# How to choose automatic voltage levels from a battery

1. Aug 10, 2017 at 6:12 AM

### Nikhil N

Hi..
I have a 40 volt (2.5Ah) , Lithium Ion battery. I need to use this battery for 20V application as well as 40V application. How to make an automatic voltage level chooser module for respective application?

2. Aug 10, 2017 at 7:10 AM

### sophiecentaur

To convert the 40V to 20V efficiently, you can't just use a Resistor in series (sorry if this is blindingly obvious to you). You will need a Switch Mode voltage converter which will lose very little energy in the conversion.
I found this link (one of many) that is possibly along the lines that you need. (Dropping the extra few volts could be done with two or three power diodes without too much power waste, if you can't find a unit with possible 40V input. Have a rummage on Google.

3. Aug 10, 2017 at 10:05 AM

### Staff: Mentor

What is your current requirement for the 20V application?

4. Aug 10, 2017 at 11:04 AM

### sophiecentaur

I just read this again. What could you possibly mean by that requirement? Even a Human needs to read the rating on the side of equipment in order to know what supply voltage it should be provided with. This would truly be Machine Intelligence

5. Aug 10, 2017 at 11:52 AM

### Staff: Mentor

I was thinking the same thing. I built a little power supply box for my telescope that can take an ac (to dc psu) or direct dc input and provides 12VDV out for the telescope and 5VDC out for a USB hub: a simple double-throw toggle switch selects the active input and a voltage converter like the one linked takes 12V down to 5V for the hub. The trick was that "12V" isn't 12V, so i had an additional one for that.

6. Aug 10, 2017 at 1:15 PM

### sophiecentaur

But you had an 'intelligent demand' circuit. This guy wants an 'intelligent source' circuit.
Perhaps the easiest way would be to have two different plugs and sockets, one for each voltage so the 20V system couldn't access the 40V supply and vice versa. That's so simple that I have only just thought of it.

7. Aug 10, 2017 at 5:39 PM

### Staff: Mentor

Yes. And still manual.
I have that too; and with different style plugs so I can't mix them up.

8. Aug 11, 2017 at 2:19 AM

### Nikhil N

Current required is 1.5A

9. Aug 11, 2017 at 10:55 AM

### Staff: Mentor

So at 30W output, you should be using a Buck DC-DC voltage regulator. You can probably find one off-the-shelf that will do what you want.

10. Aug 16, 2017 at 11:08 AM

### gmovercl

Like berkeman said, you can use one step down converter to supply both your applications. Adjust the duty cycle to adapt to either of your applications. However, you need to dig into some more design aspects if this is being designed for some application outside academic course/project work.

11. Aug 16, 2017 at 11:16 AM