# Over discharge problem



## jhuebner (Apr 30, 2010)

Hi everyone,

whilst I was away 3 weeks my girl friend drove my electric Ligier. Its controller has an undervoltage throttle feature which is really a last resort with LiFePo4 batteries. When it comes on, the pack is EMPTY.
So it did come on but she decided to crawl home. I'm currently looking at the damage done:


A number of cells are sitting at 0V, probably saw reversed voltage during the crawl
These cells are leaking some liquid, probably electrolyte
I charged the pack for an hour, the batteries are back up at normal voltages but leaked even more.
The bodies look bloated but are held together by the strapping


Has this occurred to anyone and how much damage has been done to the cells in terms of internal resistance and capacity judging by your experience?


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## Ziggythewiz (May 16, 2010)

Time for a new GF, then a new battery pack.


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## JRoque (Mar 9, 2010)

Ziggythewiz said:


> Time for a new GF, then a new battery pack.


Ha!! I nearly fell off my chair laughing. Good one.

Yeah those bloated/leaking cells are gone. Sorry for the after-the-fact line but I would consider raising that minimum voltage allowed setting and perhaps keep a button somewhere that lets you crawl home - but only you know about.

JR


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## kennybobby (Aug 10, 2012)

*Re: bloated capacity*

i've tested bloated (but non-leaker) cells to measure capacity using CC charge and discharge. In addition to expanding and getting warmer than good cells, the capacity was only about 60% of original spec. i think leakers would do even worse.


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## jhuebner (Apr 30, 2010)

Ok, not sounding good  It's staggering how much damage is done by driving one extra kilometer after the low voltage warning comes on.
It is an advocate for bottom balancing.


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## Roy Von Rogers (Mar 21, 2009)

jhuebner said:


> Ok, not sounding good  It's staggering how much damage is done by driving one extra kilometer after the low voltage warning comes on.
> It is an advocate for bottom balancing.


 
Yupper, and also using a JLD404 to shut it down when the pack reaches 90%.

Tut mir leid.

Roy


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## Red Neck (Feb 1, 2013)

Or, simply that your Ligier has bad settings. If the controller started crawl sooner and shut off a bit earlier, you would not have this issue.

If it was already equipped with these batteries and was set as is, they made bad settings. Proper ones would not allow overdischarging.


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## jhuebner (Apr 30, 2010)

No, the Ligier was shipped with Lead Acid batteries. The firm that made the controller no longer exists and I don't have a configuration utility.

So yesterday I charged up the batteries and went for a test drive today. Amazingly, the car did 45km, little short of what it normally does.

I'm now bottom balancing the cells and will see how much longer they will serve me.


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## Siwastaja (Aug 1, 2012)

Just make sure your charging is up to the task of a bottom balanced pack. Be careful, watch all the cell voltages. You may need to reduce the charging voltage.

Bottom vs. top balance just moves the risk from discharge to charge. Charge happens more often but OTOH it is easier to control.


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## Red Neck (Feb 1, 2013)

If you can't set the charger's end voltage, you can damage your cells even more now. You should first see the top voltage it charges to.

If you can't set the voltage, you will overcharge the weakest cells quickly, especially since they likely have even less capacity.


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## onegreenev (May 18, 2012)

> Bottom vs. top balance just moves the risk from discharge to charge.


Well, not really. The risk is top balancing and continuing to drive below our safe levels as happened here. It is not IF but WHEN as seen here. The so called risk during charging is really very little because you stop the charge at a specified voltage which will be the overall average of the pack. Yes some cells will have higher voltages and some lower. However you do need to be smart about your setup. Be sure you know what your low capacity cell voltage will be just before termination of the charge. You don't want it in the 4 volt range while charging. If you have one you either replace it or reduce your overall pack voltage to bring it down to a safe level. Your JLD volt meter can shut off at any voltage you specify. 

The only risk during charging is if your setup fails to stop the charge. It could happen with any system implemented. It won't be a problem if it shuts off as expected. I like my Elcon charger.


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## jhuebner (Apr 30, 2010)

No worries, there is a "BMS" in place that shuts off the charger if any cell reaches > 3.7V. The LVC feature is unusable due to the EMI when the controller is running.
Also the charger (Zivan NG3) switches to CV mode at 190V which is 3.5V per cell (54).

I agree that the charging process is the better controllable one and thus I prefer bottom balancing.


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## Red Neck (Feb 1, 2013)

If your bms can be made to simply swithch off the charger once a cell reaches 3.7v, without shunting or otherwise altering SOC of any cell, then OK, but if it shunts to any degree, it will destroy the balance (bottom balance). 

Find the cell that gets full first and only keep the BMS on that one and
have it not shunt. And set the charger to cut off when it reaches 3.6v.


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## jhuebner (Apr 30, 2010)

Yes, thats what it does. It is unable to shunt.

Not sure about removing the BMS since its there anyway. But you're right, whenever I observed which LED came on first it was the same one.


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## Red Neck (Feb 1, 2013)

I meant removing any of its hardware from other cells.

The reason was that micro drainage of any current by BMS might also remove balance.. So just a thought.

And if bottom balancing, deplete cells to 2v or even less under light load and let them settle. Even going to 1v for shor period is nothing drastic, if for purpose of properly balancing them for the long run. 2.7v is not nearly low enough. Trust me, tried and saw. They were still very apart when only balanced at 2.7v. So go lower when balancing them at the bottom.


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## Siwastaja (Aug 1, 2012)

Red Neck said:


> I meant removing any of its hardware from other cells.
> 
> The reason was that micro drainage of any current by BMS might also remove balance.. So just a thought.


That would just make the situation worse as now there would be a significant difference between the cells. (Unless the BMS is very low power.)

Just make sure that the difference in drain between the modules is low enough. It probably is, but measure it. I think the difference should be < 10 uA or so. The drain should be < 500 uA, IMO. OTOH, MiniBMS uses 10 000 uA and people are happy with it.

If you can make the drain below < 50 uA or so, you can really consider using only one cell module, but if you have all of them installed already, keep them all.


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## Red Neck (Feb 1, 2013)

I figure it is easier to simply give the one cell a bit of an extra charge for a few seconds every few months by from an independent source..


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## jhuebner (Apr 30, 2010)

Yes, I might have figured out the reason why these cells depleted-

The group of cells that were depleted so much are all in the rear battery box, the other ones are in the 2 front boxes.

So I thought that somehow the different position of the cells was the reason for the gross SoC difference.

But there is an other difference. The BMS modules on the depleted cells are a different "revision" than the ones in the front. I have to check but I think they use different voltage dividers. The cross current on both is similar to MiniBMS but there has got to be like a 1mA difference. Over the 1.5 years that the BMS is installed that difference sucks about 12Ah more. Being 20% for my 60Ah cells.

So another sample of how careless application of a BMS breaks stuff.


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## Red Neck (Feb 1, 2013)

In a way, it may also make them easier to replace by same cells then and simply
make sure you give them an extra charge once every 3 months.

But it is things like these I don't use BMSes myself.


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## Siwastaja (Aug 1, 2012)

Red Neck said:


> But it is things like these I don't use BMSes myself.


It appears that it was not a case of "using" a BMS but _designing_ one's _own_ BMS, which is on a completely different level. Clearly, not all necessary design considerations were taken into account.

I'm not saying that designing a BMS is impossible, quite the opposite, but there are a few things that somehow tend to go wrong in many cases.

In this case, it seems that the BMS was not working at all, and the designer ignored the fact, on purpose.

So, he was practically running a non-BMS system, knowing it all along. BMS didn't damage anything, but the lack thereof did. Of course there might have been some false sense of security even though it was known that the LVC is not working.

Ruining the cells by overdischarging is so typical that it's a bit embarrassing to design a BMS and still miss it. OTOH, talk is cheap, and when you do advanced things (such as design your own electronics), things happen. It would be more embarrassing if the BMS had failed and destroyed the pack by itself. Now it just didn't work as a BMS at all and it didn't come as a surprise.


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## jhuebner (Apr 30, 2010)

Let me make some corrections to the above post.
So, yes the BMS offered virtually no LVC so I always relied on the odometer to estimate the SoC. I usually drove no further than 20km where 60km would have been possible.

The HVC function always worked as supposed.

Now comes the worst part: the first revision used different voltage references than the second. This results in the first revision to have a standby current of 6mA while the second revision has a standby current of 1mA. This lead to a catastrophic runaway caused by a poorly tested BMS.

So its a simple example of trial and error that adds to future experience 

Traffic lights don't cause accidents. Poorly tested traffic lights might cause accidents. No conclusion about traffic lights in general should be made from that.


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