In article , The Natural Philosopher wrote: | Normen Strobel wrote: | | > Why wouldn't you do it, is there some scientific reasoning or are you just | > guessing it is bad? It is for a small foam electric plane, the risk is | > minimal to the plane, I just don't want to ruin a good pack. | | Problem is asymmetric discharge.
Actually, all cells will be discharged at the same rate, because they're all in series. Charging will be at the same rate as well, for the same reason.
| The 500 cells will be flat earler.
Of course. Of course, your motor will cut off when your total voltage drops below 6v or so, so nothing will ever be completely flat, unless you have one cell that's much weaker than all the rest.
| Then on recharge the delta peak circuits will get confused as some | cells will peak before others.
No, it won't get confused by this. As you charge your cells, the voltage needed to keep a constant charging current goes up slowly. When the cell fully charges, the voltage drop is a good deal larger than the slow increase, so the large voltage drop caused by six cells peaking will be much larger than the slow increase from the one cell, and the charger will stop.
| Some won't be fully charged, others may get damaged by overcharge.
Now, if the 600 mAh cell peaks first, then it may be damaged a bit by overcharging, as it's voltage drop will be drowned out by by the slow increases of the other six cells. But it shouldn't be overcharged for long, and if this overcharging does damage it (which is not certain, because these cells are designed for a little overcharging), it'll decrease it's capacity a bit and make it closer to that of the other cells.
This setup will probably benefit more than most packs from a little trickle charging after peak charging, but I wouldn't expect any problems.
However, adding one 500 mAh cell to a 600 mAh pack, *that* is asking for big problems. Don't do it -- the 500 mAh cell is likely to get reverse charged every flight and will get weaker and weaker. It wouldn't last long.
| Get a new pack.
Well, you've now got two completely different opinions on this matter. Isn't Usenet grand?