How to Experimentally Determine C Rate?

[katou]

As many of you know, I am trying to build a pack from used Moli cells from toolpacks.

I am now testing cells to find the good/bad/ugly.

According to manufacturer's spec, these things are good up to 10c, but I highly doubt that my used cells will ever be capable of that.

In addition, others have stated that from their experience, the moli cells are more like 2-5c at best (new cells)

So, I can measure the capacity of my cells at whatever C rating I like.

How do I figure out what the C rating is for an unknown cell?

Katou

 

[liveforphysics]

Load it up with some current level, and multiply this by the amount of voltage drop you see. This tells you the amount of thermal energy heating the cells. Then you can estimate the cell surface and delta - t and airflow and make a guess of where you will reach equilibrium... but this is kinda the stupid way.

Get a handle on the voltage drop amount under load, and use this to calculate when the v- drop is going to be causing the LVC on the controller to trip. If its going to start tripping under load while the pack still has more than 20-30% left in it, then you know you're at about the limit of the useful c-rate.

 

[katou]

If I understand correctly then:

1. run multiple discharges at 1c, 2c, 3c, etc.
2. stop when voltage hits 2.5v (LVC for A123 and Moli cells) at 60-70% of cell capacity*
3. one more run at just over this C rate to check that capacity has now dropped below 60-70%


* by cell capacity, I mean the capacity of the cell tested at a discharge rate of 1c or less

Do I have it right?

Katou

 

[liveforphysics]

I was thinking even more crude and more practical. Use the controllers LVC point that it trips off as the voltage to ensure your cells stay above for at least 70-80% of the cells capacity. This way you know how many cells you need to parallel (or what current to set the controller for) to end up with a useful C- rate for your application. The way you described would be fine too. Or, since the c limit is really only thermal (for practical levels at least), just build the pack, and if it starts to overheat, then you know you've gotta turn down current or group more in parallel.

Continous C-rate really depends a lot on the packs design and cooling in its final application more than an mfg's spec.

If I understand correctly then:

1. run multiple discharges at 1c, 2c, 3c, etc.
2. stop when voltage hits 2.5v (LVC for A123 and Moli cells) at 60-70% of cell capacity*
3. one more run at just over this C rate to check that capacity has now dropped below 60-70%


* by cell capacity, I mean the capacity of the cell tested at a discharge rate of 1c or less

Do I have it right?

Katou