"Zephyr" - Finally, the "v4" Fechter/Goodrum/Hecker BMS...

[megacycle]

Hi guys enjoying the thread.
Like your lite for a A123, 20AH, 24S putting together, just got a question.
I'm concerned about short circuit protection from the battery pack,noticed many bms involved in battery packs with high s/c currents are generally mounted atop the battery, was hoping to look at a mod to your lite setup for the A123's that would suit the Al tabs as well, but can't find your circuit diagrams, only pcb layouts, i suppose it would, understandably,be proprietary to you, but would like to look at keeping any connections leaving the pack short and protected at the source end, don't like the idea of external wiring or a pcb track acting like fuse wire if a fault occurs.

[fechter]

Unlike most other BMS, the discharge line does not pass through the circuit. You need to use your own fuse/wiring to the controller and the controller does the current limiting. Only the charge line goes through the circuit.

The low voltage cutoff acts on the throttle or key line going to the controller to stop discharge.

[SamTexas]

Unlike most other BMS, the discharge line does not pass through the circuit.

To me, that is one of the best feature of your BMS.

When are you guys going to have a website or a sale thread for the many versions of your BMSs? I want to be able to compare different versions against one another to decide which is best for me. But wading through several long long long long threads is not my cup of tea. May I suggest that you start with a comparison matrix?

[megacycle]

Thanks Fechter and Sam for the endorsement.
I'd like a setup to connect to the 24S pouches was it yourself to pm for it fechter?

[fechter]

When are you guys going to have a website or a sale thread for the many versions of your BMSs? I want to be able to compare different versions against one another to decide which is best for me. But wading through several long long long long threads is not my cup of tea. May I suggest that you start with a comparison matrix?

This whole process has been agonizingly slow but we can't sell stuff that doesn't work properly.
Gary will be reviving his web site once we have product to sell. This should contain a description of the various options.
The latest revision boards are getting made now, and once again we have to build/test to make sure there aren't any mistakes in the layout. We'll post here once things are up and running.

[GGoodrum]

This whole process has been agonizingly slow but we can't sell stuff that doesn't work properly.
Gary will be reviving his web site once we have product to sell. This should contain a description of the various options.
The latest revision boards are getting made now, and once again we have to build/test to make sure there aren't any mistakes in the layout. We'll post here once things are up and running.

I have started offering the "Lite" version, which uses 6-channel or 8-channel boards that can be embedded in with the packs, and uses a separate charge controller that is mounted in a small box, but these are really geared towards LiPo setups because each of these boards also acts as a parallel adapter for the individual 5s, 6s or 8s LiPo packs that make up the completed battery. The reason these are called "Lite" is because they have lower shunt currents, which means they don't generate a lot of heat, like the full Zephyr circuits can do, which is why they are mounted in an aluminum box. Healthy LiPo cells/packs tend to stay prett well balanced, so large shunt bypass currents aren't really required. Also, since the Lite 6s and 8s boards are typically embedded in with the packs, there aren't any individual LEDs on each channel. All of this reduces the complexity of the boards and lowers the parts count, which makes them much easier and faster to put together. Here's what a typical 12s LiPo setup would look like:

For LiFePO4 configurations, I'd really recommend the full Zephyr setup. It will be available soon, hopefully before the end of the year. The exception might be for a123-based setups, because it has been my experience that treated properly, these high-C LiFePO4 cells also tend to stay pretty well balanced.

In any case, I do like the comparison chart idea, and I will try to get this done soon. Also, I hope to get a new site up before the end of the year as well.

-- Gary

[SamTexas]

In any case, I do like the comparison chart idea, and I will try to get this done soon. Also, I hope to get a new site up before the end of the year as well.

Thanks. I look forward to reading both.

The reason these are called "Lite" is because they have lower shunt currents, which means they don't generate a lot of heat, like the full Zephyr circuits can do, which is why they are mounted in an aluminum box. Healthy LiPo cells/packs tend to stay prett well balanced, so large shunt bypass currents aren't really required.

This is one of the areas I don't fully understand. So "healthy" cells tend to stay pretty well balanced, but "healthy" cells age and become "less healthy". Will the Lite version still be capable of balancing those cells? In other words, is the Lite version capable of balanced charging a drastically unbalanced pack right now? How many hours does it take to balanced charge this hypothetical 6s 5Ah pack: 3.3V, 3.5V, 3.7V, 3.9V, 4.0V and 4.0V with a 5A power supply? 3 hours or less? More than 5 hours?

[GGoodrum]

This is one of the areas I don't fully understand. So "healthy" cells tend to stay pretty well balanced, but "healthy" cells age and become "less healthy". Will the Lite version still be capable of balancing those cells? In other words, is the Lite version capable of balanced charging a drastically unbalanced pack right now? How many hours does it take to balanced charge this hypothetical 6s 5Ah pack: 3.3V, 3.5V, 3.7V, 3.9V, 4.0V and 4.0V with a 5A power supply?

Yes, both the Lite, and the full Zephyr circuits will automatically balance even a grossly imbalanced pack. The difference between the two is simply the amount of time it takes. The full Zephyr circuits have shunts that will bypass about 600mA of current, while the Lite boards will bypass about 120mA of shunt current. So, it theoretically would take the full Zephyr board 1/5th the amount of time, to do a complete balance. In reality, it is probably more like a 4:1 difference. By comparison, most of the imported BMS boards cn only manage 50-60mA.

Anyway, when I say healthy cells, I mean ones that haven't lost a lot of their capacity yet. With LiPos, even as they age, the capacity numbers might start dropping off a bit, but generally, I've found this to be pretty uniform across a pack. One thing that will get even a healthy pack out-of-whack is when you discharge a pack down close to LV cutoff. The protection will save a cell from killing itself, but as the voltage gets close to the proverbial "cliff", the resting voltage will start dropping at a much quicker rate. this is where even very slight differences in capacities will show up. Anyway, when this happens, you can have some pretty significant imbalances. On the other hand, if you only discharge down to about the 15-20% level, the LiPo cells will stay pretty close.

I've had quite a bit of experience with a123 M1 LiFePO4 cells, which tend to act very much like LiPos, if you don't discharge them to LVC. Actuall, I had lots of times where a cell went ahead, and jumped off the cliff, ending up basically with a voltage under 1V. When this happened, I could recover them, by doing a very slow charge, but generally these "stressed" cells ended up losing about 10% of their capacity. From that point forward, a pack with stressed cells would still perform about the same, but it would need balancing with every charge, for sure.

As for your example, I can only make an educated guess. It looks like, with those voltages, overall the pack is about half empty. At 5A, it would probably take 1/2 hour, to 45 minutes to get to the point the shunts for the high cells start turning on. With imbalances that large, however, he shunts alone will probably not be able to hold the voltage for the high cells at 4.15V, so the upper HVC signal will start tripping, causing a cycling of the charge current. Because the full Zephyr shunts can pass more current, I'd guess this HVC cycling won't last all that long before the low cells have caught up enough that the shunt circuits alone can keep the high cells in check. Eventually, though, the Lite shunt circuits will get there as well, but how much longer, I don't know. Offhand, I'd guess maybe an hour?

In any case, both will get there eventually. BTW, all RC balancing chargers that I'm aware of, would not charge a pack with imbalances over 200mV, or .2V. Your example is four times that. Personally, if I had a pack that badly imbalanced, and it wasn't as a result of discharging to LVC, I'd keep a close eye on it, or take it out of commission.

-- Gary

[megacycle]

I have started offering the "Lite" version, which uses 6-channel or 8-channel boards that can be embedded in with the packs, and uses a separate charge controller that is mounted in a small box, but these are really geared towards LiPo setups because each of these boards also acts as a parallel adapter for the individual 5s, 6s or 8s LiPo packs that make up the completed battery.

For LiFePO4 configurations, I'd really recommend the full Zephyr setup. It will be available soon, hopefully before the end of the year. The exception might be for a123-based setups, because it has been my experience that treated properly, these high-C LiFePO4 cells also tend to stay pretty well balanced.
-- Gary

I'm thinking of charging pouches at up to 2.5C, i expect the 50A charge current may increase required shunting for my pack if its out of wack? If this is correct i expect waiting for new zephyr would be advised as suggested.
Though if not the case with the pouches and then i still get the lite, i assume i can simply remove the parallel adapters?

This whole process has been agonizingly slow but we can't sell stuff that doesn't work properly.
Gary will be reviving his web site once we have product to sell. This should contain a description of the various options.
The latest revision boards are getting made now, and once again we have to build/test to make sure there aren't any mistakes in the layout. We'll post here once things are up and running.

It's commendable that you guys are so commited to wanting to produce such perfect art and I appreciate your time out spent answering my seminewb questions.
I would gladly purchase a lite as part experimental on my pack. I am considering logging each cell anyhow, so no worries.(Aussie coming thru ).If it's not suitable, then i will just use it elsewhere as i am building other packs.

[baerfoot]

I want to get one of these BMSs for my pack of 24 100AH Hipower LIFEPO4 cells, but the TPpacs web site is down. Is there another way to order it?

[drunkencat129]

was wondering i need bare boards u got some in stock ill send u pay pal payment for a 24s lipo bms

[GGoodrum]

I started a "for sale" thread for the BMS Lite kit combos here: viewtopic.php?f=31&t=34495#p501227

Everything is in stock, and the kits will go out the same day they are ordered, from this point forward.

The full-size Zephyr boards/kits will be available in another couple weeks. Still some testing to do, which is unfortunately a time-consuming, but necessary process.

Anyway, the LiPo-centric "Lite" combos are ready-to-go.

-- Gary

[The Mighty Volt]

Hi Gary, thanks for your time and efforts on this front, have there been any developments in the full-boards situation over the past few weeks? Many thanks. Happy New Year.

EDIT: How much do the boards figure to be sold for, and how much for the full BoM. Also, can you please recommend a particular brand of solder and soldering iron? Thanks again.

[cor]

Gary, fechter, others,
I compliment you guys on doing an amazing job of designing and testing and even selling a BMS that should be rock solid as it does not suffer from SW bugs and other issues associated with a control processor.
There are a few issues that I do not see addressed and that is why I want to bring them up, so at least people are aware of them and possibly some might even lead to improvements of the BMS. These issues are not important as long as things go well, it is looking at the brand spanking new BMS that you have finally working well and shipping and asking myself: what if...
I will post under 3 separate headers, as I see these 3 separate issues:
- Fuses
- cell undervoltage
- cell overvoltage

The most important and immediate issue is regarding the use of fuses in the electric drive system.
All the drawings that I see, have the packs connect directly to the BMS Charge Controller without even a suggestion about protection.
For safety safe - especially when working with high voltage, high power battery packs, I suggest to have each circuit that includes a battery to also contain a proper fuse. With proper I mean DC rated, at least the voltage that the pack or charger can deliver and rated to break the peak short circuit current without exploding or creating a welding arc.
In theory the fuse can be placed anywhere in the circuit; in practice every commercial system has a fuse as the first thing from one of the battery terminals - personally I like it when you have series connected battery packs as in your drawings, to have a fuse between each contact where a red (pos) wire plugs into a black (negative) wire, because this even protects for cases where battery packs get damaged and short to the box or frame.
You might even use both fuse(s) and a breaker, the fuses to protect as last resort while the breaker is rated to disconnect as soon as the max output is delivered for longer than the controller or motor can sustain - this will protect the system from overload, even if it is lower than the max (fused) capacity. If the breaker pops during a long overload (like, driving up a hill at max throttle) then it can be reset and you can continue your trip.

[Alan B]

Some specifications call for a fuse within a certain distance of the battery - fusing is to protect wiring and battery so it needs to be prior to the first opportunity for a short.

[cor]

The elegance of the Zephyr BMS is a very simple circuit for each cell, while still protecting each individual cell for over- and under-voltage; allowing full protection of each cell by shutting off charging as well as discharging (throttle) before any cell reaches its danger zone - at least in theory.
There are some limitations however:
1 - If the BMS electronics or wiring fails, then this may not be detected, so the affected cell(s) are not longer protected
2 - if a cell suddenly goes to zero volts instead of gradually dropping below 3 (or 2) volts, then there is no protection and this also leads to other problems.
3 - if a pack is not charged soon after hitting the low voltage threshold, then the TC54N continues to indicate LVC, running the cells down quickly.

The problem with wiring can happen with the inter-connection between the 6s/8s/12s boards that sends the combined LVC and HVC signals from one board to the next, if a wire breaks then all the cells connected to the boards before that point will lose protection, (whether over-charge or) over-discharge will no longer be communicated for those cells. If a wire breaks between LVC output of the last board and the throttle (or brake input on the controller) then this means that the user is no longer notified of a cell reaching the lower limit by cutting drive power. If the BMS is visible to the user, then LEDs on the board may signal to the user that something is amiss, but it is not likely that a user will see a warning LED on the BMS while riding. One possibility is to connect a warning LED closer to the field of visibility, such as on the handlebars (most eBikes already have a set of indicator LEDs there) but the problem is still that even the wires connecting these LEDs can break. Also, even if the LED lights up, that does not protect the cells unless the user decides to reduce throttle.
It is possible to make a fail-safe system, but the complexity will increase much and it is not easy to test each circuit, most likely a microprocessor will need to be added to cycle through all channels and verify that the LVC signal is enabled on each channel detecting low voltage (for example by adding a resistor between the cell and the input of the TC54N plus a resistor and transistor to ground, to simulate a low cell voltage). The throttle (or brake) signal should be clamped to ground until all circuits are verified to work. Commercial BMS'es might implement this level of self-test, but it seems over the top for a DIY system.
One thing that may be done simple and easy is to add a connector to the open LVC input on the last board in the string, so that occasionally a plug can be inserted to short the signal there, then verify that the controller cannot engage due to the trottle (or brake) signal being clamped, verifying that all intermediate LVC wiring is OK.
Another even more sneaky type of wiring failure is when one of the wires between pack and BMS fails. If one of the "end" wires fails then we lose the corresponding channel, so that cell is no longer protected. If one of the intermediate cell connections fails, then the average voltage of the two cells may be OK while the actual cell voltage is too low (or too high) and the two channels might also fail to properly indicate even when the average voltage reaches LVC, because due to the votlage dividers on the 431 references, the voltages will divide perfectly between the two channels until one channel hits LVC which will then push the other channel to the HVC condition (4.2V) while the firs channel voltage suddenly drops from 3V to 1.8V. This may be too low to reliably indicate LVC, so the two cells that share a broken wire to the BMS will then be unprotected.
Then there are cases where the electronics fails, either because a component fails or something affects the BMS board (moisture, chip of metal, loose wire) and causes the fuse to blow. This will go undetected as this channel will simply stop working, so that cell is no longer protected. Also a bad solder joint or component that fails open due to thermal, mechanical or electrical shock will render the BMS inoperable without warning.

The situation that a cell suddenly goes dead will lead to two problems: since each channel of the BMS is powered by the cell that is being protected, this means that protection fails when a cell fails. The other problem is that the total voltage of the pack drops, which can lead to problems while charging, either over-heating the BMS shunt resistors or the Charge Control FET. In extreme cases the dropped pack voltage may cause the Charge Control FET to exceed its voltage rating, since it is designed to only carry the difference between charger voltage and pack voltage, but this is not likely with a 100V FET rating. It is more likely that the increased voltage difference will cause overheating once the first channel requests throttling back the charger. The danger here is that a failure will typically lead to a short circuit in the FET (typical failure mode of most semiconductors). This means a loss of control over the charger and with one (shorted) cell less in the pack, the remaining cells will get more and will be severely over-charged...

When the pack is run down to the point where the BMS disables the discharge by enabling the LVC, the low cell will continuously see the drain of the opto-coupler LED to enable LVC. Even though the 1k Ohm resistor will limit that drain to less than 2mA, the cell will see a drain that will discharge it down to about 1.2V (the opto's LED threshold) in a matter of days. So if you forget to put the pack on charge for only a few days after running it down as far as the BMS will allow you to go, you might permanently have lost the pack. It could be wise to add a disconnect when the drain should stop, or at least add a diode in line with the LVC opto to stop draining the cell at about 2V so it has a better chance of survival, even though this change will actually cause a reduction of the reliability of the LVC, because if an already well drained pack suddenly needs to dish out a large current and the cell sags instantly from 3+ to below 2V, there will be no LVC signal because the voltage is already too low....

There probably are a couple more failure modes for low cell voltage that I overlooked, but these are what popped in my head when going through the schematic of the channel and system. In case of questions/corrections: fire away!

[cor]

Just like the undervoltage case, the over-voltage can happen with the same type of causes:
1 - electronics or wiring problem
2 - cell failure: open circuit
3 - charger failure

If the over-voltage (HVC) signals are not reaching the charger controller, then cell(s) will be over-charged without the user knowing, this may lead to fire or at least to quick deterioration of cell performance. In particular if the "Any" HVC signal is not throttling the charger back, then some cell(s) will over-charge.
Also if a wire between pack and BMS fails, it will not detect an overvoltage until the average voltage on the two adjacent cells reaches max and if it happens to be one of the "end" wires then the channel will be dead and no protection will be present.
If electronics fails, then the HVC signal may no longer be generated or, if the failure is at the charger controller, the arriving HVC signal may not lead to throttling back.

Open cell failure is a nasty type of fault, during load the cell will try to go negative (reverse) and this might completely blow the associated channel electronics, for example the TC54N. During charging the HVC signal should throttle the charger back so the result will be that the pack will not charge at all.
During regenerative braking, the throttling is often not present, so the voltage will increase uncontrolled until the zener protection diode will shunt all current. If this situation lasts for more than a very brief moment, the zener will overheat and fail shorted, which will blow the fuse. After this there will be no more indication that this BMS channel failed unless the fuse (and electronics) are checked when replacing the failing cell or pack.

The situation of a charge controller failure is already discussed above, there is still the case where the charger starts delivering higher than usual voltage due to a failure in the charger or connection to a wrong charger. It is possible to warn for such failures (as well as for excessive low pack voltage that may indicate loss of a cell) by measuring the voltage drop across the charge throttling FET. Too high voltage drop will overheat this FET and may result in loss of control, causing over-charging of the pack at may charge current indefinitely...

[heathyoung]

On regen, unless you have completly full cells or really high resistance cells, the chances of exceeding the the voltage is very unlikely - especially if you have configured your controller correctly.

[cor]

For good measure, I should say that even after writing the previous 3 posts about issues that can develop with this BMS, I am still planning to build one of these for my eBike, because I like the elegance of a minimalistic BMS and I will likely, happily, run it on one or more of my packs. I am just saying that you won't find this BMS on a commercial EV because there will be a much higher requirement for fault detection.
Still - Richard, Gary and others: cheers and thank you!

[cor]

On regen, unless you have completly full cells or really high resistance cells, the chances of exceeding the the voltage is very unlikely - especially if you have configured your controller correctly.

It is an issue that I hear regularly from EV'ers that live on a hill - either they must take care to never charge their pack to full, or they will need some way to reduce/defeat regen when they drive/ride away from home, down the hill.
I have owned and driven an EV with AC motor and controller for about 3 years. It was capable of sending 200+ Amps back into the pack, which meant that on the lead-acid batteries that I had, the regen had to be disabled until SoC dropped to about 85% or the regen would easily push the pack voltage above the emergency cutoff voltage, which would mean a shutdown of the controller until power-cycled. Not a fun way to merge onto a Freeway (this was a S10 truck) and I was living in flat Silicon Valley! After 3 different events where the controller shutdown due to unexpected hard braking in the first few miles, I had finally overcome my wish to squeeze all juice back into the pack and reconfigured the controller to disable regen until SoC was low enough to allow the pack to soak it in.

[cor]

In continuation on the BMS issues list, one thing that is almost universally a fault detector is an overtemp sensor.
Whether it is a pack getting too hot (See the measurements taken of the "fake" Leadershobby LiPoly repackaged Tekocell packs) or the charger overheating; the charge-controller getting too much voltage and current to regulate or a large bank of shunt resistors shunting too long or in too high ambient. Even a motor controller getting too hot might be a reason to trigger a BMS event and reduce throttle, though it is actually outside the BMS context.

[cor]

BTW, what is the reason to put an AP431 in the BMS instead of the usual TL431? Was that a price or availability issue or is there a technical reason to go for the alternative manufacturer and type? I guess this is a question for Richard.

[fechter]

The TL431 has a slightly lower standby drain, but more importantly we found it has a slightly different turn on point that allows us to get closer to the desired set point with the available resistor networks.

One approach to signaling I looked at before is to use cascaded transistor switches instead of opto couplers. In this arrangement, if any cell triggers it trips the cell below and so forth to the end of the chain where it activates the HVC or LVC. This does a few things; the cell drain during activation can be much lower, a few uA, and there is a fail safe for any cell that becomes disconnected as this will trigger the chain. There is also no real limit to the cell count and there are fewer issues with creepage distance on large packs as the voltage differential from cell to cell is low. The Seiko chips use this kind of setup. Downside is way more parts per cell , possibly more susceptible to noise or contamination on the board and some propagation delay in the chain.

The Zephyr, being a centralized system, has little chance of a disconnection in the outputs of the optocouplers as they are all on the same board or in the same box in larger setups. Single failures of the tap wires (other than the end cells) will result in a pair of cells acting like one cell, but this still gives a fairly good level of protection.

All of the most recent Zephyr control boards have a temperature sensor that will suspend charge if the control board overheats. There is no provision for cell temperature monitoring, but that would be a desirable feature if it didn't take too many parts to implement.

[cor]

Hi Richard,
You chose the AP431 but you mention the advantages of the TL431, so I am confused.
I have been looking at combining multiple outputs to save on optos.
For a maximum of 8s you will have a max 8 x 4.2 = 33.6V which means that usual transistors and the TL/AP431 will allow combining 8 channels into single optos, which means that instead of 8x3=24 optos only 3 optos are needed for each 8 series cells.

I also like to make the LVC signal active, meaning that the throttle clamp or brake is held active unless the wiring is intact and the battery voltage high enough (above LVC), so that a broken LVC wire does not defeat the low voltage protection and you damage your expensive pack.
What this needs is an extra transistor to invert the clamp/brake signal at the throttle signal and turn the default active signal back into a signal that only clamps when there is a problem. Also a switched power input is needed, so that the BMS does not continue to draw power when the bike is switched off. This can actually reduce the power draw of the BMS to zero.
The last thing I like to do is shrink the whole BMS and Charge controller by making it SMD technology. There are plenty PCB houses that will supply a relative small number of (populated) PCBs for a reasonable price. I have dealt with PCBfabexpress in the past, I often pass by them on one of my commute routes.
I have a few sketches and drafts of how to add this to your BMS, let me know if you like me to share them.

[fechter]

All the '431 variants are pretty close in performance but as I mentioned, they have slightly different trigger points.

To combine multiple 431's into a single opto I think you'd at least need some diodes or transistors to prevent applying negative voltage to the output. You'd also probably need different resistor values for the series resistor feeding the opto. On the LVC side, the TC54 is rated for a maximum of 12v, so again you'd need level shifting transistors to go more than 3 cells. By the time you do all that, the cascaded transistor chain will have fewer parts.

The Chinese BMS just use a single transistor on the ouput of each cell circuit that pulls up a common line on the control circuit. These are coupled by 10M resistors to keep the drain low.
This approach works and is sort of a comprimise between the extremes.

I agree a fail-safe cutout is a good feature. The latest Zephyr circuit has an option for a mechanical relay output with both normally closed and normally open contacts. While the relay is power hungry (about 10ma) it makes it very easy to tie in to most any motor controller. The NC contacts can be used to complete the throttle power circuit or controller power circuit so that a fault in the wiring causes the power to stay off. Yes, you could also do this with a couple of transistors to save power.

The BMS really only needs to be active when the controller is on (discharge) or when charging. The Zephyr senses when the charger is active and automatically turns on the power hungry parts of the control circuit. The discharge side of things is always on.

Something more like the CellLog would probably be a better direction to go. The CellLog just doesn't have a sleep mode that works and needs a provision for ganging more than one in series. Other than that, it seems to work well and a single MCU monitors 8 cells and can be programmed by the user. In theory, there should be a way to make the processor sleep most of the time during standby and take a measurment only once in a while. When the voltages start changing rapidly during charge or discharge, the MCU could stay awake.