Description of the problem to solve:
When a high enough current switching (transition to/from an on/off state) occurs in an electrical circuit, an electrical arc will form between the two switching contact surfaces, in order to oppose (resist to) this circuit change in current.
The more inductance and/or capacitance in the circuit, the bigger the electric spark will be. Capacitors can be viewed as current storage devices (with a full or empty status, like water reservoirs) : when connecting a battery on an emtpy capacitor (e.g. an ESC), the maximum amperage the battery can deliver will flow into the circuit in a very short time to fill the capacitors (until the capacitors are full), producing a spark in the meantime.
This phenomenon is thus more severe when there is more total capacitance in the circuit. This is an issue with multicopters that have lots of capacitors in their power circuit : minimum 4 ESCs (for quads) and up to 8 ESCs (for octocopters), each ESC having multiple big capacitors.
The phenomenon is also more severe when the battery has a higher voltage (4S-> 5S-> 6S), simply because a higher voltage makes it easier to ionise a thin layer of air between the two switching contacts, as required to produce a spark.
Why should we avoid sparks ? to avoid damage to UAV's battery connectors (gold plated contact areas are damaged by repeated sparks, the plastic of the connectors will be burned, etc). Some sensitive electrical circuits do not like short bursts of high currents. For example, from experience, an attopilot (voltage and current sensor) circuit has been damaged by repeated sparks using a 6S battery....
Solution to the problem:
The solution is theoritically simple : capacitors have to be charged slowly (e.g. with a small current) before connecting the full amperage power of a battery to the ESCs circuits.
Many previous blog posts on diydrones & rcgroups have mentioned various solutions. The one described below synthesizes how to use an interesting circuit : a BTS555 integrated circuit,
As described in its datasheet, its applications are:
The two last bullet points, in particular, are the ones that interest us the most.
This BTS555 IC summary is given in the following datasheet extract:
We see from this datasheet table that a BTS555 will support a current load of 165 amps ! This is more than enough for a large multicopter. It operates up to 34V which is fine for up to 8S batteries.
From the datasheet we get the following pinout description:
In our application, only Pins 1, 2 and 5 will be used.
Pin 3 will not be used as it is replaced by the IC tab (for high current uses).
Pin 4 will not be used as we do not need to measure the current in our application (rather, a 3DR power module or an Attopilot would be used for that purpose)
How does this work ?
This BTS555 IC is connected between the battery leads and the power distribution board's leads. A small mechanical switch (for example a sliding switch. This switch interrupts a very small current, so no spark issue there) must be used to activate ON or OFF this circuit.
Here is the step by step process to power your multicopter using this circuit (assuming everything has been wired up as described below):
-Connect the BTS555 circuit to the power input leads of your power distibution board
-Ensure the activation switch is on its OFF position
-Connect the battery leads to the BTS555 circuit
-Activate the BTS555 circuit by flipping its switch to the ON position. Your mutlicopter receives now full power from the battery and the spark is avoided.
-If you use a double battery power feed on your drone, follow this same procedure when connecting a first battery. Then connect simply the second battery (no spark will be produced because all capacitors were loaded when the first battery was connected). You can then, optionally, remove the BTS555 circuit from the drone (to remove weight and to not depend on its reliability during flight. Simpler is better and more reliable)
How to wire everything up ?
As reliability is critical in flying multicopters, therefore to not depend on only one BTS555 circuit, the wiring scheme below uses two parallel BTS555 ICs...
The input and outputs of the overall circuit are XT90 connectors (safer than XT60 connectors for bigger multicopters). The elements (ICs, connectors) are soldered on a piece of PCB protoboard. XT90 connectors are soldered directly on each side of this PCB (more compact, no extra wires).
The wiring scheme is the following (for clarity, one side of the protoboard is used as a shared ground plane; the other side is the positive side divided in two zones: the top side where the IC tabs are soldered together and to the battery +Vin. Then the bottom side where pins 1,2 and 5 are soledred together and to the positive PDB output connector, but not to the TAB zone !) :
This circuit has been tested succesfully on octocopters (8 ESCs) , 6S batteries with APM, Pixhawk, Attopilot module.
Cheers,
Hugues
Comments
I would say no, why switch out all your connectors for these high price connectors, specially when they are the same (10 cent) resistor solution (all though yours use very low power SMT resistors)? And for $2 you have a lipo alarm monitoring every cell for 1/5th cost and more protection, win, win, just imo.
Isn't it better to use good quality connectors like Jeti Anti-Spark http://www.espritmodel.com/jeti-afc-anti-spark-connectors-5-5mm-150... ?
It is cheap, light and safe solution.
As said in my post (but burried in details that are probably not read) such an IC (bts555) is removed once the second battery has been connected. So no risk taken for an IC component failure (and less weight to fly with). This is obviously only possible if using two at least two batteries.
@Jared, yes I can confirm you that I literally exploded a XT60 metal pin because of a large spark using 6S batteries on one of my previous drone. Not only did it destroy the connector, but it also destroyed the attopilot that was in series near the connector.
@Jared
I have not had any totally burnt EC5 however when I began the hexacopter construction I connected at least five or six times the batteries before including the JST and resistor stuff
With only these five to six connections you could see in the border of the EC5 connector that it was black and burned
At that point I realized how fast it degrades when using high voltage
Has anyone actually had to replace a decent connector due to too much sparking before it was time to replace the battery/esc anyway?
We build small electric vehicles at 48volt and 350amp capacity and all our controllers use the pre-charge system with a load resistor and its a proven and reliable method of operation to avoid sparks. the resistor and balance connector is the way to go for reliability.
Regarding the Anderson power pole connectors the big advantage is the the initial contact is made on the front of the contact blade and as they slide together the current carrying contact transfers to the rear of the contact so even if the front does get burnt up over the years then the contact remains good. I have never had one fail and without all the fancy electronics. Keep it simple
@Nick, thx for the extra complementary info on the BTS555!
@Erik, note that bts555 uses a switch which is also solution you can use to not fiddle with connectors and quickly be able to switch off (or on) your drone.
Thanks Nick !
That's making it way more interesting, as I had a look at these before....
So at an average of 25 Amps for me it wil be around 0.075V which is neclectable...
Switching the main power off quickly (switch on top of the drone) may have benifits over fiddling with the XT60 connector which is always out of reach (at the bottom) when you need it...
No, it is V-Power MOSFET. The Vdson depends on current . It's Rdson is between 2.5 and 4mOhms and depends on junction temperature. If you take roughly 0.003 Ohms Rds, you can calculate the voltage drop at different currents.
This device has many advantages - high current switch, overload protection, current limitation, but the main one is it's current source output which is proportional to load current of a ratio of about 28000-30000. Thus you can simply measure the load current.
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