18 Replies Latest reply on Sep 22, 2020 2:26 PM by jw0752

    3.7 to 5v ?

    o.thornwall

      Hello,

       

      I have a device that has a power requirement of 5v, 1a. I wish to build a power source to make it portable.

       

      I have several 3.7 lithium Ion 18650's. But I'm not sure if I should put them in series to make 11.4v then reduce down to 5v or paralle to 3.7 then boost up ?

       

      Can anyone recommend a build sheet or video on this with parts list ?

       

      Thank you. OT

        • Re: 3.7 to 5v ?
          jw0752

          If you have an old auxiliary backup battery for a cell phone there is a circuit inside that takes the 3.7 volts of the battery and boosts it to 5 volts. This circuit will also allow you to charge the 3.7 Lithium battery with a standard phone charger. The only catch is that it may require a certain level of output current to remain on. In other words the 5 V device that you want to power with the battery might have to draw 20 or more mA to keep the circuit engaged.

           

          John

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          • Re: 3.7 to 5v ?
            jw0752

            Hi Again OT,

             

            Here is a picture of one that I took apart to use in a similar fashion to what you describe.

             

             

            John

            2 of 2 people found this helpful
              • Re: 3.7 to 5v ?
                o.thornwall

                Hello John,

                I ordered the boost converter and I'm waiting for it to show up. But I have another question concerning a different device: if I have a larger capacity batt with the same voltage output as the device requires, do I necessarily need a protection board of any kind between the batt and the device ?  Thank you, Oscar.

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                  • Re: 3.7 to 5v ?
                    jw0752

                    Hi Oscar,

                     

                    This is a great question.

                     

                    There are two main dangers involved with Lithium Batteries. #1 is over charging them and #2 is allowing them to discharge below a certain point. These two conditions can damage the batteries permanently and in some cases cause them to overheat and catch fire. We have all seen the videos of devices exploding into fire. That is the purpose of the battery protection devices. They know when the battery charge is getting too low and they disconnect the battery from the load or they sense when the battery is fully charged and they stop more charge from going into the battery.

                     

                    Now if you have a way for the your device to stop working before the battery gets too low and if you are removing the batteries to charge them on an approved charger then you probably do not need the protection circuit. I for example use lithium batteries in many of the LED flashlights that I have around the house. When the light beam from the flash light starts to get weak I remove the batteries from the flash light and put them into a special Lithium battery charge that has its own battery protection circuit and recharge them. I do not have separate protection circuits for these flashlight batteries.

                     

                    We all like to use the Lithium batteries because they have what is called high density energy storage. Lots of energy in a small and light package. Unfortunately the side effect is the danger that all this energy presents if something goes wrong. Since the lithium battery is fussy about how it is charged and its intolerance of being fully discharged, battery protection circuits are important. All devices that use a lithium battery have the protection circuits built right into the battery packs.

                     

                    Another way that protection circuits help the charging of lithium batteries is the situation where we want to charge two or more lithium batteries that are in series. Here the problem is called a balancing problem. If the batteries are not perfectly matched one battery may get more of the charge than the other. This is caused by a difference in the internal resistance of the batteries. This allows the charger to put a higher voltage across one battery than across another. Ultimately this damages the batteries and can cause failure of the battery pack. The protection circuit can sense the condition of each battery and treat each one as if it is the only battery that is being charged.

                     

                    If you see the protection circuit as a way to protect the battery while it is being used and while it is being charged you will better be able to decide if a protection circuit is needed in your application.

                     

                    John

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                • Re: 3.7 to 5v ?
                  genebren

                  The boost concept is a solid approach for generating 5.0V from a Li-Ion battery.  As John has pointed out there are a lot of available and cheap devices out there to do that.  I have chosen to build my own.  They are easy and affordable and you have a higher degree of flexibility in functionality build your own.  Here are a couple of links to some of my designs.

                   

                  Walky the Biped Robot - Power pack

                  Walky the Biped Robot - A new hope (actually a new body and battery)

                  Choosing the correct Inductor for a DC-DC step-up regulator - Part 2

                  Choosing the correct Inductor for a DC-DC step-up regulator - Part 3

                   

                  Good luck!

                  5 of 5 people found this helpful
                    • Re: 3.7 to 5v ?
                      charlieo21

                      I agree with the boost converter scenario. Using the batteries in parallel implies self balancing and that could help with their life cycle, also you can upgrade to more batteries to achieve longer capacity or less batteries to achieve a more compact design.

                      1 of 1 people found this helpful