# 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

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

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

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

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.

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

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.

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

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.

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.