On our quadcopter, we're carrying a switching circuit that would allow the switching between two batteries. When using the 2 Channel Relay Module, the quadcopter is having a hard time flying with it. When directing the battery to the quadcopter, the quadcopter doeesn't have a hard time with flight. Is there such a big power loss from such a small relay? Are there any other options? or is there a solution?

The reason for switching the circuit is because one battery will be used by the quadcopter, the other one will be charging. So after the first battery is drained, the other battery will then be used for the quadcopter, the other one will charge. So on and so forth.

This is the relay used by our thesis.


  • 1
    \$\begingroup\$ Why not just put the batteries in parallel and use them both? What's the purpose of switching? \$\endgroup\$ Jul 19, 2016 at 4:45
  • 1
    \$\begingroup\$ Can you add details about the swap battery procedure? Are you just trying to retain settings when swapping batteries? I'm guessing you land, stop the motors, swap batteries but want to not have to reboot the quadcopter during the swap? I have an idea but need to know your swap process. \$\endgroup\$ Jul 19, 2016 at 4:56
  • \$\begingroup\$ We have a thesis that involves solar recharging. So the idea is that the first battery would be used by the quad, the second one will be charging, if the first battery is drained, the second battery is used by the quad and the first battery is charged. So on and so forth \$\endgroup\$ Jul 19, 2016 at 5:00
  • \$\begingroup\$ Please use a relay with a less VA , if possible may I know the relay VA and your circuit current consumption? \$\endgroup\$
    – user117154
    Jul 19, 2016 at 6:14
  • \$\begingroup\$ Sorry I was not able to put that in @Nick \$\endgroup\$ Jul 19, 2016 at 6:43

1 Answer 1


While there are relays that will do what you want, these are not the ones.
Better would be either a suitable low Rdson MOSFET per battery, or a properly rated relay.

Relays of this type can generally be expected to be junk for practical purposes at LV DC and dangerous junk at mains voltages. See the pinout footprint in the datasheet below - isolation is much reduced by placing the common contact between the coil contacts. OKish on LV, terrible for mains.

You need to say what Voltage and MAX current you are switching.
The relay is dated at 28V DC at:
10A in SPST mode with resistive load.
7A in SPDT mode with resistive load.
5A in SPST mode with inductive load.
3A in SPDT mode with inductive load.

What current MAXIMUM does your copter draw?
What is your operating voltage? What voltage drop do you measure across the relay contacts during operation?
[If you have not measured it, why are you asking questions about it?]

At 10A the contacts MAY drop 1V. At 50A = 5V !!! :-( (and the relay would die)

While the 100 milliOhm contact resistance specification (see below) is a .

I found this data sheet - the same in details as Edsign links to but not as nicely presented.

The contact resistance is shown as "not more than 100 milliOhm" - Agh !!!!!!!!!!!!!!
At 10A that would drop 1V at maximum allowed resistance.
At 50 A = 5V drop.


OP advises.
Ioperating ~= 50A
Battery = 3S LiIon.
Vbattery = 3.6V x 3 = 10.8V nominal. ( 9V min, 12.6V max)

The chosen relay is totally unsuited to the task. You either need

  • A 50A relay. This is not hard to achieve at your voltage but somewhat power hungry. A car horn relay may work depending on operate period allowed. OR

  • A low Rdson MOSFET per battery.

A MOSFET at 50A will dissipate
I^2 x R = 2.5 Watt per milliOhm of Rdson.
So even a 5 mOhm FET dissipates 12.5+ Watts - not undoable but almost makes a properly rated relay attractive. (But, see below, a 0.4 milliOhm Rdson MOSFET - 1 Watt at 50 A.

You can buy 30V, 300A, 0.4 milliOhm Rdson MOSFEts for $US3.43 each i nquantity 10 (!!!). Infineon IPT004N03L. Utterly awesome. Interesting package . 8 power SFN - looks unusual but has drain tab on 1 side, 8 pins on other. Pin 1 = gate, 2-8 = source.

Good starting search here

  • \$\begingroup\$ Wow, that's a pretty awesome mosfet, it can even function usefully at logic voltage levels :) \$\endgroup\$
    – marcelm
    Jul 19, 2016 at 15:15
  • \$\begingroup\$ Hi I'm sorry for my late reply @RussellMcMahon . Thank you for this very detailed explanation. I've already gone on experimenting MOSFETS too. All my setups failed to actually pass on the right current to the quadcopter. I fail to see why. This is the current MOSFET we're using. datasheetz.com/data/Discrete%20Semiconductor%20Products/… The MOSFET switches right like what I want it to. The problem with it, the quadcopter does not even fly when connected to this MOSFET. \$\endgroup\$ Jul 25, 2016 at 13:19
  • \$\begingroup\$ @HenryLachica it would be a really good idea to see the circuit you are using - even if it SEEMS trivial. And what voltage are you using on the FET gate? That FET should achieve about 0.5V drop at 50A IF turned hard on. As it's an N channel FET it MUST go in the NEGATIVE lead to the motor / controller if Vgate is not above V+. . Source = battery negative. DRAIN to motor controller. Gate is switched to +V to turn on. There are ways to allow it to be in the +ve motor lead but then the gate needs to be switched to ABOVE V+ motor by 5 to 10 V. \$\endgroup\$
    – Russell McMahon
    Jul 25, 2016 at 13:31
  • \$\begingroup\$ Better versions of datasheet here and here \$\endgroup\$
    – Russell McMahon
    Jul 25, 2016 at 13:32
  • \$\begingroup\$ @RussellMcMahon The gate of the FET comes from the arduino as it controls the switching of the gate. The Vgs has 5.2V. The Drain of the FET comes from the negative of the quadcopter, the source of the FET goes to common ground of the battery, arduino, and the quadcopter. \$\endgroup\$ Jul 25, 2016 at 13:35

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