# Bottom balancing thread - safer? more useful?



## ewert (Sep 5, 2009)

I read through the thread here:
http://www.diyelectriccar.com/forums/showthread.php/cell-balancing-options-no-more-volt-38339.html

Which brings up the (now that it is said out loud) intuitive (now that it IS said ) idea of using bottom balancing instead of topping off balancing for batteries.

In short:
Balance batteries on the low voltage range instead of high voltage range. This aims for making all the cells have their low voltage curve begin at about the same time during discharge. This gives you useful relevant full pack voltage based "fuel gauge" effect. It also may be safer, because with the fuel gauge effect you could be better informed of when you are running low on juice, and you can more reliably use that pack voltage to trip controller limp-modes or alarms and thus not blaze through at 200-500 amps while your lowest cell is going way down while your better cells still can pump out power well enough.

It does not lower total Ah's either, since if you imagine the cells as sticks of a little different length each, it just means they are evened in the other end, naturally your total Ah will always be your weakest cell. But this way you will have the low voltage curve uniform across the cells, instead of the high voltage curve uniform (and useless as it disappears soon when discharging anyways).

Why I posted this thread is because I want to read way more about this idea, I'm just a doodler, so can't bring much info on my own in this, thought I'd just make it's own thread for it since it triggered a "man this makes so much sense it is silly it was not noticed earlier" effect in me. 

Please discuss. 

I would think that devices that discharge to some voltage at some amps are in the market already in some form? Initial / yearly maintenance bottom balancing done with those, and then a high voltage / low voltage cell level sensors that cutoff charger / poke controller? No need for expensive BMS? Cheaper EV's for everyone? Birds in the sky, and fish in the water, and happily ever after? =P


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## ewert (Sep 5, 2009)

Okay and to reply to myself with further ideas:

How about increasing the steadyness of bottom balancing by doing some parallel + serial in the cell pack? Instead of doing 160Ah x something, do 2 parallel 80Ah x something, or even 4 parallel x something? Measure IR in the initial bottom balancing, and match the parallel cells with 1-40, 2-39, 3-38 etc format so that the average for each parallel match is nearly identical? Probably more hassle than worth it maybe, more connections etc. required than going just with the right size Ah cells straight up?


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## JRP3 (Mar 7, 2008)

I'd say you're over thinking this. Once you bottom balance your pack voltage is a good representative of your SOC near the bottom. Why complicate things?


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## dexion (Aug 22, 2009)

I work to my weakest cell. I have a small volt meter on my 2 weakest cells on my 2 vehicles. These cells go down first and go up first so my situation may be unique or easier. Thats all I do, no bms or ballancing aside from the first ballance when I took the cells out of the box since they were of unknown soc. 
I love the idea of ballancing somewhere, but it just doesnt seem needed and indeed ballancing at the top or the bottom both have merits in a perfect world and problems (but still arent needed)

Top ballancing will keep some cells from discharging to 80% dod (unless they are all very close in cap.) so they could last longer (however filling them up full might cancel this out so it could be a zero gain.)

Bottom ballancing stops many cells from being topped off thus extending their life but they may (depending on your driving habbits) be discharged to 80% often thus killing them quicker again a zero gain is possible.

Also, while bottom ballancing is best for fuel gages, it is worst for peopleness. What I mean by that is people will usually push the envelope. If you push the envelope on a pack of 50 cells all at 85% dod and push it a bit too (for whatever reason) far how many cells would you lose as opposed to top balancing where 2 or 3 cells may be at the bottom how many would you lose. LVC would help here for people with this tendency, but metering the lowest cells do as well. 

Now, without knowing your at the bottom of those 2 or 3 cells (top ballancing) its more apt to happen (killing those cells) BUT all of this could be done away with if you just find and identify your 1,2,3 lowest/highest cells when you are:

1. At the furthest you will normally drive (end of a discharge cycle.)
2. At the most you would charge (End of a charge cycle.)

Assuming none of the cells are over max voltage or under min voltage for 1 and 2, stick a meter on your weakest cells and thats it.

Now, while this has worked for me for a year, there are some variables that can affect things.

1. bolts working lose (i tighten every 3 months)
2. corrosion so far not an issue, but im on the lookout
3. Cells aging faster than others too new to tell but I check to make sure the cells that have the meters on them are still the ones that should every 3 months as well. 

So, im still a fan of doing nothing after the cells get in line from when they are new aside from metering the weakest cells.


I do have a plan that I havent had to use yet in case i do go to lcv on the metered cell. I have a small 16 cell charger that takes 1200watts and a small generator that does 1800 watts. With those I can get 6 miles in range for each hour I run the generator with the charger. So, if im 3 miles from home I go get it and charge for 30 minutes or so (10 minutes on each 16 cells.) If its the bike, I will just push it home but the generator will allow me to charge the bike at a rate of about 12 miles an hour as well if needed. Also at the half way point I have permission to use a 20 amp outside charging plug at a local gas (irony) station if needed as well. 

So, I suppose a well thoughtout and executed plan could be as good as the perfect bms. But, im sure there are those that dont want to think it out or have access to what I have access too on my drive that need a bms. But there isnt a perfect bms yet.


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## JRP3 (Mar 7, 2008)

dexion said:


> I work to my weakest cell. I have a small volt meter on my 2 weakest cells on my 2 vehicles. These cells go down first and go up first so my situation may be unique or easier.


That's how it should work for everyone. The lowest capacity cells will fill up first and empty first. If you bottom balance then all cells should empty at the same rate. This is likely to save more cells than top balancing since you won't have healthy cells pushing current through drained cells. Your method of monitoring the weakest, (smallest capacity), cells should work fine, as you've seen.


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## ewert (Sep 5, 2009)

> Also, while bottom ballancing is best for fuel gages, it is worst for peopleness. What I mean by that is people will usually push the envelope. If you push the envelope on a pack of 50 cells all at 85% dod and push it a bit too (for whatever reason) far how many cells would you lose as opposed to top balancing where 2 or 3 cells may be at the bottom how many would you lose. LVC would help here for people with this tendency, but metering the lowest cells do as well.


Yeah there is that fact, but for that I reckon putting in hard settings in the controller since the pack voltage is representative of total Ah left if the pack is bottom balanced. Of course stupidity will kill batteries, and I agree if you run a bottom balanced pack one could kill the whole pack easier by being really really (stupidly) careless, whereas on the other one could kill single cells MUCH easier by being just somewhat careless with top balancing.

Dexion what you are doing is sort of the way one could roll with bottom balancing most cheaply I guess. Do a manual cell check & bottom balance during installation, pick 1-2-3 or so of the weakest cells, and just pop in high and low voltage monitoring on those.

And yeah JRP3, definitely overthinking it, if cells do not age with different speeds (and if a pack is done from the same batch of cells from same manufacturer, I guess they should not).

Okay I figured a question about bottom balancing and high voltage cut off in charging: does a battery nearing it's full charge slow down in charging speed (more heat, less energy stored)? If it does, then should a bottom balanced pack have a more strict high voltage cut off for it's weakest cells, so the better cells don't over time slowly gain noticeable amounts of extra Ah in them? And thus possibly cause the weak cell at some point to have so much less Ah stored after a charge, exposing it to being dropping while most of the batteries still can keep solid voltage, and thus possible causing a cell death?

Aaaaand stupid me figured out the answer while writing: since there would be low voltage cutoffs on the weakest cells, and the pack voltage would just be "fuel gauge"ish, it wouldn't matter because it would be caught by the LVC. =P


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## dimitri (May 16, 2008)

At a risk of being named a vendor or "Apple cart pusher" ( whatever the hell that means  ) I will keep my opinion on this subject short and simple.

In my personal humble opinion both top and bottom balancing are just means to have a reference point for cells SoC, nothing more. Its much easier to do top balancing and less risky if done correctly. Bottom balancing would require special "current sinking" tool and utmost attention to the process.

Overall, I can describe top balancing as playing Russian Roulette with one bullet in the gun, i.e. if you push your pack beyond LVC you kill one or two cells. But bottom balancing is like playing Russian Roulette with all 6 bullets in the gun, i.e. if you push beyond LVC you kill entire pack or most of it.

Bottom balancing only works if you rely on Controller LVC cutoff and you have strict cutoff rules, since you have a huge risk of quickly destroying your pack. Not all controllers have reliable LVC.

But IMHO, why not just use a simple and reliable BMS with HLVC, at the minimum on the "weakest" cell, at the maximum on all cells? This way you are protected from all angles.

Let the flaming begin....


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## ewert (Sep 5, 2009)

Yeah I just read that poll-thread on HLVC bms, much relevant discussion going on it.

So, re-re-rethinking it. If we drop a HLVC at say 3 weakest cells similar to your current idea and use the weakest cell as a SoC reference point, then:

Top balancing: SoC works, overdischarge kills single cells, LVC/HVC for controller/charger trigger if any of 3 weakest cells trigger it. Seems safe.

Bottom balancing: SoC works just as well, but overdischarge (lets say failure somewhere even with controller being set up well) may kill manymany cells or even whole pack. LVC/HVC works the same just like SoC.

I agree dimitri, considering from this point of view the usefulness of bottom balancing is pretty much gone, and due to some mishap or other the extra risks stay.

Now this is fun, discussion and opinion changing with new thoughts all the time! ... Wish it was always like this on the internet, haha. 

So, bottom balancing would be maybe a fringe thing, when you run based on pack voltage without any cell level LVC. Investing for HLVC for a couple cells though ... much more easily justified than for full every cell shunting BMS that may cost almost as much as the controller. What would a 3 cell HLVC circuit cost... =P Heck as you could really do the same with 1 with pretty much as good results as with 3 (as long as you properly identify the weakest cell I guess), instructions for DIY becomes a real option for most.

I mean for example myself, there would be no way in youknowwhere that I would do like 40 cell's worth of any kind of boards with soldering (I'd be so much better off just buying something and working my real job for the time required to solder that much), but a few boards ... that's more hobbylike and doable.


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## tomofreno (Mar 3, 2009)

> If you bottom balance then all cells should empty at the same rate.


 They will all empty at the same rate, charge/time, no matter where you balance. I think you mean they will all have the same amount of charge remaining at any point in time. Not the same soc, but the same amount of charge remaining, so be the same relative to fully discharged. 

Jack slammed top balancing as being dangerous, causing destruction of cells if you drive too far. He could equally well have said bottom balancing is even more dangerous because you will destroy all cells simultaneously if you drive too far. No matter how you balance you obviously need to stop before over-discharging cells. LVC gives you some help to do so, and HVC helps you stop charging in time at the other end.

One advantage I see for bottom balancing is if you use a meter that tracks Ah discharged, it indicates charge left in all cells as JRP3 pointed out. Whereas if you top balance, it indicates remaining charge in only one cell that you calibrate it to (eg your weakest cell), or a rough indication of some average state if you just put in the nominal cell capacity. The advantage with bottom balance in this case is it doesn't matter which cell is weakest and whether that changes with time. The Ah remaining indication on the meter will always be a measure of the charge remaining in the weakest cell, since it is a measure of this for all cells. Which I'm guessing is what JRP3 was implying. Of course at the other end, during charging, the cells will have different soc depending on their relative capacities, and different voltages as a result, so now you have to determine when to stop charging before one over charges. And overcharging can destroy cells as some here have witnessed. No matter how you balance, it is much safer to have HLVC.

Something that hasn't been mentioned explicitly is failure rate. It is one thing to come up with a system that works for you most of the time, another to come up with one that has a very, very low failure rate, for example by designing it to be robust against component failure. Dimitri has pointed out some things to make a HLVC more robust against failure in his "cheap bms" thread.


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## JRP3 (Mar 7, 2008)

tomofreno said:


> They will all empty at the same rate, charge/time, no matter where you balance. I think you mean they will all have the same amount of charge remaining at any point in time.


Yes, I should have said they will all approach the same slope on the discharge curve at the same time.


> Jack slammed top balancing as being dangerous, causing destruction of cells if you drive too far. He could equally well have said bottom balancing is even more dangerous because you will destroy all cells simultaneously if you drive too far.


Yes, but no  See my reasoning below.


> One advantage I see for bottom balancing is if you use a meter that tracks Ah discharged, it indicates charge left in all cells as JRP3 pointed out. Whereas if you top balance, it indicates remaining charge in only one cell that you calibrate it to (eg your weakest cell), or a rough indication of some average state if you just put in the nominal cell capacity. The advantage with bottom balance in this case is it doesn't matter which cell is weakest and whether that changes with time. The Ah remaining indication on the meter will always be a measure of the charge remaining in the weakest cell, since it is a measure of this for all cells. Which I'm guessing is what JRP3 was implying.


 Exactly. Bottom balancing does two things for you, it allows pack voltage to accurately reflect where your cells are on the discharge curve when you get near a problem area, and because all the cells are at the same place you won't have a few low cells dragged even lower by the other healthy cells pushing current through them. So theoretically you could kill the whole pack but you'd really have to be driving well past the the lower pack level to do it, where with top balancing you could easily be showing a safe pack level and have individual cells be dangerously low, which is exactly what we've seen happen.


> Of course at the other end, during charging, the cells will have different soc depending on their relative capacities, and different voltages as a result, so now you have to determine when to stop charging before one over charges. And overcharging can destroy cells as some here have witnessed. No matter how you balance, it is much safer to have HLVC.


Yes, though as Jack says it's easier to monitor charging at relatively low amps at home than discharging at high amps on the road.
I would suggest, possibly incorrectly, that if one stops charging when the first cell hits HVC you probably come pretty close to bottom balancing the pack, unless the cells are at varying SOC's before charging, which does not seem to be the case. This seems to be essentially what Jack has been doing with his Speedster pack and is probably why he's had no problems with it.


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## JRP3 (Mar 7, 2008)

Here's how I see a bottom balance pack behaving. I'm cruising along with my 36 cell pack at 3.2 per cell, 115.2 volts. I see 3.1 volts per cell, 111.6 volts I know I'm getting low. 3 volts per cell, 108 volts I know the party is over. 2.9 per cell, 104.4 volts, I've stayed too long at the party and am about to be thrown out. After that I'm lying in the alley in varying states of disrepair  Point being those are meaningful voltage differences that I can see and deal with. Combined with a reasonable understanding of my pack range and miles driven should keep me out of trouble.


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## icec0o1 (Sep 3, 2009)

So how would you bottom balance if your daily comute is only a few miles and you never get below 30% discharge? Discharge the remaining 70% to get to the fall off point? Do you do it every time? Once a month?


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## JRP3 (Mar 7, 2008)

In that case I wouldn't worry about it at all. If you never come close to the pack limits I can't see any reason to deal with it.


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## cycleguy (Oct 7, 2009)

I'm of the opinion that, whether you top or bottom balance, your driving blind and always taking a risk without a cellular level monitor. It's like driving your ICE vehicle around without an accurate gas gauge. A simple LV/HV cell monitor will keep your cells safe regardless of how you balance them.


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## JRP3 (Mar 7, 2008)

cycleguy said:


> It's like driving your ICE vehicle around without an accurate gas gauge.


Only if you drive around near empty a lot. Most gas gauges don't differentiate between 10 miles left of range or 1 mile left. Pack level voltage on a bottom balanced pack would probably be more accurate than most gas gauges I've seen.


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## Dolphyn (Nov 17, 2009)

Bottom-balancing requires discharging the cells just to balance them. That's an inherent risk right there, if not done carefully.

I think top-balancing is easier and less risky, _assuming_ that you have protection in place to make sure you _stop driving_ or reduce the throttle before any cell approaches under-voltage condition.


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## Tesseract (Sep 27, 2008)

Dolphyn said:


> Bottom-balancing requires discharging the cells just to balance them. That's an inherent risk right there, if not done carefully.


I don't think it's any more risky than top balancing, per se, but it does require a specialized device to do so. I wonder if it would be worth designing such a product that could then be rented out? I'm guessing it wouldn't be a very popular product to buy unless you ran a conversion shop...




Dolphyn said:


> I think top-balancing is easier and less risky, _assuming_ that you have protection in place to make sure you _stop driving_ or reduce the throttle before any cell approaches under-voltage condition.


I agree that it is easier, and that if some way is used to take the LVC signal and either have the controller cut power, or else reduce the top end of the throttle, that this would be a good combination. Maybe not ideal, but better than simply monitoring the total pack voltage unless the pack was _bottom_ balanced.


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## tomofreno (Mar 3, 2009)

> In that case I wouldn't worry about it at all. If you never come close to the pack limits I can't see any reason to deal with it.


 I think that is key. It may be lost in all this talk about ragged edges of states of charge that the raggedness of that edge is typically small. Typically smaller than a 3 or 4% change in soc I think. So if you stop at 30% soc, there is little chance of harming a cell unless you have a bad cell that has way less capacity than the others. But I still think it best to have HLVC to warn you of a good cell going bad as they age as in Brian's case.


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## Guest (Nov 21, 2009)

dimitri said:


> At a risk of being named a vendor or "Apple cart pusher" ( whatever the hell that means  ) I will keep my opinion on this subject short and simple.
> 
> In my personal humble opinion both top and bottom balancing are just means to have a reference point for cells SoC, nothing more. Its much easier to do top balancing and less risky if done correctly. Bottom balancing would require special "current sinking" tool and utmost attention to the process.
> 
> ...


Ok. They're not the same thing from different ends.

1. You charge at 15 amps.
2. You drive at 300-500 amps.

You want to be balanced where you can do the most damage. Where do you think that might be?

Let's take a quick look at bottom balancing, so we're all talking the same thing.










I believe you have to do this about once a year. And it's a pretty manual process. Drive your car until it's nearly empty. Then go around and bleed cells with a 1/2 ohm load until they all reach the same value - ideally 2.8 vdc. Then charge until the first cell hits your top voltage and quit. In fact, now you CAN charge to 4.2 vdc on the first cell, all others should be below.

Back to the why. We charge at low current values, and we do so by voltage. But when we drive, we're really looking at CURRENT and TIME. When the cell with the least capacity reaches the knee of the charge curve first, it goes over and begins a quick dive to zero volts - rather driven by those who are still above 3.0 vdc on the FLAT part of the curve.

What kind of time zone are we talking about? I've done some calculations on 160 AH cells. They're not test data. They're just calculations and I'm quite wrongly assuming 3.0 vdc as the END of the capacity. I'm kind of doing this on purpose, because we are interested in what happens after. If you want to be really accurate, do the same calculation to 90% or whatever you think appropriate, it comes out the same way.













Here's the problem, we have 376 seconds between when the FIRST cell hits 3.0 and the last cell hits it - across 8 cells. And this is at a steady 100 amps. In that period, you would take 11 AH out of the small cell. It doesn't have 11 amp hours. It has 2. Maybe 3.

Bottom balancing reduces this time period to zero seconds ideally. 

The object is to get all the cells balanced at the point where:

1. You're running out of pack.
2. You're draining the pack at high current.

Why would I EVER want to balance

1. When my pack is full
2. When I'm in the garage not moving
3. When we're dealing with 15 amps.

My speedster uses 540 amps at 75 mph.

Hello.....

Jack Rickard
http://evtv.me


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## dimitri (May 16, 2008)

You keep repeating same story over and over ( I think in your own terms this is called "typing yourself smart" ). We get it, line up at the bottom instead of top, we get it already.

No one is arguing that bottom balancing is a good idea if one chooses to skip on cell level BMS and rely on controller LVC, in fact its excellent idea. There are 2 problems with it:
1. Without special equipment its not easy to bleed few AH from a number of cells, too time consuming and too much heat produced in the process and you have to pay lots of attention. Sure it can be done once, but if you need to do it regularly its a hassle.
2. How do you know that bottom balanced cells stay balanced? What prevents them from drifting apart? Just because you don't see it in your handful of tests and one year of use, doesn't mean its not there. If you can convince us that cells don't drift, that would be good. So far I am not convinced. Unless convinced otherwise I believe cells do drift apart slowly, but surely. Since they are not all equal to begin with, their differences will cause them to drift, unless there is a process in place to counteract the drift.

Top balancing was never about top balancing at all, no one ever said that all cells have to be perfectly lined up at the top. The name is misleading, so let's call it "anti-drift process" or ADP for short, instead of "top balancing". If you spent hours top balancing your pack, then you were surely wasting your time.

ADP does only one thing, it provides ability for some cells to get a little more charge than others, to compensate for natural drifting away at each cycle. 

So, assuming ADP is needed, how do you propose to automate it at each cycle to avoid major rebalancing work on semi annual basis? How can we give some cells more charge , yet stay bottom balanced? Perhaps this means giving other cells less charge? But how to do it in a simple and effective way?

Bottom line, if your pack use is very conservative and you don't want to spend money on BMS, do bottom balance and rely on conservative charger shutoff voltage to prevent overcharging and controller LVC to prevent overdischarging, you SHOULD BE FINE, as long as you don't believe in "pack drifting apart" theory.

I can even say this, bottom balancing is much safer than top balancing *if you don't have cell level HLVC protection*. I'm not talking audible or flashing alerts, I mean PROTECTION , as in "taking action on HLVC events".

However, if you do have HLVC protection, then you don't need any balancing, as long as you have some ways to gauge your fuel tank and you don't believe in "pack drifting apart" theory.


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## JRP3 (Mar 7, 2008)

How about bottom balancing by adding charge? So drain you pack till the first cell hits 2.8 volts, then charge that cell back up to your next lowest cell. Drive again till you hit 2.8, hopefully on a few cells this time. Repeat process till all cells hit 2.8 at the same time. You could do this with the car up on jacks instead of driving around. Somewhat cumbersome but requires no special equipment other than a single cell low amp charger since you aren't putting in that many AH's at that level. Might work as a one time experiment at least.


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## Guest (Nov 22, 2009)

dimitri said:


> You keep repeating same story over and over ( I think in your own terms this is called "typing yourself smart" ). We get it, line up at the bottom instead of top, we get it already.


Clearly you DON'T get it and so I'm trying to find a more effective way to communicate it to you. The graphs are a way to SHOW you something since reading is clearly a problem. 



dimitri said:


> No one is arguing that bottom balancing is a good idea if one chooses to skip on cell level BMS and rely on controller LVC, in fact its excellent idea. There are 2 problems with it:
> 1. Without special equipment its not easy to bleed few AH from a number of cells, too time consuming and too much heat produced in the process and you have to pay lots of attention. Sure it can be done once, but if you need to do it regularly its a hassle.


I did it with a piece of "special equipment" that consisted of a 1/2 ohm 50 watt resistor and a 3 amp 12vdc supply with two clamps. You can actually do this with EITHER the 12vdc supply OR the 1/2 ohm resistor, but you ALWAYS need the two clamps. It's faster my way, using both and what the hell, we can put the $30 on a credit card and pay it off over a period of months.

I'm having a hard time keeping a straight face here. Is your idea of no "special equipment" 72 little circuit boards, each with two wires, sticking up on the top of your cells, such that even YOU broke one of them causing a cell death from its failure? Or did I misread that. MY special equipment cost about the same as two of your cell killing boards.

Why would I need to do it "regularly". I have a Speedster that I've never done it to AT ALL. I drove it 107 miles on a single charge. The 80% part is 85 miles. ALL 72 cells were between 2.8 and 2.9 volts. They had NEVER been balanced. TOP or bottom. 8 of them were kind of "married in" when added. I've had it a year. I've put 5000 miles on it. And you dismiss this as "not yet there." No yet where? 



dimitri said:


> 2. How do you know that bottom balanced cells stay balanced? What prevents them from drifting apart? Just because you don't see it in your handful of tests and one year of use, doesn't mean its not there. If you can convince us that cells don't drift, that would be good. So far I am not convinced. Unless convinced otherwise I believe cells do drift apart slowly, but surely. Since they are not all equal to begin with, their differences will cause them to drift, unless there is a process in place to counteract the drift.


Says you. What evidence do you have that they DO drift. You don't even have the handful of tests and the year. If you can convince ME that cells DO drift, (and I"ll let the others speak for themselves) that would be good. Unless convinced otherwise, I'm convinced your confused. So far I'm not convinced there's anything to this drift theory. Even your THEORY is so vague I can't follow it. There actually IS one about drift, but you clearly don't understand it. Since I don't agree with it, it's a little upside down for me to describe it to you.

"Since they are not all equal to begin with, their differences will cause them to drift, unless there is a process in place to counteract the drift."???? I love it when you talk teknikul like that Dmitri. What differences caused what? Counteract the drift? That assumes there IS a drift. Since they are different they are obligated to become more different? This is just nonsense. 

YOU have a drift theory. YOU prove it experimentally. Until then, I'm not only "unconvinced" but amused. You want me to prove that YOUR theory doesn't hold water, and you cannot even describe the theory beyond "their differences will make them drift"????? Blue Elephant Gun. Which is what you've been selling all along (literally).



dimitri said:


> ADP does only one thing, it provides ability for some cells to get a little more charge than others, to compensate for natural drifting away at each cycle.


"Natural drifting away at each cycle."???? Why do they "naturally" drift away at each cycle? If they do so at each cycle this should be readily apparent and easily measurable. How did you measure this? If it takes more than a year, how is it happening each cycle? What's natural about it? Why does it do that at all? How much does it drift? What have you done to measure this? Do you test capacity of all cells after each charge? Do you log how much energy each cell takes to reach a fixed voltage each time and note how this varies from charge to charge, season to season, temperature to temperature? Do you log how much energy comes out of each cell and what voltage it reaches after each drive? How do you know that they just "naturally" drift away from each other unless something stops them from just naturally doing so because it is their natural nature to wanna do that naturally?



dimitri said:


> So, assuming ADP is needed, how do you propose to automate it at each cycle to avoid major rebalancing work on semi annual basis? How can we give some cells more charge , yet stay bottom balanced? Perhaps this means giving other cells less charge? But how to do it in a simple and effective way?


How easy or hard it is to deal with, will not alter the physics involved one whit or tittle. I'm not sure it needs to be done at all. I have suggested that there MAY be some salutory good to bottom balancing. It makes sense. But I've actually demonstrated that it is not necessary. We drove the GEM two years without it. We drove the Speedster a year without it. I TOP BALLANCED for a WEEK and killed three cells with two others among the walking wounded.

To counteract my top balancing, I've bottom balanced, and the beastie is back running uneventfully again.
***************** READ THIS PART AT LEAST *******************

Now let me ask you a serious question Dmitri. If driving the car a year on a daily basis, never having balanced at the top or the bottom, not only hasn't show a drift problem, but in a demonstrated extreme event of a 107 mile 100% discharge, with ALL cells manually measured between 2.8 and 2.9 vdc, isn't a good indication that cell drift doesn't happen, that implies to me that you think sooner or later it WILL be a problem. And I just havent' measured enough or driven the car enough. (The Blue Elephant WILL eventually get you Jack). Do I understand that thesis correctly? You don't find it persuasive? You're not convinced? Because of insufficient data? And inadequate measurement? A handful of tests you said? A mere year you said?

OK. If that is so, why do you think you need to correct this by balancing every time you charge, or at least monthly?

It would seem your qualification for persuasion is rather on a longer time frame than your need to fix it. Or do I misunderstand something here.

If I can't make it show up to your satisfaction in a year, why do I need to fix it nightly? Or at least monthly?

***************OK YOU CAN GO BACK TO SLEEP NOW***************

Bottom line, my pack use is NOT conservative. I've driven an $85,000 car to the utter limits of what could be expected, right at 100% discharge, with no harm no foul without balancing at all.

I don't want to spend money on BMS. I don't want to spend money on batteries. I don't want to spend money on electric cars. I want to spend money on whiskey, women, and however reluctantly, daughters.... in that order. And I am fine, thank you for inquiring.

And currently, I do NOT believe in the pack drifting apart theory. Actually I have nothing against the theory. But I not only have seen no evidence of it with my own eyes, I haven't even read HERE a persuasive, or even cogent explanation of what it IS. 


If pressed, in a rational discussion, with an intelligent sentient, I probably believe 
1. there is drift, 
2. it is random in various directions 
3. largely cancelling errors, 
4. at the noise level 
5. nothing necessary.

It would only be an interesting debate in person over whiskey, with somebody pretty good at talking about these things. And if they did start talking about naturally doing this and that because it was their nature, and they naturally want to do it, I would cut off their whiskey and have their wife come drive them home.

Till then, I've got a pretty open mind on the topic. It hasn't hurt me. And it hasn't cost me. And if there is anything persuasive, I'll look at it. Till then it's not even a problem. If it was a problem, since it hasn't come up in actual operation yet, I can be pretty assured that bottom balancing about once per year would handle it. I'll save up for a "fresh" 1/2 ohm resistor, but probably use the same clamps.

Jack Rickard
http://evtv.me


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## Guest (Nov 22, 2009)

JRP3 said:


> How about bottom balancing by adding charge? So drain you pack till the first cell hits 2.8 volts, then charge that cell back up to your next lowest cell. Drive again till you hit 2.8, hopefully on a few cells this time. Repeat process till all cells hit 2.8 at the same time. You could do this with the car up on jacks instead of driving around. Somewhat cumbersome but requires no special equipment other than a single cell low amp charger since you aren't putting in that many AH's at that level. Might work as a one time experiment at least.


Of course you're correct. And I cannot even describe how trivial this is. Since I'm off of the triple digit accuracy quest. On that part of the curve, on a 160 AH cell, it takes 2 or 3 minutes per cell to add a little at 3 amps or take a little out at 5 amps to move the voltage to pretty much where you want it. On 24 cells, true this can run an hour or so. 

Ergo the GEM. On 72 cells, it would likely be three or four hours. And on the GEM they're easy to get to. But seriously JRP, spot check a representative sample of cells every month or so and bottom balance ANNUALLY more than likely. If you do this twice a year, you're wearing out your cell terminals worrying them to death.

If you HAVE been top balancing, I would recommend you get to work with it immediately before driving the vehicle. This is to UNDO what you were doing. However difficult it is. But a three amp 12vdc supply and a 1/2 ohm resistor is OVERKILL. You can do either one quite effectively. ANd on that steep part of the curve, it takes NOTHING to move those voltages all over the place. Up around 3 volts it gets to be a chore. At 2.8/2.7 it's trivial.

And there is NO danger. If your standing there bleeding a cell with a 1/2 ohm resistor you are NOT going to hurt anything unless you leave it hooked up and walk away and get hit by a truck before you come back. It does get hot though. Hooking up a small 12v charger does EXACTLY the same thing in the other direction. Get a dumb one and a 2 or 3 amp one and I think they're about $12. And it doesn't matter to the pack if they are bottom balanced at 3.00 or 2.9 or 2.8 or 2.7 or whatever. It's important to be at 3.0 or under to be on the knee.

Oddly, while the upper side of the curve is at a different voltage on the Blue Sky's, it appears to be at the SAME place on the bottom side as the Thunderskys. No idea why. One says 2.5 v and one says 2.0. I cannot find a difference on the bottom side.

Jack Rickard


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## dimitri (May 16, 2008)

Its OK if 2 intelligent people don't agree on some theory while neither can prove it either way, that should not be a reason for them to get testy and call each other names. Let's just continue to respect each other and not slide into arrogance, afterall we share same hobby and agree on many other things.

At a risk of being laughed at, my believe in "drift" comes from the fact that most Lead Acid packs get severely disbalanced, especially sealed type packs like AGM. Since all batteries are essentially chemical devices with common operating principles I just don't see why LFP battery would be any different.

Surely you are not going to argue that AGMs get disbalanced? Right?

I'm not trained in chemistry or battery technologies enough to KNOW the processes and be able to argue them one way or another, it just seems logical to me that "drift" SHOULD occur and if its not , then there should be logical explanation for it. I am simply staying on a side of caution until I KNOW what's going on. Can you blame me for it?

You haven't killed a cell by not balancing and I have not killed a cell by balancing, so that alone does not make either of us right or wrong.

Also, you seem to indicate that cell level BMS is not needed in some of your posts and yet seem to be interested in it in other posts, so which one is it? Why are you interested in it if you don't need it?


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## JRP3 (Mar 7, 2008)

It's quite possible that LA cells have more balancing issues because they are really designed as individual units and individual differences aren't tightly controlled, whereas lithium cells are designed to be used in groups and are built and tested to closer standards.


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## JRP3 (Mar 7, 2008)

dimitri said:


> Also, you seem to indicate that cell level BMS is not needed in some of your posts and yet seem to be interested in it in other posts, so which one is it? Why are you interested in it if you don't need it?


Probably the same reason I'm interested. I don't think I need it, but if it's available, reliable, and cheap enough I might want it. Like a voltmeter in an ICE, I don't need it but it's nice to have.


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## Guest (Nov 22, 2009)

dimitri said:


> Its OK if 2 intelligent people don't agree on some theory while neither can prove it either way, that should not be a reason for them to get testy and call each other names. Let's just continue to respect each other and not slide into arrogance, afterall we share same hobby and agree on many other things.


Ok, agreed.



dimitri said:


> At a risk of being laughed at, my believe in "drift" comes from the fact that most Lead Acid packs get severely disbalanced, especially sealed type packs like AGM. Since all batteries are essentially chemical devices with common operating principles I just don't see why LFP battery would be any different.
> 
> Surely you are not going to argue that AGMs get disbalanced? Right?


No, I'm not going to argue that. Are you going to propose that LiFEPo4 cells sulfate?

If you don't see why they would be different, why would you see why they should be the same. Baking soda rockets are chemical devices. Do they operate like AGMs?



dimitri said:


> I'm not trained in chemistry or battery technologies enough to KNOW the processes and be able to argue them one way or another, it just seems logical to me that "drift" SHOULD occur and if its not , then there should be logical explanation for it. I am simply staying on a side of caution until I KNOW what's going on. Can you blame me for it?


I can. And I cannot resolve it for you. And I get back to the same problem, if it's not a problem you can demonstrate, and it's not a problem that shows up in a year, why do we need to solve it nightly?

The lead acid lessons were hard won. I'd love to just carry them over. But unfortunatley, MOST of it doesn't apply. And so we have to learn LiFePo4 lessons on their own terms. 

As this goes slowly, over time, and over incidents, and experimentation is diifficult and can be expensive, these will be hard won too.


Jack Rickard
http://evtv.me


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## Guest (Nov 22, 2009)

dimitri said:


> Also, you seem to indicate that cell level BMS is not needed in some of your posts and yet seem to be interested in it in other posts, so which one is it? Why are you interested in it if you don't need it?


[/quote]

Not sure how to respond to that. BMS covers a multitude of sins. I've never opposed it conceptually, and in fact, spent a fortune on piles of them here in the garage - come pick out one you'd like. If they need a BMS, then I'm onboard.

The problem is, all the BMS I've seen have been more trouble than they are worth, and don't do the job they purport to do. If I could have found one I liked, we'd have never learned this stuff. If I liked it, installed it, and it didn't blow anything up, I'd be going along with my $3000 BMS thinking it was protecting me from something, and never know the difference.

I don't exactly know what you MEAN by BMS. Battery Management System. Managing what? Monitoring System. Monitoring what?

I'm not having a problem with balance. I'm having NO problems with charging. If that's what your BMS is intended to do, it is a solution looking for a problem in my garage.

On the other hand, I think the CENTRAL technical issue with battery electric vehicles is "managing" energy and how we deal with reaching the end of it in a car. This comes from an experiment and experience you can quite duplicate.

1. Charge your car.
2. Go drive it until it quits rolling completely.
3. Go get checkbook and buy new batteries.

If I run out of gas in an ICE vehicle, no harm no foul. Just add gas. Vehicle is 1. Non operational without gas 2 Perfectly operational once gas is added.

And that is how an electric car should work. If you run out of charge, it should coast to a stop. Add charge, you're back in operation. No damage to the car.

How do we do that?

My CURRENT thinking, that is, something I would spend money on, install, and experiment with, has to do with LV detection at the cell level. I can do all this at the pack voltage level, but the TOP charge disaster has keyed me that nothing will be certain unless I can get an alert that any cell in the pack is "drifting" /varying/descending into non goodness. If I had that, I could implement any of several strategies.

1. Cut top speed to 25%
2. Light an indicator
3. Disconnect contactor
4. Initiate Martin Baker MKIV ejection seat launch sequence to remove driver.
5. Sound a klaxon.
6. Set off tiny explosive charges to blow tires.
7. Phone home.
8. Radio for air strike
9. Throw vehicle into reverse.
10. Turn on emergency flashers.
11. Cut off drivers satellite radio until they agree to recharge.
12. Disable the Itunes.

You get the idea. This is a problem that needs to be worked off. How to gracefully run out of gas without destroying a car or doing 4 years at Renneslauer Polytechnic.

In this, I keep hearing "how do we deal with drift." I don't have drift. How do you know you don't have drift. I've driven the car for a year. That's not long enough. Long enough for what? You know, drift. If you don't balance DAILY you're going to get drift. In a year? Don't know. We haven't had it yet either.

So we have to balance. We do that to counteract drift and prevent destruction of our cells. Ok, so how come if I DO balance I destroy cells and if I DON'T balance I don't destroy cells. Because you haven't waited long enough. What?

Yes, I call it typing yourself smart. I'm open to any concept, but accepting as a given because it "makes sense" doesn't work for me. I know of a LOT of things that make perfect sense. Turns out they're not so. But it made sense.

I'm not at all wed to "bottom balancing". I just made up the term a few days ago. I'm not opposed to TOP balancing. But the data seem to indicate that it does more damage than good. The DATA. RESULTS. THings YOU can verify EASILY. And if not true, it shouldn't be too hard to REFUTE easily. 

And actually, the entire INDUSTRY of ALL manufacturers top balance. So I was pretty sure this was all going to be a little embarassing when somebody finally said look - HERE's what your looking for and why and THIS is how you can demonstrate that empirically, even if its a little tedious. And HERE is what you did wrong in your experiment do it again and try THIS and you'll see.

INSTEAD I get a blast of stuff about how it all really works differently because it always has and we all voted and you're wrong and it just naturally does this because it naturally makes sense and BOY are you going to be sorry LATER.


So I'm a little frustrated with people "typing themselves smart." But if you have any indications, data, experiments, etc. to report in support of anything, I'm wide open. I have had an epiphany of result, and have devised a theory to explain it It appears to explain it. But there might be other things that would explain it just as well. Not much has bubbled up on any value I can find. 

IF it is true, it kind of upsets a LOT of applecarts. Not just shunt designers. THere are EV component houses that REQUIRE these devices to warranty the batteries. A LOT of people have spent $1200 or $2000 grand to put them on their cars - to apparently no avail.

So I'm actually NOT CERTAIN about top balancing/bottom balancing. The INDICATION IS:

1. No balance is better than top balance.
2. Bottom balance appears to offer some efficacy. If nothing else, it appears NECESSARY to undo the top balance effect.

Lower voltage cell monitoriing looks valuable. 

Does a deeper discharge actually shorten cycle life? I don't know. The manufacturers tests of cycle life imply that 70% leads to less degradation than 80%. Is it linear? What happens if some cells are more deeply discharged than others over the long term?

Well it implies their capacities will diminish faster. So they run out sooner. That would imply BOTTOM balancing becomes more important. What if they don't DRIFT longitudinally, but the capacity of the smallest cell decreases at a faster rate and the "ragged" bottom edge caused by TOP balancing becomes more pronounced over time. Can this be counteracted by bottom balancing? 

Here's a strategy. Let's BOTTOM balance. Then note which cell reaches 4.2 volts first when charging and mark it. Let's run for six months. If it's the same cell, replace it, re bottom and continue.

In this way, you replace the cell with the least capacity every six months with a new cell to "expand" the range of your pack. Do this forever. Bottom balance at cell replacement. Your pack should maintain the same range and last forever, at the cost of two cells per year. 

Its just a thought. But that is a more likely scenario of "drift" with a CAUSE and an EFFECT, that could be tested, although it would take a lot of time. Take a 170AH cell and a 160 AH cell. Bottom balance them. And charge them serially until the 160AH cell reaches 4.25 volts. Drain and recharge 100 times. The 170AH cell should have diminished slightly in capacity. The 160 AH cell, being more deeply discharged each time as a percentage of total capacity, should have decreased in capacity MORE as a percentage. But we're only interested if it decreases more ABSOLUTELY. If it is by percentage, the cells are actually growing INTO balance. If it is absolute, they are growing OUT of balance. 

But if OUT of balance, the cells would NOT be sliding around as you describe. The shortest cell would simply be getting SHORTER. And you range diminishing faster because of this cell. 

BTW. This concept of variously diminishing capacity based on varying percentage of discharge is the drift theory you were grasping at. It's interesting, but as I pointed out, without doing a very long and tedious experiment, I can't tell if they are drifting CLOSER together, or FURTHER apart, or even at all. It is IMPLIED by the manufacturers cycle life data.

My anecdotal observation is they actually grow closer together. The short cell diminishes as a percentage of its total capacity, but the larger cell diminishes as a function of ITS total capacity. Both are influenced by depth of discharge, but total capacity overrules. To illustrate, no data I'm just making this up, picture a 160 Ah cell diminishing to 158 AH, and a 170 Ah cell diminishing to 165AH. Growing closer together. 
Second case, 160 AH cell diminishes to 155 Ah, 170 Ah cell diminishes to 167 AH. Further apart. Third case. 160 Ah cell diminishes to 155 Ah, 170 Ah to 164.78 AH - they stay the same.

As you can see, this is a little more complicated than "they naturally just drift apart because that's naturally what they want to do." They could just as easily be drifting together, or not drifting at all. They're changing in capacity, in internal resistance, in response to temperature, in response to current load, in response to time of year, in response to how you drive it, in response to how deeply you discharge it, and a dozen variables I don't even know about. 

THIS is why you have to actually DO things to make these determinations, instead of typing really really hard into a screen of what you think makes sense. IN FACT, what you say can be TOTALLY TRUE as far as it goes, and completely swamped to the OTHER result by a variable you didn't even know was in the game!

I fell on my sword in public in part to DEMONSTRATE how this thinking can lead you in precisely the wrong direction all the while making sense. And the response is more "it only makes sense."

So yes, Im very interested in your lv monitor circuit. IT's not a trick. I'll buy some. IF they work, I'll buy more. You don't have em done yet? I need them YESTERDAY.


Jack Rickard
http://evtv.me


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