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under-dimensioned connectors

Frank BFrank B Club Members Posts: 38
Your 1000mAh 20C Turnigy batteries are preinstalled with red JST connectors that are rated at max 3A. Why do you put 3A connectors onto batteries that can handle 20A continuously? They are totally useless, and I replace all of them with properly dimensioned connectors. I would rather have my batteries shipped without connectors at all, because then I don't have to take them off and throw them away (and probably pay for them as well).Frank B2016-08-20 18:30:08


  • WTWUKWTWUK CornwallClub Members Posts: 13,255 ✭✭✭
    Never had a problem with them in over 5 years of hammering the living daylights out of several 250 class helis that run flat out at a little over 17Amps. Never once have had a little red JST even get warm. Dunno what the problem is ...
  • SootySooty Christchurch NZClub Members Posts: 6,110 ✭✭✭
    There are lots of things in the circuit that will create smoke before the JST connectors give out.
  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    "Why do you put 3A connectors onto batteries that can handle 20A continuously?"

    Don't know where you got that crap from. I could be wrong but I'm pretty sure those connectors are rated to 20 amps. That could be off a little maybe but they absolutely, positively and definitely are rated at well over 3 amps.
  • WTWUKWTWUK CornwallClub Members Posts: 13,255 ✭✭✭
    He is quite correct with the 3A rating. The little red blighters are JST type RCY and are rated 3A 250Volts.

    Matters not though if they are not heating up.
  • DaithiDaithi Belfast,IrelandClub Members Posts: 8,708 ✭✭✭
    3 Amps @ 250 Volts would mean (to me anyway) that they can handle 750 Watts. Now assuming the supply voltage is 20 Volts, to sink 750 Watts you could draw 30 Amps without the things cooking (you're only pulling 600 Watts)
  • WTWUKWTWUK CornwallClub Members Posts: 13,255 ✭✭✭
    Same thinking here. Never had one get even close to being warm, even when I have had motors almost hot enough to fry eggs. And bacon on occasion ...
  • SawdustSawdust Club Members Posts: 6,763 ✭✭✭
    Now I am confused image . I was wrong about their rating. I get the point about the watts being drawn, and that watts are the real total power but, welding, I can run 120 amps at about 50 - 60 volts (rough pure guess) brought down through a transformer from our 240 volts mains power, or inverter setup these days pulling it from a very thin wire fuse and yet melting a whopping 3.5mm steel rod at the welding end. Same total power (except for some losses if anything) yet the high amps makes thick steel melt while the same supply power is happily running (sort of) through such a thin piece of wire it looks like a sewing thread image .

    Ok, I know I asked the question but don't bother trying to explain it, at least not to me, it'll go straight over my thick head for sure image .
  • __Evan__Evan Club Members Posts: 1,122

    Unfortunately that's not how it works. The limiting factor for the connector is the heat generated. Power loss (as heat) in the connector is P = I²R, where I is the current and R is the connector resistance.

    The voltage of the system has no effect at all, as long as you stay below the voltage rating of the insulation. Similarly, the total power going through the system (as opposed to the power lost in the connector) has no direct effect, other than its relationship to the current going through the connector.

    I thought the whole point of the XT30 connector was to eliminate the red JSTs for everything except really tiny models. XT30s are much nicer connectors anyway; easier to deal with by hand (no crimping tool required) and pretty much impossible to plug in backwards (the red ones can be plugged in backwards with a little bit of ingenuity).

  • DaithiDaithi Belfast,IrelandClub Members Posts: 8,708 ✭✭✭
    Umm I squared R is Voltage (I x R) times Current (I) - which is what I said - P = V x I.

    If you want to juggle the equations, V = I x R, I = V/R and R= V/RDaithi2016-08-23 01:39:35
  • airwaveairwave Club Members Posts: 3,563
    I would have to go with the others,the # don't tell all.I have run them on 20 amp ESC a lot with no trouble.go to 2MM on 20 ~40 amp.if you look at the NEC you would need 12 AWG wire for 20 amps,there are very few of us that run that size wire for 20 amps.
  • __Evan__Evan Club Members Posts: 1,122
    Daithi - that's correct, but if you're looking at heat generated in the connector then the only relevant voltage is the voltage drop that occurs in the connector (due to contact resistance). The system voltage is irrelevant. Since the voltage drop in the connector is proportional to the (fixed) contact resistance and the current through the connector (but not related to the system voltage) just using P = I²R is generally easier.

    Taking the JST-RCY connectors as an example:

    -> Maximum contact resistance is 20mOhm (from the datasheet).

    -> Maximum current is 3A (from the datasheet).

    -> Maximum power dissipation per contact is therefore 180mW. This is the maximum amount of heat that the plugs can safely dissipate in all expected operating conditions.

    Based on your earlier proposal, 3A at 250V would imply up to 30A at 25V (because this is the same amount of power in total). 3A at 250V means that the voltage drop across the connector (from Ohm's Law, V = IR) is 20mOhm*3A = 60mV, and the power dissipated in the connector (from P = IV) is 60mV * 3A = 180mW. This is the safe maximum, as expected.

    30A at 25V means that the voltage drop across the connector is 20mOhm * 30A = 600mV, and the power dissipated in the connector is 600mV * 30A = 18W. That's not going to be a very happy connector! And you'll be throwing away a huge amount of power that could be used to power the plane/helicopter too.

    This is the same reason that long-distance electricity cables are always very high voltage. High voltage means less current, and less current means that for any given cable/connector resistance you waste far less power. Otherwise power companies would just generate 110V/240V at the power plant and transfer it at that voltage, avoiding all those expensive transformers and sub-stations.
  • DaithiDaithi Belfast,IrelandClub Members Posts: 8,708 ✭✭✭
    Evan, surely the main point is that it doesn't overheat at 20 Amps as proven in actual use, which would tend to suggest that the power rating is actually higher than you are assuming

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