Internal Resistance in question with Discharge rate

[shizzzon]

Unless I am forgetting something, isn't Internal resistance directly related to the max potential discharge rating of a cell and it's potential voltage drop under load?

IE- a cell with lower internal resistance will have a higher burst rating and lower voltage drop under load, correct?

If so, then can someone explain this to me-

A cell rated for- </= 3mohm has a 30C burst rating but another cell of the same capacity with a resistance of </= 2mohm has only 8C burst.

Burst for both cells are rated for 10 seconds.

Luckily, the cell with the 8C rating has a discharge curve so i can see it's expected voltage drop based on C.
The other one does not so i have to assume or guess what i would be buying.

But does this make sense? IS it just a case of lying specifications or is there another variable which may make both specs true and i'm not factoring in something?

Both cells are LiFePO4 20ah

 

[amberwolf]

I dunno...maybe you could link to the cells datasheets, or attach them? Or tell us where they are from? Etc.?

 

[shizzzon]

Ok, here is one- http://www.electriccarpartscompany.com/b-20Ah-32V-64W-5C-b-br-Aluminum-Encased-Battery-br-ISO9001-CE-UN383-MSDS-certified-br-Lithium-LiFePO4-LFP-Custom-Built-Battery-br-h3-4000-Cycles-Continuous-Charger-Cycles-br-Click-Image-Gallery-to-see-Cycle-Graph-h3_p_113.html

And the other, i will copy n paste as it was sent to me from A&S Power Technology

No. Item unit Parameters Remark
1 Nominal Capacity Ah 20.0
Capacity according to
standard discharge,
After standard charge
2 Nominal Voltage V 3.2
Average Voltage
according to standard
discharge, After
standard charge
3 Charge Type / CC/CV /
4 Charge Cut-off Voltage V 3.65 /
5 Discharge Cut-off Voltage V 2.0 /
6 Charge current A 20 constant current
7 Discharge current A 20 constant current
8 max. discharge current A 400 constant current
9 Max. instantaneous discharge
current A 600 ＜10S
10 Weight g 700±35 /
11 Inner Resistance mΩ ≤3 /
12 Dimension（L×W×H） mm 70*27.5*185 /
13 Working
Temperature
Charging ℃ 0～45 /
Discharging ℃ -20～60 /
14
Storage
Temperature
1 month ℃ -20～45 /
6 months ℃ -20～25 /
Relative Humidity RH 25%～70% /

 

[markz]

What about Internal Resistance as a factor of Capacity. Stock mine should be 2.4Ah, now with 3x the IR, I have no clue what the capacity is without yet another test.

My IR should be 10mOhms when new, but I bought mine used so most are 30mOhm of Internal Resistance.

 

[dogman dan]

I would guess that your capacity is down to well below 80% with that resistance. But when you do measure your capacity, know this. If you run and find it's 60%, you might find that lower the c rate enough, and you get a lot of capacity back.

I learned to run very tired, very old lipo in at least 30 ah packs, because then discharging at .25 c or so, I could milk quite a few slow miles from them. Or at least decent miles, at full speed.

It's really just the same ol thing. Battery happy if it discharges cool. Increase capacity enough and you don't throw away watt hours as heat. Hammer that pack hard and make heat, less capacity for sure.