DBL brushless W/ two batteries

[SQRT(-2)]

Crazy Idea.

Has anyone tried doing a double brush less using two batteries, basically plugging the tail ESC into it's own battery, eather a second stock battery or a smaller one. I would think it would allow more current for each motor, not having to share it's draw with the other battery, and somehow offset the weight. Maby the tail battery would last a couple main batteries.

[mikejbb]

Howdy SQRT, You made me think about it so I tried it. I figured that I could test if the main or tail motor was working correctly. See my bird wouldn't fly more than 2 minutes with a 500mah15c. I can't find it now, but someone here (I think) said it wouldn't work straight up and they were right. They said you'd have to connect the grounds of the 2 batteries and again they were right. It works great, albeit a little heavy. I'm using a 500mah15c for the main and a 400mah10c for the tail. I got 8.5 minutes before the main started to pulse a little. It put back 586mah into the 500mah battery. The tail put back 140mah for that flight. So I kept the tail battery and changed the main and flew another 8.5 minutes two more times. Then I charged everything and very similar results re putting back more that 500mah. AUW 90g, a little heavy like I said earlier. So it appears everything is working well and I can finally get along with my practicing in my basement instead of always tuning. I finally soldered a wire between the 2 battery ground wires so I didn't have to worry about that connection.

And another note is that the red turbo ace doesn't get as hot when wired like this. I'm only guessing, but maybe it's because the amp draw it wants is now available where it had to share amp's with the 2.9 tail motor. Boy does that 2.9 tail really sing, good hold.

[SQRT(-2)]

Cool. I am glad I inspired you and that it works for you. Clearly this needs further experimentation.

When my brushless tail come in next week I will have to try it myself.

I'll have to compare

one battery powering both
one battery for each esc
and the two batteries in parallel.

I'll make some wiring harnesses so I can swap out.

I'll be using factory bl main and tail. Keeping it simple and cheap. Don't know if they will benefit the same as the Turbo ace, but from what I have heard the factory bl main is no wimp, and with a 16t gear does pretty good (will have to try that eventually). Maybe my plastic head will save me some weight to help compensate for the battery, who knows.

Where did you secure the second battery? I am thinking of putting one under the nose like a lot of people have done for balance and one in the default location.

How does it fly with the extra weight?

[AtTheCross]

how 'bout a 70mah for the tail esc and the 400 for everything else?

[SQRT(-2)]

from Mike's test I would think 150 mah would be minimum for the tail. Just more than a blade MCX battery, don't know if that will have the amperage, but I'll add that to the experiment list

400 mah main and 150 mah tail

there

you know if that one works best it will make my battery charging a pain in the behind, I just made a charger to charge three batteries at wonce, will I have to make one for the tail batteries? lol.

[mikejbb]

Originally Posted by SQRT(-2)

Where did you secure the second battery? I am thinking of putting one under the nose like a lot of people have done for balance and one in the default location.

I have the MIA tough landing gear so I just piggybacked them onto that. The MIA battery holder is much bigger than the stock and uses a small rubberband to hold the battery onto the bottom. It works out great. And the tough landing gear is wider so it holds her upright way better. However it is a total of 2g heavier than stock.

Originally Posted by SQRT(-2)

How does it fly with the extra weight?

I'm still hovering because I was stuck trying to figure out how to get more than a few seconds of flight before the battery... gave out. Now I fly for 8+ minutes and my education should accelerate.

Originally Posted by AtTheCross

how 'bout a 70mah for the tail esc and the 400 for everything else?

Well I think a 200mah would work well. I just remembered I have a 240mah from my mcx I fly with. I think a 70 would only last about 4mins. I want to be sure to have the tail hold even more than the main. The main gives some warning by pulsing, but the tail just stops and you are screwed. But I definitely will try the 400 with the 240 and expect 6-8mins. That will be 8g's lighter. That's a lot. But the weight doesn't seem to bother her too much. I'm at 90g AUW with the 500+400 on board. I do have some 600, 740, and 850's coming, all 20c so they may have enough power by themselves, we'll see. If not then the 250 might become my permanent tail battery.

[mikejbb]

Originally Posted by SQRT(-2)

you know if that one works best it will make my battery charging a pain in the behind, I just made a charger to charge three batteries at wonce, will I have to make one for the tail batteries? lol.

I charge all my batteries in parallel now. Over in the MCX forum a guy disagreed with my saying that creating a balancing harness for 5 batteries was safer than charging in parallel. So I did the math again and he wasn't wrong. My charger checks each cell as it charges and if one of the cells gets disconnected, the charger stops. Many balancing chargers don't have that safety net. So he wasn't right either. As long as you do 3 or more batteries in parallel and at most one disconnects inadvertently, then you are fine charging at a combined 1c rate. So charge 3 500's at 1.5A and if one disconnects then you are charging 2 at .750A. It's not great, but it's only 1.5C and won't kill your cells one time. There's just no warning if one cell disconnects. I've never had it happen, but some have. So parallel is fine for different capacities and different charge levels all at once. In fact you can start one, then add one later, and add another... and then start taking them off to use. However, you won't know it's charge state. That's ok for the main, but not for the tail.

[SQRT(-2)]

One thing I am thinking about. A smaller battery for the tail will give less amps. a 400 mah 10C battery can give 4 amps, a 200 mah 10 C battery can give 2 amps. Now a 20C 200 mah can give 4 amps, but a 20C 400mah can give 8 amps! The question is, how many amps does the tail need? Indeed, experimentation is required.

[mikejbb]

Well I experimented further and the 400mah10c used for the tail gives 3.25 batteries of flight time (at 8min per 500mah15c battery) or 26 total minutes of flight time and puts back 438mah. So that's an average of 134 mah put back per 8min battery flight. I can get more, but stopped at 8min to protect battery and motor.

Extrapolating out the details:
Red turbo ace uses 533mah/8min or 67mah/minute
2.9 tail uses 134mah/8min or 17mah/minute
For a TOTAL of 667mah/8min or 84mah/minute

I will be trying the 240mah20c zippy/hobbycity battery from the mcx for the tail and then the 138mah20c outrage battery from the mcx for the tail. I'll try the 500mah15c and 400mah10c with each of these. I'll bet the 400 and 138 gives a good flight because of the lower weight. We'll see. I have 650mah20c, 750mah20c, and 840mah20c on order at hobbycity. Unfortunately the mini-deans weigh 1g a pair, but that's what I'm going to next. I might try the 240 by itself because it's 20c, might get 3 mins if 20c works by itself. I could do 2 or 3 of them in parallel.

138mah20c outrage 3.84g
240mah20c zippy 5.91g
400mah10c stock 10.34g
500mah15c stock 13.77g