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Connecting packs in Parallel & Series together.

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Connecting packs in Parallel & Series together.

Old 03-21-2005, 06:56 AM
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nedyah
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Default Connecting packs in Parallel & Series together.

Hi all,

I'm helping a guy build a scale electric Cat (about 43" ) with twin 700BB Turbo motors (12v). He has 4 x 6 cell NiMH packs (2700mah), which he plans to use in it. So I was wondering if there is any problems in hooking these up, 2 packs in series first, then connecting the two lots in parallel. So it is 14.4v, 5400mAH?

Cheers, Hayden.
Old 03-21-2005, 08:41 AM
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Default RE: Connecting packs in Parallel & Series together.

Possibly an RC urban myth but I have heard that Nicads and NMH's should only be used in series. That is why the industry strives for higher and higher mAh ratings so that the power will last longer.

It is my understanding that the internal resistance of these cells is soooo low that if they are in parallel and one cell has a lower voltage than the other, then the higher voltage cell will try to charge the low one and you end up with over temp and thermal runaway.

At least that's what I've always heard

Series only
Old 03-21-2005, 10:28 AM
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Default RE: Connecting packs in Parallel & Series together.

I thought the same thing, but then Red said it was ok to discharge them in parrallel, but not to charge them. His explanation made sense.
Old 03-21-2005, 04:55 PM
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Default RE: Connecting packs in Parallel & Series together.

Hi,

Connecting packs in series is no problem, parallel needs a bit more TLC, as the packs need to have similar charges (as close as possible) otherwise you can get nasty things when one pack is discharged before the other, most of the time resulting in fumes or worse...
If your friend want's to spend the extra money, I'd go for an extra ESC and use two packs in series per motor, runtime will be the same.

Regards, Jan.
Old 03-22-2005, 09:13 AM
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Default RE: Connecting packs in Parallel & Series together.

Thanks Guys for your input.

No, I defently don't think he would go for an extra ESC, he reackons he has already spent about $50 more on the ESC than he was expecting. So I would say that he would just want to go for the best setup possable with the stuff that he has already purchased for it.

We were told to try hooking 3-4 packs (18 or 24 cells) in series and then hook the 2 motors in series also. So that way, it won't pull so many amps and should get a longer run time, with more power. But personally I don't like the idea of this setup and don't think the motors wouldn't last 5 minutes at these high voltages. But I may be wrong, please tell me what you think of this setup instead. As we plan on testing it out tomorrow, if it's ready in time.

Cheers, Hayden.
Old 03-22-2005, 12:27 PM
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Default RE: Connecting packs in Parallel & Series together.

It may be a good way to do it, it will keep the batts balanced better and the motors will get only half the voltag. If they are tied together with a gearbox it should be ok. If they are seperat props then well maybe a little squirely? Just my thoughts.
Old 03-25-2005, 03:30 AM
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nedyah
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Default RE: Connecting packs in Parallel & Series together.

It has a twin Graupner Z-drive sytem.

We got it all hooked up, but the right hand side motor seems to be running much faster than the left side. The motor on the right looks to be an older one the the left, they are both new motors. But the one on the right must have been old stock, as the label looks a little faded and it has a black fan in it, where the left side motor has a white fan in it.

Is it possable that the older looking black fan (R/H side) motor has more power than the other or would it be that there is more resistance in the left drive system or just the way it has been wire in series?

Cheers, Hayden.
Old 03-25-2005, 03:34 AM
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Default RE: Connecting packs in Parallel & Series together.

The motors must not be tied together? there is nothing to keep them going the same speed.

I think you could be right about the motor haveing power or resistance. Series wiring will allow that to happen.
Old 03-25-2005, 09:39 AM
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Default RE: Connecting packs in Parallel & Series together.

There are a number of things that can make the speed of simular motors different, internal resistance is only one of them. The first thing to do is to make sure that the 'slow' motor is clean and properly oiled (if possible, sometimes it isn't). Make sure the 'Z' drives are properly assembled, no 'extra' friction from being out of alinement. After doing all that, start saving money for that second ESC.
It's possible to change the resistance of the motor as seen by the battery, but it isn't exactly simple, and will mean higher current usage and shorter run times. You do this on the faster motor, not the slower one. It envloves using a resistor with very high current capabilities, which means it is physically large, and developes heat in quantities you really don't need. (Basically, you can't speed up the slow motor (much), you'll have to slow down the faster motor.)
Since each ESC will only have to handle half the current, a dual system should be cheaper than buying another ESC that can handle the current for both motors. Not much consolation, bub?
- 'Doc
Old 03-28-2005, 07:23 AM
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nedyah
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Default RE: Connecting packs in Parallel & Series together.

So really I would be better off, connecting the motors in parallel. But these motors wouldn't handle 24 cells, would they?

Cheers, Hayden.
Old 03-28-2005, 09:19 AM
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Default RE: Connecting packs in Parallel & Series together.


ORIGINAL: nedyah

So really I would be better off, connecting the motors in parallel. But these motors wouldn't handle 24 cells, would they?

Cheers, Hayden.
Twentyfour cells would provide a very nice "poof" from the 700 motors....

Well cooled in a well set-up boat 16 cells is the max.

Regards, Jan.

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