Instructables

Make a Rechargeable Dual Voltage Power Supply for Electronic Projects

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).

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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.
raslani6 years ago
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..
Derin raslani6 years ago
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
Derin Derin6 years ago
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.
computo raslani5 years ago
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
bobulator6 years ago
Do all 9V batteries work like this?
Biotele (author)  bobulator6 years ago
No, just the rechargeable NiMH 9V like in the picture.
mensmaximus6 years ago
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.
eviltechie6 years ago
You should build a project using the cells of a 9 volt battery and then sell replacement batteries at a high cost.
pridepro7 years ago
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.
pridepro7 years ago
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.
Biotele (author)  pridepro7 years ago
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.
I've never fulyl understood what degetive voltage is - I always assumed it was "ground".......mind explaining?
Biotele (author)  James (pseudo-geek)7 years ago
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.
oh I see. but why then, do you need a +3.6 volts, a -3.6 volts, and a ground? why not just ground and +3.6?
Biotele (author)  James (pseudo-geek)7 years ago
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?
Biotele (author)  James (pseudo-geek)7 years ago
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.
ok. I think I kinda understand it now, I'll look into it later lol.
rc jedi7 years ago
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.
Biotele (author)  rc jedi7 years ago
I was surprised to find them inside the 9V package. believe it or not, the battery in the picture is a Chinese knock off Sony's. They don't have the current ratings on the label, but they work. So be careful when you buy them of the internet or in discount stores. You are right they are pretty cute and beat the clumsy clusters of AA/AAAs.
Myself7 years ago
It's a center-tapped battery! How cute! Seriously, this goes way back to the various A and B batteries used by vacuum tube radios. It's cool to see the idea still serving a use.
Biotele (author)  Myself7 years ago
You still need a dual supply for high performance analog amplification. Although manufacturer have designed single supply amps, the dual supply amps are still better. The trick these days is to use a charge pump chip (DC to DC converter) that generates the necessary voltages from a single voltage like one AA battery. This instructable saves the project builder the hassle to build a power supply or two put together a lot of batteries.
josh921767 years ago
cool
Biotele (author)  josh921767 years ago
I am glad you appreciate it.
Its a great idea, but it needs to be repackaged. Place it back into the case and add double male post caps to the top. That way you don't accidentally plug it into a 9v device. Then make some double negative plugs for your projects.
Biotele (author)  upriverpaddler7 years ago
I am a very messy inventor, don't have much craftmanship skills. You ought to see my "lab". But you need the 9V plug to stick it in the recharger. If you plug it in a device by mistake, nothing happens the LED blocks the current (the charger current is in the reverse direction). Ok i will spruce it up. Put it back in the can and make a little window for the LED. It should be obvious it's not a normal battery with wires sticking out of it.