I've tested my own 3.2v LFP 26650's against the GBS and they both compare favorably! What the test reveals is simply that these cells are "energy cells", and not "power cells". Or more simply, the "3C" vs the "10C" cells.

In our application for solar, the lowly "3C" energy-cells, aka the GBS, Winstons, lower-end CAlb's or KingSolar 26650's, have *fantastic* capabilites that even under our so-called "sub-c" application won't even tax them! Heh, not to mention totally overboard for solar garden lighting!

So in the end, anyone buying CALB - go right ahead - great LFP batteries, but for OUR application, their low end models are still far more capable than we'll ever need. The older "SE" blue cells would be fine if they still make them (lurkers - don't get OLD stock however!). The gray "CA" line is of course just fine too. BUT, does our application need the "CAM" series, with the aluminum case instead of plastic?? NO. (and lurkers, shorting the case(s) with each other or a chassis is a bad time waiting to happen. Know what you are doing.)

Long story short - fit the battery to the application. Our "energy cells" do just fine for solar-housebank use.

[Sunking]

PN your sub-C thesis is a false narrative. Solar users charge at much higher rates. Us EV guys charge at C/10 or less. Solar users charge at C/6 to C/2. On the discharge side you can make an argument that EV's have higher discharge rates briefly for 10 seconds while accelerating, but cruise at Sub-C. But that is not the point. Makes no difference how fast you discharge, if you over discharge, game over. EV users use the exact same batteries you use. CALB, Winston, Sinopoly, and GBS. I do not know anyone wealthy enough to use cylindrical cells for an EV or house batteries. Do you?

That is the reason I say to do the test with a $3 cell rather than a $200 cell. It is a destructive test. One of the two cells will die from over discharge. The point of the exercise is to demonstrate there is another way to protect your investment. One in which eliminates the risk to over discharge to the point you do not have to worry about it or spend a fortune protecting them.

I understand and happily accept if you just say I do not want to do it and be done with it. But to say sub-C application inexpensive GBS batteries does not apply to cylindrical cells is a false narrative as they are the same type in a different package. If C-Rate is your argument GO SLOOOOOOOOOOOOOOOOOOOOOW with 10 to 20 Ohm's. It will kill the one battery just as dead.

I think you know the outcome without doing anything.

[PNjunction]

That is the reason I say to do the test with a $3 cell rather than a $200 cell. It is a destructive test. One of the two cells will die from over discharge. The point of the exercise is to demonstrate there is another way to protect your investment. One in which eliminates the risk to over discharge to the point you do not have to worry about it or spend a fortune protecting them.

We're on the same page, but approaching it at different viewpoints, that's all. No need for the destructive test, I know what will happen. Instead of resistors, I married a 20ah cell into my 40ah bank (removed one cell to substitute) and had a lot of fun proving to myself just how dangerous to the runt cell this could be, and also how relying solely on voltage alone to indicate top "balance" in capacity is a fool's game. Heck, I could even repeat this by substituting one of my 26650 lfp cells - same result. Dead cell if I'm not on top of it.

Is this the overall synopsis:

In a top-balanced system, due to differing real-world cell characteristics, the weakest cell will eventually be driven into polarity reversal and destroyed under deep discharge. This is true - no argument there.

With a bottom balanced system, all cells will arrive at full discharge at the same point, thus there will never be any cell reversal. Still, there are small real-world differences in cell characteristics such that the "runt" or slightly weakest cell will be your trigger to stop charging as a pack if you like!

Again no argument and the procedure has been documented fully here. One way to skin the cat. And nobody should actually arrive at zero in the first place! An LVD should be active long before that anyway. Anyone reaching that point deep into the discharge knee has definitely NOT properly sized his bank for autonomy!

And yes, I have done the full bottom balance thing - discharge each to 2.5v, wait a day, discharge again since they will recover a little, wait again for like a week to see if any drop precipitously lower than the rest indicating high self discharge, yada yada (although like some EV'ers, at that point I charged back up SLOWLY to avoid an angry ion-storm of having no place to intercalate due to excessive current deep down in the knee.)

Lurkers: This might seem especially important for "energy cells" rather than "power cells" - power cells are designed to intercalate faster (nano tech and all that). Moral - don't exceed 80% DOD, and if you DO, take it easy coming back out of the knee to the flat part of the charge curve. This is a best-procedure type of recommendation. The discharge knee is trying to tell you something - only in reverse when you charge!

Yet despite all this, for convenience sake, I don't even do the traditional top-balance with either vampire bleeders, NOR with individual cell links to a specialized charger. I purposely run very conservatively at the top and bottom after initially charging each cell individually to full which gives me wiggle room so I don't have to be so exacting.

Although I have measured my cell's capabilities to be within about 2-percent of each other, not 10% like your example, I am fully willing to run this way for convenience, setting both my upper charge limit, and lower lvd to conservative values.

So it's a case of YMMV. Works fine for me. I keep an eye on things. Maybe not for others.

[Sunking]

Is this the overall synopsis:

In a top-balanced system, due to differing real-world cell characteristics, the weakest cell will eventually be driven into polarity reversal and destroyed under deep discharge. This is true - no argument there..

Yes Sir and it is easier to over discharge than most folks know realize due to the Flat Discharge Curve of LFP, as you and I know it is very flat. Basically 3.4 volts at full and 3.0 to 10/20% SOC. The ride down from 3.0 is a steep and slippery slope.

However 99.9% fail to understand the point of the drill and the unique characteristic of LFP Cells. If you Bottom Balance meaning all cells arrive at 2.5 volts at the same time, or 0 AH, you cannot over discharge the cells and drive them into Reverse Polarity. There still has to be sufficient energy in the rest of the cells in a series pack to drive the weak cell into destruction. That is why I specifically stated put a load on a single cell and walk away. It will not destroy the cell. Do that with any other battery type and you have a boat anchor. I hope everyone reading this at least take that part away because it is critical to understand when considering Bottom Balanced systems.

What flipped me from Top to Bottom was Jeff Dahn's Lecture at Batt Com a couple of years back when he stated he was not a fan of Top Balance for this very reason. He went further to say if you are a manufacture of EV's Power Tools, Laptop's or whatever industry is they used Capacity Matched Cells within 1%. With tight tolerance cells it makes no difference if you Top or Bottom Balance because it is then technically Middle Balanced. He added a point which I already know is EV manufactures do not allow users to ever fully charge or discharge batteries and run 20/80 because that is the only way they can offer such long warranties. Then he gave the great nugget of advice. He said you can mimic what commercial EV manufactures do and eliminate over discharge. Yep Bottom Balance and only charge up to 80 or 90%. Set charge voltage to 3.4 vpc and LVC to 3.0 vpc.

With a bottom balanced system, all cells will arrive at full discharge at the same point, thus there will never be any cell reversal. Still, there are small real-world differences in cell characteristics such that the "runt" or slightly weakest cell will be your trigger to stop charging as a pack if you like!

Again no argument and the procedure has been documented fully here. One way to skin the cat. And nobody should actually arrive at zero in the first place! An LVD should be active long before that anyway. Anyone reaching that point deep into the discharge knee has definitely NOT properly sized his bank for autonomy!.

I agree with 90% of that. The actual point of no return for LFP is 2.0 vpc. Set your Inverter LVD for 3.0 vpc or 12, 24, ,and 48 volts respectively. If you load heavily (high C-Rate on discharge) go down to 2.75 vpc and you still have plenty of headroom to play with. It is a hell of a long way from 12 to 8, 24 to 16, or 48 to 32 volts. No cell level monitoring or additional automation required. Where I disagree with you and is a minor point, is a LFP system autonomy is 3 days vs 5 days to be equal. Not sure about you and others, but I am not a hermit and I frequently leave home. If you Top Balance and leave for some time without active monitoring/control, you run a risk of over discharge. You cannot intervene if you are not there if something goes wrong. I prefer passive protection and with Bottom Balance you get 2 layers of Low Voltage protection. 1 from the physics of BB. 2. Inverter LVD.
]

[PNjunction]

I knew we were coming at this from opposing directions to meet in the middle!

Long ago, I think I mentioned that I thought bottom balance was the most technically correct solution. But, top-balance can be tolerated IF one knows the risks, and uses a dash of conservatism.

Still, bottom balance on my frankenstein test bank of three 40ah cells, and one 20ah cell (4s / 12v config) means that even IF I bottom balance, that 20ah cell is going to take a beating under normal use - not to mention being the one cell that is going to control the whole show. So if one has a widely varying cell characteristics, bottom balance is no guarantee of quality. It is ONLY to achieve arriving at the bottom at the same time - which is absolutely necessary for some applications. Perhaps not for ours with dual-levels of LVD which can be accomplished with top balance too. (see below)

Will the average reader have a way to *carefully* monitor the discharge down to 2.5v without getting some sort of distraction and killing a cell in the process? Will they take the time to let it rest for a couple of days just to see who the runt is, discharge it again, and find the one that has a much higher level of self-discharge by resting some more? Will they be using a $5 un-calibrated beat up POS voltmeter to do so and make the whole thing moot - or worse endanger the cells?

The other problem with bottom balance is does the reader have a solar charge controller with very fine customizable voltages when charging at the overall pack level? That is, with bottom balance, when the FIRST cell reaches say 3.45v, the voltage of the entire pack should be measured, and use THAT pack voltage as the stopping point for the controller. Some might not have the resolution in selectable charge voltages to accomplish that. Obviously we are not getting into the world of individual cell monitoring, which most controllers that are put into place may not actually have.

This is kind of what I'm getting at - there is technical purity and there are real-world variables that if done wisely, can make a so-called balanced bank work satisfactorily.

I like the double layer of LVD with a bottom balance. In my case I'm also running two layers with a convenient top-balance. The first is my programmable one, set for about 3.15v per cell under under load. If THAT were to fail, the LVD in my Pb targeted inverter (about 2.7v per cell, aka 10.8v total) is my secondary backup. But no, if both of those were to fail, then yep, a cell would go reverse polarity eventually. I'll take that risk.

Problem - we talk about bottom balanced cells arriving gracefully at a fully discharged point worst case without reverse polarity damage. Ok, BUT if you are remote and leave them this way long enough - the subsequent recharge, which is usually with too much current, blows out the copper in the anode into the electrolyte, with gassing, swelling, bloat and general expensive unpleasantness to follow. SO, I suppose if one were remote, you'd have to have an app or some sort of global alarm to let you get to this fully discharged bank PRONTO with a SMALL recharge current like C/100 to get out of the steep knee safely - if they aren't too far gone already.

Heh, it's all these variables that lead to endless threads...

[Sunking]

Still, bottom balance on my frankenstein test bank of three 40ah cells, and one 20ah cell (4s / 12v config) means that even IF I bottom balance, that 20ah cell is going to take a beating under normal use -

Well yeah but who would do that in practice.

Will the average reader have a way to *carefully* monitor the discharge down to 2.5v without getting some sort of distraction and killing a cell in the process? Will they take the time to let it rest for a couple of days just to see who the runt is, discharge it again, and find the one that has a much higher level of self-discharge by resting some more? Will they be using a $5 un-calibrated beat up POS voltmeter to do so and make the whole thing moot - or worse endanger the cells?

PN you cannot kill a 1S Cell, that was the whole point of the test. Regardless if you Top or Bottom Balance, you have to connect all cells in parallel to 1S initially to perform the initial Gross Balance. It takes the exact same equipment to either Top Charge, or Bottom Discharge. The actual voltage is not all that important on the Bottom. Anywhere from 2.0 to 2.5 volts. You do not find the runt at the Bottom, you find it when you charge the first time after the initial charge to 13.6 volts on a 4S pack

As for charge controllers, I do not know of any that cannot be set for 13.6 volts to 14 volts. I will say BB is not for everyone. I always have said that.

[Living Large]

I probably shouldn't wade in here, since I don't have a system and don't plan to anytime soon. I also have not been following along every post or could say I understand your philosophy 100%. Since it is like 6000 degrees below zero and I have nothing else to do, I will. I was planning to use LFP with bottom balance.

The other problem with bottom balance is does the reader have a solar charge controller with very fine customizable voltages when charging at the overall pack level? That is, with bottom balance, when the FIRST cell reaches say 3.45v, the voltage of the entire pack should be measured, and use THAT pack voltage as the stopping point for the controller. Some might not have the resolution in selectable charge voltages to accomplish that. Obviously we are not getting into the world of individual cell monitoring, which most controllers that are put into place may not actually have.

I was in fact planning to use individual monitoring with a B Monitoring S, and interrupt charging when the first cell in the bank hit a threshold, if the threshold in the CC for the entire bank had not already been triggered. Not all CC might support this, but the Classic does IIRC.

I like the double layer of LVD with a bottom balance. In my case I'm also running two layers with a convenient top-balance. The first is my programmable one, set for about 3.15v per cell under under load. If THAT were to fail, the LVD in my Pb targeted inverter (about 2.7v per cell, aka 10.8v total) is my secondary backup. But no, if both of those were to fail, then yep, a cell would go reverse polarity eventually. I'll take that risk.

I was also planning for a double backup, and was willing to take the same risk.

Problem - we talk about bottom balanced cells arriving gracefully at a fully discharged point worst case without reverse polarity damage. Ok, BUT if you are remote and leave them this way long enough - the subsequent recharge, which is usually with too much current, blows out the copper in the anode into the electrolyte, with gassing, swelling, bloat and general expensive unpleasantness to follow. SO, I suppose if one were remote, you'd have to have an app or some sort of global alarm to let you get to this fully discharged bank PRONTO with a SMALL recharge current like C/100 to get out of the steep knee safely - if they aren't too far gone already.

When you say "fully discharged point", are you referring literally to totally discharged, or a point before the knee you aim to never go below? You are several levels above me in both thinking and even more in experience, but I thought the idea was to never get anywhere near fully discharged. On this topic, as someone with a remote cabin, I was planning for monitoring having the LVD trip, and either send someone to the rescue, or make a trip myself, to intervene before "unpleasantness" crept in.

Sorry if my post is off topic or my boundary conditions don't match yours. Just trying to understand. Now I need to throw 4 more logs on the fire...

[Mike90250]

....... Since it is like 6000 degrees below zero and I have nothing else to do, I will. I was planning to use LFP with bottom balance......

At those temps, you will have problems recharging LFP below 0°C - the chemistry will not tolerate it and destroy the cell.

[Sunking]

When you say "fully discharged point", are you referring literally to totally discharged, or a point before the knee you aim to never go below?.

You would never ever want to fool with a deep discharge.

[PNjunction]

When you say "fully discharged point", are you referring literally to totally discharged, or a point before the knee you aim to never go below?

Ideally you strive never to go below 80% DOD for the longest life. When you start the quick descent beyond that, you really run the risk of having a cell being driven by the others into cell reversal with top-balance - especially if you are pulling EV like heavy duty currents and only doing pack monitoring, and not individual cell monitoring.

But consider the application - are you running EV like currents with no lvd whatsoever in a solar housebank setup? No.

Do whatever you feel comfortable with, and whatever risk / maintenance level you intend to perform. Personally, my bank stops at roughly 3.15v per cell under load. The individual cells might actuall read 3.15 / 3.14 / 3.16 / 3.13v for instance, but I'm not racing to the bottom with heavy duty loads. This level is far above the inverter's pb-based lvd cutoff. And I don't always go down to this point in the first place.

Mike is right - colder than 32F is not recommended. True afficianados will move to a more temperate climate, or put their batteries inside or some other more favorable temp environment.

Or just don't use them at all and use a battery chemistry more suited for really cold conditions.

[Living Large]

Mike is right - colder than 32F is not recommended. True afficianados will move to a more temperate climate, or put their batteries inside or some other more favorable temp environment.

Or just don't use them at all and use a battery chemistry more suited for really cold conditions.

With my application in the lower Adirondacks, the first thing I considered was the temperatures the batteries would see. Initially, I didn't want Pb batteries in the house, so I asked here about burying the battery bank below ground level, within the foundation of a garage. Once everyone had a chuckle about that idea, and I became aware of other chemistries, moving to the house became acceptable. LFP in the basement became an attractive idea, for multiple reasons not the least of which was moving batteries in and out of the basement when replacement was needed (300 lb+ Pb out of the question without a crane)

I believe the temperature a few days ago there was -20*F+/- ambient. One of the considerations in giving up on PV and choosing to invest in POCO was not having to babysit a PV system, and have one really serious worry about what happens if the heat goes off for some reason and the place was unattended. I already have one dog - it felt like going off-grid would be like having another - on the scale of a Great Dane.

[Sunking]

I believe the temperature a few days ago there was -20*F+/- ambient.

Enough said. That alone eliminates LFP option. Things like EV's have built in battery heaters to keep the batteries above freezing. Not something you want to waste power with solar.