Running Batts in Series

[Mich. Maniac]

Well, heres a newbie question for everyone. Is it possible to run 2 different size batterys whether it be NiMh cells or LiPo's? For example a 3S lipo in series with a 2S. I dont think it is possible since they will be at 2 diff. voltages and it would screw with the esc. I just wondered if anyone has tried. Im too chicken to try.

[Flying Scotsman]

Yes, you can run 1 3S1P and 1 2S1P in series The batteries must be from the same manufacturer and the same C and MAH rating. The same applies to NIMH. The ESC sees total voltage.

Doug

[ED66677]

YES!
that's what it's needed to be done in Graupner's Mega DragStar for example (6+8 or 4+8)

[SJFE]

That is very interesting. So 18.5v or 26.9 systems are possible...hmmm.

[ED66677]

Sure!!! that's the reason why there are more and more High Voltage ESC's,

The reason is simple, we all need more and more power to run bigger/faster boats, planes, heli... because it is more difficult to increase available amp (cost of 25C capable Lipo's for example) than voltage, having a configuration running 16cell is definitely better than 12!
At a given output power, a motor under 16cell drain less amp than under 12cell, so run time is longer P=UI where P=Watt, U=Volt and I=Amp (for those who dont have electric basic)
Another good thing is, once again at a given output power, less amp and more volt require smaller wires, smaller connector...

[SJFE]

Ok so the packs should be from the same manufacturer & the same c rating. What about age? If the stats are close does it matter?

[SJFE]

So what your saying ED is running more volts lowers the amp draw if you run the same configuration?

[Mich. Maniac]

Im still a little weary of doing this. I dont think it works. I guess if your saying you have actually done it thats one thing but I just dont see how you can have pos 11.1v and neg 7.4. just would not work in my mind, you would end up feeding more than drawing through esc.

[Mich. Maniac]

or I should say once you started drawing power you would actually be feeding juice into the lower voltage battery from higher voltage battery. doing this escpecieally with lipo's is not recomendded at all

[Doug Smock]

or I should say once you started drawing power you would actually be feeding juice into the lower voltage battery from higher voltage battery. doing this escpecieally with lipo's is not recomendded at all

With the cell count you mentioned in your OP in SERIES, it would be the same as having a 5s pack.
There wouldn't be any "juice feeding" from one pack to the other unless you wired them in the parallel configuration, which in this case would be a no no.

Doug

[ReddyWatts]

I think the reason the HV setups use less amps is because it is using a higher number turn motor for the setup. It has more resistance and uses less amps. If you add more cells or higher voltage to a motor, its amp usage will increase.

IMO if you use the exact same brand, mah and C rating batteries, it would be ok to use different volt packs in series. The cells just need to have the same ohms (impedance). If they are not, the higher ohm batteries will get too hot during discharge.

[ED66677]

to SJFE if you increase number of cell you cannot keep the same configuration, that is not what I said!!! I said at same output power

if you increase cell count for a given motor, it will spin faster, but spinning the same prop faster means more amp, so use a gear, as ReddyWatts says in his "Reliable BL System thread" gears lower reliability, although is it more than common use in real car and are certainly the most reliable part of it, so it depends on what we call geas!!!

Hey ReedyWatts that's why I posted on your thread

Gears has the avantage to be another possible adjustement as the prop is
Using gear gives the possibility to get more output power without increasing amp, the power P is also = to rpm*torque

I'm currently running a cheap 36" mono with an 8XL under 12cell, this is my first boat ever, I will soon go for a gear because of high amp/too small prop, I will then increase cell count to 16, gear the thing to have same prop RPM and use bigger prop, in the worst case it'll drain as many amp as before, but will spin a bigger prop at the same rpm so the boat will go faster!

An important point SJFE, when you build your packs use cells that have the same age, same number of charge/discharge! if they are very close in number of use then it's ok not recommended but, I've done a 12 cell pack from a 10 cell pack plus 2 new cell, but the 10 cell pack had only 2 run, end I cycled the new cell to approach the state of the older ones!

to MichManiac, sorry but I don't understand what you're saying but Dsmoke is totaly right, one 2S in serie with one 3S is just the same as one 5S as 4 Nimh cell in serie with 12 equals a single 16 cell pack
ReddyWatts is also totaly right adding cell will increase the amp usage (under load)

If a sytem is designed to work under 100V, it will use less amp than the equivalent system designed to work under 24V, look at your hand drilling machine, in Europe they use 240V in the US they use 110V isn't it, so at the same power, US tools use more than twice the other!