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I brought 6F22 9V battery. One's output voltage became nearly 6 volt after some use.But when I connect it with a MCU ckt , It gives only 3.37V as output ( voltage tested directly from the battery's nodes ). Then I connect a new battery which gives 9V as output when it is not connected with circuit. But when I connected it with the ckt, it gives around 6.7 V .

=> Why this output voltage of battery changes after connected with the circuit? As I know Battery is a constant voltage source.

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  • \$\begingroup\$ How much current is your "MCU ckt" drawing? A 9V rectangular battery can't supply more than a few hundred mA. \$\endgroup\$ – Dave Tweed Apr 10 '14 at 16:24
  • \$\begingroup\$ around 15mA @Dave \$\endgroup\$ – Anklon Apr 10 '14 at 17:05
  • \$\begingroup\$ In that case the battery should have no problem. There is something else seriously wrong with your setup. \$\endgroup\$ – Dave Tweed Apr 10 '14 at 17:06
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As I know Battery is a constant voltage source.

A battery is not a constant voltage source - it has an effective series resistance (ESR) that means on high current loads the voltage on the terminals drops noticeably.

So, if your 9V battery drops to 6.7 volts and the circuit is taking 2 amps then the ESR is 1.15 ohms.

If a battery were a perfect voltage source it could provide infinite power and that is obviously incorrect.

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