Make a Rechargeable Dual Voltage Power Supply for Electronic Projects




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Mod a 9V rechargeable battery to give you +3.6V, Ground and -3.6V.

You will appreciate this idea if you ever had to clobber together a bunch of AAs or AAAs to get a project working.

This instructable was meant to be part of a larger project, but I decided to submit this part alone to the 9V battery group.

You need to start with a 9V rechargeable battery (actually 8.4V but in a 9V package).

Check out my other Instructables:
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Step 1: Crack the Case Open

Take a long nose plier and gently apply pressure at the top seam. The case will crack open revealing a surprising inside.

There are seven 1.2V NiMH batteries soldered in series.

To make the power supply even you need to get rid of one or add one.

I ripped off the one near the top. Don't try to unsolder it because these batteries are welded together and the batteries might explode if heated too much (so wear goggles).

Step 2: Add a Replacement

Ok, so know you have a 7.2V battery, big deal!

...and you can't even recharge it because the charger delivers 9V and might explode the batteries.

Ok so what to do?

Well, replace the missing battery with an LED. An LED always maintain a voltage drop of around 1.2V (depending on what type you use). So you made a battery "prosthesis".

Make sure that your LED is pointed in the right direction when you solder it (LEDs only conduct one way).

Solder a wire on the first battery (red +) that will be the +3.6V of your power supply.
Solder a wire between the 3rd and 4th battery, that will be ground.
Solder a wire where you soldered the positive leg of the LED and that will be the -3.6V supply.

Step 3: Recharging

Look at the pretty light when your battery is charging.

It is possible to recharge the battery while connected to your project. However make sure you have big capacitors between the voltage rails to absorb the charger's pulses.

Do not recharge the battery when your project is connected to your body. (coming soon on another instructable).

It is a hazard to be electrically connected to a device that is plugged into the power main.'''

Step 4: Make It Nice and Tidy

You can put back the assembly in the original 9V package. I made a hole for the LED and a hole in the back for the wires.



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    27 Discussions


    11 years ago on Introduction

    i just a have question on about this batteries.

    well i have my very old phone, Nokia 3310 the battery is too weak.. so, i replaced it in an BL-5C which is the standard batteries of Nokia Phones... so my question is now can i DUAL A BATTERY? I mean 2x battery... is 3.7v + 3.7v = 3.7v if i connect the two batteries? or will become 7.4? reply pls..

    4 replies

    Reply 11 years ago on Introduction

    both may happen.if you connect them in parallel,(+ to +) you will get 3.7V but when in series(+ to -) then it will be 7.4V


    Reply 11 years ago on Introduction

    and BL-5C is not the standard for all nokias,I still have my Nokia BBH-8H NiMH


    Reply 9 years ago on Introduction

    BL5C is in the new cheaper Nokia phones. There's one in my 2610. And my mom's phone (6102i) had a BL4C, which is the standard battery. 5C is the extendedededed battery.


    Reply 10 years ago on Introduction

    Well it matters which way you connect them:
    parallel will = keep it at 3.7 volts and will be 2x as strong
    series will= make it 7.4 volts and not as strong


    Reply 11 years ago on Step 1

    No, just the rechargeable NiMH 9V like in the picture.


    11 years ago on Introduction

    My try: Re a split power supply. Giving a speaker positive voltage only uses half of the excursion of the cone. Giving the speaker a negative voltage as well allows the speaker to utilize the reverse excursion in the opposite phase of a typical AC signal.


    11 years ago on Introduction

    You should build a project using the cells of a 9 volt battery and then sell replacement batteries at a high cost.


    12 years ago on Introduction

    OOPS, maybe there would be a problem with the extra cell in some cases, but I've never seen the explosion problem. A point to remember "never let out the magic smoke". On the questions about ground...I was always taught that ground was 0 Volts. If there is a voltage potential inside a circuits ground point relative to "earth ground" then that would be referred to as a floating ground. This is very common in transformerless items like cheaper televisions. Always unplug or be very careful when working on this type. On the question of why you would need both a +3.6 and -3.6 Volts...most operational amplifiers (Opamps) require a split voltage in order to function.


    12 years ago on Step 1

    Maybe I'm lazy. Why not leave all the cells on and save an LED? Just attach the positive lead between the first and second cells. Then your charger will be happy! But no pretty light.

    1 reply

    Reply 12 years ago on Introduction

    If you leave that cell in it will explode when you recharge the battery because it will be fully charged while the rest are empty.

    negative voltage can be ground. Ground can be any voltage, it is set by the designer, it is a voltage point of reference for the circuit. initially ground was a wire that went to a rod stuck in the ground in radio equipment. It was thought that that radio current propagated in the air and returned through the ground. The negative of the radio battery was attached to the ground because it was thought that the battery generated current in the air, and that current had to be returned to the battery, or form a circuit with the battery. So the terms negative and ground became interchangeable. In AC circuits, ground, is halfway between the most positive voltage and the most negative voltage.

    Well like I said before it's for reference. Without ground you can only have positive voltage and the negative is 0V. With a ground you can have a voltage more positive than ground and a voltage more negative than ground. In the past, before digital computers, people used analog computers made with special amplifiers called op-amp. If you don't have a negative integer (represented by negative voltages), a zero and positive integers (represented by positive voltages), you are limited by the math you can do. So you needed a dual voltage supply with ground (0) to do analog math. Op-amp are still used today in many devices like audio, video and precision instruments.

    so you can build a circuit that uses say.....2v with a 3.6v power supply and not burn up the battery since you have the -3.6v AND ground? is that the idea?

    I am not sure I understand your question. If I wanted to feed 2V to a circuit, I can use 2 cells which furnish 2.4 volts, you'd need maybe one germanium diode in series with the battery to drop the voltage to 2. but if you put 3.6V to a circuit that requires 2V (like a white LED), it's the circuit that burns up, not the battery, and if the circuit requires more than 3.6V, well then nothing happens except that the battery gets discharged.

    rc jedi

    12 years ago on Introduction

    i do r/c planes and these 9v batts are a good source for small cells. I used 4 cells from 1 to power mini r/c reciever in a styrofoam toy glider converted to accept a .049 motor. I dubbed it "valuejet" do another project with these batteries, they are too good to pass up.