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I've been reading a lot, here as well, and from what I understand, your garden variety battery meter isn't truly accurate for telling you the voltage of a common household use battery at any given time (AA, AAA, C, D, 9V, and button cell). This is due to no load being placed on the meter while you test a battery. The load, if I've read correctly, gives you a more accurate understanding of what a battery can do in an actual real-world application.

So, I'm trying to buy a device that cheaply tells me what any battery I have is able to output but I don't want to buy something that only gives me a superficial inaccurate reading. Does the device I want exist or is it something I'll have to assemble out of different components like a resistor, etc?

Thank you for any help and for anything I can get from you to learn this better! You'll have to go easy on electronics fundamentals if you're going to explain concepts to me. :)

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    \$\begingroup\$ To those voting to close this question, I don't think this qualifies as asking for specific products or where to buy them. I think the question is more about whether or not these devices exist. \$\endgroup\$ – DerStrom8 Oct 4 '17 at 17:01
  • \$\begingroup\$ Yes, and thank you. So there are devices that will measure more accurately and yes, it is essentially useless to just buy a cheap meter on amazon that tells you the voltage remaining of any battery (if you want to check for dead batteries or what's left in a battery) because that's something that requires more information to determine properly. Am I correct? \$\endgroup\$ – tinpanalley Oct 4 '17 at 17:10
  • \$\begingroup\$ Yes, that is correct. \$\endgroup\$ – DerStrom8 Oct 4 '17 at 17:46
  • \$\begingroup\$ @tinpanalley we go easy on people that review the guidelines on writing questions and write good questions. electronics.stackexchange.com/help electronics.stackexchange.com/help/on-topic \$\endgroup\$ – laptop2d Oct 4 '17 at 18:26
  • \$\begingroup\$ Use a voltmeter and a small selection of resistors. Measure the open-circuit voltage of the battery, write it down. Load it down with the highest value resistor, measure, write. Continue with lower value resistors until you notice a significant voltage sag or things start to get hot. Now you have the data you need in order to get an idea about where the battery is at. I can't tell you how to interpret the data, because that depends on the battery and what application you intend to use it in. Play around with this, you'll eventually get an understanding and/or feeling for what the data means. \$\endgroup\$ – Dampmaskin Oct 4 '17 at 19:06
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You are correct in that simply measuring the open circuit voltage across a battery is not an accurate indication of whether it is good or bad.

As most batteries begin to fail their equivalent series resistance (ESR) increases. As the ESR increases, it creates more of a voltage drop on the output of the battery proportional to the current being drawn. For example an ESR of 500 mOhms will drop only 50 millivolts with a 100mA load, whereas an ESR of 1 ohm will drop 100 millivolts with a 100mA load. As the battery continues to fail the ESR often continues to increase, and the chemical composition begins to break down making it more difficult for the battery to supply the power necessary to drive the 100mA load.

Measuring the open circuit voltage across a battery draws practically no current (the meter is a very high impedance) so the drop across the ESR is barely seen. You would not be able to tell the difference between a 200 mOhm ESR and a 1000 mOhm ESR simply by measuring the voltage without drawing current. By connecting a fixed load across the battery you will get a better idea of how high the ESR is, and whether or not the battery is good or bad.

To answer your questions, yes: Meters do exist that apply a load before measuring the voltage. Also, yes: You can build your own. We cannot tell you where to buy existing meters because that is considered off-topic for this site. I can tell you, however, that they are out there and you just need to do your research to make sure that the tester you end up buying does apply a load to the battery under test.

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The method by putting it under load with a resistor won't give you great results but will give you some. As an alkaline battery discharges over time from a constant resistive load, it's voltage will slowly dip. At some point it hits a knee where it falls-off and starts to dip very sharply. (My daughter and I did an extensive report on this - it is very predictable across all alkaline battery types and brands). Some devices might consider it "dead" at this point because it's no longer holding up the voltage it usually does during its operational life. Other devices (electronics) may have a charge pump which may help it extract every last coulomb from the battery (even at a lower voltage) - but this would be a very short-lived effort. That said - you'd be able to tell the battery was low at this point but it would already be at or near death.

The "correct" way to do this is to use a circuit like a "coulomb counter" in-series with the battery. It basically consists of a very low-value, high-precision resistor of which a circuit monitors the voltage across, and then can see the current leaving the battery. Constantly monitoring this over time you could tell how much has left the battery. Knowing the specs on the battery, you could determine life left. Taking other factors like temperature into account could give you much more accuracy. This is what things like phones and laptops do - but they do it also because they need to control and monitor not just the discharge, but also how they are charging, and it's part of that circuitry.

So long story short - you can get a good answer, but it's pretty complex. Simple answers aren't very accurate.

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