Battery discharge ?

[crazytom]

Im designing a circuit that will monitor a battery condition as its discharged how ever have some problems in getting the load to draw what i want.

Battery is 3v i want the 300ma to be drawn as discharge current so i put in 4 x 10ohm 0.5w 5% tolerance carbon film resistors in this configuration across vs to 0v.


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[]r10 []r10
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[]r10 []r10
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The simulator tells me this draws 300ma with resistance equivalent of 10ohm but when i measure it with a multimeter it draws 165ma which would suggest resistance equivalent of 16.5ohms how can this be ?


Does a resistor resistance change much depending on heat ?

 

[#12]

No. A battery changes voltage with amps. Batteries have internal resistance.

 

[Reloadron]

The simulator is doing the math. Three volts divided by .3 amps does equal 10 ohms. However, while that looks good on paper (or in simulation) it depends on a few things. The battery must have an amp hour rating or current capacity to deliver .3 amp. For example a coin cell (tiny battery) will not be able to deliver .3 amp. With a 3 volt battery like this you will never get .3 amp. You need a source battery that can deliver well above .3 amp if you expect it to deliver .3 amp.

Also, the battery must be new and fully charged if you expect it to deliver its rated current.

Ron

 

[crazytom]

When i hook up a ac-dc regulated supply of 3v 1 amp current limit across that resistor load i see more ma than before but still its below 230ma and no where near the 300ma that ohms law says.

Is internal resistance of a dc power supply calculated by v(no load)-v(load)/current ?

Taken in a power supplies internal resistance in to equation how do I recalculate the resistance that a load should be for a desired current flow ?

 

[#12]

Measure the actual voltage the power supply is delivering under load. Measure the actual current that is being delivered and look up the internal resistance of your amp meter. Start doing math. You are going to find some ohms you had not considered, somewhere.

 

[Reloadron]

When i hook up a ac-dc regulated supply of 3v 1 amp current limit across that resistor load i see more ma than before but still its below 230ma and no where near the 300ma that ohms law says.

Is internal resistance of a dc power supply calculated by v(no load)-v(load)/current ?

Taken in a power supplies internal resistance in to equation how do I recalculate the resistance that a load should be for a desired current flow ?

If you have an accurate 3 volt source capable of 1 amp and if your load is an accurate 10 ohms you should be seeing .3 amp of load current. The only way you will not see .3 amp is if one of the two (voltage or resistance) is not the correct value. Numbers when measured accurately don't lie.

V = I * R So 3 / 10 = .3

Therefore as suggested you may want to measure the power supply output voltage accurately with a good meter and the same for the current. Voltage across the load and current in series with the load.

Ron