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capacitor help please

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gjpollitt

New Member
Please take a look at the enclosed circuit of a basic battery tester

B2 is battery that I want to test. The 12ohm resistor is to load the battery to draw current. I want to add a capacitor to this that will take a charge from the battery then pass it to the -terminal of the lm311.

Is this possible and how can i do it? I want to make sure that I am giving the B2 battery a good test and figure the addition of the cap will help this.

Thanks for looking

Graham
 

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Roff

Well-Known Member
I can see no reason for adding a capacitor. Why are you doing this circuit? If you have a lot of batteries and you want a go/no go tester, or if you are just wanting to learn about comparators, then I can understand.Otherwise - you have to have a voltmeter to set the pot (VR1). Why don't you just use the multimeter to measure the battery voltage?
 

ljcox

Well-Known Member
Basic Battery Tester

Graham,
I don't see the point of inserting a capacitor. The 12 Ohm resistor will load the battery and the LM311 will campare its terminal voltage with the porportion of the 9 Volt battery as set by VR1.

An improvement would be to add a voltage reference (eg. the LM336-2.5) to stabilise the voltage applied to VR1. With your current circuit, the reference will decrease in porportion to the voltage of B1 as it ages.
 

gjpollitt

New Member
I am just playing around with components, figuring out how to use them etc

I know I need a voltmeter to check the voltage, later I am going to add an ADC for the ref volts,, just to see how it works

Will try adding the lm336 and seeing how that affects things

Reason I ask about the capacitor is because I have 4 AAA batteries that have come out of my digi camera, they failed to operate the flash as they have been in there for a while. Yet when i put them into this circuit (which i have proto on breadboard) there was no difference between those batteries and fully charged ones.

Im just a little confused with regards to finding out if the batteries are fully charged or not.

The point of the circuit at the mo is to set the ref voltage and then compare that to the battery voltage with the green led for ok and red for low battery. Then i can stick it in the charger!!

Thanks for your input
 

ljcox

Well-Known Member
Battery charge

I'm not an expert on batteries, but I recall that the terminal voltage does not change much as they discharge and therefore is not a good indicator of the state of charge. I think the voltage only starts to take a nose dive when the battery is almost dead.

I suggest you try to find a battery manufacture's web site. They may provide a graph of battery voltage versus charge state.
 

Russlk

New Member
The problem with your camera most likely is that even new batteries are marginal with respect to the voltage required by the camera. I have a Toshiba camera and it will not work with Copper top cells, I have to use Lithium (1.7v) cells and they last a long time.
 

jem

Member
Drawing about 90mA may not be enough to really test the capacity of the battery. A more reliable test is the ESR (Equivalent Series Resistance) of the cell. That may be why your AAA fail to properly power the flash. A way to test ESR is to pulse the battery under test with a high current (try around 0.5A for an AA) for 1 ms or so. The voltage is then measured just prior to removing the load. The terminal voltage delta (unloaded, and loaded) divided by the pulse current will give you the ESR.
 
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