Mismatching Batteries.

I have a 6cell 500mah Nicad pack. I'd like to make it a 7 cell pack, however I can't seem to find any 500mah cells in that size. All I can find are

600mah cells. What would happen if I added the 600mah cell to the pack? Would it damage the pack, or just only be under utilized?
Reply to
Normen Strobel
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Curious . . . whaddaya goin' do with an 8.4 volt pack ? Cheers, Fred McClellan the dash plumber at mindspring dot com

Reply to
Fred McClellan

I wouldn't do it. Why put your plane at risk?

Morris Lee

Reply to
Morris Lee

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.

Reply to
Normen Strobel

Currently the plane I'm flying has just enough power to stay in the air, I'm looking for a few more rpm to make it a little more fun.

Reply to
Normen Strobel

Ah.

Yes, you can add a cell.

Flight duration will be shorter.

Increasing the pack voltage will enable a higher current, but without increasing the pack capacity the pack will be drained more quickly.

Better to increase the pack capacity, if you don't mind the cost.

Avoids the risk of frying the motor on over-voltage. Cheers, Fred McClellan the dash plumber at mindspring dot com

Reply to
Fred McClellan

I don't think you understood the question, I have no problem making the 6 cell pack into a 7 cell pack. I currently fly the plane with another 7 cell pack. However I want to know if I can add a 600mah cell to a pack comprised of 500mah cells. Will the mismatched capacities cause a problem when being used or while charging, or will the pack be fine except that the 600mah cell will only hold 500mah?

Reply to
Normen Strobel

I understood the question.

Yes, you can add the higher capacity cell, and you will have a higher voltage pack with grossly mis-matched cells.

As to whether that is a 'good' idea, I will simply point out

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and recommend you fathom the section entitled "Matching". Cheers, Fred McClellan the dash plumber at mindspring dot com

Reply to
Fred McClellan

Then I guess I mis-understood your answer. But the article you provided answered my question. Thank you. It's a good site and I book marked it for future use.

Reply to
Normen Strobel

| I don't think you understood the question, I have no problem making the 6 | cell pack into a 7 cell pack. I currently fly the plane with another 7 cell | pack. However I want to know if I can add a 600mah cell to a pack comprised | of 500mah cells. Will the mismatched capacities cause a problem when being | used or while charging, or will the pack be fine except that the 600mah cell | will only hold 500mah?

Giving it a few minutes thought, I think it'll be fine.

Suppose you've got 6 500 mAh cells, and 1 600 mAh cell. You charge them all up, and top it off so that all cells are fully charged for the first time. You've overcharged the 6 cells by 100 mAh which won't hurt them.

You now go fly. Assuming that the 6 cells are pretty closely matched, they'll all drop at about the same time, and you'll drop to 1.2 volts pretty quickly. Your motor will turn off before then, so you won't reverse charge anything.

Then you charge. 6 cells are approximately empty, one cell has 100 mAh. All will peak at approximately the same time, and the charger will cut off. And even if the one cell doesn't peak at the same time, the other cells peaking will turn the charger off too.

At worst, you're going to reverse charge the 6 cells a tiny bit, but since it's just a cheap battery pack, I wouldn't lose much sleep over that. And that's the worst case scenario -- and it's unlikely.

Since it's an electric plane, the motor will shut off long before you don't even have power for the receiver and servos, so there's really no danger of a bad battery pack crashing the plane. (Adding a 5th cell to a 4 cell receiver pack in a glow plane, I'd say that's a lot more risky.)

If you agree with my reasoning, try it :) If you don't, go buy a new 7 cell pack :)

Reply to
Doug McLaren

Problem is asymmetric discharge. The 500 cells will be flat earler. Then on recharge the delta peak circuits will get confused as some cells will peak before others. Some won't be fully charged, others may get damaged by overcharge.

Get a new pack.

Reply to
The Natural Philosopher

I'm not the world's greatest battery expert, but I'd suggest that the difference between 500 & 600 mAH is trivial, and I suspect the natural variation in capacity of the different cells may make the effective difference even smaller.

The only real problem I can imagine is if the delta peak charger completely cuts out - ie, doesn't drop to trickle charge, the 600mAH cell may not get fully charged, and as it is starting from less than full capacity, the nature of its discharge curve means it may in fact give less than 500mAH.

You might like to consider a periodic sustained C/10 trickle charge of the pack to ensure all cells are at maximum capacity.

Russ.

Reply to
Russ

The internal resistance may also vary - wuite a lot from e.g./ 50AR to

600AE. This menas the AE will get more volts across it and probably get much hotter, and may peak differently leading to false double delta peaks

That is certainly worth doing, but maybe I have a prejudice. I don't like the thought of mixing different batch batteries in a pack, let alone different age and different capacity and indeed different technology - 600AEs are not built at all like 500ARs.

It may work...but is it honestly worth it? I can get a 7xx600(AE?) pack of non sanyo Nicads for less than $16..or $30 for sanyos. he sanyos are way beter, bit teh cheapos are certainly good enogh for low p[woered planes.

Reply to
The Natural Philosopher

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?

Reply to
Doug McLaren

The usenet is great because you get both sides of the story. There rarely is an obvious yes or no answers in this hobby so it's helpful. I guess I'm just going to have to give it a shot.

Reply to
Normen Strobel

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