How do LiPo's Handle Current Spike? Does the voltage drop?
- Thread starter HacknMod
- Start date
liveforphysics
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V=IR
R is the internal resistance of the cell. (often called Ri)
For example:
If your cell is 15mOhm (0.015Ohm), and you draw 30amps from it, it will drop 0.45v.
If your cell is 1.5mOhm (0.0015Ohm), and you draw 30amps from it, it will drop 0.045v.
The cells resistance changes with SOC to a fairly mild extent (generally +-50%), and greatly with temperature, cold vs hot can be 300% difference or more.
If you're trying to discharge at 3.2v, I sure hope that's a LiFePO4 cell, because modern RC LiPo is dead empty by 3.5v, and you're just wrecking the cell to go lower.
R is the internal resistance of the cell. (often called Ri)
For example:
If your cell is 15mOhm (0.015Ohm), and you draw 30amps from it, it will drop 0.45v.
If your cell is 1.5mOhm (0.0015Ohm), and you draw 30amps from it, it will drop 0.045v.
The cells resistance changes with SOC to a fairly mild extent (generally +-50%), and greatly with temperature, cold vs hot can be 300% difference or more.
If you're trying to discharge at 3.2v, I sure hope that's a LiFePO4 cell, because modern RC LiPo is dead empty by 3.5v, and you're just wrecking the cell to go lower.
3.2v/cell is actually ~99.8% drained so i doubt you'd want to be discharging at that point!
Anyways all batteries have internal resistance and will drop voltage pretty bad when used at their maximum C rate.
The amount of voltage drop is dependent on the C rate of draw VS the C rate of the batteries.. i have found to be to be quite linear.
For example... with a 40A load..
5AH 10S 20C pack @ 8C draw = 3V of drop across the pack.. ( .3v per cell )
10AH 10S 20C pack @ 4C draw = 1.5V drop across the pack.. ( 0.15v per cell )
20AH 10S 20C pack @ 2C draw = 0.75v drop across the pack.. ( 0.075v per cell )
Always run batteries far below their C rate for best performance. I never run my 20C lipos above 8C. They can take spikes, sure. But look at the constant amps you're runnin 'em at ..
Anyways all batteries have internal resistance and will drop voltage pretty bad when used at their maximum C rate.
The amount of voltage drop is dependent on the C rate of draw VS the C rate of the batteries.. i have found to be to be quite linear.
For example... with a 40A load..
5AH 10S 20C pack @ 8C draw = 3V of drop across the pack.. ( .3v per cell )
10AH 10S 20C pack @ 4C draw = 1.5V drop across the pack.. ( 0.15v per cell )
20AH 10S 20C pack @ 2C draw = 0.75v drop across the pack.. ( 0.075v per cell )
Always run batteries far below their C rate for best performance. I never run my 20C lipos above 8C. They can take spikes, sure. But look at the constant amps you're runnin 'em at ..
Thanks for the great info everyone.
I thought I'd read that you can discharge LiPos down to 3v without harming them, but I just looked at some curves and it looks like the voltage plummets after 3.5v. If they're "dead" at 3.5v, there's no point in going to 3v then. Thanks for pointing that out.
liveforphysics said:
neptronix said:
I thought I'd read that you can discharge LiPos down to 3v without harming them, but I just looked at some curves and it looks like the voltage plummets after 3.5v. If they're "dead" at 3.5v, there's no point in going to 3v then. Thanks for pointing that out.
If we're talking about RC lipo.. they are about 95% dead once they hit 3.5v/cell.. that extra 3% is not worth it as a cell or two can easily drop into the 3.0v zone as you approach 3.3v/cell average. If you have a particularly weak cell, you can have a cell go into the 3.0v zone even quicker by the time you hit 3.5v.
So the meat of the discharge is between 4.15v and 3.6v in reality.
So you get it. That last 5% is just not worth it at all.
So the meat of the discharge is between 4.15v and 3.6v in reality.
So you get it. That last 5% is just not worth it at all.
neptronix said:
Yes, I'm working with a pack comprised of 12s 3p of these cells to be specific.
neptronix said:
Does the chart represent one specific RC LiPo battery or is this typical of all RC LiPos? Some other charts I saw indicated 3.5, so I just want to be sure. I guess it doesn't matter a whole lot since it's only 5% as you mentioned.
HacknMod said:
That's a turnigy 10S 20C 5AH pack being discharged at 8a-9a.
Same results with the Zippy brand lipo; cliff starts around 3.6v.. Notice the cliff it hits.... yes, that's your warning to stop discharging
But the most important thing to see is how disbalanced the cells get after 3.5. Now, this pack could easily be discharged to 3.3v/cell average if i liked. But later down the road.. a cell gets weak.. or i neglect balancing for too long.. and i could have a lipo fire on my hands.
That's the same reason i charge to 4.15v at the top. Good insurance policy that will allow your lipos to survive many many bulk charge cycles with a very high margin of error. Plus, it theoretically extends the cycle life of these 3.6-3.8v nominal cells.
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