Chaining 2 8-cell BMS together for 16-cell?

[cycle9]

I have a 16-cell LifePO4 pack here from a customer that I'm trying to fix. The problem is a flawed BMS design, and I need a new option(see below if you want gory details*). It would be great to get one of Gary's BMS boards, but the new design doesn't appear to be available yet, and I have an enthusiastic guy who is without his ride in the meantime.

Having failed to find any other reasonable 16-cell solutions, I am thinking one option might be to chain two 8-cell BMS units together, such as these from Batteryspace: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4631. It seems that I could just have each board monitor and control 1/2 of the pack, then internally connect them together in serial, while making sure to give proper clearance for the balance resistors. However, this brings up a few questions in my mind:
- Are the Batteryspace units reliable? I've only used a few of their Li-Mn balance boards, no problems yet, but I haven't tried their LiFEPo4 (they label it Po3!) units.
- I would prefer to charge these two banks in series (at > 48V), because this guy already has 2 16-cell (62V) chargers in hand (one for work and one for home). However, I am a bit leery of that, because it seems that the two halves might not properly balance with separate boards. Thoughts on that?
- I don't see any obvious problems with serial connection for discharge, unless maybe the FET's can't handle the potential inductive voltage spikes. I would actually just prefer to bypass the discharge limit circuit and use the BMS for charge balancing, since the guy's eZee controller will take care of the low voltage cutoff. However, I'm not sure whether this BMS will function as a balance-only board.

Anyway, before we dump $140 into a pair of boards from Batteryspace, it would be great to hear whether this is a crazy idea, or not. Alternative (and currently available) solutions are very welcome. I also tried to track down 16-cell balancing chargers, but didn't run across any.

TIA,
Morgan

* BMS design: The programming on the BMS is screwed up so that balancing does not properly happen. Normally this BMS is supposed to provide a bleed-down function to a programmable set point (e.g. 3400 mV) to get all cells to the same voltage. The problem is that the built-in BMS uses a dual microcontroller setup, since each of their PICs can only handle 12 cells each. So they divided it into 2 banks of 8 cells each, each one monitored by a separate PIC (on the same board!), somehow loosely tied together (e.g. they both share an "over voltage" signal for any single cell). Crazy setup, just bound to generate timing/noise/interference or other issues.

I can program the bleed setpoints, but often one of the PIC's just glitches out and ignores the setting, and arrives at a seemingly arbitrary voltage set-point, independent of the other bank. So of course with half the cells at one voltage, and half at another, balancing is far from optimal, and hence capacity drastically reduced. So far, in order to get the BMS to behave properly, it is necessary to hit the "reset" pin repeatedly. I don't want to be telling this guy he has to reboot his battery pack on a regular basis in order to ride his bike! I continue to play with the programming to try to get it working reliably, but I am about to give up.

 

[methods]

I have been thinking about this.
I would use hobby balance boards and overlap them by one signal like this:

(Assuming common 6 cell balance boards from RC Hobby w/0.5A discharge)
(Letter represents $20 6cell bms, 3 used)
(Number represents battery cell, 16 in series)

Batt. Bal.
1 A
2 A
3 A
4 A
5 A
6 A B
7 B
8 B
9 B
10 B
11 B C
12 C
13 C
14 C
15 C
16 C

Cells 1-6 will match each other
Cells 6 - 11 will match each other
Cells 11 - 16 will match each other
The overlap on cell 6 will cause 1-6 to match 6-11
The overlap on cell 11 will cause 6-11 to match 11-16

The key is the overlap. The INCORRECT way to do it would be as follows:

Batt. Bal. WRONG!
1 A
2 A
3 A
4 A
5 A
6 A
7 B
8 B
9 B
10 B
11 B
12 B
13 C
14 C
15 C
16 C
- C
- C

Which is what I think your BMS does. This would leave the three banks out of balance with each other.

$60 solution. Does this solve your problem?

Balance, discharge, and monitor for $40
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6748&Product_Name=MKS_3in1_Balance/Discharger_System_6S

Dig through here for cheap $20 balance solutions. Read reviews to make sure the product is quality.
http://www.hobbycity.com/hobbycity/store/uh_listCategoriesAndProducts.asp?catname=Chargers&idCategory=216&ParentCat=85

-methods

EDIT: Of course this does not protect your cells, only keeps them in balance. I guess if end user wants worry free operation (like my wife) then you need more aggressive BMS to stop user from doing something stupid like deep discharge or wrong charger.

 

[cycle9]

I like your idea for overlapping the cells between each BMS/Balancer circuit. You're right that in theory that should keep the two separate banks balanced, which is what my current system is not doing. It does make me wonder whether there could be problems at the cells where the overlap occurs. For example, one pack bleeding the overlap cell down to a certain voltage, then the other one trying to bleed its whole bank down to that voltage, and so on...a sort of lowest-common-denominator effect leading all cells to become discharged. The overlap cell would be balanced at twice the rate as other cells. But maybe that is not a big deal. I'll have to think about this more.

Thanks a bunch for the ideas, and also for the link to the store with all those balancers. It is definitely a useful link to have!

Morgan

ps - regarding the Edit, no big deal, we are using a controller that has LVC to protect the batteries.

 

[methods]

I have been trying different versions and it works great!

1S overlap
3S overlap

The key is having a smart(er) balancer that has good hysteresis. A dumb balancer could run into oscillation like you suggested. I used this balancer, which is very damped:

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7443&Product_Name=CY-_B6_plus_balancer_(2-6S_series)
(I did not use the silly cutoff circuit. I just remove those wires)

What happens is exactly what you would expect, the lowest cell voltage gets propagated to the entire pack.
On my 24S2P pack the 4 groups of 6 cells looked like this:

4.15V
4.14V
4.17V
4.14V
(obviously the individual packs had been balanced already in this case)

I ganged up the balancers and within 10 minutes each 6S pack was sitting at 4.14V. Watching the balancer I could see that the 4.14V cells were never discharged at all. First the two packs at 4.15V and 4.17V, then near the end only the pack that was at 4.17V

Very reliable and very repeatable.
I will report back as I learn more.

-Patrick


EDIT: Keep in mind that prices on that website are dependent on the number of orders you have made. I commonly get 20% or greater discounts. After 10 orders you are "platinum" and receive maximum discounts. I think I only get a few dollars off that charger but I also have only 7 orders.

 

[cycle9]

Thanks again, I am definitely going to try that approach. I'll report back when I do.

Best regards,
Morgan