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Post by weber » Wed, 14 Sep 2011, 22:25

Nevilleh wrote:The optocouplers are Vishay VO615A - 6 which have a CTR in the range 100 to 300 %.
The resistor driving the emitter is 330R and the load resistor on the output is 2k2. You will note that the wires between boards are on the driver side (the 330R side) which means that the impedance is pretty low and hence there should be very little interference pickup - if any.
You would of course be aware that ours are the same, except we use differential drive and so have more noise immunity in the off state, and we split the resistance into two 150R so both outputs are short circuit limited.
On the cell modules, the Vs is as low as 2.5V which means the diode current could be under 4 mA. Works OK, but.
And on the control unit, the Vs is 5.0V so lots more current available.
No significant difference there either.
In the car, I have a pair of wires running from a cell in the boot to the control unit inside the car and there is no problem with that. All my comms wires are twisted pairs too.

Do you mean the wires between adjacent cell-top units are twisted pair as well, or just the ones to and from the master?

Ours between cell-top units are PCB tracks in most cases and twisted pair if they go more than 10 mm. Our master comms wires are optic fibre.

Thanks for that, but it would still be good to have readable schematics. Image
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Post by Nevilleh » Thu, 15 Sep 2011, 12:42

Johny wrote:
Nevilleh wrote: Yeah, I still have a bunch of un-assembled circuit boards. I never throw anything away! Do you want some?
Can you tell me the physical size. W x L and populated height? From memory they do 8 cells each and I have 16 cells per box so I'd have to house 2 of them in the space Headway allowed.

Weber - did you see in my blog that the VFD output interfered with my BMS in the closest pack. The EMF that these things put out is wild!

I didn't try to make them small! They are 125 x 93 and the shunt resistors stick up to 30mm.
You might be better to go with my new one. I have spare pcbs for that too. I bought 100 and I need 61, plus some spares, say 68 so you could have 32 of those. How many cells do you have? Also, my cell modules are designed to fit 40 AH cells with 64mm terminal spacing. But easy to modify for different cells.

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Post by Nevilleh » Thu, 15 Sep 2011, 13:10

weber wrote:
Nevilleh wrote:The optocouplers are Vishay VO615A - 6 which have a CTR in the range 100 to 300 %.
The resistor driving the emitter is 330R and the load resistor on the output is 2k2. You will note that the wires between boards are on the driver side (the 330R side) which means that the impedance is pretty low and hence there should be very little interference pickup - if any.
You would of course be aware that ours are the same, except we use differential drive and so have more noise immunity in the off state, and we split the resistance into two 150R so both outputs are short circuit limited.
On the cell modules, the Vs is as low as 2.5V which means the diode current could be under 4 mA. Works OK, but.
And on the control unit, the Vs is 5.0V so lots more current available.
No significant difference there either.
In the car, I have a pair of wires running from a cell in the boot to the control unit inside the car and there is no problem with that. All my comms wires are twisted pairs too.

Do you mean the wires between adjacent cell-top units are twisted pair as well, or just the ones to and from the master?

Ours between cell-top units are PCB tracks in most cases and twisted pair if they go more than 10 mm. Our master comms wires are optic fibre.

Thanks for that, but it would still be good to have readable schematics. Image


They are all twisted pairs.
I find it quite surprising that you are experiencing dropped or mangled characters with your setup. It seems unlikely to be interference picked up by radiation though. Unless you have an extremely high level of something well up in the rf region. A thing I used to do when designing switch mode power supplies, a rough-and-ready thing, was to get a portable transistor radio tuned to a blank spot in the broadcast band and wave it around the vicinity. You could tell where the most noise was coming from and also see how effective changes that you made to the layout, etc were in reducing the noise.
I'll email you a copy of the schematics.

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Post by Johny » Thu, 15 Sep 2011, 15:13

Nevilleh wrote:You might be better to go with my new one. I have spare pcbs for that too. I bought 100 and I need 61, plus some spares, say 68 so you could have 32 of those. How many cells do you have? Also, my cell modules are designed to fit 40 AH cells with 64mm terminal spacing. But easy to modify for different cells.
I have 192 cells pairs (384 cells) arranged 32 to a box as 16S2P. That's why I was interested in the grouping of 8 PCBs. It sounds like I would have to do a new board layout anyway. I have looked pretty hard and can not find the details of your older BMS - I know you posted schematics etc. You don't happen to remember where it was posted do you Neville?

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Post by Nevilleh » Thu, 15 Sep 2011, 15:45

Johny wrote: I have 192 cells pairs (384 cells) arranged 32 to a box as 16S2P. That's why I was interested in the grouping of 8 PCBs. It sounds like I would have to do a new board layout anyway. I have looked pretty hard and can not find the details of your older BMS - I know you posted schematics etc. You don't happen to remember where it was posted do you Neville?


Yes, I did, but I can't find them either. I think it was under "Home-grown BMS" or something like that, probably late 2010.
Anyway, if you send me a PM with your email address, I'll send you the files direct.
In some ways it is better than the current one. I implemented CCITT CRC checking with the comms and it has been very reliable.

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Post by woody » Thu, 15 Sep 2011, 16:30

Is this it?

The default search only goes back 6 months, use advanced search and change it to "Ever" :-)
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Post by Johny » Thu, 15 Sep 2011, 16:52

Yes, that's it! Yes I usually change the search to 'Any Date' but I missed have whizzed past it.

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Post by weber » Thu, 15 Sep 2011, 18:29

Johny wrote:Weber - did you see in my blog that the VFD output interfered with my BMS in the closest pack. The EMF that these things put out is wild!

Yes I did. Thanks. But unfortunately we can't deal with ours in a similar way, as _all_ our comms are right near conductors carrying the traction current since our boards are on top of the cells with the copper links beside them. Maybe we need to layout the comms tracks on the boards more carefully so they form a twisted pair. But maybe we only need to increase the opto currents and add some filtering with a cap at the micro's RX input. We'll get back to that after the Brisbane AEVA AGM and show.

Neville, I think those bypass resistors are running too hot by a factor of about 2. You said 100°C at ambient of 25°C, so 75 K rise. I assume that is at 3.6 V. Don't forget that, as Coulomb alluded to, at some time in their career, when charge current is not cut back below bypass current when it should be, they may find themselves operating at 4.1 volts or indeed any voltage up to that at which the TVS and/or fuse will blow. Dissipation at 4.1 V is 30% higher than at 3.6 V, so that would be about 100 K rise above ambient. And after a drive, ambient at the top of the cells in a closed battery box could easily be 50°C. Sure the resistors may be rated to handle it, but they or their solder joints may not last very long and such temperatures won't be any good for the micro or voltage reference or any wire insulation in the vicinity.

James rightly suggested more copper, and I'd suggest spacing them apart more too, but it sounds like you're committed to those boards, in which case I'd reduce the bypass current to about half what you have now, by using higher value resistors.

Also, I'm not sure an open "fuse" (thin PCB track) is a good idea in an environment that could contain an explosive gas mixture. And are you sure the TVS can actually blow your fuse? The TVS might go open circuit first. Have you tested it, e.g. on a board with nothing but TVS, and applied a few cells in series (not a current-limited lab supply) to it?
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Post by Johny » Thu, 15 Sep 2011, 18:35

weber wrote:Thanks. But unfortunately we can't deal with ours in a similar way, as _all_ our comms are right near conductors carrying the traction current......
I'm not referring to battery cables - the main motor cables were the culprit. Any closer than about 150mm and they caused trouble.

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Post by weber » Thu, 15 Sep 2011, 18:42

Johny wrote:I'm not referring to battery cables - the main motor cables were the culprit. Any closer than about 150mm and they caused trouble.

Oh right! Sorry. That's certainly not our problem at the moment. Our small test pack is at the rear of the car, well away from the motor and controller.
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Post by Nevilleh » Fri, 16 Sep 2011, 17:14

Yes, the bypass resistors could get too hot if they were to be turned on at those voltages. When I get time, I will test them up to the point where the solder melts just to see.
My intention though, is to use them for bottom balancing where I bring all the cells down to the same low level as I am firmly convinced that this gives a much better safety margin than top balancing.
That means they will be turned on at around 2.5 - 3 v so the dissipation will be 70%- or more - less than at 3.6v. However, the next iteration I will give them a bit more heat sinking and space.
I have the control unit pretty much done now. Need to add a cell balancing feature and that's about it. Still need to do a serial boat-loader program for the cell modules and then I'll feel confident about installing all 45 in the car.
I must say my oven soldering has worked a treat. I stuck slightly more solder paste on and was also very careful to position the parts accurately and the resistor 'movement' almost disappeared with only a couple happening in the last 18 boards.


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Post by Nevilleh » Sat, 17 Sep 2011, 15:26

Just some further thoughts on balancing, and I'd be glad to hear anyone's opinions on the subject too.
I quite like the analogy of a big bottle being filled through a small spout so the level goes up very quickly as it approaches the top. Probably could say the same for discharging, which all means that only a small amount of energy can shift the voltage quite a lot at both "full" and "empty". I guess the whole point of balancing is to ensure that all the cells will hold the same amount of energy so that when you discharge them, they all reach empty at the same time. So you can either start at empty and fill them up by the same amount or fill them all to capacity and stop discharging when the first one reaches empty. One way you are putting in only the amount of energy that the weakest one can hold, the other you are taking out only the amount that the weakest one can hold.
I guess the open-circuit voltage is pretty much the only measure we have of the state of charge, unless you were to exactly measure the capacity of each cell and then monitor the energy flows precisely. That sounds a bit hard, and measuring the voltage is much easier. And probably a fairly accurate measure of the state of charge at the full and empty ends of the spectrum.
Has anyone destroyed any Li cells yet? - running in a car, that is. And if so, what conclusions were reached?
At the present, my bms controller is set to bring all the cells down to 2.8V (programmable)when balancing is initiated, but I won't really know how "good" that is until I get it all installed in the car.
weber and coulomb will be interested to know that the shunt resistors rise approx 70 deg C with the cell voltage at 3.28.
Next board iteration will have increased heat sink capability!

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Post by coulomb » Sat, 17 Sep 2011, 16:27

Nevilleh wrote: ... So you can either start at empty and fill them up by the same amount or fill them all to capacity and stop discharging when the first one reaches empty. One way you are putting in only the amount of energy that the weakest one can hold, the other you are taking out only the amount that the weakest one can hold.
Right, nicely summed up. But the symmetry breaks down with the convenience of top or bottom balancing. For bottom balancing, you have to get to a low state of charge to do it; if you happen to be at about 40% SOC, say, then you either have to wait till it gets closer to 20%, and risk running out of energy, or waste a lot of energy, or postpone balancing until you happen to end up closer to 20% SOC (or wherever you choose to do the bottom balancing). By contrast, with top balancing, no matter what the SOC, you can charge to the top and balance every charge, and always have a full pack, or a 95% or 90% full pack if you choose to charge to lower than maximum voltage. You can even choose between a "short charge" to 90%, if you don't expect to need maximum range, or 100% if you expect to need maximum range, without planning in any way other than setting the switch on your charger or BMS master.
I guess the open-circuit voltage is pretty much the only measure we have of the state of charge, unless you were to exactly measure the capacity of each cell and then monitor the energy flows precisely. That sounds a bit hard, and measuring the voltage is much easier.
True, and I also worry about drift of the measured cell capacities, and the effects of temperature, how "hard" you drive, and other factors. Voltage seems more "foolproof" to me: regardless of what the actual capacity of the cell is right now, when it gets towards the low voltage knee, you can detect it with the cell voltage. If you assume a certain capacity, you might get caught out if you just count amp-hours in and out.
And probably a fairly accurate measure of the state of charge at the full and empty ends of the spectrum.
Right - so it's good for protecting the cells. The thing that voltage is not good for is predicting the range to empty; for that we have little choice other than to count amp-hours and hope that the capacity is about what you measured last time, or what its nominal capacity is.
Has anyone destroyed any Li cells yet? - running in a car, that is. And if so, what conclusions were reached?
Don Saxby (mcudogs) has had to replace 24 of his 45 (?) Sky Energy cells. I think his conclusion was "don't use the dodgy Chinese BMS that I used". It was a bizarre design whereby only eight of the cells could be in bypass at once. Apparently, this works OK as long as the cells are reasonably balanced to start with, but somehow his weren't.

But don't trust my poor memory, send Don a PIM to get the information first hand.

I've also seen a DIYelectriccar post from someone who had some problem with a few (five?) of his cells. He chose to bottom balance and not use a BMS, and can't get his cells replaced because the terms of use are that a suitable BMS has to be used.

Ah, here it is: 5 defective CALB cells

I'm sure there are more if you look hard enough. Lithiums are now common enough, and the idea of not needing to use a BMS is widespread enough, that there must be plenty of ruined packs out there to learn lessons from. There are also a few fires documented; one of them at a school in the US promised details after an investigation was completed, but I never saw the results published (I could easily have missed it).
weber and coulomb will be interested to know that the shunt resistors rise approx 70 deg C with the cell voltage at 3.28.

Yes, I thought it might be of that magnitude. And of course hotter again at 3.6 - 3.65 V, which is where we want to use our bypass resistors. At an ambient temperature greater than 20 degrees Celsius, that's hotter than the 90 degrees that typical wire can handle. I would not want bypass resistors melting the insulation of pack wiring.

The other issue is that the heat is right there on the PCB (even more so if you use vias and copper on the other side), which is close to the cells. Cells hate heat (apart from performing better in extreme cold once warmed up, but that happens automatically as they are used). Hence we prefer our ceramic resistors, even though they are very much larger and a little more expensive. We space them about 2mm above the PCB, so that air can circulate under them, cooling them, and keeping the heat away from the PCB and the cells. Of course, the heat may largely not escape the battery box, so it remains to be seen whether our approach is really better than SMD bypass resistors.

[ Edit: formatting; responded to 70C resistor temperature rise ]
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Post by Nevilleh » Sat, 17 Sep 2011, 19:21

All good and valid comments. I'm quite happy to "bottom" balance though, as it is not difficult to arrange - ie I know when my battery is going to need charging and it should not take all that long to get the cells balanced at that low state of charge. Its not necessary every time and it wastes less energy.
As for the heat being close to the cells, there is only 1.9 watts even at 3.6V and very little of that is transferred to the cell even with my board bolted directly to the top, so I don't believe you would see a significant change in the cell temperature due to the balancing "act". As for melting the insulation of "pack" (I assume you mean battery) wiring, the only such wiring I have is the inter-module comms pairs and they are a long way from the resistors.
So I now have an bms that monitors both cell voltages and temperatures, give the total battery voltage and allows me to turn shunts on and off to get the cells balanced. I also have an instrument that measures and displays motor current, battery current, battery voltage (by direct measurement), amp-hours used and range to empty. What else do I need?!!!

I'll have a look at those links you posted, always interested to see how other people have damaged their stuff.

I wonder what Mitsubishi (and others) are doing in their EVs. I saw Jeremy Clarkson driving a couple on Top Gear recently and they - of course - ran out of energy before reaching their destination, so they then laboured the point of being unable to charge the things.
Its going to be a long time before the electric car replaces the ic engined one as I think to myself every time I load my ute up with gear and do a 1000 km round trip with only one 5 minute stop for fuel. But then I drive my BMW into town and back and think "horses for courses"!

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Post by weber » Sat, 17 Sep 2011, 21:01

Nevilleh wrote:I'm quite happy to "bottom" balance though, as it is not difficult to arrange - ie I know when my battery is going to need charging and it should not take all that long to get the cells balanced at that low state of charge. Its not necessary every time and it wastes less energy.

I find myself disagreeing with almost everything above. Of course I can't dispute that you're quite happy Image , and I agree that neither top nor bottom balancing is necessary every time.

But I don't see how bottom balancing can waste less energy than top balancing. The only way bottom balancing can avoid wasting more energy than top balancing is if you wait until the first cell hits bottom during the normal course of driving. This is unlikely to happen just as you are turning into your driveway, and when it happens you shouldn't drive any further. Fairly inconvenient I should think, even if you can predict it.

So if you're to avoid that inconvenience, you will arrive home with significant charge still in the lowest cell, which must then be wasted to achieve bottom balancing. Even if it's only 10% full, a 120 Ah battery with a 0.5 A discharge will take 24 hours to get that cell to hit bottom before you can start balancing. And all this has to happen before you can start charging, so you will not have the use of the car in that time. Another inconvenience.

I think we all feel the attraction of the fail-safe nature of a bottom-balanced battery, as pointed out by Jack Rickard, but are not willing to live with its inconvenience or waste of energy when we can achieve similar protection using feedback to the motor controller based on monitoring individual cell voltages.
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Post by coulomb » Sat, 17 Sep 2011, 22:03

weber wrote: But I don't see how bottom balancing can waste less energy than top balancing.

It does, in this sense: when you get to actually do the balancing (having wasted a lot of energy to get there, if bottom balancing), you balance at a lower voltage than when top balancing. Suppose you bottom balance at 2.6 V (as I have seen advocated today; others mention 2.8 V) compared with 3.6 V for top balancing. The bypass resistors will waste 2.6/3.6 = 72% of the energy that top balancing will (assuming that the balancing process takes a comparable number of amp-hours, which seems reasonable).

However, wasting 28% more energy at the top when bypassing half to one amp (half to one watts, or half to one amp-hour per hour) would seem to me to be dwarfed by the energy expended in getting a pack down to 2.6 VPC. Suppose you have a 20 kWh pack (ours is closer to 30) and need to blow away 10% of the energy (you could easily have to blow away up to say 50%, depending on what the SOC happened to be at the time you were ready to do bottom balancing. If your pack was over 50% full, presumably you'd just postpone bottom balancing, even if the time was right). That's 10% x 20 kWh = 2 kWh, or 2000 Wh. Suppose it takes half an hour to balance, and you balance at one amp, and almost every cell needed the full amp of balancing (worst case). That's less than half an amp-hour; less than 1.8 Wh at 3.6 V. 28% of that is nothing compared to the 2000 Wh you need to blow away to start bottom balancing.

Maybe I'm missing something here.

You need to prepare a pack for top balancing too, but that's just normal charging, which you will do anyway after bottom balancing. Granted, bottom balancers won't charge their packs as full as top balancers will. There might be 5% energy difference between a bottom balancer stopping at 3.85 VPC and a top balancer who waits for all their cells to get to say 3.65 VPC. 5% of 20 kWh is still 1 kWh, and it usually gets done every time a top balancer charges his pack. Maybe this is where the claimed energy saving is achieved. If a bottom balancer only balances once every 10 or 100 charges, then even blowing away 30% of the pack's energy every bottom balance will average out to less than the 5% extra energy that a top balancer uses extra.

But then it comes down to how to know when to bottom balance. Is one in 100 charges enough? Is 1 in 10 enough? I guess the BMS should be able to tell you. But it could happen that the BMS tells you it's time to bottom balance when it really isn't convenient; Murphy's law says it's a certainty. It seems to me that a bottom balancing takes a lot more owner interaction than a completely automatic top balancing.

[ Edit: But as Weber pointed out above, you don't have to top balance every charge either. Many top balancers would, I think, because it's effortless, unless they know they don't need much range next trip, and they want to prolong the life of their packs. But they don't have to, so to save energy they could top balance as infrequently as bottom balancers. The difference is that if the BMS says it's time to top balance next charge, well it's no effort, and in fact, the BMS can just do it without the owner even knowing. From that point of view, we're back to bottom balancing wasting a lot more energy. ]
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Post by weber » Sat, 17 Sep 2011, 23:09

coulomb wrote:
weber wrote: But I don't see how bottom balancing can waste less energy than top balancing.

It does, in this sense: when you get to actually do the balancing (having wasted a lot of energy to get there, if bottom balancing), you balance at a lower voltage than when top balancing.
But surely it's charging-source energy we're worried about wasting. Those amp-hours all went in at around 3.3 V. It seems irrelevant to this question what voltage they come out at.
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Post by T1 Terry » Sun, 18 Sep 2011, 00:31

I top balance mine is a sense but don't use any method of power wasting. The difference between 3.85v and 3.4v in a cell doesn't amount to very much energy storage so as long as I limit my discharge to 80% DoD and switch to float charging when a cell reaches 3.85v, the float charge voltage being the sum of all the cells at 3.45v eventually the cells all come together at the top.
I tried to record this happening with a cell logger on a 360ah pack but I didn't catch the logger in time and it over wrote all the previous data with all the cells in balance. I will do a daily log with the next pack to demonstrate the effect. Here is a log graph showing the amount of additional energy stored above 3.4v
Still can't figure out how to load pictures that aren't huge, when I reduce the size the text can't be read. I will try again later

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Post by weber » Sun, 18 Sep 2011, 23:22

T1 Terry wrote: I top balance mine is a sense but don't use any method of power wasting. The difference between 3.85v and 3.4v in a cell doesn't amount to very much energy storage
Hi Terry, I think we all agree with that (assuming the charge rate we're talking about here is something like 0.1C to 0.2C).
so as long as I limit my discharge to 80% DoD and switch to float charging when a cell reaches 3.85v, the float charge voltage being the sum of all the cells at 3.45v eventually the cells all come together at the top.
This is the part I'm having trouble with. Everything I've ever read on Li-ion battery charging says that if you keep charging a cell after it is full you damage it. Other battery chemistries have relatively harmless secondary reactions, but not Li ion. Once the graphite anode has taken all the interstitial Li ions it can, Li metal starts to plate out onto the anode. And once the LiFePO4 cathode has lost all the Li ions it can it starts to react with the electrolyte solvent (or something).

So it seems to me either:
(a) it just _looks_ like it is balancing the SoC of cells, but it is really just operating so low down on the curve (3.45 V) that voltage is a poor reflection of SoC, or
(b) they really are balancing, but only at the expense of a little damage to the higher cells each time, or
(c) some combination of (a) and (b).

See http://batteryuniversity.com/learn/arti ... _batteries
although the voltages they give appear to be for Manganese or Cobalt oxide type cells. I think they need to be lowered by about 0.4 V to 0.5 V for LiFePO4.

[Edit: Corrected "LiFePO4 anode" -> "graphite anode" and "graphite cathode" -> "LiFePO4 cathode"]
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Post by Nevilleh » Tue, 20 Sep 2011, 01:25

Hhmm! I think one of the things that is apparent here is the fact that you guys aren't actually driving a car.

I find that I discharge my car battery down to 100% now and again, just because I don't really know exactly how far I can go in spite of all my monitoring.

Balancing does not seem to be something that needs to be done very often as they stay pretty close ie they don't seem to change much in actual use, if at all. At least that's what monitoring only 16 cells out of 45 told me.

An 80% dod is all very well, but if you are only monitoring voltage it is pretty hard to tell. I haven't had my AHr meter in use long enough to give me more than an idea, but still have - a few times - run the thing down to only 2.5V per cell and that really has to be close to 100% discharge. What else do you do when the voltmeter says 3v per cell and you still have 5 kms to go to get home? That last bit really drops the voltage quite alarmingly!

Anyway, I've done over 2500kms so far without any problems and without any balancing and I've kept (tried to keep) the voltages to a max of 3.6 per cell (162V) when charging and 2.5 per cell (112.5V) when discharging. That's 28 charges so far. I also found that my charger won't start up if I allow the battery to drop below 120V!

The bms will give me more info on the actual spread of cell voltages and I will feel a lot more knowledgeable then. Maybe even as knowledgeable some of the people on these Internet forums!

Last edited by Nevilleh on Mon, 19 Sep 2011, 15:28, edited 1 time in total.

T1 Terry
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Post by T1 Terry » Tue, 20 Sep 2011, 12:55

weber wrote:
T1 Terry wrote: I top balance mine is a sense but don't use any method of power wasting. The difference between 3.85v and 3.4v in a cell doesn't amount to very much energy storage
Hi Terry, I think we all agree with that (assuming the charge rate we're talking about here is something like 0.1C to 0.2C).
so as long as I limit my discharge to 80% DoD and switch to float charging when a cell reaches 3.85v, the float charge voltage being the sum of all the cells at 3.45v eventually the cells all come together at the top.
This is the part I'm having trouble with. Everything I've ever read on Li-ion battery charging says that if you keep charging a cell after it is full you damage it. Other battery chemistries have relatively harmless secondary reactions, but not Li ion. Once the LiFePO4 anode has taken all the interstitial Li ions it can, Li metal starts to plate out onto the anode. And once the graphite cathode has lost all the Li ions it can it starts to react with the electrolyte solvent (or something).

So it seems to me either:
(a) it just _looks_ like it is balancing the SoC of cells, but it is really just operating so low down on the curve (3.45 V) that voltage is a poor reflection of SoC, or
(b) they really are balancing, but only at the expense of a little damage to the higher cells each time, or
(c) some combination of (a) and (b).

See http://batteryuniversity.com/learn/arti ... _batteries
although the voltages they give appear to be for Manganese or Cobalt oxide type cells. I think they need to be lowered by about 0.4 V to 0.5 V for LiFePO4.

From what I remember of the Jay Whitacre LiFeP04 over view 60degC and 4.3v were the danger numbers. 140degC is where the electrolyte boils and a "rapid venting" potential occurs but 60degC was the real critical to prevent lithium carbonate coating of the anode and between 1v and 2v min. to stop lithium coating of the cathode (think I got those the right way around)
One of the very interesting bits he mentioned was the conditioning phase for a new cell and that creating an initial light coating on the anode help prevent further degredation during service life because this first light coating resists future coating. From what I could gather this is a A123 proprietory trick. An amazing amount of information in that guys head regarding these cells but it comes out so fast it hard to absorb it all.
I accidently took a 16 cell pack up to 4.35v per cell and the intial 4v per cell first charge conditioning really seems to have paid dividends, all the cells stayed within 25mv of each other even at that extreme. Further testing since shows no signs of any loss of capacity or discharge rate although I'm limited to around 1.5C discharge testing at the moment (need a bigger inverter)

T1 Terry
Last edited by T1 Terry on Tue, 20 Sep 2011, 02:57, edited 1 time in total.
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Post by Nevilleh » Mon, 26 Sep 2011, 13:29

Well, I must say that there are as many opinions on balancing as there are people on these forums! All to the good, I think. I well remember Jack Rickard getting really upset when people didn't regard his revelations as words from on high!
Anyway, my bms is about done - "all" I have to do now is build another 25 cell modules, install it in the car and see if it all still works!
Here's a picture of my 4-cell test set-up on the bench:

Image

You can see my awful twisted pair connections.

Here's some shots of the available displays:


Image
Image
Image

The "No Comms!" one actually has the led flashing, a bit hard to see in a still shot!
The cell voltages are stored to 3 decimal places, but only displayed to 2 and the resolution is about 4mV.
The Vt means total battery volts and the h and l are the cell numbers of the highest and lowest voltages. A bit squashed as my 2 x 16 LCD is pushed for space, but adequate. Who needs the extra cost of a 20 char LCD? Note that my cell numbering starts at 0 so in the 4-cell module above, cell 3 is the last one. I deliberately ran it down a bit so its voltage would be definitely lower than the others just to test everything.
You can also see the thermistors which I have decided to just push down into the corners of the cells.
The TO92 devices are my LM385s converted in smds by cutting the legs off short and bending them over. Only 2 legs are connected, so its not as onerous as it might be. If I wasn't so mean, I'd have put them in the drawer and waited for proper smd versions.
The pushbutton scrolls the display and there is a switch to initiate balancing. At present it is set up to do bottom balancing at 2.8V. This could easily be changed to top balancing at 3.6V, but I am going with this for now. The process is initiated by closing the switch. The way I envisage it working is that when you get home and the battery is right down, say at least one cell below 3.0v, then you close the balancing switch and walk away. It turns on the shunt for every cell above 2.8V and then turns them off one by one as the voltages come down to 2.8V. The display shows "Balancing..." while this is happening (I didn't take a photo of that). Should comms fail, all shunts are turned off automatically.
Once its finished, the charger can start up. Not automatic at the moment, but another line of code would do it. I'll see how it goes before deciding what's best. I think I would have to increase the heat-sinking of the shunts to be truly happy with top balancing at 3.6V. Never mind, next iteration will do this so either option is readily available.
Can't add too many more features as the ROM space in the controller is 90% full now. Amazing how lcd display calls burn up space even though I abandonded "printf" and wrote my own routines pared down as much as I could. Maybe I need the advice of an expert programmer!
Last edited by Nevilleh on Mon, 26 Sep 2011, 03:40, edited 1 time in total.

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Post by evric » Mon, 26 Sep 2011, 13:45

Hi Neville, Looking very nice... You haven't shown any connecting links between the cells. Is there enough clearance around components or do you have to stand these boards off with spacers/nuts ?
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Post by Nevilleh » Mon, 26 Sep 2011, 14:07

There is enough clearance provided you put a flat washer under the board - I used the ones supplied by TS. I just didn't bother with the Cu strap links on my test setup. I didn't allow for the "hump" in the strap in the pcb design and the boards ended up a smidgeon too wide to just go straight on. But they fit OK with the washer and I think that is not a bad thing as it ensures even pressure on the pcb material.
I've also just been looking (again!) at the discharge curves for the 40 AH cells and it looks like it takes just 1 AH of capacity to drop from 2.8V to 2.5V while it takes just over 2 to go from 3v to 2.8v. This is at .1C discharge. The shape of the curves is similar at .3C and 1C, just shifted a bit to the left, so I expect that at 1/2A or .01C it'll be the same, which means that balancing by taking all cells from 3v down to 2.8v wastes a lot of energy. Say 2 AH on a 40 and I have 3 in parallel so 6 Ah per cell times 45 cells at average of 2.9V is 783 watt-hours. Conversely, to drop from 3.6v to 3.3v looks like about 1/2 AH per cell, so it is going to be a lot less wasteful to do top balancing. ie about 233 watt-hours.

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Post by Nevilleh » Thu, 06 Oct 2011, 17:29

It is really quite a big job to design and build a bms! I'm glad I have only 45 cells to worry about as it has taken much longer than I expected to build and test 45 cell modules and the control unit, but that job is now done and I have installed 15 units in my car, one battery box out of three.
I bought a whole lot of IDC connectors for my comms cables thinking that would be less work than the usual crimp terminals, but they have turned out to be unreliable so I am going over all of them and soldering them as well. This actually means more work than if I'd used the crimp terminals! We live and learn.
Only 30 more cells to go!
Here's a picture of the first battery box:

Image

The boards are bolted to terminals with only a single copper strap connection, that's why the apparent random arrangement. My bolts were too short to allow them to be fitted to terminals with more than one strap.
The Al box at the bottom centre is a Meanwell charger for the 12v system and the little black box to its left is the driver for the power steering pump.
This is the control unit showing the highest and lowest cells with the total voltage for these 15 cells, plus the cell numbers of the highest (#11)and lowest (#0. My cell numbering starts at 0.)


Image
Last edited by Nevilleh on Thu, 06 Oct 2011, 07:38, edited 1 time in total.

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