# Going pure ZENER on battery management system?



## mk4gti (May 6, 2011)

Not an expert in diodes (it looks like a feasible idea) but it's nice to have a way to balance the cells while shunting (i.e fat resistor)

imo, there needs to be a way to tell the charger to stop charging (via a cell loop, like the mini BMS)

Also nice to have a way to alert the driver that one cell is below 2.x volts.

my 2c


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## ga2500ev (Apr 20, 2008)

One problem. Zeners at that low a value have very soft knees. So it's unlikely you'll get sharp shunting action exactly at 3.7V.

This would be needed only if you wanted to get the entire pack completely full. The problem is that since each cell has a different capacity, that by shunting, you will unbalance the cells in the pack.

A better idea is to monitor the cells after balancing them, and to cut off the charger when the first cell tops out at 3.7V (or whatever cutoff you plan to use). If the cells are balanced, then they have equal energy. The cell that reaches 3.7V first will have the least capacity. By shunting, you bypass putting more energy in that cell, while continuing to put energy in the others. So that unbalances the pack. If instead you cut off the charger, then all the cells will have the same amount of energy, and therefore will remain balanced.

Hope this helps,

ga2500ev


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## Tesseract (Sep 27, 2008)

Electron Power said:


> ...
> Enter the 3.7 volt zener diode. With a cell max rating of 3.65V, it would seem that shunting anything exceeding 3.7 would be a "fairly" good way to keep the cells balanced....


The main problem with this idea is that zener diodes have a fairly soft "knee". That is to say, they start conducting a significant amount of current well below their stated breakdown voltage. For example, the graph below shows the 3.9V zener will allow 30-35mA to flow with 3.5V across it, and even at 3.0V will still let 15mA through!

There are some other issues which you overlooked (you still need a resistor to limit current through the zener when it conducts, for example) but the above issue is so bad it kind of renders all the other issues moot.


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## PStechPaul (May 1, 2012)

About the closest I could find is a 1N5334 which is 3.6V nominal and 5 watts for only $0.43 each, but the tolerance itself is a variation from 3.42 to 3.78 volts. It has an ESR of 2.5 ohms which is 0.875 V at rated current of 350 mA. And it has a leakage current of 150 uA at 1 volt, and a lot more as it approaches rated zener voltage, so it could not be left on the cell. So it's pretty much useless for this application. Here is the spec sheet:
http://www.mouser.com/ds/2/308/1N5333B-D-102336.pdf

You can get a 3.6V 1.3W 2% zener for about $0.10:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtQ8nqTKtFS/KE3JtglsWdknZRUxEb2hjw=
http://www.mouser.com/ds/2/427/bzx85-103138.pdf

But it can vary from 3.4 to 3.8V, and it has a temperature coefficient of -0.08% per degree C, which may or may not be good.

I had considered using a TVS diode but they also have very soft "knees":
http://www.mouser.com/ProductDetail...=sGAEpiMZZMuNo3spt1BaVwOSolHqlxKA021px23vbP0=

OTOH you can get a PIC10F320 for about $0.50, and it has an internal 5% voltage reference and a 10 bit ADC and PWM and all sorts of possibilities for a shunting BMS. You can add a few cents worth of resistors and a 30 cent 6A MOSFET and make a very capable BMS for a couple of bucks. But it does take a lot more than just adding a simple component across the cell.


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## Siwastaja (Aug 1, 2012)

Another issue is that even if you could get some kind of imaginary optimal zener diodes, or if you built an active "zener-like" circuit out of a zener and a transistor and a few resistors, or even better, a voltage reference and opamp, you still have the problem that you have only built a top balancer which is only one of the functions a usable BMS needs. Most importantly, you would need a cell-level low-voltage cutoff.

It's not a completely stupid idea to use zeners to maintain a top balance, however. As we know, li-ion cells drift so little that many use no balancer at all, and a zener could provide very low-current balancing. In this case you'd select something like a 5V6 zener. If any cell would ever get over 4.0 volts or so, the current through the zener would be more than the current over other zeners, and the minor unbalance would be fixed. But here lies another problem; zener diodes have a considerable temperature coefficient, so while the pack sits unused at about 3.3 volts per cell, some zeners drain the cells faster than others due to small temperature differences, ruining the balance. This may or may not be a problem.

Anyhow, expect the following limitations:

- Your zener "BMS" will only be a top balancer, and you need careful charge control like no-BMS top-balance folks,
- Your zener "BMS" has no LVC, so you need even more careful means to stop discharging
- Your zener is _very weak_ as a top-balancer, expect it to balance at 1 mA max, probably less.
- Therefore, do a very careful top balance manually first.
- Measure the actual zener at different temperatures to make sure it really isn't taking much current at below 3.5 volts. OTOH you want it to take some current at 3.8V and over. I'm not sure whether you can find a suitable zener at all, but you might want to experiment by putting zeners, regular diodes and/or schottkys in series in different combinations.
- Not worth the hassle IMO. Just go without a BMS or get a decent one.


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## Malevolence (Jun 5, 2013)

First of all, I would suggest you look up "Lee Hart" with the terms zener, bulb, regulator and/or balancer and you should find several links to his design of what you're describing for use on lead acid batteries (there are many names for it). 

I built pretty much what you describe for my 24 cell lithium pack and the results were less than stellar in my opinion. The regulation is too soft for lithium as others describe. It got me close with my old pack, but honestly wasn't worth the effort. Something with better tolerances than a zener is needed. Even though the zeners were from the same set, there was a fair amount of variance from part to part. Not to mention, that my soldering skills may not have been quite up to the task! I think this would have worked great on a lead acid pack, as per Lee's design, but it's probably not a good use of your time to build one for a lithium pack. 

If you do decide to build something like that, here are a few pieces of wisdom, if you can call it that:

First, you need to put a resistor in line with the zener. I used a .1 ohm resistor in front of two parallel zeners to allow for enough heat dissipation for a 1 amp charge with some safety factor. This maintains a fairly steep cut-off curve. Not only does the resistor allow for higher charging amperage by dissipating a lot of the heat, it helps protect the zener from thermal runaway. As I understand it (someone correct me if I'm wrong) the zeners have a negative temperature coefficient within the voltage range of a single lithium cell (I also understand that the temperature coefficient is close to 0 at 5.6V, and positive above that, which works nicely for +6V balancing charge on flooded batteries but not so nice for 3.65V lithiums). This means that the hotter they get, the lower the voltage that is needed to cause shunting. This is bad because this can cause thermal runaway in the zener. The zener gets hotter, meaning that more amps are shunted across it. It's being fed now by not only the charger but the battery itself as it tries to bring the battery to a lower voltage than it's already charged to (read massive battery amps available, not just charging amps). The more it shunts, the hotter it gets. The hotter it gets, the lower the voltage and the more it shunts... until poof, your zener burned out and now you're overcharging the cell. The resistor picks up a lot of slack so that this doesn't happen quite so easily, so long as you keep your charge amperage down to what the resistor and zener can handle the amperage together. This still makes me uneasy and I'm not sure how reliable this design is. 

In my design I also paralleled the whole zener resistor combo with a 68 ohm resistor, two forward biased diodes (for ~1.3V of drop) and a white 20mA LED bulb. It's hard to tell the difference between a half lit and a full lit LED (both are really bright), but much easier to see it between, say, a quarter lit and half lit LED. So that's why there's more total voltage drop than it seems like there should be. The LED should never be fully lit in this design (you need well over 4V across your battery for that). The bulb should be lightly lit through most of the charge and gets rapidly brighter as you reach full voltage. This helped me easily identify the batteries that were charged the most. Honestly, this part of it was probably more useful than the zener shunt, since I could then manually shunt those high cells with a 12V taillight bulb and individually charge the low cells. This was good for getting my old distressed pack in relative balance. I haven't found the setup useful for my new pack where things were in pretty good tolerance from the get go (at least in better voltage tolerance than the zener setup itself). 

Finally, as also implied by others, I did not permanently attach the setup as it would bleed everything out of balance quite quickly. I used 23 of the 24 pins (I left off the smaller gauge green ground) on a 24 pin motherboard power supply extension cord (from eBay), cut it in half and used it as a connector to plug in while charging. One side of the cord is permanently attached to the bike. This was combined with the pack positive and pack negative terminals on another connector to give the 25 connections needed for 24 shunts. This worked fairly well and allowed me to easily connect and disconnect the shunt system as needed with two disconnects. Again, this part of the system seems more useful than the shunts themselves. 

Long story short, my advice is don't bother! You're not going to be happy with the results. The voltage regulation is just too sloppy. I think the other suggestion about using true temperature corrected, higher accuracy voltage regulators is a better way to go. Of course at that point you're basically building a full-on BMS, which completely misses the point of what you're trying to accomplish! Oh well, that's just how these things go, I guess! At least you get to learn from my mistake!


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## Electron Power (Jan 2, 2013)

I'm not so sure about the statement that the cell with the least capacity will be the first cell to reach Vmax, under conditions other than previously having received a bottom balance. And with TVS devices not camping down on the voltage until it is WAY above the operating voltage, I don't see them as being feasible for use as charge-controlling devices, stand-alone or otherwise. My solution to the tolerance issue would be to buy like a reel of 1K of them (or is a reel 10K?), but only use the most closely matched 100 of them for the 100-cell pack that constitutes MY scenario, That should especially help in minimizing the concerns (like runaway) associated with having an NTC Anyways, there will not be more than 365V, nor less than 315V, across the pack as a whole. But 35ma leakage translates to a bit over a continuous draw of 10 watts (for a 100-cell pack), which seems reasonable if daily cyclic usage . The main reason I left out any current-limiting resistor, aside from simplification reasons, is that the usage of one WILL soften up the zener''s knee. Conversely speaking, it definitely would help make it less vulnerable to suffering the fate of an over-current blow-out. At that point, the whole idea of slap-em-on-and-yer-done, IS compromised though.

Other question regarding zeners is: 

Do they blow HARD (dramatically), as in shorting, by first causing a huge current spike, and THEN opening up)?

Or do they die EASY (silently), as in just giving-up the ghost at the git-go, by opening right up without shorting?


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## Malevolence (Jun 5, 2013)

Hmm, I hadn't thought of how the zener would actually fail: open or short. I just assumed it would fail short circuited since thermal run-away would push it in that direction. But I suppose it could fail open, which would be better for your proposed zener-only design. As a short the resistor is critically important without a fuse inline. My understanding and experience is that resistors tend to fail open. In my design, I made the resistor by far the weakest link. Because of specific lot quantities available on eBay on the day I was buying, I ended up with a lot more resistor spares than zener spares, so the resistors were what I wanted to fail if the smoke got let out the first time I plugged it in! As the weakest link, I assumed it would act like a fuse failing open first, regardless of what the zener did. My design was to protect the zener before the resistor for no other reason than that I didn't have many zener spares - not some well thought out engineering! 

As for the paralleled zeners, I knew they wouldn't be perfectly matched so either one could potentially be taking all of the load through it. The two were more for redundancy than any realistic expectation that they would split the current 50/50. In hindsight, I probably shouldn't have bothered and would have had a lot more zener spares as a result (I had them so I might as well use them, right? - another not so well thought out plan). The maximum current was limited to 1 amp by the charger through two 5W zeners, so even with only one passing 3.6V of current, thats only 3.6W through one and 0.0W through the other. If the zener did go into a runaway condition, the resistor would quickly fail open - most likely before the zener was damaged, but definitely once the zener failed as a short circuit.


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## ga2500ev (Apr 20, 2008)

Electron Power said:


> I'm not so sure about the statement that the cell with the least capacity will be the first cell to reach Vmax, under conditions other than previously having received a bottom balance.


The bottom balance was exactly the point. Bottom balancing ensures that each cell has equal energy regardless of their actual state of charge. If you bottom balance all the cells, then put in an equal amount of energy into each of them, then they will run out of energy at the same time if depleted. But the only way to make sure that equal energy is given to each cell is never to overcharge any of the cells. So as soon as the first one reaches full charge, you have to stop charging all the others to keep the energy balanced.

I cut the rest. My only comment is instead of trying to shunt the charge and continue charging, when the first cell is full, because its voltage starts to rise, turn off the charger. Then there's nothing to blow, because there is nothing to shunt.

Try it as an experiment on a handful (4-5) cells. Connect them in parallel. Drain to 2.7V. Put in series. Charge as a string until the first one hits the cutoff (3.6, 3.65, 3.7V, the choice is yours). Stop the charger. measure the pack voltage and the voltages of the other cells. The others will be close, but not over the voltage of the first cell (otherwise they would have been the first cell). Drain (still in series) until the first one hits 3.1 or 3.0V per cell (virtually empty). Note this pack voltage as the low voltage cutoff. Then recharge back to the pack voltage noted on the first charge. Measure again. You'll find that the first cell still has the highest voltage and the others are close.

At this point if you set your charger to that marked voltage, and you set your LVC to the marked LVC voltage, that you can cycle this pack 100 times and it'll stay balanced. 

Top balancing a pack provides more total energy no doubt as the pack voltage stays higher longer because each cell starts at nearly 100% SOC. However, from a pack safety standpoint it's just to difficult too manage. Bottom balacing makes the bottom edge of energy capacity even, while top balancing make the top edge of SOC even. The problem is that each makes the other edge ragged (bottom has unequal SOC, while top has unequal energy). By keeping the pack bottom balanced and limiting the energy capacity to the 100% SOC of the smallest capacity cell, you won't overcharge the pack. Also by keeping the same energy, even a catastrophic overdischarge will empty all the cells at the same time. So there's no cell reversal.

But all of this is based on the same premise: when the first cell of a bottom balanced pack is full, then stop charging.

ga2500ev


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## Malevolence (Jun 5, 2013)

As a side note, after reading many threads here over the last several months, I'm pretty convinced that bottom balancing is the right way to go, as ga2500ev suggests, even if it comes at the cost of slightly reduced total energy storage. 

The bottom balancing idea was new to me, having no EV for the last couple of years and a lead acid one prior to that. I had pretty well planned out the project for the lead EV, but didn't apply it until I got the lithium one. My thinking hadn't been "updated" based on what people have figured out about lithium in the years in between (i.e. you don't have to top balance lithiums ever, like lead, so maybe there are better ways to balance them). That's why I've pretty much abandoned any efforts to improve the aforementioned project. I'm just waiting to have the time to go through the bottom balancing exercise to try it out. Still, I thought my experience with the zener regulators may be helpful for others.


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## jk1981 (Nov 12, 2010)

My experience of parallel zeners (I'm forced to use them for regulatory reasons) is that one of them smokes while the other barely gets warm. The tolerance (5% typically) is not good so one carries more current as they begin to conduct and there's a negative temperature coefficient on the Zener voltage so once one starts to leak and warm it hogs the current.

I'm sure it's already been mentioned that they have very soft switch on characteristics and tend to leak quite a bit below Vz.

jk


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