What's resistance in a lipo batt?

[superek4]

Hi, I got a new charger that can tell me the resistance of my lipo. I know the Older n more usage the resistance will go up but what's the benchmark?

I have 2 packs of zippy 6s. Both read abt 70. Is that high?

What's low? Thx guys

[BrianG]

The benchmark is 0 ohms of resistance. Seriously, you want as low as possible obviously, but every battery is going to have something.

And battery resistance does change with current draw. Is it measuring the resistance of the whole pack, or each cell? If it's each cell, that's pretty high. If for the whole pack, the per cell resistance is 11.6, which is still a bit higher than I would have expected (around 3-6 m ohms). And that's even if the charger is accurate.

[superek4]

Oh the cells are about 6-9 months old raced n bashed. I think the resistance is calculated as a whole.

[JThiessen]

Great question - I was wondering what mine should be also (I use a Hyperino Duo) - I seem to recall that my Neu 5S packs were showing something around 100 m-ohm, but I'm really not certain now.

[simplechamp]

That reminds me my new iCharger 106B+ can measure internal cell resistance. It will be interesting to see what kind of measurements I get with my balanced packs, and also the new pack I just got that has a suspected bad cell (only charges up to 4.09V).

Maybe I'll make a spreadsheet and record the internal resistances of the cells over the lifespan of the packs, might get some useful data there.

[BrianG]

I personally calculate the internal resistance by the values in the graphs. The process is a bit involved, but it shows actual values for that current draw instead of assuming the resistance at the charging current (generally at 1C) is the same as at running current (20C+). The process goes something like this:

1. Run the vehicle with a logger attached.
   
   
2. When the run is done, download the data to the computer.
   
   
3. Generate a graph with the data.
   
   
4. For most accurate values, zoom in on a fairly large section of the graph that is at least 1/3-1/2 way into the run where the lipo discharge curve is the flattest.
   
   
5. Pick an arbitrary current spike to work with.
   
   
6. Note the pack voltage just before the current spike. We'll call this "V1".
   
   
7. Note the current draw just before the spike. We could assume 0A, but that's not generally the case since we're running around drawing some current. We'll call this "A1".
   
   
8. Note the pack voltage at the peak of the current spike. We'll call this "V2".
   
   
9. Note the current draw at the peak of the current spike. We'll call this "A2".
   
   
10. Apply the following formula to get pack resistance at that current spike: ( V1 - V2 ) / ( A2 - A1 )
    
    
11. Then, to get individual cell resistance, simply divide that number by the number of cells in your pack. Yeah, we're assuming all cells are evenly matched, but without a logger that monitors each cell, this is the best we can do.
    
    
12. Repeat steps 5 to 11 for a spike of a different value until you get a decent set of calculations for various current draws. Try to take these repeat samples fairly close together to help reduce errors due to changing resistance because of discharge cycle.

Example: Let's say we have a 4s 5Ah pack, and we zoom in on an area of the graph with the following values:

- V1 = 14.8v
- A1 = 5A
- V2 = 13.7v
- A2 = 85A

The formula: ( 14.8 - 13.7 ) / ( 85 - 5 ) = 0.01375 ohms of resistance of the pack.

Divide that by 4 (4s pack) to get 0.0034 ohms per cell.

One of the reasons why I don't like using whatever resistance reading a charger provides is that it doesn't seem accurate. For instance, take that 0.07 ohms value someone noted earlier. If that was a true value, that means the pack voltage would drop 7v at 100A, which, on a 6s pack, would bring the cell voltage down to around 2.5v per cell. That doesn't seem right at all, so it has to be lower.

[simplechamp]

I'll check the internal resistances on my packs with the iCharger when I get home. Then we can see if it's reading way too high or what the deal is. Even if the values are off I'm interested to see the difference between the bad 4.09V cell and other cells.

EDIT: My Zippy 2s 30C 5000mah hardcase packs read 2 milliohms per cell when fully charged and 3-4 milliohms per cell when in storage state (3.8V per cell). That seems pretty close to your calculated values BrianG.