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# 9v battery discharge test

## 9v battery discharge test

(OP)
Currently, i am making a simple discharge circut to discharge 9 volt batteries at a constant current. Using a resistor won't work because the close circuit voltage will be different depending on the health of the battery. The question is how can i achieve this constant current draw from the battery for the whole lifespan. Is there a simple solution to this? Thanks in advance.

### RE: 9v battery discharge test

You should be able to use three resistors. A voltage divider to set the voltage and then the third resistor to a separate ground.

+----res----res-----~
|
Res
|
Ground

### RE: 9v battery discharge test

Not sure how a voltage divider will keep constant current from the varying battery voltage?

Brad, if you spend a little more, you can use a simpler circuit, see http://www.ti.com/lit/ds/symlink/lm317.pdf figure 14. These have a minimum load current of a few milliamps, but you should be targeting current draws above that level I think?

### RE: 9v battery discharge test

A simple constant current can be made with a 316 Voltage regulator.
The regulator will hold the Voltage between output and reference to 1.2 Volts so all you need to do is calculate the resistor you need to drop 1.2V at the current you want.

e.g. 50 mA 1.2/0.05 = 24 Ohms

### RE: 9v battery discharge test

Did you mean LM317?
LM316 is an OpAmp.

### RE: 9v battery discharge test

A LT3092 is a nice solution I recently used. One IC and two resistors is all you need.

Z

### RE: 9v battery discharge test

I did a test on many different makes of 9 V batteries once. I was running them simultaneously and the data I was interested in was for rather low discharge currents. So, I used 10 kohm resistors and recorded voltage from start to en of test. I then put the data (.txt file) in an Excel sheet and calculated the total charge from voltage, resistance and time. No big deal and real simple if you want to test many batteries at the same time. Cost for the set up: around 1 USD.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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