A warm connector means high resistance at the joint or the contact. Resolder it fully and check the fit.
What the message means
A poor solder joint or a worn plug adds resistance. At high current that turns into heat and can melt the housing.
Fix it in this order
- Check the plug and socket fit firmly, with no wobble.
- Resolder each cup with enough solder, with the wire fully inside.
- Cover each pin with heat shrink to avoid shorts.
- Match polarity: the connector is keyed, but check the markings on both sides.
- Do not use a connector below the current of your robot.
- Replace plugs that are discoloured or loose.
Why it happens
XT60 connectors are commonly listed at 30 A continuous and 60 A peak, with contact resistance often given as about 0.55 milliohm; these figures come from retailer listings, not an official sheet, and the listed resistance ranges from 0.55 to 1.2 milliohm.
Heat in a joint is current squared times resistance. As an illustration, at 30 A each extra milliohm from a weak solder cup, a loose fit or a worn contact dissipates 30 x 30 x 0.001 = 0.9 W. That is enough to warm a small plug, which is why a poor joint heats at high current while a good one stays cool.
If you see this, do this
| You see | Do this |
|---|---|
| Housing soft or discoloured | Replace the plug. |
| Warm only at high current | Resolder and check the rating. |
Related
cold solder joint, wire gauge.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.
Why this happens
Heat in a joint follows power equals current squared times resistance. A connector has resistance in the contact between plug and socket and in the solder joint between each wire and its cup. On a good connection this adds up to about a milliohm, which wastes less than a watt at the current ratings these connectors are sold for. A poor joint or a worn contact can have several times that resistance, and because current is squared, the heat rises quickly with load.
The heating is also a loop. A hotter contact oxidises faster and loses spring force, which raises its resistance and makes it hotter. A solder cup that is only partly filled, or a cold joint that holds the wire only mechanically, starts that loop from the first day. The housing is plastic, and heat softens it until the pins can shift and short.
How to confirm the cause
- With the robot running at its normal current, measure the millivolts across the connector: one probe on the wire just before the plug, one just after the mating socket. Divide the reading by the current to get the resistance of the joint.
- After a minute under load, feel or measure the plug temperature. If the plug is warm to the touch, work out the power from your measurement.
- Inspect the solder cup. It should be full, bright, and wet along the wire, not a dull ball with a gap.
- With the power off, check the fit. A plug that wobbles, or comes apart under a gentle pull, has weak contact.
- Look for brown or blackened plastic on the housing. Those units must be replaced.
Worked example
Estimating the heat from a measured voltage drop (example figures):
Current 15 A, measured drop across the plug and joints: 30 mV
R = 0.030 / 15 = 0.002 ohm = 2 milliohm
P = I^2 x R = 15 x 15 x 0.002 = 0.45 W
Compare a good connection at 0.55 milliohm: 225 x 0.00055 = 0.12 WCommon mistakes
- Heating the cup too long, which melts the housing and loosens the pin.
- Not tinning the wire first, giving a thin, patchy joint.
- Soldering with the wire only half inside the cup.
- Mixing up polarity because the plug ‘only fits one way’. Check the markings on both sides.
- Pulling on the wires to unplug instead of the housing.
If this did not fix it
- Wire gauge: a wire thinner than the load needs heats and softens its insulation near the plug.
- Plug and socket from different makers can fit loosely. Try a matched pair.
- A real fault in the load, such as a partly shorted motor or ESC, can draw more than you think.