I have encountered a circuit that totally confuses me:

I have a simple LED flash light that came with my electric bicycle. The flash light is designed to work with a range of 24V-36V so it can be powered by the bicycle's battery which is a 32V battery. As I assumed there is a circuit inside the flashlight which reduces the voltage to a voltage suitable for the LED

With the help of a voltmeter I got the following data:

  1. When I power the flashlight with 24V the current measured is 36.7mA (around 0.9mW) and the output voltage of the step-down converter (the input of the LED) is 2.9V
  2. When I power the flashlight with 30V the current measured is 30.4mA (also around 0.9mW) and the output voltage of the step-down converter (the input of the LED) is 2.7V

Let's say I want to power this flashlight with alkaline battery and not from the bicycle's battery. I know I can get a flashlight that receives a much lower input voltage or connect the battery directly to the LED but for the sake of the question, let's say I will use the same flashlight as it is and will feed it with 24V

I have 2 options as I see it:

  1. The absolutly not reasonable option is to use (24 / 1.5 = ) 16 alkaline batteries in parallel to reach 24V (which will then be stepped-down and light the LED)
  2. Use a step-up (boost) converter to reach to 24V from the input of a single alkaline battery of 1.5V

My question is how will putting a lot of batteries in parallel will make a difference opposed to stepping up a single battery. The capacity should not change because as I know when connecting batteries in parallel the capacity stays the same and you only get a higher voltage.

The second question is let's say I use a single alkaline battery with a boost converter: For how long can this battery will be able to power the LED? How can I calculate that?

I probably missing something in my understanding of connecting batteries in parallel...


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