Easy Battery Hack Using a DC Power Supply

Intro: Easy Battery Hack Using a DC Power Supply

If you have questions, watch the video, it has extra detail as well.

Safety first

When it comes to safety, it pays to double check your work and error on the side of caution. I can give some guidance on how to choose equipment safe for your application, but it doesn’t mean it is safe for your person if you get in the middle of your electric circuit. if you have concerns about doing this on your own particular project, ask for help.

Some things to keep in mind for safety:

Voltage does not kill, current kills, but voltage is required to push the current along. So, while a 5 Volt 1 amp power supply may be reasonably safe to work with, a 100 volt .1amp is potentially lethal if it ‘bites’ you in the wrong spot. Ask for advice if you do not know. At all times, practice safety.

Batteries/power supplies have specific direction/polarity (positive or negative) that the moves the current along. If you wire your circuit backwards, the current will attempt to flow backwards through your device and could damage it. Check the polarity of your batteries and power supply to ensure they match. There will be more on this in a later step.

Before replacing batteries with a power supply, consider where the device or toy is used. Will it be sitting on a desk or near the bathtub? Would your kids put it in the bathtub? Will it be a shocking experience for them if they do?

The problem:

In general, batteries make my life a little more mobile and less tangled. Sometimes however, there are certain devices that I wish came with a plug. Most recently my kids got an awesome night-time toy that projects stars, the moon, galaxies, etc on the ceiling. The problem is the toy is battery operated only and there’s no automatic shutoff so when kids use it like a night-light it’s dead in a few days. I don’t like buying or replacing batteries. (because I’m a cheapskate).

This toy just sits on the desk, so it’s a good candidate to modify to accept a DC power supply instead of batteries. This idea is not well suited to something like an R.C. Car, but in a pinch, you can use it on the remote control for your TV.

STEP 1: Some Theory

Wall outlet power is generally alternating current, or ‘AC’. Batteries are direct current ‘DC’ and only push the current in one direction. An AC to DC power supply can change AC wall power to DC power. Many common devices that have batteries (laptops, smart phones, etc) only accept DC power. They use a AC to DC power supply to allow us to charge the device by plugging it into the wall.

Ohm’s law is a formula in electronics that relates the voltage (V, volt), current (I, amp) and resistance (R, ohm) of a circuit. Ohm’s law tells us that Voltage in an electric circuit is equal to the product of the current flowing through the circuit and the resistance of the circuit.

V=I*R

For a given circuit, in my case, the toy, the resistance R is a constant. If I replace my batteries with a power supply of equal voltage, then the current in the system also stays the same. This project uses this relationship to replace Voltage, V supplied by a battery with voltage supplied by a DC power supply – nothing else is changed.

Another way to think about this is that voltage is how much the power supply pushes and current is how much the device pulls the energy. If your power supply says 5 Volts and 1.5 amps, it can power a 5 Volt circuit and it won’t burn up until the device pulls more than 1.5 amps from the power supply. In a later step, I will show you how to verify the current draw of your device.

Pro tip: AAA, AA, C and D cell batteries all have the same voltage rating and are interchangeable. That is, they are interchangeable if you can make them fit. My brother and I used to wrap AA’s in paper strips to make them fit the place of a C cell ?. The difference between the batteries is just the physically larger batteries last longer/have more ‘juice’, they don’t push more current/amperage through your system.

STEP 2: Understand What Voltage Your System Uses and How the Electricity Flows

My device has 3 AA batteries which are 1.5 volts each. All added together (assuming they are connected A to D to E to B to C to F) I should see 4.5 volts. The first thing to check is how the batteries are wired and find the beginning and end of where the energy flows.

Consider the above battery diagram. If you put your volt meter such that one lead is on A and the other D you will see either +1.5 or -1.5 volts (or something close unless your batteries are dead), Now check point A and point B do you see +/-1.5 volts or +/- 3 volts? If it is +/- 1.5 volts, then point A and B are essentially the same point and must be electrically connected. If A to B is +/- 3 volts, then you are adding the voltage from batteries #1 and #2 together. Use this same logic to trace the batteries to the end and determine if points A to F or points D to C incorporate all 3 of the batteries (+/- 4.5 volts with full charge batteries). This example are for batteries in series (connected A to D to E to B to C to F for example) Most of the kids toys batteries seem to come this way, but I’m sure there are exceptions. Let me know if you need some help troubleshooting the batteries for a different circuit.

Keep note of which points show you the 4.5 Volts or -4.5 volts and which voltmeter lead you are using at each point. You will need to maintain the +/- polarity when you hook up your new power supply.

To measure the current draw of your system, connect your multimeter inline with the batteries. You’ll have to disconnect one end of a battery and use your multimeter to bridge the gap between the disconnected end and where it should be. Note that current draw may change for your device if it has multiple modes of operation. For example, a computer at startup draws more current than one that is hibernating.

STEP 3: Find Your Power Supply

Now find a power supply that supplies the needed voltage you measured and has a high enough current rating that you won’t burn it up with your device’s amp draw.

I’ve had good luck finding power supplies at thrift stores. Some power supplies even have multiple voltage settings. In my example, I am using a 5V 1.2 Amp power supply – it is a little over-sized, but for my application, I think it will be just fine. Often times batteries at full charge put out a little more than their rated voltage as well, so that’ something else I’m considering when choosing a slightly higher voltage power supply.

Unplug the power supply, snip the plug end off (unless you are going to use it!), divide the wires and strip them at the tip. Plug in the power supply and check the voltage using your volt meter making sure to note which way gives you +4.5 or -4.5 volts (like in the previous steps example). So, if you put your red voltmeter lead on A and your black on F and saw -4.5 volts, then you want to want to see -4.5 coming out of your power supply and then whichever wire the red lead is on will go to point A.

STEP 4: Assemble

You could solder, or otherwise attach the power supply to the points determined in Step 2, or like I do in my example, I fit a plug to the power supply and a jack to the toy. You may consider if you want this process to be easily reversible if you would like to add batteries later. ***NOTE*** Don’t do this with batteries in the device, take them out.****

Solder and assemble and you are done, no more need for batteries!

Thanks for reading and happy making!
-instructodad

Comments

[dstuartlampen]

I have a different question but hoping you can answer. I have to replace a broken power supply for a bluetooth speaker/light that uses rechargeable batteries. The power supply I want to use is not the same. The picture attached has the battery along with both power supplies. The original
supply was the one with the 100-240v Input and 15v output. Is it safe to replace with the
100 v input and 12 v output. Also will the change affect my battery performance. They are brand new batteries. Thank you for any info.

[brentjair]

Hi, total newbie here. I have the opposite problem that I’m trying to find a workaround for. I have a small 4 channel sound mixer that uses a 12v power source. I was wondering if it was possible to use batteries to power it instead, and how to do that.

[Ibejoblo]

Hey thanks for the post. Im trying to power on a unihertz jelly android phone. The battery for the phone got lost and has 3.8 volt rating. I was going to build a AA battery back for it but I saw this post and wondered if I could just make this work for the phone. What do ubthink the only charger I have though has an output of 5v 2.4 amps. What do u think in should do?

[King Midas]

Ok I have a portable DVD player it requires an 9v-12, 1 a ac adapter if I want to o plug it in. Since it doesn't have any place to put reg batteries would it be possible to hook it to a small 300 CCA mower bat. So the kids can watch movies in the cloud b house with out having a o run a cord to the back back yard?

[NojiRatz]

You mentioned "Voltage does not kill, current kills" but if the supposed lethal voltage x current is applied to your body for a very short period of time, it might not do much harm at all. Therefore, time is the missing property, and what kills is voltage x current x time, which equals energy. You can prove this by looking at P = VI, and power is measured in watts, which is Joules (energy) per second. Watts times time equals energy. Therefore, energy is what kills.

[elstudsanchez]

Would adding a fuse make it safer? My Ring door bell can be either battery powered or hard wired to the house, But it came with a fuse if choosing the hard wired option. I want to do te same to my smart door lock. Thinking of adding a fuse to prevent over current situations? Any other safety features you would recommend?

[randalosgood]

How did you determine the current drawn by the toy? I expect 1.5A is enough but it might need more if it uses motors. Shouldn't that have been factored in?

[instructodad_]

Yes, absolutely, good point, if you draw too much current for the power supply it could fail/overheat/catch fire or some other undesirable thing. An ammeter placed in series with the batteries could give you some idea as to the current draw. (However, I tried that and only the motor turned on, no lights... 0.15 A) otherwise, if you have some experience with the battery powered device you are trying to switch to a power supply, consider AA batteries are typically about 3000mAh, so if it runs for 3 hours on new batteries, it's about 1 amp current draw. (my batteries were in series so it was still 3000mAh and 4.5volts). This particular toy runs overnight on batteries, so it draws well below 1 amp. Note that this approximation based on run time only works if your device runs at some constant electrical load.

[ZafarS8]

In your reply above you stated "series" but in the instructable you had stated "parallel," I believe by accident.
Might want to make a correction and change it to "series"?

[instructodad_]

Thanks ZafarS8 - I made the correction in 'Step 2' to say series.

[davis1413]

I have a basic understanding of house wiring, etc. My latest project is to try to replace 4 AA batteries powering a display on our exercise bike. The display goes through batteries very fast and by pedaling we have an alternative power source. I'm trying to connect a cheap DC motor to the flywheel and have the pedaler provide the energy to drive the motor and the motor in turn power the display. Any thoughts or advice would be appreciated.

[rohopo]

Hello. Thank you for this informative article. My concern is using a power supply that has too high of an amperage rating for the circuit. I don't know if that's even possible or an issue. The circuit consists of around a dozen tiny LEDs powered by two 3v button cells. From what I can tell there are in-line resistors mixed in with the LEDs. I am less concerned about ruining a power supply than I am ruining the LEDs.

[instructodad_]

Hi rohopo -
I'm not an expert, but...I don't believe it's an issue - you want the amperage rating on the power supply to be larger than what the device is drawing. Over-sized is safer.

Consider in a typical house with electric power there are outlets. If your outlet is rated for 15 Amps, you could plug in any device from a small 0.5 amp desk lamp to a 12 amp vacuum cleaner. Now, if you plug a very large piece of equipment into this outlet, one that draws 30 Amps (like a large space heater or clothes dryer) the 15 amp outlet will be overloaded and trip a breaker/fuse or burn up. You want the current draw of the load/device/LEDS to be lower than what the power supply can safely provide...which it sounds like you have.

[lightningstrikestwice]

Yes , I believe you only use the amps that the device is rated for. When in doubt use an inline fuse?

[lightningstrikestwice]

I have a question... I am replacing a 6 vdc ,4 d-cell battery supply to an appliance. I am using a dc buck booster to step down power from a 24 vdc solar battery bank. Can I go with a 5v 1.5 amp (buck) supply? or should I use a buck booster set to 6 vdc @ 3 amps ? Will this even work? I know d-cells are rated at 8000 to 12000 mAh so my concern is to fugure out the amp use of the 4 D-cell batteries!
I have successfully used a step down buck module to supply my router which uses 19 vdc @ 2.4 amps , The buck module is set for 19.3 vdc and can do 3 amps and it works wonderfully! Any thoughts/input or advice ?

[alfredmoses]

A productive thought

[ronanry]

I have never thought to use it this way, very clever !

[instructodad_]

Thank you, it certainly comes in handy sometimes!