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DaveAK
26 November 2010, 1138
Now I know there are a lot of opinions on how best to balance a pack of cells, and several different ways are being used here on this forum, but I hope we can avoid those discussions on this thread because my questions are somewhat different. I'm not looking for the pros and cons of any particular method, just some understanding of what's going on.

First off the way I see it in an out of balance pack of say 8 cells you might have one or two that reach full capacity before the others, or you might have one or two cells that lag behind the rest. And I guess you could have the situation that one is ahead of the pack while another lags behind. So what conditions would cause a cell to reach full charge significantly before the others, and what are the conditions of a cell that causes it to lag behind?

Secondly, (and opinions might come in to play here), what constitutes a poorly performing cell? Plus or minus 5% on voltage, (absolute or working range)? Ah? When would it become unusable and need to be replaced? And when might it be recoverable, albeit at a lower life expectancy than the rest of the pack?

I have my eval board and some A123 cells on their way for testing some of this stuff out, and I have a power supply I'm ready to turn up to eleven.

chef
26 November 2010, 1145
Age of the cell, # of cycles, manufacturing differences are some of the factors that contribute to variations in performance. But you probably already know that :)

Voltage is an indicator on the extremes (low charge and full charge). In the meat of the charge, voltages will be very, very close regardless of how good/bad the cell is. At least that's my experience with this pack of Thunder Sky 40Ah LFP cells. 1 of the cells is very bad, 2 are kinda bad, yet in the middle of the charge they all are happily 3.3v within about 1% of the other cells. However at full charge they shoot up before the good cells, and upon discharging they're the first to drop below 3v.

DaveAK
26 November 2010, 1206
So keeping within reasonable operating conditions, (whatever they are :D), those 3 cells still perform adequately? You're not ready to rip them out and replace them just yet?

Did I make a wrong assumption in my theoretical pack? Do you just have good cells and bad cells, and bad cells will race ahead of the other cells on both charging and discharging? Can you get the situation I described where just one or two cells might lag behind the others on charging?

Nuts & Volts
26 November 2010, 1523
Correct me if I am wrong but I believe it all comes down to internal resistance. All the things mentioned by chef will affect the resistance of a cell as will temperature of that cell. So some cells will charge faster (lower R) and some slower (higher R).
Kyle

DaveAK
26 November 2010, 1634
This brings up another question. I've read a little about internal resitance, insofar as I know it differs from one maufacturers cells to another. From memory it seems like people who are really in to getting the best are looking for as small an internal resistance as possible. I never looked at the specs for the TS or SE cells, simply as it wasn't a consideration for me. So, having said all that, is this something that changes over time, through "wear and tear" for want of a better term? Through cell abuse? (Prolonged heavy discharge for example.)

chef
26 November 2010, 1716
Yes, lower internal resistance is better. Internal resistance increases power dissipation (P=I^2 R) which ends up as heat in the cell. This does change as the cell deteriorates over time & use.

Internal resistance is a simplified way of modeling the chemical properties in a cell. It can be affected by the terminal material, plates and the electrolyte. Usually degradation occurs primarily in one of the terminals, though I can't claim to understand the complex chemical behavior in a cell. People with PhDs spend their careers working on those things.

At least one of my cells is below 50% of it's rated capacity, so it's pretty bad. Haven't decided whether I want to take my chances and replace the bad cells in hopes that no others are in questionable shape.

Nuts & Volts
26 November 2010, 1725
"Internal resistance increases with both cycling and age.[40][35][41] Rising internal resistance causes the voltage at the terminals to drop under load, which reduces the maximum current draw. Eventually increasing resistance means that the battery can no longer operate for an adequate period."
^^from wikipedia
So yes over time internal resistance increases. I believe it has to do with imperfections in the chemical reactions taking place. The materials in the cell most likely breakdown causing points of higher resistance. If resistance increases then less current can be pulled at the same voltage.

Dont quote me on all of this, but this is my understanding of the cells
Kyle

DaveAK
26 November 2010, 1748
Great stuff guys! Thanks! Starting to put it all together now.

Richard230
26 November 2010, 1904
Speaking of internal resistance, the Hi Power batteries are checked for internal resistance at the factory and the value is placed on a sticker on the side of the cell. Attached is a photo of one of my batteries showing this sticker. The second number from the top on the sticker is the measured voltage, 3.30. Below that, the 2.1 represents the measured internal resistance, I assume in ohms. I have no idea if that is good or bad, I assume that it meets the factory specifications.

DaveAK
26 November 2010, 1906
Yeah, anyone know what's considered to be a good internal resistance? Or a particularly bad one?

chef
26 November 2010, 2300
Good/bad IR depends on the manufacturer and the specific chemistry & materials they use. Generally speaking, the higher performance batteries will have lower internal resistance. Remember P=I^2 R, the power dissipated in the battery is directly proportional to its IR (Internal Resistance). Double the IR, double the heat (conversely, halve the IR halve the heat).

Back to your original question of cell balancing, it's the relative IRs in a pack that are important. Assuming you're using the same make/model cells in a pack, ideally the IRs should be very tight. If you use rechargeable AA's, notice how some cells drain more than others after use. Those are poorly matched cells.

It's been a while since I've had to measure battery IR. This guy explains it well
http://www.madsci.org/posts/archives/2003-11/1067871870.Ph.r.html

DaveAK
27 November 2010, 1018
Thanks chef. Great link!