Hi RIP,All my new Enerland 4350 LiPO's have their Internal resistance written on a spec sheet. Most of them range from 0.009 to 0.012.
should I group them in series or parallel.
Im guessing in series ?
Hi Tess,Ideally, you want the equivalent resistance of each "cell" in series to be identical. So you parallel cells with various internal resistance figures to result in the same overall resistance as every other bank of cells that are paralleled together to make up a series string (ie - a battery).
Who are trying to kid, you crusty curmudgeon! You love to disagree with me!?You know I hate to disagree with youAnd Woodsmith says we should not resort to insults, namecalling and threats.
Yep, I do agree, but I saw this as the lesser of two evils at really high currents (relative the the cell's capacity) because if a paralleled group consisted of all high internal resistance cells it might trip LVC prematurely. I was keeping in mind what RIPPERTON here would likely be using the cells for - namely, in racing motorcycles. Of course, you have some practical experience in this matter whereas mine is entirely fabricated, so...This is called charge redistribution. It is undesirable because it is current flow within the system which does no useful work for you...
... If you intend to use the battery hard, you might try to keep cells having higher resistance on the outer edge of the pack as they would heat more. ...
Hi David,Major, I don't have any empirical data for this, so not disagreeing, but I don't understand why that would be the case. It seems end batteries would lack warm neighbors on one side, and would have a longer battery cable to draw away heat. Why would end cells run hotter despite more ability to shed heat?
If you intend to use the battery hard, you might try to keep cells having higher resistance on the outer edge of the pack as they would heat more.
Yeah, I'd like to know this too, since I think major is full of crap...And David,
Do you agree about matching internal resistance on the parallel sets?
The terminal probably fans out internally and so you are seeing increased heating because of current crowding... that's my guess, anyway... Keep in mind I've never even seen one of these cells, so......But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}
I guess that things that are electrically isolated from each other are also thermally isolated. That means that the higher temperature at the positive terminal indicates that whatever process is responsible for the internal resistance of the battery is active at the positive plate. For the manufacturer, it means if you want to reduce resistance, look at the positive plate structure and reactions.ps...
{On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}
It is just the positive terminal on cell #14 which is at the positive end of the 14 cell pack. All cells are identical having a positive and negative foil type terminal. Except for the cells on the ends, the foil tabs are compressed together with the adjacent cell tabs with aluminum bars. The same aluminum bars are used on both ends, just compressing a single cell tab with a cable lug screwed to it. And from memory, it is like 20 or 30 degrees hotter than anywhere else on the battery. I have some notes about it in the lab I'll look for. Here is a photo of the battery.{On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}
Got it, no confusion now. Mr. Literal here read "keep cells having higher resistance on the outer edge of the pack as they would heat more" as cells on the outer edge would heat more than in the middle, and that was desirable for some reason. Here's a clarifying restatement for the Literal Readers like me in the crowd: "keep cells having higher resistance on the outer edge of the pack since they heat more."major said:Hi David,
Maybe I didn't state that clearly, or you didn't read it clearly, or I don't read your post clearlyAnyway... ... I think it is common to see cells in the middle of densely packed batteries get hotter during heavy current discharges. That has been my experience.
I have not tested this myself, but my intuition is it is better to match resistances for the paralleled cells. Here's a roundabout way to show it: A local EVer mixed cells of different capacities, I forget the exact capacities but it was in a 2:1 ratio (like 50 and 100 Ahr). He figured the 50 Ahr cells would have twice the resistance, and so would supply 1/2 the current of the 100 Ahr cells. He figured the cells would reach empty at the same rate. He was right, it worked for him. His daily commute almost completely emptied the cells each day, so it wasn't a case of shallow discharges. BTW his drive home was a long uphill with a big elevation gain.major said:And David,
Do you agree about matching internal resistance on the parallel sets?
I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.major said:{On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}
Interesting theory David. The Li cells have one aluminum and one copper terminal. Can't remember which is which, positive negative. But between the end cell terminals and the copper cable lugs I have an aluminum clamp bar. Hopefully I'll get back into testing this month. I think I will make a copper bar clamp to replace the aluminum one on the positive end and see if it makes a difference.I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.
as? since? I know what I mean when I type it. Sorry, but all you have to do is ask. If I confuse you, chances are others are confused also. I'd rather be questioned than leave something misinterpretedGot it, no confusion now. Mr. Literal here read....
...I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.
Maybe so Armin but power isnt what I wantFor racing you should get considerably higher discharge power from your LiFeTech X1P cells since they all should be under 3 milliohms