Charged capacity vs Discharged capacity 18650

[Raylinguo]

Hi there, I'm new to this forum and also to building ebikes/batteries and I have a question which I can't seem to find the answer to anywhere online.

It may be obvious but I just want to make sure. I have a bunch of recycled Panasonic ncr18650bd cells, and have been using the liitoKala lii-m4s battery charger to measure capacity.

After the charger is finished measuring the capacity of the cells it displays both a charged capacity and also a discharged capacity. And as the cells are second hand these figures are usually different, sometimes up to 300mah apart, for the charged vs Discharged capacity of one cell.

So my question is which figure is usually used as a Guage of the total packs capacity? If I am grouping batteries I want to know which figure is best to use so that each parallel group is as close as possible in capacity.

The industry standard would be for discharged capacity right? And the older chargers like opus etc when using to test cells would also display discharged capacity?

As it is a relatively new charger I think it displays both charged and discharged capacity.

And also a secondary question is, is there a point where the difference between the charged capacity and discharged capacity of one cell is big enough that it renders the cell not worth using?

Thanks in advance

 

[lnanek]

I've heard of people using charge capacity being much higher than discharge capacity as a proxy for measuring high internal resistance. You have to put more energy in than you can get get out, essentially. So there's some value to having both numbers.

Unless you only have discharged batteries on your ebike for the purpose of regen braking, though, discharge capacity is what you are actually using when you are riding under power and most accurately predicts your range.

 

[DogDipstick]

Its called collumbic efficiency... and is probally just a product of your testing machine if it is cheap. CE is the ratio of the total charge extracted from the battery to the total charge put into the battery over a full cycle.

I always measure capacity in vs capacity out, divide one by the other, and figure te columbic efficiency. Should be .92 or greater. Usually around .95-.98.

A cell that wastes power on discharge and charge cycle in-contingencies is on its way out.. life lost.

Read this.

https://batteryuniversity.com/article/bu-808c-coulombic-and-energy-efficiency-with-the-battery

 

[Sajeel]

It may be obvious but I just want to make sure. I have a bunch of recycled Panasonic ncr18650bd cells, and have been using the liitoKala lii-m4s battery charger to measure capacity.

And also a secondary question is, is there a point where the difference between the charged capacity and discharged capacity of one cell is big enough that it renders the cell not worth using?

Thanks in advance

I'm not sure I would trust a capacity tester/charger like that. I've bought the Xstar Dragon VP4 which was sort of expensive, and it's off!
I would use something else to test for capacity. There are some nice looking ones on eBay with a heating-element, a fan and a display. In regards to using old cells I would measure for internal resistance (IR), before measuring for capacity. IR is well correlated to battery health and is probably the first thing you should base your picks on for batteries in a pack, second to that would be capacity IMO.

Its called collumbic efficiency... and is probally just a product of your testing machine if it is cheap. CE is the ratio of the total charge extracted from the battery to the total charge put into the battery over a full cycle.

Read this.

https://batteryuniversity.com/article/bu-808c-coulombic-and-energy-efficiency-with-the-battery

Thanks - that's very useful! Might also be a good layman-way to gauge IR?
If collumbic efficiency is low, a lot of energy would have been lost through heat output. More heat means higher IR in the cell.


- Sajeel