|Aug 17, 2010, 11:01 AM|
Round Rock, TX
Joined Apr 2010
PL8: Cell Count for NiCd/NiMH
When charging NiCDs or NiMHs, the PL8 frequently starts the charge with the correct cell count, but then changes to a higher count a few minutes into the charge (i.e. shown by a drop in the displayed volt/cell; the PL8 doesn't directly show cell count for this; one has to infer it from the pack & cell voltage numbers).
This may not be much of a problem for a simple charge based on overall pack voltage, but when trying to do a cycle, the incorrect cell count is carried forward into the discharge cycle. So for an 8 cell NiCd pack, the PL8 often assumes 9 cells with the initial charge. If the discharge is set to 0.9 v/cell, the actual cutoff is occurring at 0.9(9/8) = 1.013 v/cell. This, of course, reduces the discharge mAh values.
--> So, for 1 cycle on an 8 cell NiCd pack that already has some charge on it: First the PL8 does a top off charge that correctly assumes 8 cells initially, then changes to 9 cells. Next, the discharge cycle occurs with the 9 cell assumption as described above. Finally, the PL8 starts the final charge cycle on the battery assuming 7 cells, then correctly bumps up to 8 cells after a few minutes. Since the battery wasn't really discharged down to the desired 0.9 v/cell, the final charge amount is also depressed.
Has anyone figured out how to get this to work for doing a single cycle?
Other Details: working with preset 20; testing with an 8 cell 700mA NiCd pack primarily with 1.00A charge & 1.00A discharge; tried lower charge & discharge rates; tried setting the Increment 5mA to 'Yes'; tried lower Conv. Frequency; tried to directly define 8 cells (instead of Auto Detect), but got a wrong cell count safety error.
|Aug 17, 2010, 02:57 PM|
The reason this occurs is because the preset is set up so that per-cell volts on NiCd will work best on a new battery pack. When NiCd start to age, they require a higher per-cell voltage in order to peak correctly, and they will also heat up more during fast charging as they age. This is normal and unavoidable. At some point, however, when the pack is too old, this anomoly will tend to indicate that the pack will not return much in the way of capacity during discharge. Then it's time to replace the pack.
Here are some things you can try to see if you can fix the problem you are seeing:
1) Reduce your charge current. The per-cell voltage should not rise so high with a slower charge rate. Try 1/2C or lower. Perhaps these NiCd are not intended for fast charge?
2) Try copying the factory preset you are using over to a new preset and then change the detection to be 8s fixed. This parameter can be found on the Detection Tab. The parameter is called cells and is set to Auto Detect by default.
3) You could always fool the PL8 during discharge by reducing the cutoff voltage. If you're looking for 0.9v cutoff, revise this to 0.8v, then when the PL8 counts the pack as a 9s, it will revise the per-cell cutoff back up closer to the 0.9v you are looking for. This may not be ideal since you indicate above that it only counts the pack incorrectly on the first discharge in the cycle.
Let me know how you make out.
|Aug 17, 2010, 03:41 PM|
Round Rock, TX
Joined Apr 2010
Thanks for the reply & suggestions.
The pack I'm working with is 1 year old, but it hasn't been used much except for an occasional maintenance cycle. I think I'll discharge it and then give it a long low C charge - maybe it will respond better then.
I did try your suggestion 2), but the PL8 would return an error within the first few minutes of charging (I believe #34) when it counted a different number of cells.
In any case, you've given me a better understanding of what's happening, and that is helpful.
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