Finding Maximum Battery Discharge

[Trisorion]

I am having a bit of a problem finding the maximum discharge of a battery. This maximum discharge occurs when the battery is shorted out with such a low resistance wire that the limiting factor becomes the chemistry of the battery. The battery in question is **broken link removed**. Wikipedia says that AA sized NiCds can reach up to 18 amps discharge.

I put my multimeter on the 20 amp range, switched the leads to the correct unfused slots and tested it. It produced 4.5 amps for 15 seconds without getting appreciably warmer. I was very disappointed by this poor performance but I thought I would do a few more tests just to make sure. I then tried a 14 AWG wire and the battery become hot to the touch in four or five seconds. This leads me to believe that the resistance of my multimeter is too high for this application. I will not be able to buy a new one.

Do you guys know of an alternative ways to test this?

 

[ericgibbs]

I am having a bit of a problem finding the maximum discharge of a battery. This maximum discharge occurs when the battery is shorted out with such a low resistance wire that the limiting factor becomes the chemistry of the battery. The battery in question is **broken link removed**. Wikipedia says that AA sized NiCds can reach up to 18 amps discharge.

I put my multimeter on the 20 amp range, switched the leads to the correct unfused slots and tested it. It produced 4.5 amps for 15 seconds without getting appreciably warmer. I was very disappointed by this poor performance but I thought I would do a few more tests just to make sure. I then tried a 14 AWG wire and the battery become hot to the touch in four or five seconds. This leads me to believe that the resistance of my multimeter is too high for this application. I will not be able to buy a new one.

Do you guys know of an alternative ways to test this?

I would recommend you dont short out a SLA battery, it warns you in datasheet that the battery could explode, you will get covered in boiling acid gel.

Exactly why must you know the max SAFE discharge rate.?

 

[Willbe]

With 1.2v and 18A this is an internal battery resistance of 0.07Ω.

Use a current sense resistor for this, .01 Ω or less, and measure the voltage across it.

The fuse in your ammeter is costly to replace.

 

[Trisorion]

I was going to try submerging them in 0 degree Celsius mineral oil so that they don't explode, additionally they will only be on for 30 seconds. And the case will be sealed so no one gets boiling acid on them. But if they still explode, I will need the maximum safe discharge. But lets not get caught up in the application just yet, that is a different question.

Id guess I would like to know how to find both, flat-out maximum and SAFE maximum.

 

[Trisorion]

From the battery's data sheet: 16.0mΩ Internal Impedance

Is that the same as DC resistance?

 

[Willbe]

Forgot:
with resistance this low you need to use a four terminal Kelvin method, i.e., the wires that carry the current through the resistor need to not touch the wires that measure the voltage across this resistor.
With this method and >10A of current you can measure the resistance of a nickel.

 

[audioguru]

Why do you want to blow up little batteries?
Most won't blow up but some will.

18A x 16m-ohms= only 0.288V. Almost nothing. For only a few seconds.
Why??

 

[Trisorion]

It is for a contest to make an electromagnet using 2 AA batteries. If they only last 1 try that is OK. I will be using pretty think wire so .2 volts will be plenty. But please, before this thread gets sidetracked to another tangent:

Is there an alternate method for finding the maximum discharge?

 

[Hero999]

What voltage is the battery?

Assuming it's 12V and has an impedance of 16m, 750A will flow and the power dissipation would be 9kW which would cause the battery to meltdown pretty quickly.

 

[audioguru]

On this website or on another there is another kid in your class doing the same thing. Carrying as many paperclips as possible for a short distance.

 

[Hero999]

To measure the internal resistance attach known constant current load to the battery for a short length of time, measure the voltage drop and calculate the internal resistance using Thevenin's Theorem.