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Self Discharge on Powerwall Cells
#1
Hey All,

I'm wondering about the importance of removing self-discharging cells from those going in a powerwall. I have small battery packs that I use for mobile charging solutions or portable ham radio equipment, and those are not cycled much. Obviously I remove any cells with self-discharge issues from those packs. However, it seems that a powerwall would be different. I am building my powerwall for an RV that will be used 24/7. It will always be charging or discharging. That being the case, is it important to remove cells that self-discharge? Removing cells that drop like a rock makes sense, but if a cell holds voltage for a day or two it seems like it would be fine in a powerwall.

Am I missing something here?
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#2
If a cell is self-discharging, it gets removed from production. A cell that self-discharges will continually get worse over time. It might be loss of a few 100mA in a week, to draining 1000's in a week over a period of time. Do you really want an energy sucking tick in your powerwall? I sure don't
Loquist and Crimp Daddy like this post
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#3
(07-21-2018, 08:32 AM)Loquist Wrote: .... but if a cell holds voltage for a day or two it seems like it would be fine in a powerwall.

I tend to agree ...but  it's very rarely you find a cell self discharging quicker than 0.01V/day  ..

How many of these dischargers do you have? ... even discharging at 1 volt a day would not be an issue for your situation , or for most powerwalls , once such a cell is put in a 100p pack the discharge of the pack will only be 0.01V/day ....

the truth is we just don't know how a bad self discharger will behave in the future.

If you have a lot of these it might be worth purring them all in a small pack together where you can keep an eye on how they develop
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#4
Thanks Korishan. That makes sense.

Ozz, I don't have any measured out at the moment. My current setup that I've been running has premade packs from some used medical equipment, and they've been fine. The cells I am processing now are either waiting to cycle for testing or waiting for a bit to be tested for self discharge.
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#5
One consideration is if a self discharging cell is draining only one parallel group in the series string, your BMS will constantly be fighting, draining the other cells to keep the pack in balance. I suppose it isn't as much of an issue in a pack that is cycled every day, but in my pack I had a very slight self discharging cell and my BMS with only 30mA of balancing current just couldn't keep up and the pack drifted further and further out of balance over the course of a week.
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#6
The problem with self dischargers isnt really loosing energy on 1 pack but the fact you need to balance all OTHER packs to the same level so you dont overcharge them!

So as rev0 said you will in the end up with a BMS that cannot balance fast enough and thats when you hit overvoltage protection instead. If you see that when you fully charge the pack and the pack cant stand even any more you should consider change out the pack with the faulty cell. This is my recommendation.

You need to draw the line where this becomes an issue that is. I have packs like that with old crap cells that do drift and they work just fine but I need to understand whats going on and such.
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#7
I had this issue this weekend. One of my packs was much lower in voltage after resting overnight than the rest. Took the time to tear off the fuses on one half and let sit for a couple hours then found 4 cells in the 56p pack. I keep a small assortment of cells handy for issues like this. But upon reading another thread this morning about having an extra pack for the just incase factor is making much more sense to me now.
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#8
Yeah always run n+1 and have a spare. You will get self discharging cells sooner or later and thats when you need a BMS or monitoring to tell you what pack is causing it and be able to swap it out. If self dischargers wouldnt have existed we most likely could have skipped the balancer in most cases... (Yes IR will cause it to...)

Can add that with active balancers self discharging packs are less of a power to waste issue but in the end you should not have the self dischargers in the system for to long.
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#9
Hi all !
I have a related question: from which discharge value per day do you ditch your tested cells ? What would be a so-so value ? and what is a ok value ?

From my measurments, I have a huge range of value from 0.001 to 0.033 v/day.

Thanks
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#10
I would say 0.15-0.2V per month which is 5-6.6mV/day is my limit for good enough to use.
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