Nano tech lipo's, different batches different sizes

[Punisher 67]

My batteries are brand new and are the larger cells....................

I am interested in what HK say's about the smaller cells

[northernmike]

Well, if the rI of one pack is different than the other, in parallel at least you are only dividing the resistance... it's another problem when you're going to draw current in series, if one pack (or cell!) is drastically different some spectacular results can be seen...

My charger reports my Turnigy 45-90 2200 3s to have an rI of 7mOhm, what are you guys seeing on your cells?

How would mismatched packs affect an ESC?

[Boaterguy]

They just wouldn't be great for caps, since they would be getting a constantly changing input as the cells battle over getting their discharge rate.

[northernmike]

... since they would be getting a constantly changing input as the cells battle over getting their discharge rate.

I don't understand. How do they battle?



I would think the lower rI pack would just "win".

[Boaterguy]

It's a bit of a metaphor, they don't battle, but the current would change more than running two cells of the same type. Also, you could discharge one batt quicker than the other therefore over discharging one.

[northernmike]

...the current would change more than running two cells of the same type.

How and why? We're talking parallel, right? I'm still learning how this all works..

[Boaterguy]

I'm mostly using guesses, I'm stumped exactly how it would change, but I was told NEVER run different discharge or capacities in parallel or series

[Punisher 67]

There is no battle , The lesser cells would just drain faster having less capacity . The cells with less internal resistance would be able to supply more current ( bigger cells ) . All in all providing you did not go past the safety threshold voltage wise on the lesser of the two batteries you would still wake up the next morning to live another day..... Everything will still be all good

Do use both of the same batteries in parallel though just to keep things even , but if you didn't it wouldn't be the end of the world .

[Boaterguy]

well, I guess a few mAh difference wouldn't be a problem, but if you were running a 4000 20C and a 5000 45C, it wouldn't work

[northernmike]

I'm mostly using guesses, I'm stumped exactly how it would change, but I was told NEVER run different discharge or capacities in parallel or series

Here is an explanation of why capacitors are necessary (especially with longer battery wires)

First a watery analogy, water running in a pipe and through a tap. Now turn off the tap quickly. You'll hear a loud knock/shock sound in the pipe. The water wants to continu flowing but it can't, for a moment the water pressure is much higher than the static water pressure. It's the same for a current that's switched off, because of the inductance it wants to keep on flowing, voltage gets higher. This is also what causes sparks (brush fire) in a brushed motor.

The controller is like a watertap that's switched off (and on) very fast (8,16, 32kHz PWM) to get the desired current. Turning off the current, in combination with the battery wire inductance, causes voltage spikes because the current wants to continue on its course (ref. inertia of the moving watercolumn). Those voltages are higher than the battery voltage. The input capacitors (cylindrical) takes care of these spikes (they reduce the wire inductance). The longer the wires, the higher the voltage spikes induced in the wires, the harder on the input capacitors. They will get warmer, heat up and explode. This is caused by the wire inductance, not by wire resistance. Therefore, using thicker wire will not help much, it's not a bad idea either, but extra capacitors are the solution, thus reducing/compensating the wire inductance. Or longer motor wires instead of long battery wires.

from: http://www.rcgroups.com/forums/showthread.php?t=952523

Please do not guess when people ask questions about using power at such levels.

It's ok if you don't understand everything, just let people know where you heard it and share your own experience.

Sorry if I'm rude, I just hate to see "answers" confused with knowledge.

If running questionable cells in parallel, I would consider using a board from these guys: http://www.tppacks.com/products.asp?cat=26

This would prevent large currents from being drawn through higher rI cells or cells approaching their LVC.

[Punisher 67]

well, I guess a few mAh difference wouldn't be a problem, but if you were running a 4000 20C and a 5000 45C, it wouldn't work

That is correct , even batteries that are the same are not identical capacity and internal resistance wise . there are always some subtle differences but well within tolorable limits .

Putting a 4000mah with a 5000mah is asking for problems , the 4000's safety threshold will most certainly be exceeded . what would be safe for draw on the 5000 at higher amp draws would probably put the 4000 over and compromise the battery .

this would be the end result........... and then.........

[millzee]

After about three back and forth emails, they wanted a range of different photo's each time, I think they were hoping I'd give up but I finaly got another email telling me that the matter is "under investigation in several internal departments"
Not sure what's going to come of it but I'll keep at them.

[m4a1usr]

The construction method is why some packs are thinner and others are thicker. Thats the most likely reason. The cell maker is probably using the same production line. The assembler is most likely the reason. He could be using double sided tape to stick each cell to each other ( so there would be a cumulative increase as cell quantity increases) or they were using a certain size/ piece of end insulator that again has changed dimensions. Pack construction is horrible when you take apart the cheaper packs. I know. I have taken apart quite a few 6 and 5S packs due to bad cells. You guys would be amazed at the balsa wood and cheap tape holding the packs together under the heat shrink. Totally amazes me each time I look at the guts of our packs. And I wonder how the heck they can get away with some of the "C" ratings when you look at the solder job done on the cell tabs. Sure has opened my eyes!

John

[Boaterguy]

if you watch some "how it's made" shows, the assembly worker taps the joint with an iron and solder, they just heat it enough to get solder on there, no concern about quality.
i'm gonna email that show and tell them to do one on lipo's; see what we are really getting.

[sailr]

The HK packs are cheap. Ya gits wut ya pay fer.