1. Feb 22, 2013

### johnsmith12345

Right now I'm doing an experiment for my EE about battery capacity. How would I be able to monitor the battery's true emf while discharging the battery at a constant current?

Also, when I connect the battery in a circuit and discharge it at a constant current, I notice that the voltage across the battery decreases. Since I am discharging at a constant current, will this difference in voltage from when its in a circuit and the true emf be constant?

2. Feb 22, 2013

### Staff: Mentor

Connect a voltmeter?
Right.
Which difference? The measured voltage IS the true voltage the battery delivers at that current.

3. Feb 22, 2013

### Simon Bridge

Also see:
OP feels that the "true" emf of the battery is the open-terminal voltage.
He wants to keep track of it for the discharge curve... most people seem happy just to use the terminal voltage on-load.

4. Feb 22, 2013

### Staff: Mentor

Hmm, well, can you interrupt the discharging process for the measurements?

5. Feb 22, 2013

### Simon Bridge

That's what I thought - but doesn't the open circuit voltage for a battery take a bit to "relax" after the load is disconnected, like a hysteresis effect ... possibly altering the discharge curve? It would depend on the battery of course...

I also thought that the early measurements could be used to work out the internal resistance and, after that, the open-circuit voltage would just get calculated.
However, the internal resistance may change during discharge - it would certainly be affected by temperature.

If the load resistor was very large compared with the internal resistance, then the loaded voltage would be close to the open-circuit...

6. Feb 22, 2013

### Staff: Mentor

I would be surprised to see any effect on the scale of a human intervention (~1s), but that would be easy to test.

7. Feb 22, 2013

### Simon Bridge

I was thinking that repeatedly switching the circuit may be different enough from a continuous discharge, over the entire discharge time, to affect the shape of the curve - but like you say: easy to test.

We don't know how accurate things are either... any effect could be small compared with the sensitivity of the instruments - especially seeing the battery in question is quite high tech.

8. Feb 22, 2013

### jim hardy

@ johnsmith

That would be interesting to test. I never tried it with an exotic battery.
Published curves are with battery under load.

Tinkering with automobile batteries, indeed there is a time delay when you release the load. Voltage bounces back to 12.6 in about a minute.

I've seen car batteries that'd indicate 12.6 volts but were too run down to illuminate even the dome lamp.