How to calculate C-rating or safe constant discharge current

[Zerts]

Such as for LiPo, the manufacturer always claims unrealistic C-rating and allowable discharge amps. I have an IR (internal resistance) meter, knowing the battery capacity in mAh, voltage and IR in mOhms, how do I calculate "real" C-rating or safe allowable constant discharge amperage?

 

[redilast]

I don't think there are any industry standards for calculating C-rate. Ratings vary quite a bit and often times there are notes in the data sheets such as 30A max discharge current with an 80C cutoff. Meaning you might be able to safely discharge at 30A for a few minutes, but not necessarily continuously due to the cell temperature reaching an unstable level.

What kind of IR reading do you have? DC IR or AC? To get a ballpark figure you can base AC IR off existing manufacturers ratings of other reputable 18650s for example. For example 15 mili ohm AC cells are often rated around 20A discharge. So 7.5 mili ohm around 40A.

But there is more to it than mili ohms, because you have to take into consideration both the cell chemistry and also the package type (e.g. size and shape of cell -- cylindrical, pouch etc). Different sizes and shapes of cells have different heat dissipation characteristics. Most of the hybrid cell chemistries like NMC are stable at higher temperatures at around 80C compared to more traditional LiCo which are advised to stay below 60C. LiPo, I'm not sure the max recommended safe temperature off hand, it might be 60C.

Best way to rate the cell is probably to load test it and measure temperature during the load test, and if the temperature exceeds the cells safe maximum temperature then the discharge current is set too high for continuous use.

 

[Matador]

I don't think there are any industry standards for calculating C-rate. Ratings vary quite a bit and often times there are notes in the data sheets such as 30A max discharge current with an 80C cutoff. Meaning you might be able to safely discharge at 30A for a few minutes, but not necessarily continuously due to the cell temperature reaching an unstable level.

What kind of IR reading do you have? DC IR or AC? To get a ballpark figure you can base AC IR off existing manufacturers ratings of other reputable 18650s for example. For example 15 mili ohm AC cells are often rated around 20A discharge. So 7.5 mili ohm around 40A.

But there is more to it than mili ohms, because you have to take into consideration both the cell chemistry and also the package type (e.g. size and shape of cell -- cylindrical, pouch etc). Different sizes and shapes of cells have different heat dissipation characteristics. Most of the hybrid cell chemistries like NMC are stable at higher temperatures at around 80C compared to more traditional LiCo which are advised to stay below 60C. LiPo, I'm not sure the max recommended safe temperature off hand, it might be 60C.

Best way to rate the cell is probably to load test it and measure temperature during the load test, and if the temperature exceeds the cells safe maximum temperature then the discharge current is set too high for continuous use.

Great explaination.
I think C-rating is more of an empirical value, included in the datasheet of the cells.
I does makes sense that lower DC IR cells can provide higher amps.
With DC IR and current value, you could calculate the Voltage drop of a given cell ( dV = R x I ) (R is DC-IR not AC).
With that given voltage drop, you can calculate heat losses : dP = dV x I (also dP = R x I^2)

There is a way to calculate temperature by using the value from heat losses (dP) in watts, but I don't know how to do it as it requires advanced knowledge about the capacity of metal and enclosed chemical in cells to dissipate that heat by radiation, convection, etc..., the surrounding ambiant tempertures, the size and shape of the cell, etc...

 

[redilast]

Great explaination.
I think C-rating is more of an empirical value, included in the datasheet of the cells.
I does makes sense that lower DC IR cells can provide higher amps.
With DC IR and current value, you could calculate the Voltage drop of a given cell ( dV = R x I ) (R is DC-IR not AC).
With that given voltage drop, you can calculate heat losses : dP = dV x I (also dP = R x I^2)

There is a way to calculate temperature by using the value from heat losses (dP) in watts, but I don't know how to do it as it requires advanced knowledge about the capacity of metal and enclosed chemical in cells to dissipate that heat by radiation, convection, etc..., the surrounding ambiant tempertures, the size and shape of the cell, etc...

Yep and the heat generated internally in the cell can't even be calculated very precisely based on the DC IR, because the IR actually can drop as the electrolyte in the cell heats up it actually increases chemical conductivity within the cell decreasing the IR.

 

[Zerts]

Thanks all, my meter is something currently being used by R/C aircraft hobbyists, it is a Wayne Giles ESR (equivalent series resistance) meter ... I assume it is measuring by DC. It is powered off the LiPo's own current. It retails for about $95. The meter should be accurate only when the pack temperature is at 72 degrees F.

It is Wayne Giles' contention that LiPos manufactured for hobby claim outrageously high C-ratings. For instance, I just bought some packs that claim 40C continuous, but according to the ESR meter, they run about 13C to 18C.

 

[Hillhater]

"C" rate is a simple ratio figure , but it is useless as a performance measure without several other parameters measured at the same time. Hence it should be viewed as a "Advertising" gimick since there is no standardised method of measuring it.
It really needs a voltage "sag" figure or % included at least and also a capacity or state of charge level quoted at the same time. Temperature can also make a difference.

 

[Zerts]

I understand, I just needed a ballpark method to shop for batteries and I thought the ESR meter would do it.