# Bottom Balancing: What can it really do?



## daniel1948 (Jun 6, 2009)

I have told and discussed at great length the unhappy story of my electric Porsche in this thread. When Gordy finishes building his new building, he's going to have the motor rebalanced, which will leave the bad LiFePO4 pack as the biggest problem. Theoretically it should have a range to empty of around 80 miles at moderate freeway speed, but the BMS starts complaining at about 15 miles, and starts really screaming at 20 miles. The battery cells are "all over the map," as Gordy put it. He's now talking about the idea of bottom balancing as an alternative to replacing the pack. A new pack would cost more than I'm willing to put into the car at this point. (Back in June I gave up on it and bought a Tesla, which I'm delighted with, thus making the Porsche superfluous to my needs. I don't even have garage space for it now.)

Gordy says that although the presence of weakened cells will reduce the range of the car, bottom balancing would prevent the BMS from complaining, and would prevent the weak cells from damaging the good cells, and from being further damaged themselves. If there are not too many damaged cells, maybe I'd get 60 or even 70 miles of range to empty. (I figure 80% of that for "safe" range.) Gordy also says that it would not be necessary to have a bottom balancer permanently installed, but that balancing once a year or maybe even less would be sufficient, because once bottom-balanced, the pack would function properly for a considerable time.

I do not understand batteries all that well, so the following is my understanding, not stated as fact, but rather so that I can be corrected:

A battery cell has a certain capacity in amp-hours or kWh, dependent on its size and chemistry, and dependent also on the rate of discharge. (You get fewer kWh if you discharge faster.) It also has a certain voltage when fully charged, and a lower voltage when fully discharged, and if you try to discharge it below a certain level, something called a reversal of polarity can occur, which will damage it. If you try to charge it above "full" that also can damage it.

Top balancing is done during charging, and prevents cells from being overcharged, by cutting off the charging voltage when a cell is full, allowing the other cells to continue to charge, so that the pack ends up with all cells full. What I'll call middle balancing (I've not actually heard the term used) is when a partly discharged pack is allowed to settle, so that higher-voltage cells can give some charge to lower-voltage cells, and eventually all cells are at the same voltage.

Bottom balancing would be a system that stops current from being drawn from cells when they reach a designated voltage, or draws less current from cells which are at lower voltage, so they are not over-discharged, thus protecting them. I can see how this would be a good thing, as it would allow the stronger cells to continue providing energy after the weaker cells are too low to safely do so. The voltage and current available would be reduced as the weak cells are successively taken off-line, so power would be lost, but the car would still drive without damaging the pack. Or alternatively, by proportionally altering the current draw from each cell, would assure that all cells reach discharge at the same time, even though the available current and voltage would decline as the pack discharges.

At the end of this, all the cells would be at the same (discharged) voltage. But after you recharge the pack, the weak cells are still weak, and you'd need to have the bottom balancer in place during the next, and all further, discharge cycles.

So what I particularly do not understand, is Gordy's statement that we'd only need to bottom-balance the pack once (or one series of a few cycles) and after that the pack would be good for a considerable number of cycles. 

Is my understanding correct? Or is Gordy correct? And if he's correct, what do I have wrong? As I said, I don't really understand batteries. Gordy is a very competent mechanic and builder, but in my opinion (sorry, Gordy, if you are reading this) he is insufficiently critical of extraordinary claims, and he does not pretend to be very knowledgable concerning electronics. So I wonder if he's giving too much credit to bottom balancing. I.e., its ability to correct a bad pack, rather than merely protect it.

Thanks in advance for reading this long post, and for any information you can provide.


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## EVfun (Mar 14, 2010)

Unless damaged, LiFePO4 cells seem to have nearly no self discharge. So, it is quite possible to only balance a pack once a year in many cases. If you bottom balance you will need to remove the BMS, or at least keep it from shunting to balance the pack as they function to top balance the cells. The idea is to set your charger so it shuts off before the weakest (lowest capacity) cell gets to high in voltage. Other cells will be lower in voltage but that is OK as you can't drive farther than your lowest capacity cell. For a very mismatched pack you will need a charger that allows you to choose the ending voltage and current.


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## dougingraham (Jul 26, 2011)

daniel1948 said:


> Top balancing is done during charging, and prevents cells from being overcharged, by cutting off the charging voltage when a cell is full, allowing the other cells to continue to charge, so that the pack ends up with all cells full. What I'll call middle balancing (I've not actually heard the term used) is when a partly discharged pack is allowed to settle, so that higher-voltage cells can give some charge to lower-voltage cells, and eventually all cells are at the same voltage.
> 
> Bottom balancing would be a system that stops current from being drawn from cells when they reach a designated voltage, or draws less current from cells which are at lower voltage, so they are not over-discharged, thus protecting them. I can see how this would be a good thing, as it would allow the stronger cells to continue providing energy after the weaker cells are too low to safely do so. The voltage and current available would be reduced as the weak cells are successively taken off-line, so power would be lost, but the car would still drive without damaging the pack. Or alternatively, by proportionally altering the current draw from each cell, would assure that all cells reach discharge at the same time, even though the available current and voltage would decline as the pack discharges.


Balancing can be done manually or in the case of TOP balancing by some of the BMS systems. If the cells are all full at the same time the BMS does nothing except to signal the charger to turn off.

You talk about middle balance and cells settling. There is no mechanism that will cause the cells to come to the same voltage. 

Bottom balance must be done manually at this time. There are no BMS that will do this for you and it isn't practical for them to do it. You can discharge each cell individually to somewhere between 2.5 and 3.0 volts resting and your pack is bottom balanced. (Pick the voltage and use the same for all the cells, 2.8 would be a good value). An alternate way would be to put all the cells in parallel and then discharge the whole pack to 2.8 volts resting as if it was a single cell. This would bottom balance the cells. Then put it all back into series and slowly charge the pack while monitoring the voltages of the cells. You will probably find that you have one or more cells that are incredibly bad at this point and reach 3.45 volts long before the others. I would remove those from the pack as they are going to do nothing but reduce your range.

The idea behind bottom balance is that all the cells will run empty at the same time. You have no choice but to pull off the side of the road and get a charge or a tow. But you wont have any cells ruined when you do this. A well bottom balanced pack will look just like someone turned off the switch. One moment you will have good power and the next nothing.

So in summary, Figure out the bad cells and remove the worst. Bottom balance the remainder. Dont forget to reset your charger to the new lower voltage required by the bottom balance. And don't let your BMS try to top balance as that will mess up the bottom balance.

Best Wishes!


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## dougingraham (Jul 26, 2011)

daniel1948 said:


> So what I particularly do not understand, is Gordy's statement that we'd only need to bottom-balance the pack once (or one series of a few cycles) and after that the pack would be good for a considerable number of cycles.
> 
> Is my understanding correct? Or is Gordy correct? And if he's correct, what do I have wrong? As I said, I don't really understand batteries. Gordy is a very competent mechanic and builder, but in my opinion (sorry, Gordy, if you are reading this) he is insufficiently critical of extraordinary claims, and he does not pretend to be very knowledgable concerning electronics. So I wonder if he's giving too much credit to bottom balancing. I.e., its ability to correct a bad pack, rather than merely protect it.


You would only need to bottom balance once as long as you dont overcharge the weak cells. Every cell sees the same current both charge and discharge so the fuel level is the same in all the cells and will stay that way without any outside influence. Unfortunately there is an outside influence. The BMS can place a different load on every cell and will cause the balance to drift over time. It may not be much but even a little bit will cause problems over time. When people see the BMS rebalancing their packs what they are seeing it it correcting the imbalance that it is forcing on the cells. To make a bottom balance work you are going to want to remove the BMS because it will unbalance the cells.

Your friend is probably correct that you can operate like this for quite some time. I am guessing that you have a few pretty bad cells that are causing all your concern and that they were probably damaged by a single over discharge event. Remove those and your top speed and acceleration will suffer a tiny bit but the car will be quite usable.

Best Wishes!


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## daniel1948 (Jun 6, 2009)

So if I use bottom balancing, each cell would only carry as much energy as the weakest cell. By removing the weakest cells, I'd be able to put more energy in each cell than if I left the weakest ones in. There would be some ideal number of cells to remove, depending on the actual profile of the pack.

Now, if I were to just remove those same weakest cells, and use top balancing to fill all cells, and a BMS to warn me to stop driving when the weakest cell is low, wouldn't I still have the same amount of usable energy in the pack? Instead of taking each cell down to a determined bottom point, and then putting an equal amount of energy in each, I'd be filling each cell to full, and again taking an equal amount of energy from each; in both cases determined by the capacity of the weakest cell.

So is there actually an advantage in bottom-balancing, other than that it eliminates the need for a BMS?

BTW, Gordy says he's been talking to someone who has developed a bottom balancer. It's expensive, but they'd loan it to him, and since it would be a one-time (or occasional) thing, we wouldn't need to own one. But from the above, it sounds as though such a device is superfluous. Unless maybe the advantage is that it does not require removing the pack.


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## DavidDymaxion (Dec 1, 2008)

I used a 500 A battery load tester to balance. There's no need to remove the pack from the car.

Not that I particularly recommend it, but can't you set a shunt system to a low voltage, and it'll shunt each cell down to the bottom balanced value desired?


dougingraham said:


> ... Bottom balance must be done manually at this time. There are no BMS that will do this for you and it isn't practical for them to do it. ...


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## steven4601 (Nov 11, 2010)

Hi,

Hopefully I am correct and not waffling , you had the Porsche built by a conversion shop which did a horrible job? I was quite sad after reading the whole story. Like building a house with the wrong masonry. 

about your questions.

- Physical lost capacity/range cannot be suddenly regained with top or bottom balancing. . 
- Bottom balancing with a BMS installed is a waste of time. It is very uncommon for BMS's to provide bottom balancing. (yet ill be proven wrong for the one odd fish)


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## daniel1948 (Jun 6, 2009)

Yes. The original conversion was done by Paul Liddle in West Palm Beach, FL. There was so much wrong with it that I had another shop, Gordoz E-Speed, in Hayden Lake, ID, take it all apart, re-design it, and put it together again from scratch. Gordy did an excellent job, but the pack is in bad shape. According to what I've been told (though Paul denies it emphatically) it suffered an accident on the bench in Paul's shop. According to an associate of Gordy's, I could get a new pack for $12,000, not counting labor.

But I don't want to put any more money into a car I would not drive (now that I have the Tesla). So we've been talking about taking out the bad cells and running it at higher voltage (at present half the pack is in parallel with the other half). Bottom balancing was another suggestion.


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## dimitri (May 16, 2008)

daniel1948 said:


> But I don't want to put any more money into a car I would not drive (now that I have the Tesla). So we've been talking about taking out the bad cells and running it at higher voltage (at present half the pack is in parallel with the other half). Bottom balancing was another suggestion.


Bottom balancing will not improve your range at all. It will simply put your good cells at same level with bad cells, which is absurd thing to do. You must weed out bad cells and either reduce the pack size if you refuse to replace weak cells with good ones, or spend a little money just to replace weak cells to stay at same pack size. Replacement cells don't have to be same brand, just same or very similar rated capacity. 

Its not known how many cells are damaged, could be just one, or perhaps a few. All you need is a load tester to load each cell and observe its voltage sag. Write down loaded voltage for each cell and see if there is a good baseline and find cells sagging much more than baseline. It will be obvious which ones have to be weeded out after such test. Once weak cells are removed/replaced, top balance the pack and enjoy the car.


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## daniel1948 (Jun 6, 2009)

Problem is, there might not be any actually good cells left. The whole pack was damaged in the charging snafu, as far as we know. At this point I am not optimistic. This car has never given me any reason to be. I'll be discussing options with Gordy once he finishes his new building.

I appreciate the information about bottom balancing. From what everyone has said above, it doesn't look like a solution.


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## brainzel (Jun 15, 2009)

I don't use any parts to shunt, balance or do anything during the daily drive. Just, charging, driving, counting amphours and sometimes take a closer look, just to calm myself 

About a year ago I made this graphic to visualize the bottom and top balancing issues. Hope it helps to understand or discuss.

The most important thing you have to know, is your smalest cell capacity.
This is your limit, no matter which way you go, top or bottom balance.
Balance once and than just stay away from the top and the bottom.

If you bought f.ex. 200Ah cells, but one cell within yous serial string of batteries is about only 180Ah, than don't take more than ~160Ah out of your pack and you should be fine.

Michael


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## daniel1948 (Jun 6, 2009)

Thanks, Michael.

Once Gordy has time again, I'll ask if he can map the capacity of each cell. With 96 cells, I'm afraid it's going to be a big job.

What's actually involved in determining the capacity of an individual cell? Does it require a special tool? Or can it be inferred just by measuring the voltages?


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## brainzel (Jun 15, 2009)

Voltage is neither a good nor a reliable indicator for capacity.
In my opinion, the only way to weasure the real capacity is to discharge the cell to the lower knee, charge the single cell up to the upper knee and then measure the amphours which have flowed into the cell.

The discharge/charge cutt off point are written in the manufacture datasheet.

It's a long and painful job, but if you don't know your cells and therefore don't know what you're dealin with, it's imho inevitable.


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## dimitri (May 16, 2008)

2 things matter for cells used in EV, capacity and IR ( internal resistance ). You need both. 

Low IR prevents deep sag under heavy load, which determines efficiency/internal heat losses inside the cell. Cell damage usually means IR is going up, which is a positive feedback process, i.e. it will only get worse over time since more heat is generated inside the cell under load, which further contributes to increased IR.

Obviously capacity determines range, so its also critical.

To measure both of these things in one step you need equipment to load individual cell with 1C-3C currents and count AH. You charge the cell to upper knee, then discharge at high C rate, which will reveal both IR and capacity for the cell.

Obviously this process takes time, skills and patience, in addition to equipment.


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## daniel1948 (Jun 6, 2009)

Thanks to both! I'll find out whether Gordy or his friend who helped with some of the electrical stuff have access to such equipment. If not, I may just be up a tree. I know that what the car really needs is a new pack. But now that I have a much better EV, and no place for a third car (I need the Prius for road trips) the Porsche is of no real use to me, and I'm not willing to spend the money on a new pack.

So the question becomes whether by sacrificing range I could salvage enough to give it really good performance. That might make it desirable enough to sell. And with good performance perhaps someone would find it worthwhile to invest in a new pack to recover the range. But if testing all the cells involves a lot of equipment and labor, we're getting back to my unwillingness to put a lot more money into a car that after all I might not be able to sell.

Three and a half years ago, when I began this project, the Porsche as originally promised would have been one of the nicest electric cars on the road. Today, with the Tesla and the Leaf available, and more on the horizon, the Porsche even as originally promised would only be one more very nice EV among many. And there's no guarantee that even with a new battery pack it would perform well enough to bring an acceptable price.

Buying this car and trusting Paul with the conversion of it has been the third worst mistake of my life.


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## brainzel (Jun 15, 2009)

A self made discharge/charge/measurement circuit doesn't have to be much expensive.
At the moment I try an experimental set-up with an Arduino, a resistor, an heat sink, two relais and a puwer supply. I hope to report results soon.
This set-up would be about ~ $100 I think.


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## GizmoEV (Nov 28, 2009)

Daniel,

What type of BMS system do you have? Is it one where there are individual cell boards with LEDs on them showing when a cell hits bottom? If so you could capacity test the cells in a crude way but it will give you the relative capacity of each cell. What you would do is charge the pack and let the BMS top balance it. When it is done put a load on the pack. I connected several ceramic light bulb bases in parallel for a load. As long as the pack voltage is under about 150V or so standard light bulbs will work. Get some high wattage bulbs and screw them in until the discharge current is what you want. I use a DC clamp Ammeter to check the current flow. Note this current and time when the process starts. If the current is say 20A then each hour 20Ah will be removed from the pack. Use this to estimate when the pack will be getting close to empty and make sure to check on it before that time. Note when the first BMS module signals low voltage and stop the discharge. Remove the battery from the series and then continue discharging, noting the new current and time. Continue until all the cells have reached the low voltage signal. This will give you a rough estimation of the capacity of each cell. Keep the best ones that will still work with the voltage range of the rest of the system.

This isn't perfect but it will work. Bottom balance the pack and don't use the BMS or use the BMS and let it do its thing. If you go without the BMS you might want to have Gordy build a half pack monitor like I did for my Gizmo. I documented it in my blog. It is very simple to build.

It was really sad reading about the poor job done on your conversion. I'm really glad you were able to get a Tesla, however.

Oh, I forgot to add that I have gone a year with no balancing on my pack and the cells were still very close. I have a well treated pack, however.


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## JRP3 (Mar 7, 2008)

One cheap way to see how each cell is behaving under load is to get a Cell Log 8, about 30 bucks, charge up the pack, and drive around a bit putting some heavy loads on the pack. Since the Cell Log can only do 8 cells at a time you'll have to move it around 8 cells at a time and retest. This will give you a visual graph of the voltage sag of each cell and a damaged cell should stand out. You can then test the worst looking cells for capacity individually. As others have said bottom balancing will not help a pack with a large difference in capacities, no balancing scheme will. Bottom balancing is a good way to run a pack with close capacities without using any BMS. 
At this point I think you are better off selling the car as is, needing a new pack, than to keep messing with a vehicle you really have no interest in and don't want to spend any more money on.
I just sold an old Corvette I put a lot of time and money into at a big loss because it needed more time and money that I didn't want to put into it and I had no interest in it. I hated to do it but I'm so happy it's out of my life at this point it was worth it.


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## daniel1948 (Jun 6, 2009)

Gizmo: Yes, the BMS is the kind that has a little circuit board at each cell. I'll mention your suggestion to Gordy.

JRP: The Cell Log 8 sounds like a lot of bother. Gizmo's idea sounds easier to me.

Nothing will be done for a couple of months, until Gordy's new building is finished. And in the mean time I am open to any reasonable offers to buy the car. I'd love to be rid of it.


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## caglassmith (Apr 18, 2011)

Daniel, 

Hate to see you so negative about your battery pack. If during the "charging snafu" there was not total fire and conflagration, I would suspect there are at least some good cells. My suggestion would be to continue studying up on charging methods (top balance vs bottom balance) so you are making an informed decision before plunking down $12k. 

Based on my engineering background, my personal bias is to lose the BMS and use the bottom balance technique. Even cells that are near zero volts may be able to be rescued (sometimes referred to as "precharging"). Regardless of what technique you use, it will be labor intensive and may not interest the conversion shop. 

If you want more information on bottom balancing, the strongest proponent for the technique is Jack Rickard of EVTV.com fame. He has several videos on charging and bottom balancing that might be of interest.

Roger...


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## JRP3 (Mar 7, 2008)

No balancing scheme will make up for damaged cells with lost capacity.


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## dtbaker (Jan 5, 2008)

caglassmith said:


> Based on my engineering background, my personal bias is to lose the BMS and use the bottom balance technique.



and so, without a BMS and bottom balanced, how do you end a charge cycle before overcharging a single cell at the top?

Most of the guys I have read going sans BMS top balance, let the charger do its curve to end charge, and avoid going more than 80%DOD.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> and so, without a BMS and bottom balanced, how do you end a charge cycle before overcharging a single cell at the top?
> 
> Most of the guys I have read going sans BMS top balance, let the charger do its curve to end charge, and avoid going more than 80%DOD.


Don't charge to such a high voltage. Stop at 3.45 or so. Naturally the cells can't be wildly out of balance depending on how many cells are in series.


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## dtbaker (Jan 5, 2008)

GizmoEV said:


> Don't charge to such a high voltage. Stop at 3.45 or so. Naturally the cells can't be wildly out of balance depending on how many cells are in series.



if you are bottom balanced, you don't get a choice on how wildly the top may be unbalanced.  so, HOW do you sense when its time to stop without a BMS?


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> if you are bottom balanced, you don't get a choice on how wildly the top may be unbalanced.  so, HOW do you sense when its time to stop without a BMS?


Yes you do. You check your cells at the end of the first charge after a bottom balance. If there is too much difference swap out some cells. This is easier if you can capacity check all your cells first. If you didn't purchase extra cells then you are stuck with what you have. I took Jack R's advice and bought 10% more cells for my pack. Since I buddy paired mine I did a rough match of capacities so that each cell pair had roughly the same capacity as the next.

You also get to choose by what brand/vendor you go with. Some match in sets. Get cells from a manufacturer with poor quality control and you will have a wildly mismatched set. Get better quality for a closer match.


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## dtbaker (Jan 5, 2008)

GizmoEV said:


> Yes you do. You check your cells at the end of the first charge after a bottom balance. If there is too much difference swap out some cells. This is easier if you can capacity check all your cells first.



wow, I just don't think it's realistic to use the manufacturing QA to ship matched sets. Why would anyone want to buy 10% extra to sit on a shelf?

Besides, you still havent shared your method for reliably catching UN-top-balanced cells at end of charge without a a cell-level BMS that can shut down the charger.

....this is why it seems more reliable to me to top-balance and use the charger to do its job. No BMS required if the top-balance is good, and no harm done if you avoid more than 80%DOD


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## EVfun (Mar 14, 2010)

It seems that charging a bottom balanced pack with wildly mismatched damaged cells (point of this thread) might make it difficult to sense the shut-off point. 

You can't drive farther by top balancing or bottom balancing, but you should be able to go farther on a balanced pack compared to an imbalanced pack. With an imbalanced pack you can have different cells limiting the upper charge limit and the discharge limit. 

I generally shoot for a 3.50 vpc shut-off (no BMS.) The difference in charging time for me between 3.44 and 3.50 isn't that great, but the end of charge is quicker and more sharp with the higher voltage. Charging my 60 amp hour cells to 3.44 vpc I put in about 36 amp hours at 12 amps and then 3 hours slowly tapering until full. At 3.50 vpc I put in about 54 amp hours at 12 amps then about 45 minutes with the current tapering to full.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> wow, I just don't think it's realistic to use the manufacturing QA to ship matched sets. Why would anyone want to buy 10% extra to sit on a shelf?


In case some go bad before the rest are ready to replace. You are not guaranteed to find some that match. It is a risk either way. Choose which one you want.



dtbaker said:


> Besides, you still havent shared your method for reliably catching UN-top-balanced cells at end of charge without a a cell-level BMS that can shut down the charger.


It was in the second sentence of mine that you quoted. Check on the first charge. They don't change with respect to each other by much if at all as long as there aren't any unbalanced loads so you are good to go if the first charge is ok. If not, swap cells or change methods.



dtbaker said:


> ....this is why it seems more reliable to me to top-balance and use the charger to do its job. No BMS required if the top-balance is good, and no harm done if you avoid more than 80%DOD


It think it all depends on how often the range is pushed. If it is regularly pushed then a bottom balanced set is likely to be safer. If not, then top balanced is likely to be safer.

I think the choice also would have to do with what the ending current is. If the ending current is 0.05C then I think the "ragged top" is less of an issue than if the current is really low because of the difference in "energy above the knee" that the two ending currents have. Stopping charge at 3.6vpc with something in the mA range of current constitutes more energy at the end than stopping with 0.05CA. This greater energy put in at the end by the low ending current has the potential to overcharge the smaller capacity cells by a greater amount. Now comparing 0.05CA ending current at 3.6vpc to something like 0.001CA ending current at 3.45vpc, I don't know. I had to return my borrowed cells I was testing before I could fine tune my testing methodology to find out. 

In my case, I'm really glad I bought 10% more cells because when I learned I didn't need to charge to 3.65-4.00vpc, and really shouldn't with my charger's low ending current, I installed my extra cells knowing that if I had any go bad that I could actually drop as many as 20% of them and still have a vehicle which performs better than it did with 48V of lead acid batteries.

I still wonder about end of life. With a top balanced pack and only a half-pack voltage monitor, will it give me the STOP NOW! signal in time when I push the range of my pack? I do have a CA to count Ah but I'm talking about as the capacity reduces to well below the current 200Ah spec.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> Charging my 60 amp hour cells to 3.44 vpc I put in about 36 amp hours at 12 amps and then 3 hours slowly tapering until full. At 3.50 vpc I put in about 54 amp hours at 12 amps then about 45 minutes with the current tapering to full.


I wonder if the difference in time between the two ending voltages would be smaller if the charger had separate voltage sense leads? When I charge with my wall mounted charger I notice it starts tapering back the current much sooner than my onboard charger does. I'm not talking when it starts tapering back from 40A, I'm talking about when it is below the 14A that my on-board charger would still be putting out for a given voltage. My wall mounted charger has at least an extra 20feet of DC extension cord to charge through. If there were separate voltage sense leads then the voltage drop in the cord would be negated.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> and so, without a BMS and bottom balanced, how do you end a charge cycle before overcharging a single cell at the top?
> 
> Most of the guys I have read going sans BMS top balance, let the charger do its curve to end charge, and avoid going more than 80%DOD.


I think most people going sans BMS are actually bottom balancing since Jack Rickard is probably the loudest proponent of skipping the BMS and recommends bottom balancing. Since I've had 3 very low DOD events, one to where my car stopped moving, I'm very glad I bottom balanced. If you buy your cells from CALB you'll have a very closely matched pack, recent orders have come in within less than 1% differences, so per cell end voltages are very close, close enough not to matter. You should indeed request closely matched cells, express your preference for such and it will become the norm, as it should be. My cells from over 2 years ago came in between 110-114 ah actual and I can run bottom balanced and stop charging when my smallest cells are around 3.45. I have one oddball from a different order with slightly less capacity, if I replaced that one I could get them even closer, but since I don't want to charge to 100% SOC anyway I just use constant current at 20 amps and then have the charger shut off, no constant voltage stage. This keeps the pack undercharged and well away from the steep part of the curve. If I want a full charge I just restart the charger at a lower current, or engage the timer. If I had not bottom balanced my pack I would have some dead cells. Of course the more closely matched your pack is the less it matters if you top or bottom balance, and the more mismatched your pack is you probably need to top balance since if they are really out of whack you can't end charging even close to the same time. You'll just have to stay well away from the bottom when driving. Obviously I'm not able to do that


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## dtbaker (Jan 5, 2008)

Allow me to re-phrase.... I really m not trying to start a holy war, I am just trying to understand the specifics of how a person can reliably end a charge cycle with bottom balancing and no cell level BMS that can stop the charger when the first cell hits the desired end voltage.

so.... how do you tell when that first cell hits target, and then how do you stop your charger?

My simple charger has no timer, and no control input for a voltage sensing BMS to shut it down. I just don't see how you bypass the charger's normal pack voltage sense, and even TELL when your first cell hits target. seems like if you bottom balance you have to go with SOME cell level voltage sensor and a relay of some kind to pull the plug on the charger... rather than rely on the pack voltage sense of the charger.

(which is why I went w/ top balance.... simple charger, and no BMS)

The end-of-life question is where a bottom balanced system is safer for sure. The big question that nobody can answer yet is the nature of failure of these cells after thousands of cycles..... do we lose capacity suddenly?, gradually? depending on how they are used? i.e. how quickly do my 100ah cells degrade to less than 75ah capacity.... How will I know other than if my pack voltage suddenly starts dropping as I'm driving along?


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## JRP3 (Mar 7, 2008)

I did some initial charge testing by monitoring my cells near the end of charge. I then set total pack voltage end of charging when my smallest cell hits around 3.45 V. Sometimes it's a little higher, sometimes it's a little lower, depending on temperature. The cells are close enough that total pack voltage can be set to keep an individual cell from going too high. I use 123V pack voltage for my 36 cells, which averages 3.42 V per cell, but in actuality gives me ranges from 3.40-3.45 or so. If I pulled the one outlier I'd probably be between 3.41-3.43


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## daniel1948 (Jun 6, 2009)

Most of the above is way over my head, though I will refer Gordy to this thread. However, a couple of points regarding my own particular situation:

With the Tesla happily in my garage and being my daily driver, I am not concerned about the range of the Porsche, and I will not spend money on a new pack. I'd really like to sell it as is, if I could. But failing that, I'd like to salvage as much of the pack as I can, and get the car to where it could perform at a high level, even if only for 20 miles. With the looks, comfort, and handling the car already has*, if it could accelerate respectably, it might have some appeal to a potential buyer who would either be satisfied with the range, or willing to invest in a longer-range pack, knowing that at that point range is the only issue.

From what I think I do understand of the above, and given that the car already has a BMS and top balancing, I don't think that the labor of bottom balancing would be worth my while. 

When I bought the Porsche, it was to have been my daily driver, and I wanted to be able to drive it 100 miles without taking the pack below 80% DoD.

Now all I want is to bring the car to where it can show respectable enough performance that somebody might be willing to buy it.

* In looks, comfort, and handling, the car is superb! It is a beautiful car, in beautiful condition. It is far more comfortable to sit in and drive than is the Tesla. In fact, it's the most comfortable car I've ever driven. And, being a Porsche, its handling is just amazing. It actually handles better than the Tesla. The Porsche will never match the Tesla in range or acceleration, and probably reliability. But in comfort and handling the Porsche is unequalled.


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## dtbaker (Jan 5, 2008)

JRP3 said:


> I did some initial charge testing by monitoring my cells near the end of charge. I then set total pack voltage end of charging when my smallest cell hits around 3.45 V. Sometimes it's a little higher, sometimes it's a little lower, depending on temperature. The cells are close enough that total pack voltage can be set to keep an individual cell from going too high. I use 123V pack voltage for my 36 cells, which averages 3.42 V per cell, but in actuality gives me ranges from 3.40-3.45 or so. If I pulled the one outlier I'd probably be between 3.41-3.43



ok, so lemme see if I understand what you did.... you :
- bottom balanced somehow
- manually monitored during a charge cycle to identify your highest cell, and recorded the pack voltage when the high cell hit 3.45
- set your charger to trigger on that pack voltage


A problem for a lot of people is that we cannot change the charger end voltage easily after delivery.


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## tomofreno (Mar 3, 2009)

> My simple charger has no timer, and no control input for a voltage sensing BMS to shut it down. I just don't see how you bypass the charger's normal pack voltage sense, and even TELL when your first cell hits target.


 I'm surprised you haven't seen this explained several times here. You simply monitor cell voltages with a dvm while charging to see which is the highest voltage cell and determine at what pack voltage it gets to the highest cell voltage you want. Then you have your charger programmed to charge to that pack voltage, with some cushion. On the Manzanitas, like JRP3 has, you can simply adjust the limit voltage of the charger yourself to achieve this. You also need to periodically check to ensure that cell remains the highest one, and it is being charged to about where you want. It is pretty simple if your charger charges to a limit voltage and holds exactly that voltage while decreasing current in CV mode. The voltage continues to creep up a bit on the Manzanita chargers, so it can result in charging that high cell higher than you want in warmer temperatures or at lower charge currents. More care is required in that case. That is why I changed to top balance with my Manzanita. This gives a larger voltage change near end of charge as a number of cells start going up the exponential part of the curve rather than one (the lowest capacity cell), resulting in larger voltage change which causes the Manzanita to cut back its charge current more quickly. This gives a much wider margin for cell temperature and charge current magnitudes I can charge at and not have it over charge a cell (though I do have the minibms as backup). If the cells were very closely matched in capacity it wouldn't matter if the pack was top or bottom balanced as they would all be close in voltage at both ends.


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## Jan (Oct 5, 2009)

daniel1948 said:


> From what I think I do understand of the above, and given that the car already has a BMS and top balancing, I don't think that the labor of bottom balancing would be worth my while.


Yes, you're right, unless your charger/BMS doesn't do its job properly, and your pack is very out of balance. In that case, which is not perse unlikely, a proper top or bottom balancing exercise might give you your range back. Top or bottom doesn't matter. The difference in those two religions lies mainly in savety and lifetime expectations.


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## JRP3 (Mar 7, 2008)

If he had damaged cells with reduced capacity nothing will bring the range back. All you can do is make sure the smallest cells are able to deliver their full remaining capacity, which, if they are damaged, will probably continue to decrease with use.


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## caglassmith (Apr 18, 2011)

JRP3,

"If he had damaged cells with reduced capacity nothing will bring the range back. "

Suspect that you are very correct on that point. You certainly have much more experience with these cells than I do. 

Unsaid in my original message was the hope that Daniel would get more data on the exact condition of each cell before making a sweeping decision to purchase a new pack. If the unbalance problem is simply unequal discharge, a bit of TLC on each cell could recover a substantial portion of his investment. Of course if the charging incident was the source of the problem, this could have permanently damaged all or at least a substantial portion of the pack.


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## JRP3 (Mar 7, 2008)

Range is determined by the smallest capacity cell. Balancing can only assure that all it's remaining capacity is being used. So yes if the pack is unbalanced such that the smallest cell, or cells, are not able to contribute their full capacity then balancing could improve the range.


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## dougingraham (Jul 26, 2011)

JRP3 said:


> If he had damaged cells with reduced capacity nothing will bring the range back. All you can do is make sure the smallest cells are able to deliver their full remaining capacity, which, if they are damaged, will probably continue to decrease with use.


Nothing will bring the cells back but removing the worst couple will increase the range assuming only a few are bad. The trick is to find the bad ones.


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## daniel1948 (Jun 6, 2009)

caglassmith said:


> Unsaid in my original message was the hope that Daniel would get more data on the exact condition of each cell before making a sweeping decision to purchase a new pack.


As I've said, I am not going to buy a new pack. Period. I'd like to maximize performance at whatever reduced range comes out, in order to demonstrate what the car can do, in the hope that this might make it easier to sell.


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## dtbaker (Jan 5, 2008)

daniel1948 said:


> As I've said, I am not going to buy a new pack. Period. I'd like to maximize performance at whatever reduced range comes out, in order to demonstrate what the car can do, in the hope that this might make it easier to sell.


I would suggest that your first step is to top-balance right away so you can avoid over-charging any cells since you are using a standard charge curve without BMS.

Once that is done, you may have some damaged cells with reduced capacity, but at least the first X miles will be ok, and charge cycles will not be causing further damage.

you CAN top balance individual cells (up or down) while the whole pack is installed and connected in series.... but it is a pretty time consuming process to get that last little bit evened out. The end-of-charge voltages are very sensitive in that last 10 minutes; it only takes a tiny bit of energy added or removed from a cell to bring it into balance at that top end.

the procedure I found most effective is:

- do a charge cycle, watch a suspected high cells carefully and stop early if they start shooting over 3.8v while the rest are still chugging away. If you have identified runaways, knock some energy out of them with a resistor... and repeat until you can complete a charge without any cells skyrocketing.

- once your charge can complete 'normally', wait a couple minutes for surface charge to settle a little, then measure ALL cells and record. I labelled all cells, start in the same spot and go the same order when voltage has dropped from 139 to 136; thats when it slows down enough for me to take all measurements before pack voltage settles too much to see differences.

- identify cells that are 'out of tolerance' above the theoretical average 3.65vpc, and knock down the highest ones some fixed amount of time (10 to 30 second increments at the end!) If you have a power supply like a mastech, you can also put a little energy into the lowest cells to speed things along.... fix voltage at 4.0v and give a cell maybe 30 seconds to bring up the ending voltage a little.

- apply some small load like drive around the block or headlights for 10 minutes, and run another charge cycle and repeat....


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## daniel1948 (Jun 6, 2009)

dtbaker said:


> I would suggest that your first step is to top-balance right away so you can avoid over-charging any cells since you are using a standard charge curve without BMS.


Actually, the car does have a BMS, which charges all cells to full, and sounds a warning when the weakest cell gets too low.

Understanding what I do now about bottom balancing (thanks to the folks who have replied in this thread) I am surprised that Gordy ever brought up the idea, but I'm glad he did because I have learned a good deal from the thread.

The car is presently with Gordy (since I have no garage space for it) and is being kept charged while he is busy constructing his new building. Once he's done with that, he's going to take the motor out for re-balancing. Then we'll probably remove the weakest cells and take the car for a test run. If it does well, I'll try to sell it. (Though I'll entertain any offers for it as is in the mean time.)


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## dtbaker (Jan 5, 2008)

oh....

well it sounds like your BMS is set up with shunting to re-top-balance automatically. One thing to be aware of though is that if the cells were NEVER top balanced initially, they may have been far enough out that the BMS is unable to shunt and balance. Most BMS shunting can only handle a couple amps at the tail end of charge, and NOT badly unbalanced systems when charger is chugging along at 10+ amps.

Do you have a build thread, garage description, or website with full specs on your vehicle 'as-is' ?


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## daniel1948 (Jun 6, 2009)

dtbaker said:


> Do you have a build thread, garage description, or website with full specs on your vehicle 'as-is' ?


No, I don't. I have done none of the work on the car myself. I am not a builder, just someone who has wanted to drive electric ever since experiencing those occasional brief few minutes of pure electric driving in the Prius. In May, 2007 I bought the Zap Xebra. About a year later I bought the Porsche. I was an early hand-raiser for the Nissan Leaf, and an early order-placer, but Nissan lost my order, then lost my car, then assigned me a different car, then took away that assignment, and then just gave me the run-around. In June of this year (2011) having given up on both the Porsche and the Leaf, and having driven the under-powered Xebra for 4 years, I bought the Tesla.

Next time I see Gordy I can get a list of "full specs" if you (or anyone else) is interested.


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## Mark F (Aug 13, 2011)

Hi Dan,

What size resistor do you use for knocking off some energy of the higher voltage cells? Where would you buy them?

I have a couple of batteries that take off toward the end of charge and I'd like to get them in balance with the rest of the pack.


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## dtbaker (Jan 5, 2008)

Mark F said:


> Hi Dan,
> 
> What size resistor do you use for knocking off some energy of the higher voltage cells? Where would you buy them?
> 
> I have a couple of batteries that take off toward the end of charge and I'd like to get them in balance with the rest of the pack.



I got two 1k ohm, 25 watt resistors and wired in parallel, and stuck'em in some alligator clips. more info on my site [here].


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