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I try to measure a 9volt battery and it displays "9.14V". But the battery is empty. What is happening? Does the multimeter measures it's own 9V battery?

Can someone explain to me what i observe? How does a completely empty battery acts in a circuit with a multimeter?

EDIT When I connect a LED (without resistors) the battery is able to light it a bit, and then the multimeter displays 8.8V and falling. But if I disconnect the LED the voltage measured starts rising again until it reaches 9.15V again...

The weird thing is that the battery is almost empty.

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  • \$\begingroup\$ An empty battery acts like it's empty. Stick it across your tongue. If it hurts, the meter is right. If it doesn't, the meter is wrong. \$\endgroup\$
    – EML
    Commented Nov 8, 2017 at 11:25
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    \$\begingroup\$ No, the battery is not empty. Connect a LED in series with a 1 kohm resistor across the battery terminals, the LED should light up. If not reverse the polarity. How can an empty battery make a LED light up? The battery might not be able to deliver enough current for a different application but it is not empty. \$\endgroup\$ Commented Nov 8, 2017 at 11:26
  • \$\begingroup\$ Thanks, this battery that displays 9.15V gives a small feeling in the tongue but it is not able to rotate a small motor. Another one that measures 4.5V rotates the motor fine... \$\endgroup\$
    – John Am
    Commented Nov 8, 2017 at 11:28
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    \$\begingroup\$ When a battery gets depleted, its internal resistance increases meaning the voltage drops when a (demanding, current hungry) load is connected. The battery can still drive a light load though like a LED. \$\endgroup\$ Commented Nov 8, 2017 at 11:28
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    \$\begingroup\$ So I can understand that the "chemical damage" is kind of unpredictable Only if you load the batteries too much for too much time. Look at this datasheet: d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2016/03/… Note how at 250 mA, battery life is less than 2 hours. For a 9 V battery I'd stay below 50 mA peak current and a long time average current less than a few mA. Are your batteries all same brand and model? All same age? Only then can you compare fairly. \$\endgroup\$ Commented Nov 8, 2017 at 12:44

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As pointed out in the comments, the voltage of a battery measured with a DM may be misleading, since there is nearly no power/current required. To get a better picture of the battery state, the voltage should be measured while the battery is connected to the application, in this case to the motor. Probably that battery can be still used for other applications that do not demand a high current, f.e. clocks, weather stations, remote controls etc. Sometimes if a battery is used for a high power application for a short time, it can relax and show a higher voltage than before after disconnecting and waiting some time. Also the heat that may be produced during the high load can increase the voltage if measured without the load. This relaxation process, i.e. a possible higher voltage as expected may occur under certain conditions like manufacturing/formation issues, storage history, load profile, temperature etc. In some cases, the application provides a tiny backflow into the battery if the battery power is not needed, f.e. if the application electronics has also a mains power supply or another battery and/or has a fault or is made for rechargable batteries. In that case a nearly empty battery may also show a misleading high voltage if measured with a DM without load.

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To a rough approximation, an ordinary zinc or alkaline battery looks like a perfect voltage source with a resistor in series with it. As the battery runs down, this internal resistance increases. The voltage only drops significantly when the battery is totally flat.

A digital multimeter has a very high resistance, usually about 10 Mohm. This is much higher than the internal resistance of the battery, so you read the true voltage of the battery.

If you connect a load, then the voltage at the terminals of the battery drops because the internal resistance is in series with the load.

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That’s exactly how a nearly empty battery will behave: with a very small load (a multimeter) it puts out it’s nominal voltage (slightly over 9 volts in this case); with a heavier load (an LED directly across the terminals, or a motor) the output voltage drops because the battery can’t provide enough power for the heavier load.

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