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Smoke stopper blows with 6s battery

Arty

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Hello everybody!
I am new to FPV and am building my first drone.

I ordered and assembled this setup:

- T-motor F60 1750Kv
- Succex-D MINI F7 Twing + 60A Stack
- DJI air unit
- crossfire nano RX with crossfire immortal T antenna V2
- Vifly finder 2 Buzzer
- smoke stopper with 2A fuse

I also bought 2 batteries: 4s 1500mAh and 6s 1100mAh
When I connect the 4s battery everything seems to be normal ( I could test the motors with beta flight)
The problem is that the fuse (2A) of my smoke stopper blows when I connect the 6s battery.
Normally the FC is supposed to work with both batteries and has a voltage regulator for DJI so there shouldn't be any problems. Am I missing something?

Thank you

battery 6s problem.jpg
 
A smokestopper is really just for that first plug in, you can also check continuity between the battery leads for a short. If you have already tested with 4s and all is good then the 6s should be fine too. What is the fuse rated for (volts and amps)?
 
The original one was from getfpv then I got similar ones from here

It is a 2 amps fuse, they don't mention the voltage rating though, but personally I don't think this is the problem.

I guess I will try to connect without the smoke stopper. Hopefully won't burn anything...
 
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It is an automotive fuse, so it is meant for 12v systems, a fully charged 6s is 25.2v, I think that is the problem. Also, when you run a 6s rated quad on 6s, you are running it to 100% of capability, and that often means things could blow or won't last long, think of it like running your car at full throttle all the time. If it works on 4s, it works, that is still no guarantee that something won't blow on 6s, but you have done your due diligence at least.
 
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A fuse does not blow because the voltage is too high. As long as the circuit doesn't draw more than 2 A, the fuse won't blow, no matter what the voltage is. (There is a voltage limit but it's only to prevent sparks flying over: Why do fuses have voltage rating? - Quora)

If it blows a few seconds after connecting a 6S battery, something is definitely wrong. It means the system is drawing over 25.2 V x 2 A = 50.4 W , which is way too high for an idle quad. If you weren't testing the motors, it means you either have a short somewhere or the fuse was faulty.

Do you have a large low-ESR capacitor connected to your battery leads? If so, this capacitor can draw a lot of current to charge itself, albeit for a very short period of time (much less than a second). A fast fuse could blow, but I doubt those car fuses are that fast.

Your FC has a current sensor so I wonder what Betaflight is showing you when you connect either battery? And do you see the voltage dropping fast when idle?
 
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I wasn't saying that the voltage was too much for the fuse, I was suggesting that when you double the voltage, and resistance stays the same, that the current will increase. So from what you are saying, Arty's quad isn't really working FINE per the post previous to yours?
 
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Oh ok, your post made it sound like the voltage is "too high for a 12 V fuse" while only the current really matters.

At 4S, the resistance of the system could be as low as 16.8 V / 2 A = 8.4 Ohm without blowing the fuse. If we increase the voltage with a 6S battery the current would be 25.2 V / 8.4 Ohm = 3 A, which the fuse can't handle.

I'm really not sure if Arty's quad is fine, it drew more than 50 W with the 6S battery. I just wonder when and for how long. The FC, VTX and camera together should never use that much power at any point.
 
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Perhaps, but my 40+ years in electronics, and my 4+ years in this hobby, has shown me that you can't always just run a few numbers on a dynamic parallel resistance/impedance and expect to always know what is going to happen. I also wonder (in your description) where the ESC and Receiver come into play since they are all powered when the battery is connected. The bottom line is that Arty's quad does indeed seem to be working fine based on his post.
 
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Thank you for your opinions.
I just came back from flying for the first time. Both batteries seem working normally without problems.
I see both of your points though. Since I really have no idea what I can do to fix the issue (if there is) I guess I will just let it as it is. Hopefully, nothing bad will happen ;)
Btw I didn't put any additional capacitors. Should I?
 
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Happy to hear the quad flies without issues!

I guess the system does draw a lot of startup power, and the fuse is a fast one. I wonder if the ESC's startup tune might be the culprit; I don't expect the ESC microcontrollers to require a lot of power (which is why I didn't list it earlier) but I forgot about the startup tune which does activate the FET's and provides short bursts of current trough the motors.

If the quad doesn't continuously burn 50 W when it's unarmed, there's nothing to worry about ;)

Regarding the capacitor I see that this kit includes one, and it's probably not without a reason: Amazon.com: iFlight SucceX-D Mini F7 TwinG Stack Mini F7 TwinG FC with SucceX 40A BLHeli_32 4-in-1 ESC for DJI Air Unit: Toys & Games . It's definitely wise to add it to your quad.
 
Thank's a lot for your fast answers!
Very helpful!
Going back flying for my second time. It is super addicting ;)
 
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