One for the BMS-o-philes

How do you store and manage your electricity?
Faz
Groupie
Posts: 96
Joined: Tue, 05 Jul 2011, 16:43

One for the BMS-o-philes

Post by Faz » Fri, 09 Nov 2012, 21:47

weber wrote:
Faz wrote:Waiting several days to charge the battery is exactly my point "why bother"?!?!?!
I'm not sure what you're missing here, so I'll try explaining the whole thing in more detail.

Commissioning (initial balancing) only has to be done once. From then on, a full recharge from flat can be done in around 8 hours (in our case) because our charger can then run at its full 5.5 A for most of that time. The balancing phase at the end, where the current tapers down to 0.4 A, only takes a few minutes. But it's exactly the same charger and exactly the same BMS-controlled charging algorithm in both cases.

Our BMS shunt capability is exactly the 1%C or C/100h that you mentioned as typical (0.4 A for a 40 Ah pack). Of this, you wrote:
Which is great if the pack was already commissioned cell by cell or your charger is of a ridiculously low current. Otherwise what is the point of a full blown bms?
So I've explained that even with only 1%C shunt capability you do not need to have a charger that's only capable of a ridiculously low current, and you do not need to commission your pack cell by cell.
I was thinking of a constant current then constant voltage charger. Your charging algorithm is a good idea and takes care of that issue to a degree. Though a few minutes at 1%C still only allows for a 0.1% difference in cell balance (@6 minutes). So that either means far more than "a few minutes" or there is so little imbalance what's the point?
weber wrote:If I had commissioned my 228 cell pack cell-by-cell it would have taken far longer. But even if you only have say 45 cells and so it would take less elapsed time to commission them cell by cell, that requires a different charger and a lot of your time spent moving it along from cell to cell. Whereas my BMS-controlled initial charge did not require any attention from me and so let me work on the many other jobs required to get an EV conversion on the road. So from my point of view it took zero time.
So we are in agreement that an initial pack balance is important.

[QUTOE=weber]
Um destroyed cell? Well it is clearly already stuffed if a properly commissioned pack has a corrupt cell.
You didn't previously say anything about it being "corrupt". You wrote:
I have the same thought on "lvc" if a cell voltage is dropping while driving, well either you stop or drive till the pack makes you stop. The end result is similar, but one way gets you closer to home.
I presumed it was clear that if a cell was so far out of balance that it is corrupt.
weber wrote: If you "drive till the pack makes you stop" you will by then have completely flattened that cell and begun to "charge" it in reverse. This will lead to it going high resistance (where "high resistance" here may still be less than one ohm) and thereby ending up with a large fraction of the battery voltage across it in reverse, while the remaining cells continue to force current through it.

Both jonescg and I have packs with around 200 cells. Imagine one of those cells with say 100 volts across it (in reverse) and 100 amps being pushed through it. That's 10 kilowatts being dissipated in one cell! How long do you think it will last?
True and a good point. This is still only the case if some cells are very far out of balance. I presume that most people don't drive to 100% DoD. This scenario happening at 90% DoD means a cell with 10% less (or worse) capacity than its neighbours. There will also be clear indications of problems before this occurs, ie: drop in vehicle performance and then clear indications before battery ignition ie: burning smell as the battery heats.
webber wrote:
You refer to "our bms" I wasn't referring to indi built bms's.

For the purpose of this argument, I don't see how it matters who builds them. You're talking about two of the most basic and necessary functions of a BMS for Lithium-ion batteries. You seem to be saying that an undervoltage alarm is not worth having, and that a balancing shunt is not worth having unless it can bypass far more than 1% of C/1h. I'm trying to explain why you might be mistaken about those.
Then I apologise for not making myself clear. I believe in monitoring, not automatic cutout/off. I don't believe a balancing shunt needs to be perma-mounted to the cell and I believe that if it is worth having it is worth making of a suitable capacity.
weber wrote:There is a third basic and necessary function of a BMS for Lithium-ion batteries, and that is an overvoltage alarm (or a single combined over/undervoltage alarm).
Again I don't believe this needs to be perma-mounted to the cell.

User avatar
weber
Site Admin
Posts: 2623
Joined: Fri, 23 Jan 2009, 17:27
Real Name: Dave Keenan
Location: Brisbane
Contact:

One for the BMS-o-philes

Post by weber » Fri, 09 Nov 2012, 22:39

Right, jonescg. You've described the minimum set of BMS functions being available when they are needed. Nothing says they all have to be on board.

But if you're bringing ribbon cable out of a 700 Vdc pack then you have to pay attention to the shock, arc and fire hazards that presents. There's no reasonable way you can protect it against short circuit. A 700 V fuse at every cell would be too big and too expensive. Although if they were only for voltage sensing (not balancing) then you could have put three 0805 size 250kR resistors in series with each wire on that PCB of yours. That would make them non-lethal and short-circuit proof but still allow voltage measurement. That's a trick we learned from acmotor. But no use to you.

The best you can do is try to make up for the lack of fusing with extra mechanical protection for the insulation. It should be impossible for a dropped tool (or dropped bike) to damage the ribbon-cable.

However you can ensure that the ribbon cable is insulated with an appropriate voltage rating.

You should at least be using something like the Pro Power 300 V rated 0.1"-spaced ribbon cable that Tritium recommend for use with their BMS. See http://tritium.com.au/wp-content/upload ... cedure.pdf

Don't use ordinary 0.05"-spaced computer ribbon cable.

But since even the ProPower ribbon cable is only rated for 300 V, you'll need to sleeve it. And the sleeve should be orange to remind you, and others, that it is hazardous voltage. A waterproof cap or some tape over the connector, when it isn't plugged into anything, would be essential too.
Last edited by weber on Fri, 09 Nov 2012, 11:50, edited 1 time in total.
One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

User avatar
weber
Site Admin
Posts: 2623
Joined: Fri, 23 Jan 2009, 17:27
Real Name: Dave Keenan
Location: Brisbane
Contact:

One for the BMS-o-philes

Post by weber » Fri, 09 Nov 2012, 23:16

Faz wrote:Though a few minutes at 1%C still only allows for a 0.1% difference in cell balance (@6 minutes). So that either means far more than "a few minutes" or there is so little imbalance what's the point?
There is very little imbalance. And the point of balancing very little imbalance every time, is so it can't build up into a lot of imbalance. Which is why 1%C has been found to be plenty. But we thought the same as you, 3 years ago.
True and a good point. This is still only the case if some cells are very far out of balance. I presume that most people don't drive to 100% DoD. This scenario happening at 90% DoD means a cell with 10% less (or worse) capacity than its neighbours. There will also be clear indications of problems before this occurs, ie: drop in vehicle performance ...
I don't think a drop in vehicle performance is at all a clear indication of a cell problem. It could be due to many things. And by the time it happens it is too late. The cell has been damaged irreversibly.
... and then clear indications before battery ignition ie: burning smell as the battery heats.
The "shudder and sniff" method of battery monitoring? I'm going down to the workshop right now to rig up a blower to pump the fumes from the rear battery box back into the cabin. Image
Then I apologise for not making myself clear. I believe in monitoring, not automatic cutout/off.
Me too, as I said at the start of this discussion. But coulomb and I are working on automatic back-off, i.e. the motor controller's battery current limit will be progressively reduced as required to protect the most stressed cell.

One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

One for the BMS-o-philes

Post by Nevilleh » Fri, 01 Feb 2013, 14:58

Only just saw this thread and read it with some interest! It does highlight some of the issues of monitoring a big, high voltage battery, doesn't it?

When I first started looking at battery monitoring/management, I built a "centralised" system with wires running from each cell, but when it came time to install it, I was frightened off by the dangerous voltages existing in the great clump of cables - and I have only 45 cells. I shudder to think of weber and coulomb's 228 cell battery being done like that.

Initially for this reason, I decided to build a system with cell modules. This decision was reinforced by the NZ regulations being changed to require all cell voltage monitoring wires to be individually fused, except where the module is bolted/soldered directly to the cell terminals. So I built my modular system I also looked at weber and coulomb's bms and they kindly sent me a sample of their module, but I didn't have the support for the micro they used and decided to do my own with Microchip parts.

I decided to include a shunting capability to allow cell balancing, especially as my charger can be throttled back to 1/2 amp which is the balancing current. Hence, I have a battery management system rather than just a battery monitoring system and it has proven its worth over the last - how long is it? - over 5000 kms now and that has to be at least 60 charge cycles.

As I described elsewhere, I was using bottom balancing and was quite happy with the results of that, but a few other people who have built my bms asked about changing it to top balancing, so I did and have been running that in my car ever since with good results. As weber said, the initial balance took a long time, particularly so because my cells were bottom balanced, but now it takes maybe a 1/4 hour or less and its automatic, so I don't need to worry about it.

You might note that my cell modules are permanently attached to the cells and draw their 1 mA continuously. The watchdog reset ensures that the micro is always in control and I've not had any shunt failures at all. Even if I did, the current drain is only 1/2 amp and I have 120 AH, so that's 240 hours to run a cell down - 10 days. In case of such a failure the master control would report such a thing and alert me to a possible problem with whatever cell it might be.

That's about as safe and secure as it can be made without going to ridiculous and expensive lengths.

And Chris, if you are interested, my cell module could fit into your 40 x 20 x 4 volume with only a small change in the layout!
Last edited by Nevilleh on Fri, 01 Feb 2013, 04:05, edited 1 time in total.

User avatar
jonescg
Senior Member
Posts: 2996
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.

One for the BMS-o-philes

Post by jonescg » Fri, 01 Feb 2013, 16:41

Yes, technical challenges indeed. 10 days is a very short time to run a battery flat. I haven't driven my car in three weeks, that would ensure an early demise.

EV-Power has also offered assistance on the BMS development front, so if you can come up with a prototype we can compare and contrast.

I will continue to go with a centralised monitoring system for the race bike, but it won't be 168 cells to one plug, it will be 42 cells to one plug, and four plugs. This is still be 175 V top of charge, so they will need to be isolated appropriately.

However the low voltage and high voltage warning circuitry will need to be built into the distributed cell modules. The 4 mm added to the tops of the cells also makes it a very tight fit; the total height of the cells, modules and monitoring wires cannot exceed 155 mm. Tough when you have 43 wires coming up from the pack.

I will get some more images together of the pack and their size limitations. Oh, these cells are 3.4 V to 4.2 V by the way Image
AEVA National Secretary, WA branch chair.

User avatar
Johny
Senior Member
Posts: 3729
Joined: Mon, 23 Jun 2008, 16:26
Real Name: John Wright
Location: Melbourne
Contact:

One for the BMS-o-philes

Post by Johny » Fri, 01 Feb 2013, 16:48

jonescg wrote: Yes, technical challenges indeed. 10 days is a very short time to run a battery flat. I haven't driven my car in three weeks, that would ensure an early demise.
That was only if the shunts got stuck on which is highly unlikely. Normally at 1mA it would take in excess of 10 years for a 120AH cell to run flat - self discharge would be the main culprit.

Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

One for the BMS-o-philes

Post by Nevilleh » Fri, 01 Feb 2013, 17:05

I think you are needlessly concerned about a shunt failure running your battery flat. The probability of that is vanishingly small. Modern electronics, properly designed, has a very long lifetime.

I think I explained some very good reasons for why I abandoned the central monitoring idea and I’ll be interested to see how you get on. Have you found a 43 way plug yet? I had an electric motorbike here that did what you are trying, but with only 24 cells and it was a nightmare. I certainly wish you luck with your 168 wires!

I don’t propose to go into competition with EV Power. My bms is open source and freely available to anyone who wants to use and/or modify it in any way. I just mentioned that it could fit in your available space with very little modification, basically a new pcb layout.

Edit: here's a connector! http://uk.farnell.com/itt-cannon/ca3101 ... dp/1728886
Only 146 pounds each!
Last edited by Nevilleh on Fri, 01 Feb 2013, 06:27, edited 1 time in total.

User avatar
Richo
Senior Member
Posts: 3737
Joined: Mon, 16 Jun 2008, 00:19
Real Name: Richard
Location: Perth, WA

One for the BMS-o-philes

Post by Richo » Fri, 01 Feb 2013, 20:22

and $300 locally Image
Talk about being reemed.
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

User avatar
jonescg
Senior Member
Posts: 2996
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.

One for the BMS-o-philes

Post by jonescg » Fri, 01 Feb 2013, 21:41

Oh I wasn't suggesting a competition; just expressing my delight with how helpful and supportive the Australasian EV community has been with this bike!

I was planning on using a D-sub44 HD. They are rated for 200 V between pins, but I should be OK with no more than 9 V between two adjacent pins. The layout of my pack makes it quite reasonable to manage 43 wires, but not dead easy.
AEVA National Secretary, WA branch chair.

User avatar
Richo
Senior Member
Posts: 3737
Joined: Mon, 16 Jun 2008, 00:19
Real Name: Richard
Location: Perth, WA

One for the BMS-o-philes

Post by Richo » Fri, 01 Feb 2013, 22:39

Oh no don't use the D word - you'll jinx it Image
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

User avatar
jonescg
Senior Member
Posts: 2996
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.

One for the BMS-o-philes

Post by jonescg » Fri, 01 Feb 2013, 22:46

D-sub or Dead? Either one would be jinx-worthy Image

Someone once told me "If it stinks, it's chemistry. If it's disgusting, it's biology. If it doesn't work, it's physics." Nothing more frustrating than soldering a circuit up, powering up, and getting nothing...
AEVA National Secretary, WA branch chair.

Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

One for the BMS-o-philes

Post by Nevilleh » Sat, 02 Feb 2013, 01:48

And if you can't understand it, its mathematics.

User avatar
jonescg
Senior Member
Posts: 2996
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.

One for the BMS-o-philes

Post by jonescg » Sat, 02 Feb 2013, 20:09

OK so here is the whole battery pack with labels on stuff. Any distributed BMS equipment needs to go in the space between the cell packs and the covers (not drawn for clarity). There is literally 10 mm of vacancy here, so between the boards and the monitoring wires, there's not a lot of room.
Image

The 44 mm vacancy above is where the monitoring plugs go. It's designed so you can take the tank cover off, take the top cover off and access the plugs without exposing yourself to the HV stuff below. Provided the key is off, the maximum voltage here is 175 V.

So there's a fair volume for a centralised system up there, or maybe 4 centralised systems, all connected somehow, but it's not excessive.
AEVA National Secretary, WA branch chair.

User avatar
jackhyq
Groupie
Posts: 101
Joined: Tue, 22 Jul 2008, 23:42
Real Name: Jack
Location: China
MSN: jackhyq3@hotmail.com
Contact:

One for the BMS-o-philes

Post by jackhyq » Sat, 18 May 2013, 06:05

jonescg wrote: OK so here is the whole battery pack with labels on stuff. Any distributed BMS equipment needs to go in the space between the cell packs and the covers (not drawn for clarity). There is literally 10 mm of vacancy here, so between the boards and the monitoring wires, there's not a lot of room.
Image

The 44 mm vacancy above is where the monitoring plugs go. It's designed so you can take the tank cover off, take the top cover off and access the plugs without exposing yourself to the HV stuff below. Provided the key is off, the maximum voltage here is 175 V.

So there's a fair volume for a centralised system up there, or maybe 4 centralised systems, all connected somehow, but it's not excessive.


Nice!
Portable AC/DC/Solar UPS;
3D BirdView,RearView,A.I System;
1C~90C LFP/LiPo Battery ;
BatteryShow@126.Com;
www.Ponilion.Com

Post Reply