Can choosing a 7.4 lipoly with a large discharge rate be equivalent to running an 11.1 lipoly with a lower discharge rate? From what little I know about motors, Ni (turns*current) determines how much flux is produced. From this I am assuming that opting for the low voltage/high discharge battery can result in similar performance while reducing wear from arching. Please correct me if I'm incorrect.

Sort of. The magnetic field _is_ proportional to the number of turns * current.

However, the battery is a voltage source (which can supply a maximum current). The current running through the motor is based in the voltage of the battery, the resistance in the wire, the induction in the wire and a few other things (V = IR or I = V/R).

The battery will only provide up to the maximum current. The circuit requires a certain amount of current, and the battery supplies it. It will not "push out" more current. So a lower voltage will generate a lower current (and not a higher current, which is what you would want for this to work).

-Dave

I was confused by the first reply post. Voltage gives rpm (that's what Kv means). Current (amperage) is a measure of how hard the motor is working to (try) reach the design KV.
Walt

Agreed, though the first post has some technically correct info. In most cases, unless there is a HUGE difference in capacity and C rating, the higher voltage battery of the same type are capable of higher current. ie 11.1v lipo verses 7.4v lipo. But for those reading this who are not ready for the Ohm's law lesson, the short answer is that a higher voltage battery generally means higher RPM.

That makes much more sense. I guess the people who told me that didn't really know what they were talking about. So when a battery claims 30C constant, it doesn't mean it "pushes" that constantly, it means it is capable of that if the circuit requires it? How do you determine how much a motor draws? I would think you stick an ammeter in series under normal load, but you would already have to have a power source with a discharge rate greater than what is required. Otherwise, it would just max the power source out and possible damage it.
Thanks for your help dknovick

RC ers use watt meters like this one.

[link]http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=6553[/link]

Cool. Ill have to pick one of those up.

Something just occurred to me. If a battery discharge rate is too low for a particular application, will it ruin the battery over time? I play airsoft and the standard pack is 8AAs in series. I just went through my third battery not to long ago. Its odd because they still charge to full capacity but they wont pull the spring anymore. Is that because the batteries couldn't support my setup?

Correct. So, for example, a 1000mAh 30C battery could supply a maximum constant current of 1000*30 = 30000 mA = 30A, but if the load only requres 5A, 5A is all the battery supplies.


True, the battery would need to be able to supply more then what is required, otherwise you would starve your electronics (and possibly cause a brown out).

Drawing out close to the maximum of the battery will cause it to heat up, and reduce it's lifetime.

-Dave

What is the battery chemistry? NiCD have a "memory", and in general, rechargeable batteries will "wear out". They seem to charge up, but can't supply the required current.

-Dave

There are intellect 2000mah NiMh. They have high drain written on them but don't specify discharge rate. I use a MRC Super Brain 977 at .5A and 25mV delta peak but I stop the charge at 2000 mah to be safe.

Nope, no memory w/ NiMH. Hmmm, I don't know what to tell you. Do you have to charge them frequently? Maybe there's a current draw (faulty circuit), that's causing you to cycle your batteries more often then you should need to.

Maybe something mechanically wrong with your airsoft, that increased the friction? If you put new batteries in, does it start to work again?

-Dave

I have been using it as a back up so I don't use it much. It did seem to hold the charge though. The primary still works fine. How do you tell if its drawing current?

You need to match the load to the battery. If the watts out (current times voltage) is the same, the power output is the same. The big problem here is minimizing the IR losses in the wireing between the load and source, the higher the current the bigger percentage of the available power is lost in the transmission of the power.
I see that some are still under the impression that NiCad has a memory in the way we are using them here, not true. Now both NiCad and NiMh do have the problem with voltage depression (note NiMh is not immune from this same problem) which many mistake for memory. This myth about memory has been around for several decades based on the erroneous interpretation of the results of some of the early satellite problems and just won't die no matter hard or complete the chemist's and physicist's explanations are.

Correct, not memory, but crystalline formation:
[link=http://www.batteryuniversity.com/parttwo-33.htm]Memory: myth or fact?[/link]
thanks for the correction!

-Dave

So I went ahead and got a 11.1v 2650mAh 20c lipo only to find out that the dimensions advertised were incorrect. The only battery I can find that will fit is a 1500 mah and in my experience, I dont believe this will give me the run time I need. Can I buy two of these packs and create a connector to wire them in parallel? This will allow me to balance charge them separately but I'm not sure how safe it will be to use them considering they might not have the same internal resistance. Im also curious how it will affect the C rating. Would it make it a 40C "pack?"

No, it will still be a 20c pack, and you would end up with 3000mah. Wiring them parallel is fine, but as you said, charge and balance them separately. EDF jet guys do this all the time due the space constraints on these small foamy jets. Just get a bunch of deans connectors and make a "Y" harness similar to an aileron servo harness that you can plug the batteries into. Just be sure you use female connectors on the batteries and on the single lead of the "y" harness, male connector goes on the parallel ends of the "Y" and on your ESC or rx