Is this circuit possible with two batteries?

Other Hardware General Electronics
1

I have an application where I have two AA alkaline batteries running in PARALLEL to supply 1.5v to a small electric motor. At 1.5v the motor runs at the perfect speed and I have two AA batteries running in parallel because the motor needs to run for long periods of time and C/D batteries will not fit in the housing the motor is mounted in. The issue is I need to power a standard yellow 3v LED from these batteries. If I test my LED on the two AA batteries in SERIAL the LED illuminates as it should. With the batteries ran in parallel the voltage is obviously too low to illuminate the LED. At 3v the motor runs too fast for my application.

This basically boils down to this: If I have two AA batteries connected in parallel in a circuit to power one device (the motor) at 1.5v, is it somehow possible so also have them connected serially so at the same time it can power a second device (the LED) at 3v? Also, if it is possible will the solution cause wasting of battery power?

A typical voltage regulator wastes a lot of power, and a buck converter requires at least 5v if I'm not mistaken. Using current limiting resistors seems like a poor way to achieve this and would also drain the battery much quicker. The space available for additional components is roughly 1"x1"x4".

If this isn't really feasible then does someone know if there exists some sort of rechargeable battery pack that is roughly the size of (or smaller than) an AA battery that can output voltage/current adequately to power a standard 3v LED?

2

Put another battery in series with the LED.

LED Battery.jpg
LED Battery.jpg759×550 29.1 KB

LED Battery.jpg
LED Battery.jpg759×550 29.1 KB

3

No, you cannot have the same batteries in series and parallel at the same time but you can make parallel and series combinations. That is you could add a third cell in series with the two in parallel to achieve 3 volts. The third cell does not need to be the same type. Not ideal but it can be done.

Another option would be a 3 v coin cell as a stand-alone led power source for the LED, depends on how much power is required by said LED.

If it were my project, I’d be using a 3.6 v li-ion and just PWM the motor speed. The power density of li-ion by volume is typically 30% better than alkaline. Two 14500 sized cells in parallel and you’re miles ahead over alkaline.

4

There are boost converters that will work from a single AA battery.

5

Thanks for the advice!

6

Thanks for the good advice!