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I have a multimeter with a battery test setting in addition to voltage measurement. This is marked 1.5-6V 50 ohm. I have some questions:

  • is the only difference between the voltage and battery tester increased decreased resistance?
  • why is the top voltage rating only 6V? should larger batteries be tested with the voltmeter? what would the extra couple hundred mA damage that would not be damaged be higher voltage testing in general?

Having decided to try a 9V battery anyway, I find that weaker (8-8.5V) ones have continually decreasing voltage as long as I hold then multimeter on the terminals. Stronger (~8.9V) batteries do not seem to exhibit this behaviour. I have read that weak batteries sometimes do this and should be disposed of.

  • does a continually decreasing voltage indicate a weak battery or a damaged meter (or something else)?
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    \$\begingroup\$ As Techydude says in his answer the 50 ohm load is lower than the normal input impedance (typically >1MOhm). \$\endgroup\$ – David May 10 '15 at 8:11
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    \$\begingroup\$ The typical 9V block is not designed for such a high load. Your typical datasheet will spec the battery for a substantially lower load. \$\endgroup\$ – jippie May 10 '15 at 9:03
  • \$\begingroup\$ The voltage also appears to continually fall on the voltage test setting (10 Mohm). \$\endgroup\$ – Sam Brightman May 10 '15 at 9:10
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Some battery chemistries will show a normal voltage under no load (and a multimeter is next to no load - 1 to 10 Mohm) even when they're flat. The 50 ohm load in battery-test mode is to negate this tendency and give you some indication of whether the battery is holding up to a light load, or is actually completely flat. If you connect in battery-test mode and its voltage drops as you watch, then yes, that's a flat battery. Most batteries other than little coin-cells should be able to supply a 50ohm load without sagging.

The voltage limit is probably about the power (Watts) rating of the 50ohm resistor - it'll comfortably handle up to 6V batteries. Above that, it'll cope for a few seconds, maybe, beyond that all bets are off :).

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  • \$\begingroup\$ So in principle you should use the voltmeter for testing batteries >6V, but be aware that they could give false positives? In this case the voltage falls when attached to the voltmeter also... \$\endgroup\$ – Sam Brightman May 10 '15 at 8:19
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    \$\begingroup\$ Correct. And if a battery's voltage drops when you put the ordinary high-impedance probes across it, it's definitely flat! :) \$\endgroup\$ – Techydude May 10 '15 at 8:30

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