I have the 3DR quad frame with standard motors and wondering if I can use a 1800mAh Turnigy 30-40C 3cell. I know it would decrease the flight time but just wondering if there is any problem otherwise in using a smaller battery. Didn't want to start a fire :)
I normally use 3000 or 5000mAh but am having some dead cell issues with them now but have some new 1800 I would like to try.
Actually, in this case both the 3S and the 4S battery have the same capacity (5000mAh). Increasing the voltage doesn't increase the capacity. It will mean lower amperage flowing from the battery to achieve the same wattage load, but unless your wires are undersized that should make absolutely 0 difference. A motor pulling 100W at 12V still pulls 100W at 16V.
If anything (and Davey mentions this) I would guess that you'd lose additional power in the voltage regulation circuity. A hotter ESC means a less efficient ESC, as energy is being converted to heat instead of RPM.
The only time I would consider replacing a 3S with a 4S would be if you're just not getting enough lift from the engines, and even then it would probably be better trying to shave some weight off somewhere then throwing more voltage at it. If you can't remove any weight it would likely be best to just get larger engines, anyway. :)
I am using 4s3300 and have 20min of flying time. 850kv motors 20 esc 10x4.5 props with 3s 2800 i had 8 mins. for m4s is the way to go.
how heavy is your rig Ruben? 20 min is impressive. I have just bought 2x 4s3300 as well so not sure whether to use the both yet or just one. my rig is about 2kgs and its X8 quad same motors as you are using but with 12x4.5 props
I would do the following.
Charge a 3000mAh Lipo and then fly for 5minutes. Then recharge to full and read the recharged mAh number on the display.
Then a little bit of math:
5min = "your number"mAh
xmin = 1800mAh
xmin (flight time with 1800mAh) = (1800 / "your number") * 5
lets say you recharged 2300mAh:
(1800 / 2300) * 5 = 3,91min
So instead of using a 3000mAh Lipo you can use a 1800mAh Lipo and fly 3,91min.
In reality a bit more because the 1800mAh Lipo wouldn't be as heavy as the 3000mAh Lipo.