Battery Discharge Power vs Charge Power

[heule]

Guys

Newbie questions. Pardon my ignorance.

I was playing with some LIPO cells
- Battery Capacity was 240Wh
- When I discharge @ 0.3C (72W) , I saw 238Wh from the battery (cutoff 3.2V/cell)
- When I discharge @0.5C (120W), I saw 219Wh from the battery (cutoff 3.2V/cell)
- When I discharge @0.67C (160W), I saw 205Wh from the battery (cutoff 3.2V/cell)
- In all the cases, during the re-charging, I saw 235-238Wh being pumped into the battery.


Questions.
1) From the tests, the battery capacity differs at various discharge rate.
At higher discharge rate (0.67C), the delivered capacity is approx 85% of rated.
So where did the balance 15% energy go to?

2) Also, during the recharging, we actually saw nearly 98% of rated energy being pumped back into the battery. It doesn't seem to match the output energy consumed.
So is this recharging capacity a good indication of the energy consumed?


Thanks.

 

[dogman dan]

Heat. The faster you discharge, the more the resistance in the cell heats it up.

Going back in, less resistance at slower rate, but still some, plus the warming of the charger, the plug to the battery, etc. But when you discharged fast, you didn't leave any in there, it flew away as heat, so it will take the full amount to put it back in.

On good cells, the charging number can be used. Once they are old and shitty though, they will sit there forever trying to get all the way full, vampire sucking away.

 

[heule]

Heat? As in lost to the internal resistance of the battery and wires?

 

[joss]

Heat? As in lost to the internal resistance of the battery and wires?

Yes - internal resistance of battery and associated wires and equipment (I2 x R current squared x resistance)

 

[redilast]

It is true that some is lost in heat, but the fact is the higher the discharge rate, the more voltage sag and the less of the batteries actual capacity you use.

For example if you discharge at the industry standard of 0.2C to 2.75v @ 25C you should get roughly what the pack/cell is rated for.

If you discharge at 5C to 2.75v @ 25C, you will most likely get around 5-10% lower capacity. However, if you were to let the voltage rebound for a few hours and then discharge again at 0.2C you would likely be able to discharge a couple hundred mAh back out of the pack.

So in short the capacity isn't just all lost to heat, its still there just because of voltage sag at higher rates and 3.2v cutoff voltage you are not drawing all the capacity out of the pack.

 

[heule]

I suppose the voltage sag is due to the internal resistance?
Voltage lost = current * internal resistance?

 

[dogman dan]

Exactly