Home grown BMS ideas !

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acmotor
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Post by acmotor »

weber wrote:

..... Absence of evidence is not evidence of absence. Image.....


Yep, that's also my thinking on Jack's tale.

Definitely prius is conservative in battery cycle depth but it is a toy battery pack after all and hammered to how many C ??.
However, keep in mind, I was already wondering how I would manage 2000-3000 cycles at 80% DOD (from 99%SOC) on the TS. How long might that take ? 10 years ?

Oh, and talking about absence of evidence. What actual evidence is there for running Lithium at less than 100% SOC ? Maybe this will make the weaker cells fail sooner so you can replace them as they will have been going to 100% SOC. Image
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Post by weber »

acmotor wrote:However, keep in mind, I was already wondering how I would manage 2000-3000 cycles at 80% DOD (from 99%SOC) on the TS. How long might that take ? 10 years ?
Good point. Calendar life is likely to be the determinant, unless they are kept very cool.
Oh, and talking about absence of evidence. What actual evidence is there for running Lithium at less than 100% SOC ?
If 100% SoC was the best SoC for long life, wouldn't they also store them at 100% SoC?

It's possible that there could be two minima of ageing-rate vs SoC, one at around 50% and another at around 100%, with a maximum in between. But the more complex the function, the less likely it seems, to me at least.

[Edit: "if they are kept cool" -> "unless they are kept very cool"]
Last edited by weber on Thu, 26 Nov 2009, 16:05, edited 1 time in total.
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Post by acmotor »

weber wrote: ............
If 100% SoC was the best SoC for long life, wouldn't they also store them at 100% SoC? ............


You must know so much more about the cells than me to make that call ! Image After all, I get into trouble from QLD if I make such assumptions Image

TS are shipped at around 60% SOC (mine were). But TS state that they should be charged to 4.2V and current drop before use and claim 3000 cycles to 80% with that charging method. I don't claim to be smarter than the factory ! (even though I charge to only 3.7V)

I remember asking a long time ago if LiFePO4s last for 'x' years quite independent of cycling them or not.

Its OK though, IF the bottom balance assumption is correct then as I noted, the weaker cell will soon fail for more than one reason (thousands of cycles before the rest of the pack ?) and then you can replace it and get on with life !

Actually, I've lost track of where this is heading. Image
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Post by weber »

acmotor wrote:
weber wrote: ............
If 100% SoC was the best SoC for long life, wouldn't they also store them at 100% SoC? ............

You must know so much more about the cells than me to make that call ! Image After all, I get into trouble from QLD if I make such assumptions Image
Let's not start that again Tuarn. That wasn't a call. It was a genuine non-rhetorical question. So was the last sentence of my post before that. I don't know any more about this than you do. I thought we were trying to help each other figure this stuff out. But I guess Jack would say we're just trying to type ourselves smart. He may have a point.

Those questions were an invitation for you (or anyone else) to show me where my reasoning was faulty (as vague as it is). And if you do, I won't be upset. I'll thank you. I even pointed out one way it could be faulty myself, namely two minima.
I remember asking a long time ago if LiFePO4s last for 'x' years quite independent of cycling them or not.
Who did you ask? Did you get an answer?
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Post by Mr. Mik »

Oh, so much confusion...

One thing I know for sure is this: I've watched 2/3 of the video now!

Is it a record?

It reminds me more of the Waltons than the Mythbusters, though... Image
(No offence meant!)

For the intrepid explorer: You can download the video and then watch it (with pause, stop, FFwd, reverse, etc) by using the free VLC media player.

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Post by acmotor »

weber, Yes, let's not !
Maybe the smilies don't show up on your screen ? Image It was my way of saying that the assumption of less than 100% SOC was good for long life seemed without background since storage and long life from useage are not the same thing ? We may also not be considering all the factors the factory did in choosing the shipping/storage state SOC. Some battery types are shipped with zero SOC as it is a less the better situation for safety. Remember that this is all before the initial full charge.
I have heard that some Lithium cells suffer chemically if stored at high SOC. Best thing then is to use them not store them !

I think it is fair that we question assumptions while we type ourselves smart. Image Even if someone reminds us !

Now, that other question.
April 2007 !! seems an eternity ago. I think we have learnt a lot since then ? But the question still remains, along with so many others !!
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Post by antiscab »

jrickard wrote:

Of course you wont' lose any cells if you limit your DOD to 80%. That's precisely WHY you are unaware of this.

And you SHOULD limit to 80%.

To suggest I don't know the state of the pack is idiocy. I can of course monitor the overall pack voltage, and additionally I have a system that counts the actual AH into and out of the pack.

You have spent cash money (hard come by) on a Blue Elephant Gun. ANd because, as you indicate, there are no Blue Elephants (no dead cells) it must be working and you like your Blue Elepant Gun. Probably ought to have a spare and God love ya.

For others, spending money on a series of shunt regulators that do absolutely nothing is NOT the prudent investment they thought it was. THAT's the news.

You have spent cash money (hard come by) on a Blue Elephant Gun. ANd because, as you indicate, there are no Blue Elephants (no dead cells) it must be working and you like your Blue Elepant Gun. Probably ought to have a spare and God love ya.


im always late to the party Image

my blue elephant gun came in rather handy the 7 times i used it

why did i stop? because when i whacked the throttle closed, that buzzer didn't shut up. The distance, between first alarm at sane currents, and static being too low was about 1.5km (from a pack that normally gives 40km). Plenty of time to pull over.

that BMS has definately paid for itself, though i suspect the most expensive part of it was the monitoring part, rather than the balancing.

put it this way, with top balancing:
i overdischarge 7 times, and dont lose a single cell
you overdischarge once and lose 3.

by overdischarge, i mean several cells were below 2v static.

so you demonstrated top balancing with no BMS kills cells.
i demonstrated top balancing with a BMS doesn't kill cells, not monitoring does.

additionally, as your pack ages you experience capacity degredation.
yeh i know, old news, till you get caught out.

the reason i my pack went into overdischarge was actual capacity of the pack was lower than i had previously measured.
course i was using an Ah counter.

my pack degraded much faster than everyone elses as i pulled big currents, let em get hot, didn't compress them, and a few other things.
so yours won't degrade that quickly, but it still will degrade.

so using just an Ah counter and ah half pack differential votlage sensor, you are still very much flying blind.

incidentally, i tried bottom balancing with that old pack. im glad i still monitored per cell, as on the charge cycle one of the smallest cells almost overcharged.

i could have done the test without per cell monitoring, let the cell die, and proclaimed bottom balancing kills cells (gee that sounds familiar).

neither approach kills cells, its not monitoring at the cell level that does.

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Post by coulomb »

weber wrote: Calendar life is likely to be the determinant, unless they are kept very cool.

Yes, I suspect that especially in the northern half of the country including Brisbane, cells will primarily age through heat death.

You know when you get into a car that was parked in the sun and you burn your hands on the steering wheel for the first minute? That's about 60°C (I haven't measured it, but I've read it, and believe it). It seems that Lithium cells are best kept at 55°C or lower. That seems to be a major disconnect to me.

Perhaps one saving grace is that the cells will not be in the passenger cabin, so there isn't the greenhouse effect that the driver experiences under the boot or bonnet. And under the bonnet, there isn't an engine at 90°C and exhaust at 200? °C, though the electric motor will get to perhaps 90 °C (at its case, hotter inside, perhaps? I don't know). But the motor has a smaller thermal mass than the engine, so that should help, and a smaller surface area for radiating heat to the cells.

So I don't think we have to worry about the pack lasting so long that we miss out on an excuse to get the latest battery technology. But really, we won't know for years.
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Post by Electrocycle »

antiscab wrote:neither approach kills cells, its not monitoring at the cell level that does.

what is this real world experience with actual testing using the devices in question? Image

Where are the dire warnings based on wild assumptions made from the results of vague testing? Image
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Post by coulomb »

Mr. Mik wrote: I've watched 2/3 of the video now!

Is it a record?

I think it deserves 2 stars! Image Image

No offence intended, I just couldn't help myself.
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Post by Electrocycle »

I watched the whole thing but I did make some use of fast forward, and doing other things while listening to it in the background :P
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Post by weber »

I found some real data on calendar life (but not very useful). I posted it in the thread acmotor pointed to, as it has nothing to do with this thread's topic. See
viewtopic.php?t=55&start=2

BTW, I checked with the Waltons and they are offended.

I watched one of Jack's videos once. I guess it's a kind of rite of passage for EVer's. I found that Jack has such a deep manly voice that it is still quite intelligible an octave higher. i.e. I ran the video at double speed. Image
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Post by bga »

Matt:
Late to the party but making sense.
Good point that even simple schemes can effectively protect the battery from catastrophe.

I've watched 0/3 of the video. After all the BS about bottom balancing and not monitoring the cells, I think that it'd be a waste of time and bandwidth to download it. Why so huge for a video?
TS cells are expensive, there are a lot of them and if any fail, it causes an outlading and subequent afternoon recovering a dead vehicle.

<Anecdote>
A few years ago, a friend of mine built an tyre monitoring system for heavy haulage trucks in remote areas. The truck was equipped with automatic tyre inflation control. The system monitored air demand and pressure on the 60-odd tyres in wheel groups and was successfully able to predict tyre failures between 1/2 and 4 hours ahead of the actual failure.
</Anecdote>

I believe that, more than limit monitoring, we should be discussing cell performance estimation (modelling) and consistency evaluation on a per cell basis.

The question being: are any of the (150-200 in a 600VDC system) cells weak or heading for a failure?

The workshop is a far better place to do maintenance than the roadside.

Also with limited range and a hard stop at the end, journey planning and management is very important. It's essential to know the facts in order to plan. Running the tank empty isn't a very good way to check the range remaining.
Last edited by bga on Fri, 27 Nov 2009, 04:58, edited 1 time in total.
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Post by Johny »

coulomb wrote:....though the electric motor will get to perhaps 90 °C (at its case, hotter inside, perhaps? I don't know).
Although you don't have to consider heat caused by parking in the sun, which I think will be the worst cause of aging, together with the motor heat. Once you get going it should be able to be arranged that the surrounding air about the cells is close to outside ambient. Same with the outside of the motor.

I'll have half in the boot and half under the bonnet and my plan is too have a small fan to vent the boot area. The engine compartment will not be totally sealed so should quickly cool once moving.

I have considered my own personal portable carport at work though - those tent like ones. We have no undercover parking and my car will spend most daytime hours in the workplace carpark.

My experience with car covers is that they may stop some heating but are not as effective as a "tent".

BTW I ordered my pack(s) - Headways - see blog. It has been nerve wracking - including paying for them up-front.
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Post by acmotor »

Hey Jack, if you are still there, I think we owe you a vote of thanks for raising this charging issue on the forum. Image Image Image
Don't take offence to the video feedback. It was still brave of you to put yourself in the frames. Lots and lots of them Image Nice EV !!
I wish you a BMS for Christmas !

I remember TJ's post here where his battery voltage monitoring system graphically showed the death of a battery (LA). I asked at the time, did it make any difference to the fact that the cell was terminal anyway ? In that case monitoring would not save anything, maybe just give pre-warning. But yes, as complete a monitoring and bga modelling as possible would help with the charge and discharge limits and identifying weak cells.

Matt, shame you didn't post sooner ! Image

edit: found TJ's post
Last edited by acmotor on Fri, 27 Nov 2009, 06:13, edited 1 time in total.
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Post by domEV »

Hi guys,

Recently I came over this EV forum and so far I read just few of many interesting topics - quite a lot experience, time and effort all of you put in here Image
I'm myself still in "design" phase of converting some old ICE car into EV. After (too) long time spent in "DC or AC" scruple, final decision was to go for AC (special thanks goes to acmotor who inpired it).
Since many of you spent lot of time discussing BMS design and how to avoid destruction of weakest cell, I would like to ask you for your opinion on "complete bypass of cell" idea.

E.g. let's look at long string of bateries used to achive high DC voltage. Basic idea is to completely bypass weak cells when they get discharged in few ocassions when you run out of "fuel" and you are not in front of your house. Therefore final DC voltage will be affected by only few percentage (even if we reduce it by few cells) and therefore should not affect much performance of your VFD drive, especially if you are instantly warned by your BMS of such a meassure taken.
However I'm not sure how to bypass cell (disconnect it from string) in efficient, simple ant low cost way, but you might have some good idea...
Last edited by domEV on Sun, 06 Dec 2009, 14:41, edited 1 time in total.
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Post by Electrocycle »

If it was one cell the easiest way to fix the problem (for the drive home) would be to carry a spare cell and connect it in parallel with the flat one with jumper leads :)


In theory you could have a DC-DC converter that outputs ~3.6v at high enough current to "run" the bad cell from the rest of the pack.

<over unity>If you had enough of those you wouldn't need any batteries! :P</over unity>
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Post by coulomb »

domEV wrote: Basic idea is to completely bypass weak cells when they get discharged in few occasions when you run out of "fuel" and you are not in front of your house.

Great idea, and I don't think you're the first to think of it.

The problem, especially with a high voltage AC system, is that you would need 200+ contactors or pairs of high power FETs to achieve it. Even the series resistance of all that starts getting significant, but the worst part is the complexity, space, and weight.

A compromise might be to work with groups of say 16 cells, so you only need 12 or 13 of these. But then you lose ~48 V in one hit. Not bad for a "luxury limp home".

The least cumbersome of these systems might be a BMS that could channel charge to a weak cell from the rest of the pack, as already mentioned. One way could be a ~40vAC bus that connects to a high frequency transformer with 10:1+1 turns ratio on each cell board, with just a centre tapped, two Schottky diode arrangement driving each cell. The transformers provide isolation. The 40 VAC HF bus would be powered from a small oscillator powered by the whole pack. Shielding of the HF square waves could be a problem. I suggest 40VAC since a 10:1 turns ratio is easily achieved on the transformers, and the higher voltage and lower current reduces losses (same reason you use a high voltage pack, right?).

Automatically, most of the energy would go to the cell with the lowest voltage; the others would draw almost no power unless they were nearly as low. This could replace bypassing altogether. At each point, the lowest voltage cell gets more charge, so it can't be bad. You could possibly adjust the amount of "redistributed" power, backing it off or turning it off altogether when the pack is balanced, and turning it up when the pack is unbalanced. A good BMS could do this automatically. You would not be deliberately wasting power, as you are with normal balancing.

The devil might be in the detail of distributing the HFAC bus in such a way that all cells got the same energy, despite being different distances from the master BMS. Maybe you could have trimpots on each board to compensate somehow. Or do some tricky star wiring, where most of the buses come from a central point. The main stumbling block is probably the cost of each transformer, but they would be fairly low power, like a 25mm torroid.
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Post by acmotor »

Hi domEV !
Welcome to this forum !!!

acmotor grovels as name is mentioned in thanks Image

I'm not certain just how much people should worry about weak cells and how to limp home. It should not be high in their EV design to do list.
Pushing the pack when one or two cells are flat will be expensive in terms of reduced battery life for the other cells in the pack even if a flat one can be bypassed.
The BMS has done its job by warning of the first flat cell. Preferably you don't usually go there.

Having said that, I actually like the idea of the battery pack that has 48-72V breakup modules (acmotor style !! Image) being able to bypass one module if a cell in that module has failed for some reason or even just gone flat.
So coulomb, yes to that thought. It was raised last year but nothing became of it at the time. I guess our BMS discussion was more in its infancy at that time though.

Bypass at the cell level seems too complex and expensive also potentially increasing the effective resistance of the pack due to extra wiring and contacts.

As battery packs age there will always be infancy failures, premature failures and eventually the 50% difference in overall cell life causing cells to give up and letting the EVer down. Being able to bypass them would seem to me the important thought from domEV, perhaps less so than just a flat cell ??

Image

The cells shown could be 48 to 72V modules. The changeover contactors need to be rated at pack current and pack voltage although you would not be deliberately switching them under load so the lower voltage contactors used in red suzi would be OK.

Interestingly, with all the contactors in the bypass position, all module's +ves would be connected together. Halfway to parallel charging that I do anyway.

Image
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Post by domEV »

Thanks for warm welcome! Image

Actually, I'm planning to have batteries in 48V banks (sounds familiar don't you think? Image ).
I would also like to have simple BMS that would just keep track of cells voltage, drawn Ah and temperature and warn me when things start to "change".
However, EV would be driven by my girlfriend as well, so I would like to avoid unnecessary stress of sudden alarm which would require, more or less, immediate search for parking.

But if we come up with some relatively simple and cheap solution for bypassing weak cell/bank, she (or anyone else who would like to drive my car) would have much more time to find parking or maybe even enough usable strenght in batteries to drive home or to some place where batteries can be recharged. During this period performance (acceleration, top speed, etc.) of EV is not important at all, so remaining batteries should not be overloaded.

I also had in mind simple solution of using already available contactors between 48V banks (btw, acmotor, most probably I missed that detail, but what was price of your contactors?), so I might proceed with that one.
Another aproach could be similar to coulomb's, but to have DC-DC converter (48V to HDC required by VFD). In this case 48V banks would be autonomous from each other (no need for contactors), but question is price and effeciency of relatively powerfull DC-DC convertors.

We could also go for many smaller DC-DC convertors (e.g. 12V to HDC), but this might end up being too low input DC voltage...
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Post by Nevilleh »

When I first started looking into this stuff, I found an "EETimes" article on charge balancing that used a HF transformer with multiple windings to distribute charge from the "strongest" cell(s) the the "weakest". The article describes its operation very well and I had a go at building one, but stumbled over what to use as electronic switches. I put it to one side with the idea of coming back to it when time permits.
The basic idea of it is that it balances the cells continuously, so you never have a "weak" one, ever. It's fairly simple with a transformer for say 8 cells, some switches and a micro. You'd need one unit for every 8 cells.
I have a copy of the article in pdf format and I don't know how to upload that to this forum, else I would!
If anyone is interested, either tell me how to upload it here or drop me a private message and I'll email it to you.

Further work on the "master" unit for my own BMS has resulted in a single board that can monitor the battery pack voltage and current via two ADC inputs. It can display these on a 2 line LCD. Also can calculate AHrs used and/or left and watt-hrs used as well. Or it will when I get the s/w that far! At present it just reads and displays volts and amps. I have to do the AHrs stuff with long ints and I'm still figuring that out.
I use a Tamura 500A Hall sensor for the current, but the 10 bit ADC only gives me a resolution of 1.66A. This is plenty good enough for discharging, but may be a bit inaccurate for measuring the charging AHrs.
Could be a handy instrument for those who don't want to go to the trouble of individual cell monitoring and it will not be very expensive with only about 30 or 40 bucks worth of parts.
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Post by Electrocycle »

I would just set the BMS's low battery warning threshold a bit higher, so when it goes off you can still limp home at reduced speed.
Chances are if there's one low cell, the rest won't be far behind - and if you bypass one it's only going to drain the others quicker!
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Post by coulomb »

domEV wrote:
Another approach could be similar to coulomb's, but to have DC-DC converter (48V to HDC required by VFD). In this case 48V banks would be autonomous from each other (no need for contactors), but question is price and efficiency of relatively powerful DC-DC converters.

I think it will be too expensive to be able to provide traction current for a failed or weak cell. My idea was really to provide bypass-like currents (say 0.5 to 2 A) to whatever cell might need it, avoiding a weak cell altogether. Of course, if you then get a low voltage alarm, it will mean that the whole pack (or at least one 48 V group) is about to die. So you need a good amp-hour based fuel gauge, so you can avoid alarms with a little planning.

The idea of using group DC power instead of a pack-wide AC bus is interesting. I think that the bus should still be AC, so each cell doesn't have to convert DC to AC for the transformers' inputs. The weakness of this idea is that any group that has 2 weak cells will have access to much less "continuous balance" energy than the other groups, but you can't have everything. This idea would also avoid a lot of inter-group wiring, which is a good thing.

Edit: spelling and clarity
Last edited by coulomb on Mon, 07 Dec 2009, 03:15, edited 1 time in total.
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Post by coulomb »

acmotor wrote: Interestingly, with all the contactors in the bypass position, all module's +ves would be connected together. Halfway to parallel charging that I do anyway.

True.

If only double pole contactors were more common, bypassing could be done "for free" (or whatever the extra poles and wiring cost), assuming that you are isolating at the group level for safety).

With some co-operation from the controller, you could use lower current contactors (which can't break under load); get the controller to cut back to zero power for a half a second, do the changeover in that time, then go back to 80% of the original power. That way, if the user really needed to get moving for safety, they are still able to, and are well aware of the need to pull over since the vehicle just cut out for a second (much like an ICE car about to run out of fuel).
Nissan Leaf 2012 with new battery May 2019.
5650 W solar, 2xPIP-4048MS inverters, 16 kWh battery.
1.4 kW solar with 1.2 kW Latronics inverter and FIT.
160 W solar, 2.5 kWh 24 V battery for lights.
Patching PIP-4048/5048 inverter-chargers.
Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

Home grown BMS ideas !

Post by Nevilleh »

I did manage to convert that pdf file to 3 jpg pages, so here they are:

Image

Image

Image

You will not that they used 12 cells per module, limited by the number of transformer pins. I guess that's not too bad.
I made a transformer using an ED48 core with a primary and only a single secondary, but couldn't find the opto-coupled fets they used. Didn't try all that hard at the time.
One of the nice features is to use it to also measure the cell voltages.

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