I recently added a battery analyzer mode to my capacitive discharge FET based tab welder (http://endless-sphere.com/forums/viewtopic.php?f=2&t=2633&start=690#p300329).
I used it to analyze the capacity of a 2300 mAh A123 cell that had been charged to various voltages. The cell was charged with an HP E3614A cc/cv lab supply set to a 6A current limit. The cell was charged until the current fell to 50 mA. It was then removed from the charger and discharged (at around a 1C rate) to 2.5V. Extracted power was:
3.65V -> 2293 mAh
3.55V -> 2282 mAh
3.45V -> 2268 mAh
3.40V -> 2274 mAh
3.35V -> 2157 mAh
3.325V -> 1526 mAh
3.30V -> 953 mAh
3.25V -> 611 mAh
There appears to be no significant difference in capacity when charged above 3.40V. Somewhere between 3.30V and 3.35V cell capacity drops off a cliff. A123 recommends a float charge voltage of 3.45V
Charging to a voltage lower than the standard recommended 3.65V may improve battery life, particularly if you don't trust the accuracy of the cutoff voltage of your charger. A value of 3.50V looks like a good place to set an iffy charger... midway between the recommended charge voltage and where the capacity begins to fall off. Charging at lower voltages takes longer than charging at higher voltages. Those runs at 3.30V and below took forever for the charge current to taper off.
---------------------------
Here is some capacity data on the effects of different discharge rates:
Testing on 2 different new A123 cells hot off the charger (and charged outside where it was 100F) Cell 2 was a bit hotter off the charger. It was brought in from outside and tested without any chance to cool.
Basically A123 cells have no capacity loss from 1C to 5C discharge rate. There may even be a very slight boost at the higher rates due to increased cell temperature.
Cell 1:
@1C: 2248 mAh, 6862 mWh
@5C: 2237 mAh, 6848 mWh
@7C: 2242 mAH, 6704 mWh
Cell 2:
@1C: 2277 mAh, 7243 mWh
@5C: 2337 mAH, 7249 mWh
------------------
Cell temperature rise data:
At 1C discharge, +1 degree F in open air, +3.5F insulated.
At 5C discharge, +23 degrees F in open air, +33F insulated
At 7C discharge, +40 degrees in open air
-------------------
I did some testing to determine the A123 cell voltage that indicates 90% discharge. Unfortunately it seems to depend upon the cell discharge rate (or perhaps temperature).
At 1C a voltage of 3.075V was a good indicator there is 10% capacity left.
At 5C the voltage was 2.850V
At 7C the voltage was 2.822V
-------------------
Next I popped a cell in a 5F freezer for a couple of hours, removed it, and then immediately discharged it 5C (12 amps) in an insulated, room temperature box. First plot is a normal (non-chilled) cell. The next two are the voltage and wattage curves of the chilled cell... can you spot the cold? The total cell discharge capacity was not affected by the cold, but its ability to hold up the voltage and power output was drastically affected.
Here is a plot of A123 cell voltage recovery after being fully discharged at around 1C. After discharge at 1C to 2.500V there was a 4 milliamp load on the cell.
-----------------------------
I charged some A123 cells with a CC/CV charger (constant current/constant voltage). The CC phase was set at 6 amps. With the 2300 mAh cells, the cells would ideally fully charge in 23 minutes, but the chemistry does not allow that. At 18 minutes the charger started the CV phase where the pack voltage remains constant and the charge current drops. It took another 18 minutes to reach the recommended cutoff current of 50 mA.
So just how much extra capacity does one gain in the final CV charge phase. A fully CV/CC charged cell had 2305 mAh capacity. The same cell that was charged for only the 18 minute CC phase had 2093 mAh. You gain around 9% capacity from the final CV phase (that takes as long as the constant current phase).
-----------------------------
How are A123 cells for self-discharge? I bought some A123 cell developer kits from a guy. They came with the original sales invoices. He bought them in June of 2006 and never used them! They had never been charged in over 4.5 years! The cells gotta be toast, right? Wrong! The 12 cells measured from 3.189 to 3.306 volts. Most were above 3.3V
The factory ships them at 50% charged so they should have left the factory with around 1150 mAh in them. I put them on my analyzer and they still had from 209 to 989 mAh in them. 7 of the 12 cells had over 900 mAh left, the others 209, 517, 683, 789, 810 mAh. It looks like the self discharge rate of these virgin cells (stored at room temp) was around 5 percent PER YEAR!
Can you expect these kinds of self discharge in the real world? Probably not, but you should be able to get close to them. These were virgin cells. A good way to increase a cell's self discharge rate is to abuse them. Also, the self discharge rate of a paralleled string would be governed by the weakest cell in that string. My weakest cell had a 18%/year rate (note: rates calculated using simple math... i.e. capacity drop/years, not proper exponential decays)
I used it to analyze the capacity of a 2300 mAh A123 cell that had been charged to various voltages. The cell was charged with an HP E3614A cc/cv lab supply set to a 6A current limit. The cell was charged until the current fell to 50 mA. It was then removed from the charger and discharged (at around a 1C rate) to 2.5V. Extracted power was:
3.65V -> 2293 mAh
3.55V -> 2282 mAh
3.45V -> 2268 mAh
3.40V -> 2274 mAh
3.35V -> 2157 mAh
3.325V -> 1526 mAh
3.30V -> 953 mAh
3.25V -> 611 mAh
There appears to be no significant difference in capacity when charged above 3.40V. Somewhere between 3.30V and 3.35V cell capacity drops off a cliff. A123 recommends a float charge voltage of 3.45V
Charging to a voltage lower than the standard recommended 3.65V may improve battery life, particularly if you don't trust the accuracy of the cutoff voltage of your charger. A value of 3.50V looks like a good place to set an iffy charger... midway between the recommended charge voltage and where the capacity begins to fall off. Charging at lower voltages takes longer than charging at higher voltages. Those runs at 3.30V and below took forever for the charge current to taper off.
---------------------------
Here is some capacity data on the effects of different discharge rates:
Testing on 2 different new A123 cells hot off the charger (and charged outside where it was 100F) Cell 2 was a bit hotter off the charger. It was brought in from outside and tested without any chance to cool.
Basically A123 cells have no capacity loss from 1C to 5C discharge rate. There may even be a very slight boost at the higher rates due to increased cell temperature.
Cell 1:
@1C: 2248 mAh, 6862 mWh
@5C: 2237 mAh, 6848 mWh
@7C: 2242 mAH, 6704 mWh
Cell 2:
@1C: 2277 mAh, 7243 mWh
@5C: 2337 mAH, 7249 mWh
------------------
Cell temperature rise data:
At 1C discharge, +1 degree F in open air, +3.5F insulated.
At 5C discharge, +23 degrees F in open air, +33F insulated
At 7C discharge, +40 degrees in open air
-------------------
I did some testing to determine the A123 cell voltage that indicates 90% discharge. Unfortunately it seems to depend upon the cell discharge rate (or perhaps temperature).
At 1C a voltage of 3.075V was a good indicator there is 10% capacity left.
At 5C the voltage was 2.850V
At 7C the voltage was 2.822V
-------------------
Next I popped a cell in a 5F freezer for a couple of hours, removed it, and then immediately discharged it 5C (12 amps) in an insulated, room temperature box. First plot is a normal (non-chilled) cell. The next two are the voltage and wattage curves of the chilled cell... can you spot the cold? The total cell discharge capacity was not affected by the cold, but its ability to hold up the voltage and power output was drastically affected.
Here is a plot of A123 cell voltage recovery after being fully discharged at around 1C. After discharge at 1C to 2.500V there was a 4 milliamp load on the cell.
-----------------------------
I charged some A123 cells with a CC/CV charger (constant current/constant voltage). The CC phase was set at 6 amps. With the 2300 mAh cells, the cells would ideally fully charge in 23 minutes, but the chemistry does not allow that. At 18 minutes the charger started the CV phase where the pack voltage remains constant and the charge current drops. It took another 18 minutes to reach the recommended cutoff current of 50 mA.
So just how much extra capacity does one gain in the final CV charge phase. A fully CV/CC charged cell had 2305 mAh capacity. The same cell that was charged for only the 18 minute CC phase had 2093 mAh. You gain around 9% capacity from the final CV phase (that takes as long as the constant current phase).
-----------------------------
How are A123 cells for self-discharge? I bought some A123 cell developer kits from a guy. They came with the original sales invoices. He bought them in June of 2006 and never used them! They had never been charged in over 4.5 years! The cells gotta be toast, right? Wrong! The 12 cells measured from 3.189 to 3.306 volts. Most were above 3.3V
The factory ships them at 50% charged so they should have left the factory with around 1150 mAh in them. I put them on my analyzer and they still had from 209 to 989 mAh in them. 7 of the 12 cells had over 900 mAh left, the others 209, 517, 683, 789, 810 mAh. It looks like the self discharge rate of these virgin cells (stored at room temp) was around 5 percent PER YEAR!
Can you expect these kinds of self discharge in the real world? Probably not, but you should be able to get close to them. These were virgin cells. A good way to increase a cell's self discharge rate is to abuse them. Also, the self discharge rate of a paralleled string would be governed by the weakest cell in that string. My weakest cell had a 18%/year rate (note: rates calculated using simple math... i.e. capacity drop/years, not proper exponential decays)