New 16-cell Battery Management System (BMS)

[drewjet]

I again cannot type what I am thinking. I read a few pages back that 4.2 volt was possible. What I meant was, is there a total pack voltage limitation, could the system used on a 700 volt battery?

 

[recumbent]

Nice work
Are those LED's that i see in the picture?
Does one light-up when one cell is not performing like it should?
What would be (roughly speaking) the cost for a kit, to attach to 16 Lifebatt's/48 volts?
I'm all fired-up to see these available for the summer of 2008.

 

[GGoodrum]

I again cannot type what I am thinking. I read a few pages back that 4.2 volt was possible. What I meant was, is there a total pack voltage limitation, could the system used on a 700 volt battery?

Yes, there is an upper limit, but I'm not sure what it is. Bob would know. It probably depends on whatever the zener diodes in the 5V regulator can handle. The 4310 FETs are good for at least 100V.

-- Gary

 

[GGoodrum]

Nice work
Are those LED's that i see in the picture?
Does one light-up when one cell is not performing like it should?
What would be (roughly speaking) the cost for a kit, to attach to 16 Lifebatt's/48 volts?
I'm all fired-up to see these available for the summer of 2008.

The orange LEDs are used during charging. When the cells start getting full, they start coming on. When fully lit, the shunt for that cell is in full bypass, so the cell is full. When all channels are in full bypass, the charge current is cutoff, the orange LEDS all go off, and the green LED is lit.

-- Gary

 

[efreak]

That is so great thanks a lot Gary and Bob. just like a christmas tree!! looks like spring should only be a couple of more months away i can hardly wait.
BTW I received the lvc and modules in great shape.(thanks a million) this seems so much easier just get the whole battery and bms package . and enjoy. definately on the wish list.

efreak

 

[bobmcree]

to answer a couple of questions that have come up:

the lvc can use 2.1 or 2.7v or higher voltage parts without increasing the drain of a microamp per cell. any voltage above 2.1v can be achieved by putting a pair of resistors on the input, increasing the current drain to 100 ua to maintain 1% accuracy.

the shunts can be adjusted to any voltage above 2.5v. with the values provided standard the range should be about 3.2-4.0v but resistor value changes could make it higher.

if you use the low voltage signal to trigger the ebrake signal on the controller you can eliminate the need for the bottom end cut-off fets that interrupt the power on this error condition. these fets control the maximum voltage string that can be managed, standard will be a pair of irfb4110s that will dissipate 5w at 50A continuous drain and will handle up to 100v. the board has locations for 2 TO220 fets so 150v or higher voltage devices could be substituted.

each shunt circuit is isolated to its own cell so that part of the circuit can be expanded indefinitely, and the outputs are combined with optic isolators so there is no real upper limit there either.

as gary mentioned, we use a 5v zener to produce the few milliamps needed and if the voltage were much over 100v the resistor in series would need to be increased, as well as the resistor value that is used to logically combine the shunt signals. i specifically used a zener rather than a 3 terminal regulator to overcome the voltage limitations of the latter.

the practical limit on the number of cells, other than the voltage rating of the cutoff fets, is the combined total leakage currents of the optos and diodes. i would put a practical limit at 100 cells and 400v.

 

[bobmcree]

Nice work
Are those LED's that i see in the picture?
Does one light-up when one cell is not performing like it should?
What would be (roughly speaking) the cost for a kit, to attach to 16 Lifebatt's/48 volts?
I'm all fired-up to see these available for the summer of 2008.

The orange LEDs are used during charging. When the cells start getting full, they start coming on. When fully lit, the shunt for that cell is in full bypass, so the cell is full. When all channels are in full bypass, the charge current is cutoff, the orange LEDS all go off, and the green LED is lit.

-- Gary

we have also provided for a variable delay period after the last cell stabilizes at the target voltage before the charge cycle is terminated, during which time all cells are at the set final voltage. this permits the last cells that reached the full voltage to charge at that voltage to their full capacity. we were considering setting this to 15 minutes nominal but it can be seconds or hours. the limitation of course will be the current at which this final portion of the charge cycle operates, and the size of the heat sink provided.

 

[Deepkimchi]

Whoops - reckon my question about the diode connection was off topic - never got an answer.

Maybe Bob's applying for a patent :lol:

 

[bobmcree]

Whoops - reckon my question about the diode connection was off topic - never got an answer.

Maybe Bob's applying for a patent :lol:

i'm not sure what you are talking about? you posted a link to a fast high current diode that has way too much forward drop to use for sharing battery packs. the 40CPQ100 diode i typically use is 100v 40A and has about .55v forward drop which is 1/3 that of the diodes you suggested.

there is nothing patentable about our simple bms design. i thought i answered the question with my response about using a separate bms for serial connected strings or a single unit for clusters of cells in parallel. if there is some question i did not answer i will be happy to do so.

 

[Deepkimchi]

Nah - that's why I thought I was off topic. The question wasn't about your BMS.

I couldn't figure out whether the diode was placed across the battery poles the same polarity or reverse polarity. It's an outstanding idea from you, I just don't dabble enough to be sure about the hookup.

The other question - I'm sitting on $ 600 worth of a 36V 12 ah pack, if I used your individual LVC, could I attach a different format and AH pack (ie 6 cells of 10ah - 40138) in series to my existing pack?

Would have 2 packs in series each with your LVC and diode setup, so I wouldn't have to worry about the controller LVC.

DK

 

[bobmcree]

Nah - that's why I thought I was off topic. The question wasn't about your BMS.

I couldn't figure out whether the diode was placed across the battery poles the same polarity or reverse polarity. It's an outstanding idea from you, I just don't dabble enough to be sure about the hookup.

The other question - I'm sitting on $ 600 worth of a 36V 12 ah pack, if I used your individual LVC, could I attach a different format and AH pack (ie 6 cells of 10ah - 40138) in series to my existing pack?

Would have 2 packs in series each with your LVC and diode setup, so I wouldn't have to worry about the controller LVC.

DK

if i can help explain the other issue more completely i will be happy to but i don't really understand the question. of course if you put a 10 Ah pack in series with a 12 Ah pack you will run out of power at 10 Ah. if you have the lvc on each cell the outputs are combined with optic isolators so the cell voltage could be different. each cell has its own independent comparator. if the total pack voltage is <100V you would not need any additional diodes but would just hook up one or our bms systems to each set of up to 16 cells. the fets that cut off power on a low voltage condition need to handle the full pack voltage so if it is >100V we could use different parts.

 

[Deepkimchi]

Thanks Bob - that answered it. Hope we are not bombarding you too much with questions..

Was just trying to figure out what options I had with my existing pack...

 

[Patrick]

Hi Gary and Bob:

If I wanted to use the BMS on a 72 volt pack, of course I would two of them; with one jumpered (and perhaps less populated). Would I also need to use two chargers (either isolated, or break the series connection between packs)? I was assuming I would, but then thinking about it it wasn't obvious.

Where are you at in terms of production?

Thanks for such great work!

 

[bobmcree]

Hi Gary and Bob:

If I wanted to use the BMS on a 72 volt pack, of course I would two of them; with one jumpered (and perhaps less populated). Would I also need to use two chargers (either isolated, or break the series connection between packs)? I was assuming I would, but then thinking about it it wasn't obvious.

Where are you at in terms of production?

Thanks for such great work!

with a couple of jumpers it would be possible to use the low voltage cutoff fets and the charge current regulation and control functions of one board, and just use the shunt and lvc sections of the second board to extend the serial string. This would permit use of a single charger >3.65v/cell. Alternately, the 72v pack could be split into two completely separate 36v subsystems each with its own fully functional bms for pannier or other split mounting situations.

gary has corrected a couple of errors on the first run of pcb's (my fault) and i am finishing up on the final test and documentation right now. i believe he will have kits available next week, assuming that the final testing i am doing now turns out ok. the design is very simple and is intended to be as fail-safe as possible, but there can always be something unexpected, so i am putting in a little extra time up front in the hope of reducing the need for changes later.

 

[GGoodrum]

Yep, all you would need is to populate the first 6 channels, top and bottom, on the main board, and then the last 6 channels, top and bottom, and then run two 14-16-gauge wires plus four small gauge wires to connect the opto channels etween boards.

Here's the completed first board, which I'm testing:

Here's another shot with a temporary cover:

As Bob said, we've made a few changes, mostly minor stuff, like changing resistor values and adding a cap here and there, but I think th design now is pretty well fixed. I got new boards today.

I'll post some more pics when I get this installed on a 16-cell pack.

-- Gary

 

[jdh2550]

For us lazy and/or soldering iron challenged folks when do you think you'll begin offering assembled systems for use with other battery packs? (If I read the thread right you're going to wait until you go to surface mount before offering the assembled systems)?

The board looks great and I REALLY appreciate the amount of thought you've put in to allow it to be used with the most flexible array of options (rather than insisting people buy the whole charger / BMS / battery pack from you and only you). GREAT JOB. Many thanks.

Of course, if it's going to be "a while" before the assembled system comes along I guess I'll just have to get off my butt and brush up on my soldering skillz...

 

[GGoodrum]

Actually, the building of the boards is not all that difficult, but unless you are a trained assembler, it can take quite a bit of time, as there are 284 parts. I am not a trained assembler, so until I can find a place to have these put together for a reasonable cost, I think it will have to be kits-only, for now. Who knows, maybe someone here will want to make a little extra money, and provide an assembly service. In any case, Bob and I are doing a detailed set of instructions, with lots of pictures,for assembly, testing and for making the proper adjustments. I hope to have the first kits available early next week.

-- Gary

 

[chas_stevenson]

I really like what I have seen here and think you guys have a winner. I would like to build a 36-volt pack using a123 batteries using your product.

When you guys get the instructions finished I would like to offer my time as a test subject. I have many years of soldering skills as well as 16 years of creating instruction manuals for the military. I would be happy to purchase the kits I build, maybe at cost, and test your instructions for the masses. I would then give you written feedback on the instruction sheets.

Thanks,
Chas S.

 

[recumbent]

Okay, enough teasing us with these awsome looking boards, do we just place the components in the proper holes and solder them in place?
I have a large magnifier with light attached to my work station, fine solder wire, and tweezers, do we need anything else besides the iron & a steady hand?

 

[karma]

Actually, the building of the boards is not all that difficult, but unless you are a trained assembler, it can take quite a bit of time, as there are 284 parts. I am not a trained assembler, so until I can find a place to have these put together for a reasonable cost, I think it will have to be kits-only, for now. Who knows, maybe someone here will want to make a little extra money, and provide an assembly service. In any case, Bob and I are doing a detailed set of instructions, with lots of pictures,for assembly, testing and for making the proper adjustments. I hope to have the first kits available early next week.

-- Gary

its nice to see someone offering kits, put me down for one if you cant find a assembler in your area keep me in mind. i have a room setup for this type of work. extra money? dont need it just throw me some samples


cheers

 

[GGoodrum]

Okay, enough teasing us with these awsome looking boards, do we just place the components in the proper holes and solder them in place?
I have a large magnifier with light attached to my work station, fine solder wire, and tweezers, do we need anything else besides the iron & a steady hand?

Yep, that's pretty much it.

Bob mad a couple more small changes yesterday, mainly in the area that controls the charge current, but I don't think it requires any board changes. He's also generating the test and adjustment section for the instructions. I'll post an update, later in the day. We should be very close.

-- Gary

 

[bobmcree]

no special tools are required to properly adjust and test the boards, but you do need a variable power supply that can be adjusted from <2v to >4v that will supply at least .5 amp, and any decent digital meter of 0.5% accuracy or better. It is very helpful but not absolutely necessary to have a variable voltage and current limited supply that will provide 0-60v with adjustable current limiting to be used to power the charge circuit, but we know most people will not have this capability, so we will include instructions for building and testing the kits with a minimum of test equipment. once the shunts are properly adjusted, any charger capable of providing at least an amp at adequate voltage to exceed the total voltage settings of all the series shunts can be used for testing and to charge a connected battery pack.

When the bms is operating in the current limited final charge stage the required voltage drop across the current limiter may cause the supply to see a bit higher voltage, so it is important that the supply not shut down under these conditions. for this reason it is probably best to use a supply capable of providing as much as 4v/cell without sensing a false end of charge condition and shutting down.

 

[rsisson]

QUESTION: What do you need as a Charger SOURCE for the BMS to work...

The comercial chargers are $$$ but since this SHOULD be replacing the brains of the charger, is all we now need a suitable open frame supply ?

There are some very robust 48v-72 supplys available that aren't to expensive that can sink 10-20 amps...

http://www.powersupplydepot.com/productview.asp?product=16510+PS

or do we need at least a SLA charger

http://www.infinigi.com/dls54415-battery-charger-internal-p-1516.html

 

[GGoodrum]

Actually, you can use any supply or charger that can output about 3.7V x the number of cells. For a 12-cell/36V pack, that means you need 44.4V. the 48V supply featured in the link will work fine for a 12-cell pack, as it can be adjusted +/- 10%. You would just need to dial it down a bit, to 44.4V. For a 16-cell/48V pack, you need a supply capable of delivering 59.2V (3.7V x 16...). Although usually more expensive, you can also use variable supplies, like the HP 6247B 0-60V 0-15A. Bob found one of these on ebay for me that ended up only costing me $125. It turns out the seller (OCDepot...) is about 15 minutes from my house so I didn't even have to pay shipping.

You can also use any 36V SLA-type charger that can either output at least 44.4V, or be able to be adjusted to that value. I have two Soneil chargers, one for the 36V (3610SRF...) and one for 48V (4808SRF...)that the factory "tweaked" the outputs for me to outout 44.5V and 59.5V, respectively. These will work fine with the BMS. Also, the SLA-type chargers don't really have all theat much smarts in them. They are basically just CC/CV supplies with a crossover set for somewhere between 2.40-2.45V per SLA cell. For a typical 18-cell 36V SLA setup, the crossover is usually set around 43-45V. That makes it convenient for using them with LiFePO4 cells because for charging purposes, 6 SLA cells roughly equals 4 LiFePO4 cells, which optimally need 3.65V as a CC/CV crossover point.

What the shunts in the BMS do is to basically control the constant voltage (CV) portion of the charging profile, but lets each cell do this at its own pace. It is equivalent to having individual CC/CV chargers on each cell.

The nice thing about using lab supplies is that you can charge with higher currents, so it really cuts down the charge time. The BMS doesn't do anything until the cells get to about the 85-90% full stage, when the voltage for any cell first hits 3.65V. Whenever the first cell reaches that level, the shunt for that cell clamps the voltage at that level. It also sends a signal to the BMS charger control logic to throttle back the current to about 2A. This is actually going to be adjustable from about .5A to about 10A. It totally depends on how big a heatsink is used. Once all the cells have reached the cutoff, where there shunts are in full bybass (which says that cell is full...), the charge current is cutoff completely, the individual orange LEDs all go out and the green LED is lit. In any case, 2A is a pretty good "throttle back" value, as it should only take about 10 minutes to finish, unless the cells are really out-of-balance.

-- Gary

 

[PJD]

Gary,

Is lower voltage float/maintenance charging ever done, or advisable, with liFePO4's?