Dual Lipo Setups

[ikem]

I am working on some ideas for a quad and looking at increasing flight times with different configs.

There are really 3 different configs i can go with:

1 large cap battery running all 4 motors. (conventional)
2 small cap batteries in parallel running all 4 motors. (semi conventional)
2 small cap batteries running 2 motors each. (i dont know if this has been done before. I have seen a few large builds with a battery for each motor, but those were extreme builds.)

I have a nice understanding of electronics and fabrication, but just wanted an insight from others. What would be a good choice? and what problems would I have with different configurations.

Thanks!

 

[Mustang7302]

For the same total mAh capacity, one battery is going to be lighter than two or four. Weight is the most important thing when it comes to flight times; just adding more battery is a double edge sword. While you add more capacity, you also need more power and stronger components to cope with the extra power and weight which further compounds the additional battery weight.

From my experience with a Turnigy H.A.L. frame with NTM 800KV motor spinning 10x4.5" props with a 4S 4,000mAh battery, it was four pounds (~1800 grams) of all up weight and would fly around for 15 minutes per charge. It was stable, sluggish, and weighted so much it was a hazard to itself and property in the event of a crash; for the last reason I didn't enjoy flying it anywhere but in a big open field with nothing around.

I then sought to shrink both size and weight so I could fly around the house with considerably less risk. On a budget I kept the same NTM 800KV motors and 10x4.5" props, but went down to a 3S 1,800mAh battery and a frame made of wood and carbon fiber. This new craft is just shy of two pounds (~900 grams) of all up weight and runs for 10 to 11 minutes of flight time. Half the weight, 30% smaller, and only a 35% sacrifice in run time. This smaller lighter craft is much more maneuverable, making it more fun to fly, and is no longer a threat to itself in the event of a crash.

At some point I plan on getting lighter motors, shrinking down to 7" props, and using carbon fiber for the booms to get my all up weight down to a hopeful 600 grams (1.3Lbs). Getting down to this weight should get me back to 15 minutes of flight time on the 3S 1,800mAh battery.

Multirotors work best with the KISS mentality. Extra batteries is extra weight, extra wiring, more places for things to go wrong, and have a longer turn around time to get back into the air during recharging.

 

[cranialrectosis]

If this is your first attempt to build and fly a multirotor, my suggestion is basic KISS.

Once you can fly, mod it all to heck. If you are still learning, the simpler the build, the simpler troubleshooting will be and the simpler it will be to fly.

A 2200Mah Turnigy Nano-tech 25C battery weighs 237 grams (x2 = 474). One 4500Mah Turnigy Nano-Tech weighs 459 grams. On paper that's 15 grams less for more Mah.

IMO, you can get better performance from one larger battery than you can from two smaller batteries. This does not even include dual wiring harnesses and dual voltage alarms that you would need and will add weight and complexity.

My data is not emperical evidence. YMMV.

 

[Craftydan]

looking at your third option, this is done on multi-engine fixed wings all the time, but how does that apply to multi-rotors?

For a multi-rotor, the control board doesn't know what motor/prop/cell count the motors are running -- as long as they're close enough, it doesn't need to. It'll use the gyros (and accelerometers) to dynamically adjust the ESC command to get the roll rate or attitude that the receiver is requesting.

as for effects of splitting packs (replacing 1 big with 2 small), As long as there's enough power, the control board will deal with this too, but it'll be bounded by the strength of the weakest link -- when the weakest battery is out of juice, it'll no longer be able to maintain stable flight.

Recomendations if you do choose this route:
- ALWAYS share the ground. if you don't do this, the control board won't be able to talk to all the ESCs
- put an independent battery alarm on *EACH* independent battery.

Is this better than the other scenerios?

- assuming you don't want to buy the one big pack instead of using two small packs you already have/use, yeah, its better than that.

- two seperate packs don't run the dangers of two paralled packs -- the stronger pack charging the weaker pack in an uncontrolled manner -- but you are limited by the actual capacity of the weaker battery.


BTW, Welcome to the forum!

 

[ikem]

another thing to note, there should be less of a load using the third option though, correct? (since it is only powering 2 motors)

Thanks for the responses! This is my first quad build, and I was going to learn its quirks before going all in, just had some ideas to optimize what I wanted to do with it.

 

[Mustang7302]

Focus more on keeping the weight down and the current draw low. These two factors yield flight times.

 

[Cyberdactyl]

I'll throw in some of the prop physics.

For longest flight times, and this goes directly against agility. . . look at the largest and slowest spinning prop as can be managed efficiently by the electrical power components. The prop pitch and rpm should be just shy of a stall on the slow side. You want to make a bubble of sufficient low pressure air above the rotor disk with as little effort, and with as little disturbance to the environment as possible.