Nevilleh's BMW

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Post by coulomb » Fri, 16 Apr 2010, 16:51

Johny wrote: What did you do about the end caps?

Yes, I hope you used the correct engineering material, which of course is kitchen cutting boards, available in at least two convenient thicknesses, available from your local supermarket. Image

Edit: added availability
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Post by Nevilleh » Fri, 16 Apr 2010, 17:17

I had a sheet of 6 mm black PVC from which I cut a couple of rectangles slightly over size (the thing is not square, of course) and then filed them to fit. Had enough bits of the end with the terminals to use as a template to cut slots for said terminals. Then glued them in place with a bit of silicone sealer. I wouldn't immerse the thing in water, but its dust and splash proof.
Unfortunately have relatives staying at present (school holidays) so I can't disappear and work on the thing. But they go home tomorrow, so after that will be my chance to tune it up.

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Post by Johny » Fri, 16 Apr 2010, 17:41

I hope the relatives don't know about this forum. Hmm, I wonder if I can use this knowledge into blackmailing you. One BMS 8 cell slave unit per month for 2 years should do it. Then, just when you thought you had got rid of the skeleton in your closet - I demand the MASTER unit or I'll give them the URL. Image
Didn't I see this plot on 24?
(Internet police - this is a JOKE!!!)

Anyway. OK on the black PVC. I'm always on the lookout for cunning gluing techniques but silicon and me are old buddies...

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Post by Nevilleh » Fri, 16 Apr 2010, 18:03

I hope they don't too! They've just gone down to the beach which lets me play on the computer for a while.
Back to the BMS also in a day or two. I really need to get some more slave boards built - and with properly made, plated thru boards too. The master has most of the functionality needed, just some more s/w hassles to finish off and I need to do a new board layout for it as I am using the control unit from one of Protech's battery chargers hacked about a bit. I can shrink it quite a bit if I only put in what I need.

226 cells is a lot!

That's "Silicone" not silicon! A very different product and as you know, it sort of sticks to anything. My end caps are a fairly tight fit - in places - and the Silicone mostly just provides a seal. I am hoping it will be easily removed should I have to look inside again. Although I have now made the bottom plate removable, so access is fairly easy.

This LS controller uses 600V IGBTs and diodes with 250V capacitors, so it could work off 200V no trouble - even a smidge more. The controller board gets its power from a primary regulator which is (now) A 200v bipolar configured as an emitter follower with a 26V zener off its base. This feeds a 15V regulator which provides the gate drive and a 12V regulator which feeds the other electronics. This means that the lowest voltage component is that primary regulator which has the supply voltage minus 26V across it. Still can stand 200V easily. The point of all this is that my 45 cell SE pack is charged at 162V and the controller will handle this with a considerable margin. Much better than the Curtis unit.
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Post by Johny » Fri, 16 Apr 2010, 18:45

Sorry, I type the one without the 'e' a lot and it kind of slips onto the keyboard.
I have been cogitating over IGBTs v/s FETs and I think that the saturation voltage of IGBTs is a lot more closely defined than the ON resistance of power FETs. Parallel IGBTs is way more reliable IMO.

Not 226, 192 cells pairs. 384 cells. 12 of 48V20AH.

The Headway BMS is OK but yours will be better.

Some comments about BMSs and LiFeP04 balancing etc.

For the past couple of weeks (minus one where I took the week off), I have been charging, note-taking and stuffing around with the 12 x 48V20AG Headway packs.

First thing is that the charger can't really fully balance the packs because the Headway BMS turns it off when a cell gets over 3.9V.
So I have been using 22, 10, 5, 2 Ohm resistors to keep the charge rate below 170mA so that the 170mA balancing (20 Ohm BMS resistors) can do it's job. They turn on at 3.7V. I don't like the cells going to 3.9V regularly for a couple of days, the resistors keep all cells to 3.7V or lower.

I got sick of visiting the pack every hour to check so I built a < 0.1 volt drop, 170mA current source to balance the packs. It goes in series with the charger and works a treat.

This comment is just to note that having a charge current that just equals the bypass current is the optimum way to go for the balance phase. But I bet that everyone but me knew that.


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Post by Nevilleh » Fri, 16 Apr 2010, 19:57

You are right, to some extent. But FETs have a positive temp coeff so if one gets hotter, it's on resistance rises so it takes less current which provides a pretty good self-balancing capability. IGBTs on the other hand, may have a +,- or 0 temp coeff which means they may not current share in the same way. That temp coeff may even vary in the same batch, so they tell me. The smaller current ones tend to have a + temp coeff and they are more uniform which seems to be why its better to use 20 x 50A ones than 10 x 100A ones. I'm only going on what I've read in a few App Notes over the last week or two as I have no experience in using the things at all. Tritium-James appears to have been working on that ac controller they have just released and would, I expect, know a fair bit about this. But FETs are really limited to a few hundred volts whereas IGBTs go to, what, 1000 or more? so the choice is fairly clearcut in that way. It's the grey area where the overlap that is hard.
My measurements of the gate drive on the LS controller indicate to me that the trace length is not so critical with relatively slow switching times generated by somewhat large gate resistors and these slow times mean that the devices are turning on slow enough so that one will not "hog" the current before others start to share the load.
It certainly is something I'd like to do a lot more study on.

192 cells is still a lot! 24 slave boards.

My original idea was to use 3R3 balancing resistors which give a current of 3.6/3.3 = 1.09A for SE cells and to have the charger switch down to that current when one cell got to 3.6V while the master turns on the shunt for that cell. That means that 1.09 amps continues to flow through the battery, but not through any cell that has reached 3.6V. Because I "know the man" who is designing the charger, I can get him to make that lower current limit programmable to suit whatever the BMS is doing. 1A or so balancing current seemed to me to be a reasonable figure. But after reading a bit more about balancing etc, I decided that the heat generated was going to be a bit excessive so I decided to increase the balancing resistors to 18R and drop the charger current down to .2A. It seems that this is still a quite adequate charging current to do the final balancing act and it should be if the cells are fairly close anyway. It wouldn't be much good if you had 100 AHr cells with one 100% charged and one 100% discharged!
The beauty of this scheme is that you can really make the balancing resistors as large or small as you like within your capacity to keep them cool and you can program the reduced charger current to match.
In your case, with 3.7V per cell and 18R shunts, the charger would be set to .206A. Or you could use 3R3 resistors, program in 1.21A and stick a fan on each slave board to keep them all cool.
The charger can deliver up to 5400 watts if you have 3-phase available, but it is limited to 180V which means it could deliver as much as 30A. In my case with 162V needed and only single phase power, it will be set at around 20A.
If you were to use a different charger, you could build a programmable current limiter and control that with the BMS output that says "reduce to lower current".

All this is just a long-winded way to say "yes, everyone but you DID know that"
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I have further considered that if one were to use two FET switches for each cell, one in series and one in parallel, you could turn off the series one and turn on the parallel one simultaneously. The parallel one would then carry the whole charge current bypassing the cell and series switch and the power dissipation would only be i x i x Ron so the balancing would be done at the full charger current and therefore very much faster. You'd want FETs that can carry 20 A or so with an Ron down in the milliohms region, but that should be able to be a lot less power wastage than the resistors. More expensive of course.

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Post by antiscab » Fri, 16 Apr 2010, 22:22

Johny wrote:
This comment is just to note that having a charge current that just equals the bypass current is the optimum way to go for the balance phase. But I bet that everyone but me knew that.


I used to do that, until one of my cells leaked after extended charging (3.7v never exceeded).

turns out you can overcharge without voltage going high.

my now prefered method is to bulk charge till BMS cuts out (the way headway do it), followed by going around with a ~5v current limited power supply and using my BMS to interrupt mains current (when that cell goes over).

that first balance is always a PITA.

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Post by Johny » Sat, 17 Apr 2010, 02:08

Thanks Matt - I wasn't aware that you could overcharge by holding at 3.7V. I'm only doing it this once, for about 12 hours.
The Headway doesn't just cut out and that's it - it cuts out when any cell goes over 3.9V until a the cell goes under about 3.7V then cuts in again until a cell goes over 3.9V - repeat forever.

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Post by antiscab » Sat, 17 Apr 2010, 03:11

im not entirely certain why that cell leaked, but the slow trickle charge was the only way it had been treated differently.

the pack was getting a constant 0.5A charge.
that results in an equilibrium at 3.8v with the V5 evworks BMS
that cell reached full charge first.

the balancing was going on for a couple of days.

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Post by Nevilleh » Sat, 17 Apr 2010, 13:22

I am a bit inclined to do the charging at max current until one cell reaches the voltage limit and then shut the charger off entirely and do the balancing by shunting the cells and bringing them all down to the lowest voltage one, then turn the charger on again and repeat the process until they all are pretty much at the max voltage together.
This sort of balancing I would only do every now and again, but especially the first time I am using the battery. Its speculative, I know, but it should prevent any cell being overcharged and get them all to 100% SOC eventually.
Then I wonder about whether it is even necessary and maybe just the first cycle would be enough?
If the lowest cell is at say 95% SOC when the highest one hits the voltage limit, then my BMS would bring them all down to about that level, wouldn't it? Is the voltage a good enough indication of SOC or only when they reach max volts is it good enough?
Its something that is going to need a bit of study once I get this car running reliably and can use it every day.

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Post by coulomb » Sat, 17 Apr 2010, 16:20

Nevilleh wrote: I am a bit inclined to do the charging at max current until one cell reaches the voltage limit and then shut the charger off entirely and do the balancing by shunting the cells and bringing them all down to the lowest voltage one, then turn the charger on again and repeat the process until they all are pretty much at the max voltage together.

That's an interesting idea. I'd say it's impractical for Sky Energy/CALB cells, since they don't seem to have a safe maximum charge voltage that is significantly higher than the 3.6 V that you would want to charge to.

With Thunder Sky cells, if you are willing to go to a higher voltage like 3.8 VPC, you could use that as the high limit and try the above.

The problem could be that the lowest voltage cell might be under the knee of the charge curve, meaning that it's next to impossible to match SOC with other cells. However, at least getting all cells under the knee might be enough of a rough balance to attempt another pass.

The other thing is that this is actually discharging some of the cells some, adding tiny fractional cycles to their lifetime totals. It's the cells that reach higher voltage first that you will do this to, so these will be the ones with the lowest capacity, which can't afford any extra cycling. The effect would be tiny, but it would lead me to look for other solutions.
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Post by Nevilleh » Sat, 17 Apr 2010, 18:12

"That's an interesting idea. I'd say it's impractical for Sky Energy/CALB cells, since they don't seem to have a safe maximum charge voltage that is significantly higher than the 3.6 V that you would want to charge to"

I don't see why you say that. The max voltage of 3.6V IS 100% soc and you don't want to go higher than that. If you look at the charge curve, when the cell hits that voltage, that's it, its done. And if the lowest cell is 3.4V or above, it is within at most 10% of max.
The BMS will tell me if a cell is still below say 3.5V and then I would not use this technique but use the shunt bypass/keep charging method.

"The other thing is that this is actually discharging some of the cells some, adding tiny fractional cycles to their lifetime totals. It's the cells that reach higher voltage first that you will do this to, so these will be the ones with the lowest capacity, which can't afford any extra cycling. The effect would be tiny, but it would lead me to look for other solutions."

Yeah, but a difference that makes no difference IS no difference, as they say. The lowest capacity cell sets the bar for the others.
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Post by antiscab » Sun, 18 Apr 2010, 00:50

Nevilleh wrote: I am a bit inclined to do the charging at max current until one cell reaches the voltage limit and then shut the charger off entirely and do the balancing by shunting the cells and bringing them all down to the lowest voltage one, then turn the charger on again and repeat the process until they all are pretty much at the max voltage together.
that works, however, if your BMS trips out (cell over voltage before current falls to 0.005C A at ~3.6v pack average) during normal use, somethings wrong.

Unless the car has been sitting around for some time, of the "always on" current of your BMS modules has significant variation, i'd do a pack capacity test.

Even with a brand new pack, unless I need the full capacity right away, I just let the per-charge balance sort it out (shunting at 3.6v, and cut-off at 4v gives brings the pack closer by around ~0.02C Ah per charge, YMMV)

I can't see any reason why your way (Neville)/ headways way won't work or cause problems.
My old Yesa batteries balanced the same way and don't seem to be any worse from it.

I just prefer my BMS to tell me if a cell got to the limit.
Nevilleh wrote:
I don't see why you say that. The max voltage of 3.6V IS 100% soc and you don't want to go higher than that.


not quite,
at 3.6v, the cell is completely charged when the current is ~0.005C.
or 0.5A for a 100Ah cell.
3.4v at 0.005C is also full.

3.6v with 0.2C charge is more like 95% SOC. almost there.

its just a matter of having the cell voltage above the ~3.3v reaction voltage.
the other limit is another secondary reaction which happens at ~4.3v IIRC (above which bad things happen)

so as long as current has fallen to a low value, and the voltage is within that range, the cell is fully charged.

just don't continue to trickle charge, as that also does bad things (as i found out)

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Post by Nevilleh » Sun, 18 Apr 2010, 12:56

The SE40AHA charge curve (12A CC, 3.60V CV) shows the current falling off extremely rapidly once the cell reaches 3.60V. Its down to 3A inless than 5 minutes. The SOC curve (actually AHrs)flattens right out at that point too, showing no appreciable increase from then on.
Even after another 15 mins at that voltage, the current is still about 1.5A and nowhere does it fall to 200mA.
Where did you get that .005C figure from, Matt? I assume you are describing TS cells, not SE, as you mention going up to 4 volts.

SE40_Charge_curve.jpg is the file I uploaded showing this curve way back - I'm not sure how to refer to it here.
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Post by weber » Sun, 18 Apr 2010, 14:52

Nevilleh wrote:The max voltage of 3.6V IS 100% soc and you don't want to go higher than that. If you look at the charge curve, when the cell hits that voltage, that's it, its done.

Right, but that curve is at a specific current. To give an extreme example underscoring Matt's point, you can take a cell which is only 50% charged and hit it with a 6C charge (as we did at Tritium with the power supply James had on loan) and it will go past 3.6 V instantly, but it's still only 50% charged.

So you have to qualify that by saying "at a sufficiently low charge current 3.6 V is 100% SoC".
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Post by woody » Sun, 18 Apr 2010, 15:09

Which reminds me, did current monitoring make it into anyone's BMSes ? I.E. 2.5V @ 6C discharge is OK, 2.5V @ 0C is bad?
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Post by Tritium_James » Sun, 18 Apr 2010, 15:30

Ours has the hardware for it but the software to do that functionality isn't in there yet...

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Post by Nevilleh » Sun, 18 Apr 2010, 16:08

I am referring to the recommended charging current of .3C, of course. This is what my charging system is designed to do, given that my mains power supply is up to it. It isn't, so I throttle back to 20A which is only .167C. Maybe I'll get 3-phase power one day in which case I will be able to get 33A, or .278C which is as close as I will be able to get, unless I double up on the chargers.

Again, the recommended charging process is CC at .3C with a CV of 3.6V and if you try pushing in 6C then the figures are meaningless.

My BMS master has a Hall effect current sensor connected to it, the Tamura one I have described previously. At present, it just displays the current on the LCD, along with the min and max cell voltages. My s/w is still quite rudimentary as far as processing all this info is concerned, I think that just having it available will do until I work out what should be done with it.

The CPU in the driver's head processes it for now!

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Post by coulomb » Sun, 18 Apr 2010, 16:30

woody wrote: Which reminds me, did current monitoring make it into anyone's BMSes ? I.E. 2.5V @ 6C discharge is OK, 2.5V @ 0C is bad?

I don't think it's practical to have every cell-top board (or even a board that handles 4-8 cells) attempt to measure hundreds of amperes... does yours actually do that TJ?

All cells in a BMS are typically in series (even if some cells are buddy paired or tripled or higher). So the current that each cell-top (or group of cells) board sees is the same. So the master or the controller/charger could send a message to each cell processor indicating the present current (I'll try and avoid jokes about the current current Image ). If load sag becomes an issue, that's how we intend to handle the current measurement with our digital BMS.

It will have to be handled with some care, since the instantaneous current will vary, so the last-known current could be different to the present current, so false alarms (and also false security) could be an issue. Requiring a few alarms in a row before being treated seriously could be one of those, or possibly a small (e.g. 10%) cutback could result from the first alarm, with much more severe cutbacks if the alarm persists. Or maybe there is extra hardware for peak current detection, so the present current packet can contain information like "there was no more than a 10% variation in instantaneous current since the last present current packet", so that way the "badness" can be escalated if appropriate.

This is one of the huge benefits of a digital BMS, IMHO. It means that things can be changed on-the-fly (well, on the next software revision) to adjust behaviour based on observed performance. With an analogue circuit, you're pretty much stuck with the circuit you started with; perhaps you can adjust a few parameters with DIP switches or solder pads or the like.

Of course, that assumes that the digital BMS can be made reliable. That's why we originally went with a simple analogue system; presumably op-amps never crash (they may go crazy with a glitch now and then, but return to normal behaviour immediately after, unless the glitch was a silicon killer). I have a couple of digital BMUs here on the bench, but until it goes into a vehicle with hundreds of amperes of PWM current and all the other electrical noise from a vehicle, we won't know if it is reliable or not.
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Post by Nevilleh » Sun, 18 Apr 2010, 16:55

Surely we are only interested in the battery current, as the cells are all in series.
I measure all the cell voltages and the battery current, in or out. Then I only display the battery voltage, the lowest cell voltage, the highest cell voltage and the battery current. No temp monitoring at present - should that be "currently"?
That has to be enough info, methinks!

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Post by Tritium_James » Sun, 18 Apr 2010, 17:49

Yep, same as what Nevilleh has in his system. Except we used a shunt instead of a hall - much less issues with magnetic hysteresis in the sensor and consequent offset current measurement problems for longer term Ah counting. This problem only really shows up if you have regen in the system and the sensor has negative current through it.

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Post by antiscab » Mon, 19 Apr 2010, 00:58

Nevilleh wrote:
Where did you get that .005C figure from, Matt? I assume you are describing TS cells, not SE, as you mention going up to 4 volts.


oops, added an extra 0.

should be 0.05C (5A or 100Ah, 2A for 40Ah).

that number came from trial, error and generally playing around with cells.

SE can take going to 4v, theres just not alot of use in doing so.

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Post by Nevilleh » Mon, 19 Apr 2010, 12:55

Ah, that makes more sense and agrees with the charge curve from SE.

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Post by Johny » Mon, 19 Apr 2010, 18:16

So, is it possible to charge a LiFePO4 cell too SLOWLY?
What rate is too slow?

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Post by Nevilleh » Mon, 19 Apr 2010, 18:38

I don't think so and I say that because we had an enquiry about replacing PbA in a standby situation. I asked TS and SE about this and they both said it was OK to float the cells at their recommended max voltage ie 3.6 for SE and 4.2 for TS. As long as the voltage was not allowed above this level all would be well! We didn't get the job, so that's all I know. Except that when I was testing a TS cell I left it connected to 4.0V for several days and the current was down to zero. The cell didn't show any signs of unhappiness.

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