Battery load testing?

BatteryCharger

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I have a DMM from harbor freight with a "battery tester" function. The instruction manual says "The battery amperage under a load of 370 m-ohms will be displayed to a resolution of .1ma. Normal amperage for a 1.5v AA battery = 4ma"

What the heck does that 4ma mean in relation to battery health? Can I translate that number into remaining capacity? Is the "normal amperage" of 4ma the same for other 1.5v batteries like Ds?
 

SilverFox

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Hello BatteryCharger,

EDIT: It seams I flunked math yesterday... The value needs to be 370 ohms rather than 370 milli ohms for these figures to work. I was treating the current as Amps rather than milli Amps. Oops... ENDEDIT

Let's see now... E = IR, so with a resistance of 0.370 ohms and a current of 4 mA, you end up with a voltage under load of 1.48 volts. EDIT: this only works with a resistance of 370 ohms.

At a 3 mA reading, the voltage would be 1.11 volts, and if it drops to 2 mA, the voltage would be down to 0.74 volts. EDIT: Once again we need 370 ohms.

Now, if your light draws 4 mA, you have a pretty good idea of how well the battery will work...

Tom
 
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Power Me Up

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Something is wrong with those figures - 1.5 volts into 0.37 ohms would give a current of 4 amps!

Maybe the resistance is actually 370 ohms which would give a current of 4mA at (about) 1.5V.

If that's the case though, it's not of much relevance to most devices - particularly flashlights. 4mA would be enough to give an average LED a dull glow. In comparison, a Fenix L2D CE draws about 1 amp in turbo mode - 250 times that test current. Even in low mode, it'll draw about 50 mA - still more than 10 times that test current.

I'd suspect that like most battery testing functions on multimeters that it's pretty useless - really no better than a simple open circuit voltage measurement.
 

SilverFox

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Hello Power Me Up,

Oops, I screwed up...

I have edited my post. You are correct, you need 370 ohms to get the figures to work properly.

Tom
 

Power Me Up

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Hmmm....so basically it's a worthless function and I should just test the voltage?
Pretty much.

Depending on the type of cell that you're testing, just reading the open circuit voltage may not give you much information either.

You could take a look at the ZTS Multi Battery Testers:
http://thomasdistributing.com/shop/...ee-deluxe-protective-carrying-case-p-564.html

Although they're fairly expensive.

A cheaper solution would be to get a resistor that would put a similar load on the cell as whatever it is you normally use them in and test the voltage whilst connected to that load. Keep in mind that with NiMH cells, this still isn't going to tell you a huge amount because of their flat discharge curve...
 

David_Campen

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In as much as name brand Alkaline AA cells are only about 33 cents each when purchased in packs of 3 dozen; if I have any concern about the remaining capacity of the Alkaline AA cells in a device I just replace them with new cells.
 

MrAl

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Pretty much.

Depending on the type of cell that you're testing, just reading the open circuit voltage may not give you much information either.

You could take a look at the ZTS Multi Battery Testers:
http://thomasdistributing.com/shop/...ee-deluxe-protective-carrying-case-p-564.html

Although they're fairly expensive.

A cheaper solution would be to get a resistor that would put a similar load on the cell as whatever it is you normally use them in and test the voltage whilst connected to that load. Keep in mind that with NiMH cells, this still isn't going to tell you a huge amount because of their flat discharge curve...


Hi again Power,

This is one application i hope the Battery Analyzer i made can help too.
The resolution is almost as good as a digital meter yet it logs the info
and you can save the info for a particular cell and when you test it again
at a later date you might be able to tell how far discharged it is. This
would be done on a cell to cell basis of course.
One problem i have however is that many people are going to want either
fully populated pc boards or a complete unit. If i make the boards myself
it might take too long to get them together.
 
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