Linear Tech 12-cell battery monitor/BMS

[fechter]

I have 3 units sitting here on my desk that are missing (or at some time have lost) their magic smoke. To be clear - none of these were "mysteries" - all had a clear cause and effect - sometimes a long time in the finding - the cause being either me or the customer in all cases.

Those look like pretty impressive burns. That's how you learn

Yes, that PWM stuff really raises hell with all sorts of things. At least you have the option to turn off things during the measurement. Trying to maintain accuracy to within a few mV with kilowatts of noise flying around is difficult.

Anyway, looks like you're making progress. Very nice

[methods]

I have been paying for my education alright

I have a lot of experience with firmware and electronics but I am definitely still learning about power engineering. Lately I have been trying to wrap my head around heat-sinking requirements. Luke has been helping me to try and estimate how many fets and how much heat sink I need to do a completely passive system. When you add the 50A 4khz PWM to the mix the power handling requirements go thru the roof! I think we have a handle on it and I am going to start building prototypes soon. Unfortunately with V3.0 I never had a lot of time to prototype so I ended up having to modify the circuit after the PCB's were spun out.

Having the option to turn off the Shunts and the PWM makes all the difference in the world at the last stage of balancing. Once all the cells are around 4.18V (as you know) just about any balance circuit will go into oscillation if there is any kind of noise. I chased my tail for a few weeks until I figured that out.... My shunt display used to play like a ghost piano.

I was looking at your latest creation..... Looks like you are using a similar zener approach to protect your TC-54 chips. I was not aware that you were popping those but it makes sense. I know you are the cheapest bastard on the planet so those zeners must really be necessary or you would not have them.....

-methods

[methods]

Ouch.... Built an 18S 10Ah pack for a ride with a new Space Monkey and something went wrong... I think I might have actually reversed cells for the first time. After charging for 30 minutes the lowest cell was at 1.5V. Now after about an hour I am still not even up to LVC

The two packs that were low were HOT HOT HOT

001_FUBAR.jpg (33.05 KiB) Viewed 785 times

Ignore the cells above 18
19 and 20 are short circuits, 20 - 25 are open circuit

-methods

EDIT: By "Went wrong" I mean that I did a crappy job building the pack. Nothing happened with the BMS - there was no BMS - As it obvious from my results

[liveforphysics]

Wait... are you saying the goal isn't to run your LiPo cells down low enough to reverse cell voltage?

[methods]

It is official - for the first time I have ruined lipo cells out of pure nOoBieNess
I think I might actually put a BMS on my "loaner" bike now

-methods

[methods]

I have stopped reading PM's from people asking for help on the LTC.

On a brighter note - I have been patching up those units that were damaged in the field. The protection circuitry worked great - on one unit I only needed to replace a 100 ohm resistor and on the other I need only to replace the PWM circuits. Isolating each section of my board was a wise move in hind sight..... Did not lose a single LTC in all that carnage

The vertical mount shunts are still good too - no failures yet.

My only real complaint is that I tried to stuff everything into such a small box... Makes rework a living hell

-methods

[heizal]

hi all.
I'm heizal from Malaysia.
actually I'm beginner in battery management system currently do project using LTC.
I would like to ask a question.
>May I know what is the ideal voltage different between each cell so that can start doing discharge for balancing all the cell?

tq

[amberwolf]

May I know what is the ideal voltage different between each cell so that can start doing discharge for balancing all the cell?

0V

[heizal]

I hope someone can help me.
1) what is allowable tolerance of voltage between each cells so that it can be balance? the voltage variation rapidly change and if 0V difference to be balance the cell, the discharge operation will do continously.
2) when stack more than 1 board, I found that the voltage reading in second board is different. Voltage reading is lower than actual. Is it my firmware design wrong?

Hope someone can help me as i'm beginner in this field.
TQ in advance

[methods]

I hope someone can help me.
1) what is allowable tolerance of voltage between each cells so that it can be balance? the voltage variation rapidly change and if 0V difference to be balance the cell, the discharge operation will do continously.

Damping

0V is fine - but your circuit will just be under damped or in oscillation
Anything greater is also fine - it will just drive your accuracy - right?

How about 20mV?

I do 3mV.... But then I also do a lot of things to make sure I take good readings - things like turning everything off while my A/D's are settling.... Taking the naive approach will result in very noisy readings (i.e. allowing the LTC to control the shunts while you take readings). I turn them all off manually -> Resulted in significantly less oscillation.

2) when stack more than 1 board, I found that the voltage reading in second board is different. Voltage reading is lower than actual. Is it my firmware design wrong?

Hope someone can help me as i'm beginner in this field.
TQ in advance

You need to include a lot more information here.... but right off I would guess you have a grounding issue between the boards. Could be firmware, could be hardware, could be wiring. My guess is that you are rigging together demo boards with too long of a ground lead. Is is just cell number 13 that reads low or 13 - 24? If it is 13 - 24 it would probably be a firmware issue right?

Think it thru and include all of the details. You will know that you included enough detail when you answer your own question before finishing the post

-methods



P.S. From that WIKI -> this is what you want.

Critical damping (ζ = 1)

When ζ = 1, there is a double root γ (defined above), which is real. The system is said to be critically damped. A critically damped system converges to zero faster than any other, and without oscillating. An example of critical damping is the door closer seen on many hinged doors in public buildings. The recoil mechanisms in most guns are also critically damped so that they return to their original position, after the recoil due to firing, in the least possible time.

In this case, with only one root γ, there is in addition to the solution x(t) = eγt a solution x(t) = teγt:[2]

x(t) = (A+Bt)\,e^{-\omega_0 t} \,

where A and B are determined by the initial conditions of the system (usually the initial position and velocity of the mass):

A = x(0) \,

B = \dot{x}(0)+\omega_0x(0) \,

[heizal]

Is is just cell number 13 that reads low or 13 - 24? If it is 13 - 24 it would probably be a firmware issue right?

-> all second boards cell connected (13-24) will read low reading. i also suspected something wrong with the firmware and will check again
thanks for your explanation.

[heizal]

Hi Method,
I detail my question.
I'm using DC1393B and DC590B board (programmed with own firmware).
The first DC1393B is connnected with 4 cells and second board DC1393B also connected with four cell (the connection as in Datasheet DC1393B page 3).
Connection between 2 boards are as in Configuration D Daisy chain SPI communication (refer to Datasheet Dc1393B page 12) with all the jumpers on the board set correctly.
The voltage reading for each resisitor connection is about 2.8V.
Then I run the firmware, each cell in first board shows 2.8V (same as actual), however in second board all cells shows only 2.6V each(lower than actual which is 2.8V).

[methods]

Beware - the latest version of the LTC6802-2 demo board has a retarded mistake
Schematic

The inductor BLM31PG330SN1L is inline with cell 12+ between the cell and the shunt so if you want to make a *real* bms and not just a demo board you had better move that 33ohm inductor behind the shunt...

unless you want a BMS that does X amps on every cell but only 100mA on cell 12

My V4.0 is ready for fab. All the fans and silly shit are gone. I am now using TO126 two ohm shunts in potted slave assemblies. Each slave does 4S-12S and has 2 RTD's. Slaves terminate in an 8" 25 pin flat flex cable that can be fitted with a custom board per customers request. Lots of cool new features.

I am no longer contracted by the company that asked for version 1-3.... this is a totally new and improved version that looks/acts/feels/is a totally new product. I will start a new thread soon.

-methods

[bigmoose]

Methods, isn't that part no BLM31PG330SH1 It's a ferrite bead, with 33 ohms at 100MHz, DC is 0.01 Ohm. I thinks it's an RFI/EMI filter for motor/load "trash" on the power line to the LTC part. Did you find it influencing the balance currents?

Congrats on your new design. Wishing you all the best with it!

[methods]

Methods, isn't that part no BLM31PG330SH1 It's a ferrite bead, with 33 ohms at 100MHz, DC is 0.01 Ohm. I thinks it's an RFI/EMI filter for motor/load "trash" on the power line to the LTC part. Did you find it influencing the balance currents?

Doh - you are right. I just read 33 ohms in the BOM and made a poor assumption. I moved it out of the balance balance route anyway .


Here are some pictures of a slave unit. Final form will be a 1"x1"x4" block of epoxy/Aluminum with 20cm of flat flex coming off of it. The block will have to dissipate about 120W peak so it gets bolted to the frame. It can be run without additional heat sinking and ballance currents will be limited from 2A down to whatever can run while keeping the LTC below 85C.

The other end of the FF is where the water tight connector housing sits.

Yes I am aware that the FF is only 30awg. That is fine for 2A - especially with the insulator these have - and I dont take measurements while the shunts are active - so I dont have the accuracy problems that Gary and Fechter have to deal with. I am running 100 ohm resistors inline as it is.

-methods

[bigmoose]

Very, very nice! I take it you routed the board and can get all the traces off pins 1-22 of the LTC chip? The ones close to the edge of your board.

I like how you are getting rid of the heat. We have only 3 options, convect it with air; conduct it with liquids or conduct it into structure. When you get small in electronics, heat is the problem. Just like fuel cooled avionics in high performance aircraft, the heat has to go somewhere or you are toast.

The one thing I don't like about the LTC part is if you re-power it, it resumes the factory default until you reprogram it. I heard there was a potential dash number revision being considered that would read state information from an eeprom... but don't know for sure it will be introduced. I also wish the pins were on 1mm intervals in lieu of the 0.5mm.

[texaspyro]

Brings new meaning to the words "printed circuit"...

[methods]

Very, very nice! I take it you routed the board and can get all the traces off pins 1-22 of the LTC chip? The ones close to the edge of your board.

This is actually the first PCB I have ever made totally on my own so yea - it is routed - but I used the AutoRouter. I set up an LTC ground and a communications ground on the second layer. I have two autorouting profiles - first one runs only grounds and punches a via as close as possible to the pin. Second one routes all the rest starting with the wides. I have 3 classes - 24mil for the shunts, 10mil for power, 8mil for signals. All chosen pretty arbitrarily - and I know the shunting traces are going to get quite hot. I did the calculations for a 60 degree temperature rise on a 65C ambient board and it worked out good enough.

route.png

(50.13 KiB) Downloaded 143 times

I like how you are getting rid of the heat. We have only 3 options, convect it with air; conduct it with liquids or conduct it into structure. When you get small in electronics, heat is the problem. Just like fuel cooled avionics in high performance aircraft, the heat has to go somewhere or you are toast.

Well - I tried the surface mount and radials and found that they both were totally useless unless spec'ed at 20W or blown constantly with a fan so this is the next logical step. The TO126 are 20W and at 85C they derate to 9W which is about the peak that I can run them at. So long as I can keep the aluminum below 85C I can run at full power - but even 1/2 power is 5X the balance current of most systems so it will be fine.

The one thing I don't like about the LTC part is if you re-power it, it resumes the factory default until you reprogram it. I heard there was a potential dash number revision being considered that would read state information from an eeprom... but don't know for sure it will be introduced. I also wish the pins were on 1mm intervals in lieu of the 0.5mm.

I dont understand what you are getting at here - so you are going to be putting it to sleep by powering it down? The sleep mode on these is pretty good - what in the 10uA - 100uA range? Yea... I guess that with a smaller pack that could be too much - but it wont take but a few hundred uS to reprogram it each time you wake up and over time that wont even add up to 1uA.

The way I am using the chip I am pretty much constantly sending it commands and I have it set up to use the 2 second watch dog in case the uC dies. I dont really care too much about the power it eats up while sleeping - the packs I am working with are 80Ah.

-methods

[liveforphysics]

I love the TO-220 shunt choice.
Huge improvement in thermal path.

Good work! I'm proud of you!

[grindz145]

so sick.

[fechter]

I like the direct conduction cooling too. Fans suck. I did some testing using the regular metal film axial resistors mounted to an aluminum bar with some silicone. This does a nice job of sucking the heat out and can be made water resistant.
Rule of thumb for power resistors, as far as I can tell, is they need to be rated for about 4x what you want to run them at.

[bigmoose]

Thanks for the clarification Patrick. I should have picked it up from the silkscreen. I have selected a topography where the the LTC chip is always powered, and I have the controller powering up the micro when it (the controller) is active to monitor LVC stuff; and likewise the charger powers up the micro side to do balancing and charger control; else the LTC is semi quiescent after loosing data link.

Just an aside from a micro problem I was asked to trouble shoot last week. They had a clock line 6 inches long with a bifurcation about 4 inches in with 7 nS edges on the waveforms. Were getting reflections off the impedance change in the trace so I had them add series termination impedance (my favorite 10 ohm resistor) to the clock and data lines. It damped the jitter. Might be worth considering laying down two 0805 pads with the trace through them, so if there is a problem you can scratch cut the trace and add the resistor for the clock and data lines, unless your design has them slowed and shaped through an optoisolator.

[methods]

Just an aside from a micro problem I was asked to trouble shoot last week. They had a clock line 6 inches long with a bifurcation about 4 inches in with 7 nS edges on the waveforms. Were getting reflections off the impedance change in the trace so I had them add series termination impedance (my favorite 10 ohm resistor) to the clock and data lines. It damped the jitter. Might be worth considering laying down two 0805 pads with the trace through them, so if there is a problem you can scratch cut the trace and add the resistor for the clock and data lines, unless your design has them slowed and shaped through an optoisolator.

I am using RS-485 drivers for my SPI lines.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ISL8485IBZ-ND

End of the line gets terminated at 100 ohms.

Each driver set also has a digital isolator between it and the LTC. Speeds are slow too - 100khz is plenty fast. I had too much noise @ 1Mhz with all the EV noise.

-methods

[methods]

I finally have the Master section worked out. Steve had the great idea to break it up into two boards and stack them to save space - worked perfect. Only a 5 pin header goes between the 150V PWM section and the logic section and there is 100% galvanic isolation.

It may seem that switching 150V 50A only mm away from a uC would spell doom - but on V3.0 I never had any trouble what so ever. One major aspect is the true isolation - and the fact that I am only switching at 4khz. Either way - since we are using a daughter board it will be easy to add shielding (or distance) if any noise issues do crop up.

This is a rough cut but basically what it will look like. The lower board is the PWM section - meant to have 4 studs poking out of the bottom of the board. yea... yea... I know that the fet driver is not optimally situated but at 4khz I really dont think it is going to make much difference. The fets are arranged similar to ebike controllers where they will have a vertical heatsink. The middle of the board has an isolated 100A hall effect current sense that is used to monitor charge current (or... possibly another current... that we wont speak of now).

The top board has all of logic - opto couplers, isolators, drivers, regulators, etc. I moved the relays off onto a satellite board so that they could switch higher current than my 30AWG flat flex connectors can handle. The brain has 50 pins of logic coming off of it - everything from communications, opto outputs, relay drivers, and a bunch of extra stuff. The two flat flex cables (25 pins each) are stacked one atop the other and exit from the right side of the board.

I stream an isolated USB output, isolated TTL 9600 baud output, I take in the CA data stream in, and I have a few isolated spare comms channels for talking to aux devices like GPS, accelerometers, etc. I am still using the Aruino code base but this time it is on the big-boy chip.

On the PWM board there are four IRFB4115 fets to handle the charge current along with 10uF of 250V ceramic caps (no electrolytics anywhere in the design) The two other TO220 packages are actually Schottky diodes that are unfortunately there to make up for my lack of cleverness... Basically they isolate the battery from the DC-DC so that the BMS automatically powers up if a charger is applied OR if the vehicle is turned on. This allows the BMS to sit at a very low trickle current for storage. I could have had the leakage current in the 100uA range but I got cheap at the last minute and started spec'ing zeners with higher leakage rates. The ultra low leakage zeners get expensive!

anyway...



-methods

[fechter]

Cool.
I found that the actual leakage on zeners will be much less than the rating if they are not operated too close to the zener voltage. I had some 6.8v units that were 100uA according to the datasheet, but measuring them at 4.2v I only saw about 5uA. As the voltage gets closer to the zener voltage, the leakage goes up, but it's non-linear.