Questionable Battery Answer to Keith in Bolder, CO

batteries

#1

Ray got the right answer when he said that the terminal melted because of a bad connection. He missed explaining why and the second half of Keith’s question (How can it be prevented from happening again?). Keith did not over-tax the battery. If he had both terminals would have melted. Current (amps) is uniform in a series circuit. Heat generation is not. Heat happens when current flows through resistance (ohms). The battery to cable connection should offer very low resistance but Keith’s was dirty or maybe even just loose, resulting in an “ohmic” (high resistance) connection at the negative post. The high current demand of the winch motor and the battery’s ability to source high currents (hundreds of amps) resulted in very high power dissipation (watts) at the terminal. Let’s guess and say it was a 10 ohm connection and the winch drew 30 amps. If that had been the case there would have been a 3 volt drop across the terminal connection (30 amps / 10 ohms = 3 volts). That means that there was 90 watts (3 volts x 30 amps = 90 watts) dissipated at the negative terminal. 90 watts concentrated in a small volume results in high temperature.



Since there were only 9 volts left (12 volts - 3 volts = 9 volts) to run the winch motor (it was running under spec voltage) it was probably drawing more current than normal (as under-voltage motors are want to do) so the 30 amp estimate could easily be low.



Keith can prevent this problem by making sure his connections to his battery are clean and tight.


#2

Actually it was far worse than that. The formula for watts dissipated is P=I^2R. The power dissipated in heat goes up as the SQUARE of the amps. A reasonable winch can draw 80+ amps under load. A nice clean battery to lug connection will be under .1 ohm. If the terminals were even slightly corroded, say causing a 1 ohm resistance, the heating power at the battery terminal would be 6,400 watts. (8080*1) The Lead/Antimony alloy that terminals are made of melts at about 650F and the specific heat of that alloy is low so it heats up quickly. When dealing with high amperage loads the contacts have to be super clean


#3

Man I have got to stop posting without my brain plugged in. Yes, V = I x R so at 30 Amps a 10 ohm connection would have had a 300 volt drop and the winch motor would not have run. Since the motor did run, we can guess the ohmic connection dropped a few volts, so let’s go with 3 volts for the sake of argument. If it was 3 volts and the winch was kind enough to draw as few as 30 amps then the power dissipation at the terminal would have been 3 volts x 30 amps = 90 watts but that would have meant the connection was a 0.1 ohm (V^2 / P = R) connection, not a 10 ohm connection as I misstated in my post.

If the connection had been more ohmic (i.e. worse) then the wattage would have been higher. At 1 Ohm with 30 amps of current we would have seen 900 watts (I agree, I^2 x R = Watts) and the terminal would have been quite hot.

Gashnore, thanks for correcting my Sunday brain-deadness.


#4

You guys are talking about different orders of magnitude. Any chance of laying it out with more math and fewer words? 6,400 watts is a radio station. Carriage returns = readability.


#5

If I could find a way to do physics without math I might win the Nobel Prize!

Basically, in any electrical circuit with resistance power is lost in heating the wire and connections. The amount of heating is related to the square of the amps flowing through the circuit and directly to the resistance. To move a lot of amps efficiently the resistance has to be very low.

A medium size truck winch can generate more than 4 horsepower. One horsepower is 745 Watts. In this case the caller said the winch was straining so it must have been close to 4 horsepower. At 12 volts that is 240 amps. The square of 240 is 57,600 so with just 1/10th ohm of resistance that battery post was theoretically being heated by 5,700 watts, the equivalent of a small central heat furnace.

In reality however the heating was probably much less because the winch was stalling because it was starved for power.


#6

Thank you. Kindly note that I asked for MORE math, not less.


#7

I’m curious: How are you going to push 240 amps through a 1/10 ohm resistance with only 12 volts? Use Ohms law: V=IR. At 12V a 0.1 Ohm short is only going to generate 120 amps, adding in the resistance of the winch motor will make that number go down.

You have the same problem with your previous (80801 = 6400) math. You’re not going to get more then 12 amps through a 1-Ohm connection with a 12V battery.

Lets ignore everything but the battery post and the motor.
In order to draw 240A a 12V the motor needs to have an internal resistance of 0.05 Ohm. If you want to dissipate the maximum amount of power at the battery post it should have the same resistance as the motor. When you combine the two in series the total resistance of the system is 0.1 Ohm, which will draw 120A from a 12V battery.
120 * 120 * 0.05 = 720 Watts.


#8

Would it be fair to say bazinga?


#9

good connection is close to zero ohms
loose connection might be .1 ohm
let’s say the motor is drawing 50 amps
then we’re loosing 50A * .1 ohms = 5 volts at the terminal
so 7 volts is getting to the motor
and we’re generating 5 volts * 50 amps = 250 watts of heat at the terminal
touch a 100 watt light bulb: OUCH!
take 2.5 times that much heat and concentrate it between the terminal and the cable clamp: MELT!