Battery spring overheating

Ok so i have a little issue that i could do with a little help.

I have 8 servo's, an arduino mega and a 4 AA Battery Pack.

the servos power and ground arre connected to the battery pack, signal wires going to the Mega. The mega is powered via USB.

Heres the issue. If the mega is not being powered, when i turn the power on to the servos, they give a satisfying twitch on turn on ( as they always do when things are good ). I can leave it like that with no issue.

Once the arduino is powered, the two outer springs of the battery pack, the + and - , begin to get hot. So hot they turn red and even change shape a little. 3A wires are connected to the battery pack so i know thats ok.

What could be happening?

Hi, Can you please post a comlete copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png? Also a picture of your project so we can see your layout. Links to spec of your servo units thanks. It sounds like you have a short somewhere, and it is not okay, what are your AA batteries?

Tom.... :)

ill get on that now, but before i do. when you power servos using external power, do you have to add a link from the servo->battery ground to the arduino ground? If so, why is this important?

Hi,

Yes the gnd from arduino to servo power supply is very important, the arduino signal is with respect to the arduino gnd, if you do not connect gnds then the servo will not have a reference for the signal.

Tom..... :)

Sounds like you have a cheapy battery holder with too skinny springs or poor contact, throw it away and get a better one, the springs are ruined anyway if they got red hot, the tension is gone. How much current are you drawing?

the servos currently dont have any load so i cant see it drawing much current. like i said before though, i dont have this problem if the servos are given power but the arduino isnt connected. it was a cheap box, but ive powered 10 servos with it before under load to the same arduino with no issue

did any of the arduino's pins turn red, did you notice any smoke or sparks?

Hint: sounds like you induced a direct path from +V supply to ground when you plugged in the arduino.....

You need a battery holder that is rated for 10A or so, cheap ones are not rated for heavy current and are made of steel riveted together (not soldered or brazed), so have loads of contact resistance to generate heat if you pass lots of amps through.

RC battery packs are the place to go I think, normally used with high currents so better made than the cheap ones.

If you look at this pic you'll see about the rivetted construction - expect upto 0.5 ohms per rivet before you cook them, and anything afterwards:

http://www.tandyonline.co.uk/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/4/x/4xaa-battery-holder.png

Looking apart from the obvious possibility of incorrect connections and a crappy battery-holder - you say it only happens when you turn on the Arduino. Generally the max/minimum position swing that the default servo sketch uses, is larger than most servo's can go. This leads to a servo that is rammed up against it's internal end-stop and drawing copious amount of current until something breaks.

Try changing the code a bit and see what happens.

// Per.

If he made a battery spring glow, that was not servos against thier stops - that was a short, and reconnecting it the same way is the last thing he should do! Using a bigger battery is also not the solution, the solution is to find the wiring error and fix it.

POST HOW IT WAS WIRED

I would NOT do this for testing, but after something like that happens and you have removed the power - quickly feel up the now unpowered circuit and see if anything feels warm.

Glowing battery connectors imply high current.

I would suggest that if your circuit is correctly designed something like a short is causing excessive current draw.