BMS free zone battery management

Post by **antiscab** » Thu, 25 Jun 2009, 21:51

Electrocycle wrote: It'd be fine. The cells would all be too scared to go S/C

we could call the cell chuck norris

Matt

Post by **Electrocycle** » Thu, 25 Jun 2009, 22:28

hmmmmm.... I guess we'd better stick to using multiple cells in series!

Post by **skyenergy** » Thu, 02 Jul 2009, 21:32

I think it needs a whole system, including BMS,charger,controller.

BMS can control the voltage of the batteries, also can control the charger and controller.

Charger with good quality must has the CAN Bus, can communicate with BMS.

Controller must has the CAN Bus too, can communicate with BMS. When the voltage of the battery is low, the BMS can "inform" the controller outputting lower power.

Though, all the eqiumpents each has CAN Bus and can communicate with each other, can work best together.

As I know, almost nobody knows why a controller needs a CAN Bus.

Above is I heard from a engineer who are working in a BMS factory. I am working in a very famous lithium battery manufacuturer.

Post by **antiscab** » Thu, 02 Jul 2009, 21:55

Welcome to the forum Steven.

For a more conventional BMS, signal via Can Bus is one (rather unreliable - think vectrix) way to inform charger and controller of the need to throttle back.
There are many other ways to achieve the same outcome (simpler and less susceptible to noise, and unrelated equipment failures)

Matt

Post by **Richo** » Thu, 02 Jul 2009, 23:18

The point of this thread was to do away with multiple BMS.
So a can bus system isn't really needed.
More to the point could be integrated into a controller.

Post by **acmotor** » Thu, 02 Jul 2009, 23:41

Thanks Richo,

Yes, the (far reaching) question is BMS without connecting monitoring/shunting to each cell. Thinking caps still guys.

Post by **Squiggles** » Thu, 02 Jul 2009, 23:58

Infra red imaging?

Post by **acmotor** » Fri, 03 Jul 2009, 00:04

This was done on an optima test some time back and did show up faulty cells at least. It was not in a vehicle though and access to inner cells of a pack could be an issue.
Imaging the top (terminals) of a pack may be revealing. Have to think about it.

Post by **Squiggles** » Fri, 03 Jul 2009, 00:09

Magnetic resonance?

Post by **acmotor** » Fri, 03 Jul 2009, 00:30

Sounds expensive ? and may not be compatible with high traction currents ?
What changes in MRI would you expect for SOC of cells ?

Post by **Squiggles** » Fri, 03 Jul 2009, 02:45

acmotor wrote: Sounds expensive ? and may not be compatible with high traction currents ?
What changes in MRI would you expect for SOC of cells ?

Haven't a clue.
You asked for ideas not solutions

Actually I was a bit more serious about the infra red. It is used in the power industry to locate bad connections on transmission lines etc. Maybe there are characteristics of batteries that produce different heat output.

Post by **Johny** » Fri, 10 Jul 2009, 15:49

No BMS needed - at least not one to stop over-charge. This does away with cell equalisation devices.
New development for Lithium

Post by **coulomb** » Fri, 10 Jul 2009, 16:10

I'm not sure that I like this idea: basically convert excess charging voltage to heat, thereby shortening the life of the cell. You can still ruin your cell through overcharging, it just takes longer.

Also the chemical process sounds rather less precise than an electrical balancing circuit, so it seems to me that while these cells may not die quickly from gross overvoltage, they'll still grow out of balance with each other. Worse, it may be more difficult to balance them since the chemical process will start getting in the way, necessitating more charging current for some cells, and therefore more bypassing current for others.

This balancing problem would not occur if the chemical processes started taking effect at a high enough cell voltage, say 4.0 VPC, so you can still balance as normal (e.g. while charging) at 3.65 VPC.

Post by **Johny** » Fri, 10 Jul 2009, 16:55

Valid concerns - let's see what they can do with US$3.3M.
It does mean that the sensing system can just take in the whole pack voltage and current to detect when full charge has been reached. Then maintain a nominal 3.65V/cell for a while and the cells self-balance. I imagine that the cells would be designed to behave a lot like a low current balancing system.

Post by **weber** » Fri, 10 Jul 2009, 18:09

coulomb wrote: I'm not sure that I like this idea: basically convert excess charging voltage to heat, thereby shortening the life of the cell. You can still ruin your cell through overcharging, it just takes longer.

Also the chemical process sounds rather less precise than an electrical balancing circuit, so it seems to me that while these cells may not die quickly from gross overvoltage, they'll still grow out of balance with each other. Worse, it may be more difficult to balance them since the chemical process will start getting in the way, necessitating more charging current for some cells, and therefore more bypassing current for others.

This balancing problem would not occur if the chemical processes started taking effect at a high enough cell voltage, say 4.0 VPC, so you can still balance as normal (e.g. while charging) at 3.65 VPC.

Er, are you reading something other than the above article? Where did you get all this about it producing heat and being imprecise and taking effect at too low a voltage?

I assume they are just trying to add to Lithium cells what we get for free in lead-acid cells. In lead-acid cells we get water being electrolysed (no heat, non destructive), at a very precise voltage, which is above the fully charged voltage of the cell. That's what allows us to equalise lead-acid cells without bypass electronics. We have bypass chemistry instead.

The trouble with lithium is that the existing secondary reactions at high voltage are irreversible and destructive, but if you can interpose a reversible or non-destructive reaction at a voltage in between that of full charge and the destructive reaction then you're OK. Sounds like a good idea to me.

In the case of flooded lead-acid you have to top up the water on those cells that had to go into chemical bypass to let the others finish charging. In sealed lead acid the hydrogen and oxygen bubbles stay in suspension and slowly diffuse and recombine back to water later with the help of a catalyst, when charging is long over. Yes this generates heat, but at such a slow rate it has no significant effect.

Of course if you overdo it with sealed lead-acid batteries they will vent hydrogen and oxygen and then water is permanently lost from the cell and so their life is shortened.

Post by **coulomb** » Fri, 10 Jul 2009, 18:27

weber wrote: Er, are you reading something other than the above article? Where did you get all this about it producing heat and being imprecise and taking effect at too low a voltage?

Surely it has to produce heat to "cap the voltage". The lead acid gassing happens at a precise voltage (though dependent on temperature I think?) and is integral with the chemical reactions that make the stored energy available as electricity. But the proposed reactions would presumably be independent of the cell's chemical to electrical conversion process. It might be just as precise, I suppose; it just seems unlikely to me.

Ah, I see, a reversible chemical reaction could absorb charging energy up to a certain limit, like sealed lead acid cells, and not heat the cell much at all. Sorry, I didn't consider that. Valid point.

Post by **acmotor** » Sat, 11 Jul 2009, 04:31

Chemical solution to overcharge... good.
Good to see specific money being thrown at areas we all know need attention.
But what about detecting overvoltage and undervoltage ? These still need to be detected in some way to protect cells and to detect cell failure.