Bottom balancing?

When your total e-bike costs $1100 and the BMS costs around $200, just doesn't seem like that good of a value to me. It's probably a close call but the question is similar to whether or not you should buy that extended warrantly plan for $12 when you buy a drill at the hardware store. I'm still without a BMS on my bike and I'm doing fine. I use my bike recreationally, though. I charge it to around 95 % and discharge only to 50 %. Every now and then I'll put in a 100 % charge just to make sure nothing is getting extremely out of balance. Buy more battery capacity than you need and you really don't need a BMS as long as you read everything you need to know about lifepo4. Read Read Read.
 
Jack Rickard is wrong about some things about lifepo4. I know this for a fact because I've e-mailed him questions. I like Jack but I don't understand where he gets his information or if he just likes being the antagonist. Jack told me that the C rating on a discharge doesn't matter when you are using lifepo4 and that is not true at all. That's what made me stop listening to Jack Rickard. He said you could use a 5 C discharge on a Thundersky battery rated at 2C and it wouldn't hurt the battery as long as you didn't run it to the "knee" of the charge curve. I just disagree with that. I'm quite sure that is wrong. I do agree that BMS are a lot of headaches and you can do without them as long as you set up some rules for yourself and never go against the rules that you have set. Always know how much capacity you have and where your batteries are at along the discharge curve.

I guess I can't say for certain whether a 5C discharge would hurt a 2C rated lifepo4 but I don't have the money to experiment. I'm saying can you put a 5C discharge on a 2C rated lifepo4 battery for 3 minutes without hurting the battery? That's what I wonder.
 
I like Jack but I don't understand where he gets his information or if he just likes being the antagonist. Jack told me that the C rating on a discharge doesn't matter when you are using lifepo4 and that is not true at all. That's what made me stop listening to Jack Rickard. He said you could use a 5 C discharge on a Thundersky battery rated at 2C and it wouldn't hurt the battery as long as you didn't run it to the "knee" of the charge curve.
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I think it's important to know where Jack is coming from, i think. His experience is mainly with the large Lifepo4 batteries. I doubt he's messed with a ping pack or other smaller batteries-- at least not as much as the big batteries.

Part of the problem, i think is how C rates are determined. There is the continuous discharge to 80% test over a lifetime test, but burst or peak discharge is a kind of gray area in my opinion. For example, i've heard that headways should not be discharged past 5C, even though they're rated for 10c. I routinely push my 3c thundersky to 4.5C without any noticeable ill effects so far. We can all agree that lifepo4 batteries can be pushed past their 2c or 3c rating for a short period of time, but how short? How high of a discharge? the long term effects are all a little hazy. Throw in the fact that battery manufactures tend to exaggerate specs and the various build qualities and you can see that determining the long term performance of a battery is very very difficult to do.
 
The C rating of a cell is determined primarily by its internal resistance, with the cell interconnection current capability being a close second. The usual measure of it is the cell voltage drop under a given load. A 10C rated cell might be 3.2V nominal open circuit and 2.5V (the safe lower limit) at 10C discharge. A 3C cell might drop to the same voltage at just 3C.

Obviously pushing cells beyond their limits may have consequences, depending on how often it'd done and for how long. Most of the issues probably centre around local heating generated in the cell or in its interconnects. Short burst of current in excess of the cells rating will probably do no more than cause an excessive voltage drop, longer bursts, or running cells continuously above their maximum C rate seems likely to cause damage.

As cell Ri tends to increase with depth of discharge, it is probably riskier to pull more than the rated current from cells that are almost depleted than it is when they are hot off the charger.

Jeremy
 
The "bottom balancing" idea only works if all the cells are exactly the same capacity, which is quite unlikely. Even really good quality cells, like A123s, will probably have a 5% or so capacity variation from one cell to another. If you discharge all the cells in a pack to the same low voltage, then one or two will reach full charge before the rest. If you carry on charging, these cells will get over-charged, whilst the bigger capacity cells in the pack play catch up. All told, this is a fairly dumb idea, in my personal view. It's far better to control each cell to a fixed voltage, as that way the cell capacity tolerances don't matter (apart from the pack usable capacity being set by that of the lowest capacity cell, as you'd expect with any battery).
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This sums up jacks mistake. He assumes all cells are of equal capacity. If Ri and capacity were within tight tolerances for the entire life span of the batteries, the cells wouldn't go out of balance and you wouldn't need cell level lvc and hvc. There is actually a video of this ex-nasa battery engineer giving the same advice. The only difference was he recommend a service that matches cells. Which if you think about it in the scale of which modern automobiles are made makes, may be cost efficient. But for guys running the cheapest lithium cells they can find, it's stupidity.
 
I haven't seen this touched since the infamous jack richard debacle. In short, he was very against BMS. He made some other inaccurate claims and was very arrogant to respected members of the community, but I'd like to disregard this and have a serious discussion about bottom balancing. Many people here are now bulk charging and are ironically anti-bms just as Jack was. Is bottom balancing better? It's a bit different than how our bms and r/c chargers balance:

:arrow: The cells will all hit lvc at the same time rather than the weakest cell going low first. You're risking all of your cells rather than just one.
:arrow: The weakest cell will hit HVC first, so you'll need to shut off the charger once that happens.

Here are some graphs I made:

Top balancing:

Bottom balancing:


Here are some graphs others have made. Note they are charging TS lifepo4 cells to 4.2 :shock:

Top:


Bottom:



It seems to me that bottom balancing would exacerbate differences in the pack when compared to top balancing. It will charge the weakest cell to HVC while the stronger cells are taken to lower voltage. Since charging to higher voltage reduces cell life and increases Ri and cell degradation, the weaker cells will grow weaker faster than the stronger cells and the differences between them will grow faster than with top balancing.

On the discharge side, the same could be said for top balancing: By allowing the weakest cell to go lower before the rest of the pack you increase cell degradation. However, most of us don't go near LVC. I my self rarely use more than %70 of my packs capacity, so when discharging the differences in cell degradation won't be any where near as important as with charging.

This has made me consider a way to deal with a weaker cell when bulk charging: top balance the pack, but slightly discharge the weaker cells. This way the strong cells will be charged fully, but the weaker cells will be slightly undercharged. If you go no where near LVC, over time this could help equal out differences in capacity and Ri.

To be honest though, I'm really unsure of what I'm saying, and I typed this out mainly to flesh out some thoughts I've been having recently. What are your thoughts? What is the community's consensus? Has any one tried bottom balancing?

The only advantage I could see is that'd it'd be quicker to bottom balance a mostly discharged pack. With top balancing, it can take a long time to balance a out of whack pack when using a inexpensive r/c charger or bms.
 
One issue I see is bottom balancing is possibly a little harder to implement. You have to discharge the high cells before charging the pack. This may be difficult to automate.

Another issue is by bottom balancing, you are essentially forced to run the cells through a complete discharge (I think?). Cells will last much longer if you only partially discharge them. I would seem silly to fully discharge the pack if you only used 10% of the capacity on a cycle.
 
WOW, have I been doing this wrong for the past 2 years? To be honest I've never considered what I do as bottom balancing because I don't nornally run my packs lower then 60% of capasity. But at the end of every ride (even short rides of a few miles) my packs are in perfect balance (every cell is within .o1v at rest). Is this considered "in balance"? BTY, I do not use a BMS. What I did d olest year is to install a CellLog 8 to monitor both charging and discharging. I also made a really cute little homemade charger using individual cell chargers. It's small and charges every cell perfectly at 3.65v. The only problem with it is it's only 2a so it's slow. I was thinking of installing a bulk charger to charge at the same time with the ind cell chargers and let the CellLog cut off the bulk charger at 3.60v and let the ind cell chargers top the pack off. I did a test and this actually worked very well. Just never finished building it.

I am now testing cell bleeders with a 6a smart charger. This is the standard setup sold by Elite Battery Solution. I'm really not sold on it. The bleed off is way to slow to control the over voltage of the week cells. The cells will charge in balance until about 3.45v then then 2 cells on each 24v pack (I have 2) will start to gain on the others. At 2.70v the bleeders kick in and slow the gain down but they will hit 4.0v before the other cells are full. And because the charger is reading the total pack voltage it will cut off before all the cells hit 3.65v. What is really needed are adjustable bleeders. This would allow you to raise to bleed on the runt cells so they do not over charge. This would also allow the better cells to catch up before the charger cuts off. I have considered pulling the bleeder from a few strong cells and install 2 bleeders on the runt cells to speed up the bleed. Elite claims the imbalance will improve with more charges. We'll see!!

Bob

Bob
 
Not sure why you'd want to bottom balance.

Cells are of varying mAh and discharge rates, and will have natural variances, just by nature.
That is why cell voltages start to rapidly differ at the end of the charge.
No reason for alarm, it's normal.

The extra mAh should be left at the bottom, where voltage is low, not the top, where voltage is high and useful..
 
The points that immediately jump out at me so far is that 'many' people are not anti-BMS, and those that are seem to be either refugees from the $12 'planned obsolescence' devices or are still hanging on to the persuasive pitch from a battery beginner. The 'theory of bottom balancing' was presented by someone that didn't know...jack...about cells and balancing in the real world - it was an 'initial theory' from a lithium noob that has since evolved into something a bit more realistic.

Cells change as they age - and the change starts with the first cycle. State of charge moves with temperature as capacity grows or shrinks. Monitoring voltage is an estimate of SOC and isn't exact. Bottom balancing gets us close to 'balanced' only at the time it's performed - just as a stopped clock is only right twice each day.

Assuming that we're doing manual management, or simple limits management, instead of using high-end computerized model-based management with active balancing throughout the charge or discharge cycle, real-time cell degradation monitoring, and CAN buss comms to a master processor that looks after battery health, then I don't think it really matters how we balance.

We still need a way to keep any cell from being overcharged, we still need a way to keep any cell from being overdischarged, we still need to keep current and temperature in check.

If we have a large shop, a store room with $2000 in spare cells, and a DC-3 in the hangar, we can probably afford to replace cells that we lose. The rest of us will very likely want to use either a manual system, electronics, or a combination to make sure our pack is operating within limits each and every time it's used and when it's charged.
 
"Jay Whitacre, a Carnegie Mellon professor with a joint appointment in Materials Science and Engineering and Public Policy, gives a quick overview of Lithium-ion batteries. Starting at the chemical level, he explains the properties and mechanics of the battery which give rise to macroscopic behavior"
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This lecture has lots of good stuff. He doesn't believe in BMS and top balancing though. I tend to disagree with him, but he's a professor who's designed robots for NASA so maybe he knows something about batteries I don't? A good watch none the less, and a good explanation of why you'd want to bottom balance.

The 'theory of bottom balancing' was presented by someone that didn't know...jack...about cells and balancing in the real world
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:lol: well done :lol:

The points that immediately jump out at me so far is that 'many' people are not anti-BMS, and those that are seem to be either refugees from the $12 'planned obsolescence' devices or are still hanging on to the persuasive pitch from a battery beginner.
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Your right about the refugees from cheap BMS. A lot of peoples attitudes here are a) I don't want my $$$ battery pack to rely on a $ bms and b) why spend $300 for a proper BMS when I can manage the pack reliably with a cell-log and balancing charger?

Bottom balancing gets us close to 'balanced' only at the time it's performed - just as a stopped clock is only right twice each day.
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You think so? It seems to me that the bottom balanced cells would stay in balance just as well as a top balanced pack. People here report running hundreds of cycles with out balance issues. Of course this is with high quality cells that are well matched....

Not sure why you'd want to bottom balance.

Cells are of varying mAh and discharge rates, and will have natural variances, just by nature.
That is why cell voltages start to rapidly differ at the end of the charge.
Click to expand...

The same is true with discharging, and with a bottom balanced pack the cell should be balanced near the end of discharge. So you could theoretically use a controller or a CA to do LVC. Since the cells should be in balance at the lower end of the voltage scale, you could safely rely on a pack level LVC. This is the main argument on the full scale electric conversion boards. No need for expensive cell level LVC. A $5 digital multi-meter can be used! I'm not saying it's not retarded to trust pack level LVC to protect $10,000 worth of batteries though...
 
I think it's a hell of a lot easier to stop discharging at a safe average voltage. You won't get that extra mAH that some cells have, but at the same time if you took certain cells 'offline' at the bottom of their charge, you would lose 2-3 volts each time a cell went offline, which would result in a sudden, stepped cascade of loss in power.

Bottom balancing is an interesting idea but if you did so, you would end up with an imbalanced top and slightly imbalanced middle..

I wish someone could figure out how to manufacture lithium cells in a more precise manner. These little variances create the need for BMS/balancing and all sorts of fun crap.
 
A lot of the top/bottom issues, balance issues, etc come from folks using LiFePO4 cells in giant saggy formats.

This makes it nearly impossible to judge the SOC in the pack from reading the voltage. You have to coulomb-count and/or do other wacky implementations to know where your pack is at while you're running down the road.

This means they often run them down low, even on a drive they make everyday, one day they might have a monster head-wind against them or didn't have time to fully charge before hand etc.

So, for whatever reason, it's easy to over discharge, and the weak cell falls off the cliff and gets reverse charged and damaged.

Bottom balancing helps avoid this from happening, as they all fall off the cliff together, so the other cells don't have the energy left to hurt the weak one.


The reality is, no matter how you balance, top/bottom/active/shuttle etc, you still only have as much usable capacity as the weakest cell in the string. The difference is, if you top balance, you spend that available capacity with more time spent at a higher SOC, meaning lower average internal resistance, and higher average voltage (though with LiFePO4 that's almost a non-factor).


For large format saggy LiFePO4 (thunder-sag, sag energy, etc) operated by amateurs trying to avoid damaging cells, bottom balancing makes sense.

For RC LiPo, or any other chemistry where voltage does clearly indicate SOC%, bottom balancing just means you get less performance from your pack with no advantages gained.
 
Thunder sag, lol..

Yeah Luke, you make an excellent point here. Lifepo4 sags, especially on electric car conversions where they are basically installing the bare minimum they can get by with as the project just gets rapidly more expensive the more AH you have...

It's like people trying to run 30 amp controllers on 10AH pings because they can't afford the 20ah-25ah pack.

I notice that with my 20c lipo, sag at lower voltages is massive. After 3.6v hits, it's extremely hard to know where i'm at in terms of mAH left..

You can't pedal with a car, so i see why you'd want try every obscure thing to extend your range. This would actually reduce your range :mrgreen:
 
Mr. Luke, as always you have the answer I am searching for.

The difference is, if you top balance, you spend that available capacity with more time spent at a higher SOC, meaning lower average internal resistance, and higher average voltage (though with LiFePO4 that's almost a non-factor).
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You're wasting power in the good cells with a bottom balance for sure. OTOH, since the "stronger" cells are being run at a lower SOC with a lower Ri, could it be that this would accelerate wear in them when compared to the weaker cell being run slightly higher voltages and with lower Ri? Perhaps this would counter act the exacerbation of capacity/Ri drift between the strong/weak cells caused by charging the weak ones to higher voltages?

Another thought I had; suppose you are a huge multinational company. Suppose you had access to a service like described in the video I posted. You send them 10,000 cells and they send you back X amount of perfectly matched cells. Or perhaps you can afford to pay for A grade cells and the factory does a really good job of matching them up. However you get them, lets assume you have cells matched in capacity and Ri to the .00001. Now, could you throw these cells in a product with only a pack level LVC/HVC? If you did, would you top or bottom balance them? What would be safer?

Something like this maybe?

I was looking through my LBS and saw they had electric bikes for sale with LiPo battery packs there did not appear to be any BMS in the battery box and the only input to the charger was a 2 way connector, this lead me to wonder what the bike company knew that I had missed,
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In this sort of situation where a pack is with out a bms and any sort of user feedback what would be safer? Essentially, which poses a greater safety hazard: Top or bottom balancing?
 
Many packs run no BMS because the cells don't need it.

All the Mn spinel stuff, (kinions, Emoli, LG, etc) have a decreased nesting efficiency as the SOC nears full. This functions as a natural passive balancing mechanism. It's how Doc Bass, JohnCR, and many others keep perfect balance packs doing nothing but bulk charging.
 
liveforphysics said:
Many packs run no BMS because the cells don't need it.

All the Mn spinel stuff, (kinions, Emoli, LG, etc) have a decreased nesting efficiency as the SOC nears full. This functions as a natural passive balancing mechanism. It's how Doc Bass, JohnCR, and many others keep perfect balance packs doing nothing but bulk charging.
Click to expand...

What do you mean by nesting efficiency?
I looked 'nesting efficiency' on google and found mainly poultry-related articles.

I thought that the whole reason why most lithium chemistries get disbalanced is due to small variations in the mAh they can store and different internal resistances, leading to gradual drifting over time.

Does the internal resistance rise near the top voltages on the MN spinel cells? Is that how they work?
 
I would guess that nesting efficiency is the ease which lithium ions "nest" in the anode during charging.

For typical lithium ion:
In normal charging, about 60% of lithium in the cathode active substance is extracted, and is doped into the carbon in the anode. As a result, the cathode active substance becomes Li1-x CoO2 (x≈0.6), but when overcharged, the remaining lithium is further extracted, and the lithium extracted in overcharging precipitates on the surface of the anode as metal lithium. This metal lithium precipitating on the anode surface is extremely active, and it violently reacts with electrolyte, possibly causing thermal runaway, and safety in overcharging is uncertain.

On the other hand, when lithium manganese complex oxide (LiMn2 O4, etc.) is used in the cathode, in normal charge-discharge reaction, the following reactions (5) and (6) take place. That is, in normal charging, almost all lithium in the cathode is extracted and doped into the anode carbon, if overcharged, there is no longer any remaining lithium to move from the cathode to the anode. Therefore, if overcharged, metal lithium does not deposit on the anode, and safety in overcharging is guaranteed.
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http://www.surechem.org/index.php?Action=document&docId=1680604&db=USPTO&tab=summ&lang=&db_query=0%3A%3A0%3A%3A0%3A&markupType=all

So the nesting efficiency is decreased because there is no or very little lithium left. Essentially you could bulk charge all the cells and they'd inherently be top balanced? Lithium batteries behaving like SLA?!?! Shit, sign me up!
 
Bosch Fatpacks with Konions used to be a bargain at 45 bucks apiece delivered and would qualify for your "ideal self balancing battery", Auraslip. Now they are about $100 for the same 36 volt 2ah pack. Not such a great deal anymore. Still great cells though! Doc Bass sells them as individual cells from Makita packs (slightly different cell but similar)!
otherDoc
 
I have turnigy Lipos. I drug them to the true 105% discharge. The voltages were all over the place but the voltages were still in the safe zone. I had up to 35mv of deviation. I put them on the charger and within 2-3 minutes they were all at the same voltage. They all terminated at the same voltage and were in balance. So to me bottom balancing is useless it just creates a pain in the ass on the top end because they will be out of balance because obviously the cells have different capacity therefor different votlages when full dischaged.
 
docnjoj said:
Bosch Fatpacks with Konions used to be a bargain at 45 bucks apiece delivered and would qualify for your "ideal self balancing battery", Auraslip. Now they are about $100 for the same 36 volt 2ah pack. Not such a great deal anymore. Still great cells though! Doc Bass sells them as individual cells from Makita packs (slightly different cell but similar)!
otherDoc
Click to expand...


Yep.
Kinions are a damn fine option. Safe. Brainlessly easy. Decent power/energy density.
 
icecube57 said:
I have turnigy Lipos. I drug them to the true 105% discharge. The voltages were all over the place but the voltages were still in the safe zone. I had up to 35mv of deviation. I put them on the charger and within 2-3 minutes they were all at the same voltage. They all terminated at the same voltage and were in balance. So to me bottom balancing is useless it just creates a pain in the ass on the top end because they will be out of balance because obviously the cells have different capacity therefor different votlages when full dischaged.
Click to expand...


Yep. A perfect example. Lipo is easy peasy.
 
Out of curiosity how much deviation is considered normal in mV?

BTW, I'm in the process of using cell-logs to do some testing of my year old ping. I'll post them in a new thread near the end of the week.
 
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