JohnMuchow
Posts: 64
Joined: 10/6/2003
From: New York, NY, USA
Status: offline
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How are you determining the capacity? Is it the same thingamabob that tracks votlage?
For those recent tests I used an Astroflight Super Whattmeter to track the AH coming out of the cells.
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If your discharger is a bulb discharger, how does it maintain a constant discharge? As the voltage of the pack decreases but the resistance of the discharge circuit remains constant, won't the bulbs just dim andsuck less juice?(the old 1 volt at 1 ohm = 1 amp thing). The discharger I am using sucks a constant amperage measured to three decimal places regardless of pack voltage.
All of our dischargers are constant-resistance dischargers. That is (as you suspected), the resistance of the load stays constant so the discharge current drops as the voltage of the pack drops. This better simulates the actual load put on the cells in a R/C vehicle/plane and that resistance is very stable for any possible purpose we would ever need. CC (constant-current) dischargers use a fixed current to test cells and that's a very common way to test cells. Not better, just different. We need to know which way the capacity of our cells has been determined to best understand how those cells will respond when we use them in any particular application or to compare performance of two different cells.
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Most important...when you charge these cells, what do you use to terminate the charge? Zero delta volt? Peak detection? Temperature?
For the above tests I used a negative delta-voltage peak-detecting charger, an Astroflight 112D. The charging method of the cells (and the AH put into them) have very little bearing to their discharge performance as long as you fully charge them without overheating and occasionally slow charge them. Charged is charged, no matter how you terminate the charge.
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1. Your packs are/were professionally soldered...most likely very high temp device with very little or specialized solder...even resitance accross the pack (I checked out the website).
The possibly lower inter-cell conections of my packs *might* make a difference. But in my experience, not enough of a difference to explain the results you're seeing in your packs. Unless one or more cells is damaged or the connection has an especially high resistance, the cells should track pretty closely. We're only dropping each cell from 1.15V or so to 0.9V.
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4. If you are using analog dischargers...meaning that the discharge rate is not precisely controlled by a computer, the results cannot accurately be compared to those of a device that maintains a constant rate.
Agreed, but not because of any sort in inaccuracy in these "analog dischargers" you're mentioning. Some of the most accurate constant-current loads around are completely analog. Analog or digital has no bearing here. What matters is how you're specifying the capacity of the cells; by using a constant-resistance, constant-current, or constant power load. Each gives you valid results for the capacity of the cell. But, for those results to be useful you need to know how other cells you are comparing yours to have been tested. Constant-current discharging is often used by manufacturers but constant-resistance discharging simulates what happens better in R/C. Neither is better, just different.
I never intended my results to imply that others should see those types of capacity numbers. I was only quoting voltage-at-cutoff numbers for comparison against each other to show relationships at different discharge current levels (another reason I used older less-well-known cells). And that doesn't depend on a particular load configuration. Now, if I was to say that GP3300 cells went for XX.XX minutes before dropping to a particular cutoff and averaged XX.XX volts during the discharge, it would be very important to know what type of load was used.
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If I have this electricity stuff right....the bulbs represent a fixed resitive load, a fairly high one at that. As the voltage of the pack drops, so do the amps that pack can deliver to that load. In a computerized discharger (like a RC competition discharger) the volts drop but the current flow (Amps) remains constant (less watts). With the bulbs, the amp flow would decease as well, giving the cells a rest towards the end of the discharge (unless the resitance is low enough to allow a maximum curent flow greater than the designated discharge rate). I'm just speculating on this one...I'm only 20% through my 10,000 page electricity and electronics book. 
Correct. A CC-load (constant-current) presents a "tougher" load to a cell or pack. The current is not allowed to drop as the pack discharges. It's a harsher test for a cell but make sure the results of the tests represent what you're looking for and only compare those results to other CC-discharge tests if you're concerned about capacity.
IMHO, the relatively balanced voltage of the cells I'm seeing near cutoff in the packs used in the tests above is not because the cells are being discharged by a fixed-resistance load. Those cells would track just as well with a CC-load. At any particular point in the discharge it doesn't matter whether the discharge was constant-current or constant-resistance up to that point. Only the state of the active materials in the cell matters. Your cells should be tracking better than they are....no idea why they aren't.
LOTS of tests to do with those Promatch packs you're getting. :-)
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John Muchow
CamLight Systems
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RE: Discharging to .9v/cell - 
