Duff/weak cell?

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Beemer
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Re: Duff/weak cell?

Postby Beemer » Sat Sep 17, 2011 9:51 pm

Don't forget to fit any cell back in at the same state of charge as the rest of the pack or you will be doing damage.

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Re: Duff/weak cell?

Postby GregsGarage » Sun Sep 18, 2011 11:02 pm

Beemer wrote:Don't forget to fit any cell back in at the same state of charge as the rest of the pack or you will be doing damage.


If the BMS is doing its job, it won't allow any cell to go over or under voltage. The effect will be reduced range. Cells only get damaged when they are operated outside their allowed voltage ranges.
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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Mon Sep 19, 2011 12:03 pm

I was going to say I don't really have any control over or way of testing individual cells' states of charge, but I also presumed that paired cells balance themselves anyway if they're healthy?
Ian.

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Re: Duff/weak cell?

Postby GregsGarage » Tue Sep 20, 2011 2:41 pm

Cells connected in parallel will balance/equalize themselves when charging. You need to get to the "knee" where the voltage starts rising (above 3.4 volts) and they will balance themselves
.
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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Wed Sep 28, 2011 10:55 pm

I haven't really had the time lately to do any more investigation on this cell problem, but I've now modified my charging code so that instead of one cell hitting the preset max voltage and finishing the charging process, it now just stops charging, waits for the high cell to be pulled back down by the dummy load to a preset voltage and then starts charging at low current again. Hopefully this cycle will gradually bring the low cells up closer to the high one and the pack voltage up to my preset limit. Once the all the cell voltages are within a preset range it should finally stop the charge :)

I'm just trying it now and I'll probably leave it running overnight - anyone see any problem with this approach?
Ian.

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Fri Sep 30, 2011 8:07 am

Well, after two nights of charger puffing and blowing, my cells are finally getting balanced :D

I had the load cutin voltage set at 3.40 volts and when I checked this morning they were all round about this level (and most of the loads were on), and after settling the pack showed a nice healthy 85V :)

So tonight I'll push the cutin level up to 3.45V and see where they are tomorrow morning!
Ian.

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retepsnikrep
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Re: Duff/weak cell?

Postby retepsnikrep » Fri Sep 30, 2011 9:42 am

Well done. I have my system set so that as soon as all cells have reached > 3.50v on that charge cycle it terminates.

In my master software a flag is set when a cell reaches 3.50v when all flags are set then :)
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.

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Re: Duff/weak cell?

Postby GregsGarage » Fri Sep 30, 2011 4:45 pm

Ian, you will also want to monitor temperature of the pack and shut down if it starts climbing. Peter's code monitors temp but does your code also monitor tempurature?

Greg
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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Fri Sep 30, 2011 9:50 pm

It doesn't actively check the temperature, merely display it. To be honest I've never seen it higher than about 27° so I haven't been too worried. Unless that's considered high? I'm only charging at about 5A during bulk charge before switching to low current for the balancing phase at which time the temperature always drops anyway.
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 » Fri Sep 30, 2011 10:31 pm

The problem is that lithium cells can behave very badly if overcharged (actually all batteries will behave badly if overcharged). A risk of fire is very real from a overcharged lithium pack. So if you are going to do balancing charging overnight you need to make sure that the bms will shut down if anything is amiss. For example lets say one of your cells goes to 0 volts. Will the bms detect this and shut down, or will the charger see the lower voltage and go back to its 5A bulk charge phase?

Greg.

P.S. Glad to see that the problem appears to be a balance issue.
Greg Fordyce

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