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This thread was helpful in my decision making. I wish I read it before I changed my batteries.
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I recently upgraded my board from 15s to 20s. I did not realize at the time that XT90S would not work to anti-spark 20s because the surge current eventually causes the resistor to fail. I painted myself into a corner because this is my only means of anti-spark unless I rebuild parts of my board. So I decided to rebuild the XT90S instead.
The resistor I selected is wirewound which should handle the surge voltage better than a film resistor. The linked resistor has a lot of useful information in datasheet including surge testing up to 500V.
The resistance is 22 Ohm so I expect the maximum in rush current to be 84V/22= 3.8A which is significantly lower than the 5.6 ohm resistor in the XT90S. I also compared this to the capacitance on my Little Focer V3.1 to make sure steady state voltage would be reached in a reasonable amount of time.
I cut the smaller tubes from a sacrificial XT90F. I looked for the right tube size to match the XT90 terminals but it has about 4.7mm ID which is hard to find. Easier just to use another XT90. I used a tube cutter to cut and a wet stone to sand to the final dimensions.
Soldering the resistor was a bit of a challenge. I sacrificed a XT90M to make a little jig to hold the small tubes and the 3D print. This way the small tubes stay in the correct location if the 3D print overheats. You want to make sure that the tubes stay concentric with the XT90 connector.
Terminals on the XT90F are connected via 1mm thick x 6mm wide solid copper bar. I also sanded down the terminals a bit because they don’t need to be so tall.
The two pieces were glued together with JB weld. I cleaned the inside of the connectors with IPA in case some glue got on them. I used fiber tape to hold the assembly together tightly while it dried. JB weld used again to seal the terminals. A bit of sanding to smooth the transition between the two pieces.
I put a pull string for disconnecting through the body of the connector.
Not too much testing so far. It work for anti-spark and moderate riding. Still needs testing for heavy riding.
In the future I may modify the design to use folded metal tabs for the resistor contacts instead of tubes. I think this could be easier to build.