# BMS question, prove or disprove if you can



## EVfun (Mar 14, 2010)

With a cell level BMS it may shunt on charge if its cell connection is bad. However, on discharge it will also see the cell voltage as less and alert to a low voltage condition. I know it will show as a reduced cell voltage because each cell is connected to the rest opposite the direction you would connect it to charge. 

A cell level BMS can be very good at detecting a bad connection, but it is possible to miss it if its connection through the bolt to the BMS is much better than from the terminal to either side. I don't know if any BMS board has ever has missed a bad connection this way (just that it is possible.)

My bigger concern is what happens to a BMS module if it is reversed by 10 volts, even briefly. That is something a bad connection could easily to the first time the throttle is hit. Even though the driver is likely to see the sudden sag and back off it would be to late for any parts not able to handle the high reverse voltage.


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## ElectriCar (Jun 15, 2008)

So you can see where a bad connection could cause the module to react to a condition that doesn't exist? Assuming the connection develops 1V across it, I think it would actually subtract from the battery voltage after thinking about it. 

Any voltage created I believe will be negative relative to the source voltage. IE battery post -, connectionV +, connectionV -, battery +. If this is the polarity of the voltage generated, it will indeed subtract from the cell voltage, creating a false LV cutout if the sag minus the connection voltage takes the perceived voltage low enough. 

So with a fully charged cell you take off down the road and have 1V negative acting against the cell voltage leaving you 2.3V or so before sag. Of course the higher the current draw, the higher the sag and the higher the voltage drop across the bad connection. In that case I can see the LV kicking in and shutting you down momentarily until the amps drop allowing the voltage to rise and clear the LV condition until you accelerate again.

I've never noticed the polarity of a voltage drop like that but it seems that would be the case, ie - + - + just as the batteries are connected in that fashion.


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## GizmoEV (Nov 28, 2009)

I don't see how a high voltage reading on discharge could occur. The BMS boards I have used connect to each terminal bolt on a cell so it will only get a poor reading if the bolt it self has a poor connection to the cell, independent of the quality of the connection of the strap. In a BMS system where it has the number of cells+1 wires then for most cells there is only one sense wire/cell so a poor connection could cause a faulty reading but then it would still only be a low voltage reading not a high voltage reading. The reading would then signal either a low cell or a bad connection, assuming no BMS problem.


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## EVfun (Mar 14, 2010)

Picture what would happen on discharge if the cable was connected to the BMS module but no longer connected to the cell. This could get close to happening at the aluminum negative terminal. Tin and Gold make good contact conductors, but not Aluminum. On discharge the negative terminal would go positive while on charge the negative terminal would go more negative.


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## GizmoEV (Nov 28, 2009)

Ok, so the voltage seen by the BMS board would be the voltage across the rest of the circuit, like the BMS was now the load.


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## atzi (Jun 26, 2008)

The question is a good one. I am not an expert either. I am running a Elithium BMS and when installed, one of the cell boards was bad (I may have damaged it). It read 2.5 volts while the other 47 were reading 3.2 volts. Two volt meters showed 3.2 volts on that battery, so I replaced the cell board and all went well.
As far as a higher voltage reading caused by any possibility including voltage drop because of resistance? Hold on... Let me draw on a piece of paper in front of me.
We are testing potential energy difference with voltage. With flow of current either direction and the possibility of resistance between:


 the board and cable
 board and cell
 cell and cable
Voltage would stay the same or drop as far as the brains of the BMS sees, except if the resistance increase was between the *cable and cell* while the board still held connection, while there was a current flow either direction. Even if the BMS did not control load or charge, wouldn't the increase in resistance cause all kinds of issues like; smoke, heat, noises, most chargers shutting off (if charging), power loss, damage to cell board. 

But maybe just some little resistance in just the right spot?

To cause the problem of a false voltage signal increase, the resistance would need to be sustainable. Would the heat created cause more resistance or meltdown that the few seconds or minutes of cell balance would not matter before you had a open circuit or more?

I will probably do this wrong, someone correct me......
Using Joule's first law Heat Generated = Current Squared X resistance X Time. 
With 10 amps flowing on charging, if you had enough resistance to cause your increase of 1 volt reading too high that would be from a resistance of .1 ohm. Then using Joule's law 10 amps squared is 100. 100 x .1 ohms x 60 seconds would be 600 Joules. If the cell board balanced for that 60 seconds at it's usual 200 milliamps there would not be a problem yet. If one hour went by that would be 36,000 Joules or 36,000 watts/seconds or 10 watt hours or 30 BTUs.
Load current of 400 amps instead of the 10 amp charge current would of course increase the false voltage reading or increase the heat.


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## EVfun (Mar 14, 2010)

EVfun said:


> Picture what would happen on discharge if the cable was connected to the BMS module but no longer connected to the cell. This could get close to happening at the aluminum negative terminal. Tin and Gold make good contact conductors, but not Aluminum. On discharge the negative terminal would go positive while on charge the negative terminal would go more negative.





GizmoEV said:


> Ok, so the voltage seen by the BMS board would be the voltage across the rest of the circuit, like the BMS was now the load.


In practice it wouldn't actually be disconnected from the cell, it would be a poor connection. Lets say it was 0.05 ohms terminal to cell face. At 200 amps you likely reversed that BMS module. How much will depend on the resistance of the path through the top of the terminal into the BMS module, then through the bushing to the top, through the washer(s) and into the Stainless Steel bolt where it can pass out of its threads to the post. It won't be the full 20 volts because there is a secondary path. There is going to be some serious hot going on. 

I haven't heard of this happening to anybodies Li pack for sure, but I have seen it with AGMs and regs.


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## Jozzer (Mar 29, 2009)

That's an easy problem to spot, any decent BMS will NOT be measuring cell voltages whilst it is discharging (or will be using a 4 wire measuring system) so that a high resistance connection to a cell will not cause bad readings.
Trouble is, many BMS's DON'T use this method, and what happens is that a couple of amps of balance current cause the voltage to be read low on one balance tap connection, and high on the next, as the balance current chooses it's path between the BMS and the unfinished cells and can be flowing both directions on different balance taps.

Often, a BMS owner doesn't even spot this, since they may check it's operation using a multimeter connected to the BMS end of the lead, you actually need to check both ends of the BMS cables at once to see this happening.
Unless the pack is very out of balance though, or the connections are very poor, this usually throws the cells out of balance by a few MV, until charge/balance current drops back and they level back out..

If you've got LiPo cells, and don't trust a "BMS" you can use one of these to spot LVC and HVC. http://endless-sphere.com/forums/viewtopic.php?f=31&t=32460 
Very VERY low powerdraw, about as safe and benign as it gets.
If you ask Methods nicely and there is enough interest, he may even redesign a version for LiFePo4...


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## EVfun (Mar 14, 2010)

Any normal BMS *will* be measuring pack voltage while discharging, if it didn't the system would have no way of warning against a cell going low. Anyway, detecting a bad connection as low voltage is a lot better than not detecting a bad connection. At 500 amps or more you can burn posts right off of cells with a bad connection.


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## ElectriCar (Jun 15, 2008)

As I mentioned, I don't know very much about them but enough to decide to use other means instead. I know there are several that use individual boards to manipulate the battery and signal the main control of HV/LV conditions. That's about it for me.

I did however melt down two terminals when running lead because of over tightening.


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## atzi (Jun 26, 2008)

EVfun said:


> Anyway, detecting a bad connection as low voltage is a lot better than not detecting a bad connection.


EVfun,
now that is quotable, good point.


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## Mr_tim (Dec 13, 2010)

Not sure how well it's going to work as I'm just getting it set up, but I'm monitoring individual cell temperature as well. I'm assuming I'll see a temp rise in a cell that has a loose/bad connection. I'm monitoring alternating posts on each battery. I'll see a bad connection pretty quick on a monitored post. I'm assuming it would take a little longer for the other post to heat up. I'll let you know how it works out. I've melted a couple of lead posts off as well....

_-- Sent from my Palm Pre using Forums_


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## GizmoEV (Nov 28, 2009)

I remember hearing someone say that one post heats up more than the other but I don't remember which one or what evidence there was for it. It would seem to make sense to monitor the one which heats up the most.


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## Mr_tim (Dec 13, 2010)

I read on here somewhere that one heats during charge and the other heats on discharge. Not sure if it's true, but I'll find out. I set my pack up to monitor the positive terminal on the even batteries and the negative terminal on the odd batteries. If I see one heats more than the other, I'll switch the thermisters over. Give me a couple weeks to get some cycles on the pack and I'll let you know what I find.

TiM

_-- Sent from my Palm Pre using Forums_


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## Elithion (Oct 6, 2009)

ElectriCar said:


> What I see as a possible situation is a BMS module detecting the voltage generated across a loose connection then taking corrective action.


Indeed all 3 of our BMS families (Lithiumate Pro, Lithiumate Lite, and Lithiumotive) handle the situation of a loose connection. They do take corrective action, but not by making the situation worse. Instead, they do so by reporting a low voltage, even if the battery current is 0, which will cause a Low Voltage Cut-off, and send the user to the shop to see why that is.



ElectriCar said:


> Loose connections cause an increase in resistance. ... and happen to be pulling much higher amps, a higher voltage develops across that connection...I can see the BMS bleeding a cell down and thus destroy a perfectly good.


Yes, but it's a *lower* voltage, not a higher voltage (you got the sign of the voltage drop wrong). So it would not result in an inappropriate balancing, as you suggest. So, even non-Elithion BMSs will handle that situation just fine.
(You should also see that the situation that *would* result in unwanted balancing is during regen. True. But regen is of limited duration, so the unwanted balancing is of small consequence.


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## tomofreno (Mar 3, 2009)

> I remember hearing someone say that one post heats up more than the other but I don't remember which one or what evidence there was for it.


 The positive one. There are some thermal modeling/measurement results at NREL:
http://www.nrel.gov/vehiclesandfuels/energystorage/


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## ElectriCar (Jun 15, 2008)

Elithion said:


> Yes, but it's a *lower* voltage, not a higher voltage (you got the sign of the voltage drop wrong). So it would not result in an inappropriate balancing, as you suggest. So, even non-Elithion BMSs will handle that situation just fine.
> (You should also see that the situation that *would* result in unwanted balancing is during regen. True. But regen is of limited duration, so the unwanted balancing is of small consequence.


Yes it came to mind that the voltage drop created would likely reduce the monitored voltage, not add to it as I mentioned earlier and I noted this in post #3. 

So since you're here, what happens with your systems if the current rises enough to cause an LV situation with the increased voltage drop added to the cell sag? Is there an immediate corrective action, ie shutting down of the control until the voltage rises or what?


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## Elithion (Oct 6, 2009)

ElectriCar said:


> ... as I mentioned earlier and I noted this in post #3.


Sorry I missed it: I only read your original post.



ElectriCar said:


> what happens with your systems if the current rises enough to cause an LV situation...?


I think you answered your own question there: there is an LV situation. 

BMS users who do things properly, will connect the LV output (called "LLIM" in the Lithiumate pro, and "Discharge OK" in the Lithiumate Lite) to the load (motor driver) to shut it down in case of LV. The result would be cut-outs during acceleration. 
However, clever BMS users will also connect the DCL (Discharge Current Limit) to the throttle, and get a reduction in torque as the cell voltage sags, so that they at least would still be able to drive, albeit more slowly, and avoid a heat spot at the point of poor connection.


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## tomofreno (Mar 3, 2009)

> So since you're here, what happens with your systems if the current rises enough to cause an LV situation with the increased voltage drop added to the cell sag? Is there an immediate corrective action, ie shutting down of the control until the voltage rises or what?


 On my car it would trigger the LVC on the minibms which would cut my throttle by 50%, sound a buzzer mounted by the steering column, and light the "check engine" on my instrument panel.


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