Duff/weak cell?

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Sat Aug 20, 2011 9:39 pm

Of course as Beemer pointed out the problem could be a false reading from the bms. Use a volt meter to confirm the cell readings, being careful of course as working on pack voltages can be lethal.


I see what you mean now about loose connections, I had only considered how it might affect the voltages in the pack and not how it could skew the BMS readings. And don't worry, I'm well aware of what I'm dealing with - I had some mild pyrotechnics when I was wiring up the BMS. This is probably one of the reasons I've been so reluctant to take the pack out again, besides the fact that I've got loads of wiring and bodywork to remove and it's a pretty tight fit in there :roll:
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

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retepsnikrep
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Re: Duff/weak cell?

Postby retepsnikrep » Mon Aug 22, 2011 6:31 am

Sounds like a bad cell to me. I've had a few go duff in my time with similiar symptoms.

Just for the record i have some 40+ good A123 8ah cells available at £5.00 each :wink:
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

GregsGarage
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Re: Duff/weak cell?

Postby GregsGarage » Mon Aug 22, 2011 4:15 pm

Since the pack is configured as 25S2P, it would need to have 2 faulty cells at the same position to give this fault. Not impossible, but that's what made me think it could also be a connection problem.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Mon Aug 22, 2011 5:17 pm

That was also going through my mind - if they are both duff could it just be coincidence or has one cell taken out its partner over time?
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

Beemer
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Re: Duff/weak cell?

Postby Beemer » Tue Aug 23, 2011 7:58 pm

Two cells in parallel act as one cell. If one cell is dud it will as sure as cheese appear as a half size twin cell and wreck the cell in parallel by dragging that down with it.

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Sat Sep 10, 2011 5:57 pm

Finally got around to replacing those (potentially) duff cells yesterday and tightened up the screws (there was a loose connection roughly where I expected). A few fireworks due to that damn single plate swinging round and shorting against adjacent cells :x Burnt my fingers a bit trying to swing it back, finally got it attached and tightened up every screw on the pack - there were a few other not totally tight ones as when I originally put the pack together I wasn't using a crosshead screwdriver that fitted the screws perfectly.

Finished putting the bodywork back together this afternoon and took a test ride round the block, all seems to be well :)

It's now receiving a well-overdue charge, so should be fine to start using it again on Monday :D
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

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Re: Duff/weak cell?

Postby GregsGarage » Sat Sep 10, 2011 8:42 pm

Ian,

It would be interesting to do a capacity test on the old cells. One of my projects is a battery/cell cycler that will safely charge and discharge cells and report the capacity. I've got a pcb and most of the components, unfortunately one of the critical components is on back-order... ...time! :lol: :lol:

I am using Nord-Lock washers on my pack, they seem to be keeping all the connections tight.

A few fireworks due to that damn single plate swinging round and shorting against adjacent cells :x


Glad it wasn't more serious like this incident. The Electrifying Birth of ‘Plasma Boy’ Good reading if you haven't heard how John Wayland got the nickname 'Plasma Boy'
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Sat Sep 10, 2011 9:24 pm

It would be interesting to do a capacity test on the old cells. One of my projects is a battery/cell cycler that will safely charge and discharge cells and report the capacity


Now that sounds interesting - I wanted to test the cells anyway because they may be fine, but battery testers seem to be pretty expensive. When you get yours up and running I'll be interested in building one if you have any more PCBs?

I'm keeping an eye on things though, as the new cells still seem to behave a little "flexibly" (reporting lowest voltage under load and charging quicker than the others), so I'm beginning to suspect an issue with the slave circuit. I'll see if it settles down after a few charge/discharge cycles.
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

Beemer
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Re: Duff/weak cell?

Postby Beemer » Mon Sep 12, 2011 4:15 am

I think everyone will agree your problem is not solved until you are sure all your cells sit at the same state of charge, preferably from the bottom.

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Re: Duff/weak cell?

Postby GregsGarage » Mon Sep 12, 2011 8:45 am

Beemer wrote:I think everyone will agree your problem is not solved until you are sure all your cells sit at the same state of charge,


I guess I am not everyone. :twisted: As long as the scooter has the range required to complete Ian's driving needs then the problem is solved. If Ian wants to have the maximum range possible from his pack then he only needs to insure that whichever cells trigger a low voltage alarm when the pack is completely discharged, get fully recharged. Any further balancing will gain you no further range and so is of no benefit. The range/capacity of the pack will be determined by the weakest cell in the pack. As long as you can work that cell from fully charged to fully discharged, it doesn't matter what the other cells are doing. All that matters is that your bms prevents any cell from getting over charged or under discharged.

preferably from the bottom.
Why is from the bottom preferable? I have never liked the idea of balancing from the bottom. It takes far to long as you need to fully discharge the pack and it is very easy to ruin a cell(s) if you discharge to far. Adding additional charge to individual cells is far quicker. You do it after the pack is fully charged, so if you suddenly find you need to use the vehicle in the middle of balancing, you just stop what you are doing and return to the balancing an other day. Bottom balancing is quite literally a waste of time in my opinion, time I would rather be doing something else. I am interested in hearing about anyone's actual experiences with bottom balancing and most importantly, would they bother doing it again.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


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