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I know that alkaline batteries' voltages go down when they're at low charge (from 1.5V~1.7V to 1.0V). For typical electronic devices like remote controls or bluetooth mouses/keyboards (that uses two AA or AAA batteries and typically works at 3V and tens of mA), do the batteries have to output more current at low charge in order to fulfill the devices' needs?

On one hand, I think so because \$P = UI\$, so when \$U\$ goes down, \$I\$ must go up in order to maintain the power. On the other hand, these devices usually have built-in voltage converter so even 1.0V can be converted to, say 3.7V or 4.2V, and lower total power wouldn't be a greai issue.

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    \$\begingroup\$ That depends 100% on the device. And since you haven't specified a device... \$\endgroup\$ – Harry Svensson Oct 7 '18 at 14:59
  • \$\begingroup\$ @HarrySvensson I added some information. Please check the edited question. \$\endgroup\$ – iBug Oct 7 '18 at 15:00
  • \$\begingroup\$ Your updated question is nibbling on my point. Especially the "usually" part. \$\endgroup\$ – Harry Svensson Oct 7 '18 at 15:02
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Usually not, but sometimes.

Most devices that run off of two AA cells or similar are just powered directly from the two cells in series. The components inside the device are chosen to be able to operate over the valid voltage range of the two cells. For example, there are many microcontrollers, especially those intended for low power applications, that can run from 1.8 to 3.5 V.

Using the battery voltage directly simplifies these devices. The microcontroller can be directly connected to the battery. When nothing is happening, the micro is in sleep mode drawing very little current. But, it can be woken up quickly when something happens, like a button is pressed. The sleep current of modern micros is well below the self-discharge current of typical batteries, so there is effectively no difference in battery life between everything being truly off and the micro sleeping when there is no activity.

Some devices, typically ones that handle more power, have a switching power supply that makes a fixed internal voltage from the varying battery voltage. Those devices will draw more current as the battery runs low, since they are effectively drawing a constant power, not a constant current. However, going into low power mode when nothing is happening gets more complicated. This is usually not cost-effective in cheap high-volume devices, like TV remotes, for example.

For other devices, the current goes down significantly with voltage. Something that just connects the battery to a LED thru a resistor will actually draw lower current even more quickly than linearly with battery voltage.

In the end you don't know, but again, most devices will draw somewhat lower current with lower voltage. Replace the batteries with a variable power supply and ammeter in series and see.

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Yes. True, when battery voltage is low, the device will demand higher current to maintain proper operation of the device.

All switching mode power supplies are having this characteristic. Almost all modern portable gadgets are having smps between the battery/cell and the other electronics of the gadget.

We can loosely call this as negative resistance characteristic.

You may find this on the label behind of your computer monitors too. They consume higher current at 110V and half as less when run on 240V.

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