Quote:
Originally Posted by bwilson4web
Well done! Love your charts which I can't find from the vendor.
Just a couple of questions. I've re-ordered them to look at charge and discharge:
Looking at these excellent discharge curves, I get the impression regardless of the starting voltage, the bulk of the energy starts at ~3.278-3.302V. Charging over this voltage does not appear to give any corresponding increase in total energy. So I'm wondering what is the smallest peak charge voltage that would give a starting discharge of 3.278-3.302V? Would a peak charge of 3.5V still put the cells in a state that they still start their discharge at 3.278-3.302V?
|
I'm quite happy to help. The biggest hurdle for me in using lithium was understanding the differences between it and lead. I'm also finding it interesting to compare my findings with the A123 cells vs the older and prysmatic type mottcell cells.
From the charts, it would seem like 3.5V would give you a fair amount of capacity. I will test it and post the results. If you are really interested in the raw data I can post a link to the software (which is free) I am using for my charger and upload my datalogs.
Quote:
|
At the end, it looks like 3.017-3.055V is also where the available energy pretty well falls off. You went ahead and stopped it at 2.813V but I'm thinking of cell powered, battery management. I might stop it at 3.0V only to make sure the BMS logic continues to run without external power ... for a long time. My rough estimate from your charts is about 2Ahr, 10% of the rated capacity so I might go a little lower depending upon BMS load.
|
That might be doable IF you have a constant and low load. These tests only show up to a 2C dis/charge rate. They are designed to take 10X that! The voltage sag obviously increases as you draw more current. Unfortunately, 40A is the limit of my charger. The other big factor is temperature. I've noticed with the mottcell batteries in my PHEV kit, that in winter the voltages sag quite a bit more. This is more than enough to totally screw up that low voltage setpoint. So, as the temperature changes you'd have to account for that.
Quote:
What first attracted me was the initial 3.431V on both charge cycles. Given the discharge went to 2.813V, we're looking at ~0.62V 'recovery' after the discharge ended. After the cells are discharged to 2.813V, have you measured the unloaded, recovery voltage, what they float to once the discharge ends?
The reason I ask is the open circuit voltage after discharge, what ever it float up to AFTER discharge to 2.813V, helps us understand the initial V*I, power needed to start the charge cycle. It also gives an idea of the dV needed to force charge into the cell.
In a similar fashion, what happens to the cell open voltage once the charge completes? Does it droop?
|
I haven't specifcally noted their voltage recovery after discharge, but it would be similar to what you see as the voltage at the beginning of the charge cycle. Looking at the charts it seems to be around 3.24V. The reverse is true for the voltage drop after charging. It looks to be around 3.43. When I do my the above testing, I will confirm these numbers.
Thanks for the great questions Bob!