Picture of Add an AC adapter to a battery-powered device
With a new baby, we are acquiring an astonishing number of battery-powered devices -- bouncy seats, swings, activity jumpers, mobiles, ... -- and burning through an even more astonishing number of batteries. Now I know why Costco sells those immense packages of AAs.

My wife asked me whether I could attach an AC adapter to our baby's mobile. I'd seen an article in MAKE about modifying noisy toys, so I knew it was possible. It turns out to be surprisingly easy, provided you (okay, I) don't make some dumb choices along the way.

NOTE The project I describe will void the warranty for whatever toy you modify. The manufacturer (rightly) will not support you or provide you with assistance in doing this. If it works, be happy: If You Can't Open It, You Don't Own It. If it doesn't work, take it as a lesson not to meddle in the affairs of Corporations, for they are Subtle and Quick to Anger.

Step 1: This Instructable is Incomplete

Picture of This Instructable is Incomplete
You'll notice that two of the Steps don't have useful images. I started writing this up after completing the project on one of our daughter's mobiles, and before starting the second. I definitely don't want to take the first one apart again just for some photos, and we've since decided not to do the AC adapter on the second mobile (the cord wouldn't be adequately secured).

Nevertheless, the descriptions in the Steps are accurate and complete, based on lessons learned the first time. I think this can still be useful to folks.
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Kelseymh, please educate... I have 2 battery powered LED light strings. 20 lights each string. The tag on each string reads: 4.5v DC (3, 1.5 AA's) .25W. I want to wire both together into one string so I can solder a transformer plug. Then buy the proper AC/DC transformer at radio shack or online to plug it in to house current. Now the questions ... If each string is 4.5v, and I want to connect them together, do I need a 9v, 100ma transformer? Do I worry about Ohms or wattage when crunching the math for the proper transformer? Any advice, tips or education you can provide would be appreciated!

hey, ok I'm in a lil bit of a Doosey.. Soo I have a toy that uses a 9.6 volt battery at 1600mah and I want to know what kind of power supply I would need to operate it ? Can you help me?

palach31 year ago

Hi there. I'm about to modifiy my baby's music mobile to use wall AC adapter to power it. It uses 4xAA alkaline batteries. The batteries are connected in series. My question, what is the best Voltage and mA/current adapter should i use? i believe the voltage should be 6V. But how am i going to calculate mA/current since there is no information on the housing? Any help much appreciated. Thank you.

kelseymh (author)  palach31 year ago

That sounds exactly like my own (no longer a) baby's mobile and Fisher-Price aquarium. Typically, battery-powered devices don't draw more than about 1A (above that, and the battery usually discharges very quickly, and does so at a lower than nominal voltage). I used an 800 mA adapter for my project.

NYDouc11 year ago

Hi, I need help with a Air Mattress powered with 4 D Batteries. I'm trying to use wall AC adapter to power it. I tried all kind of 6 volts with NO success. Which AC adapter do you recommend?

Thank you

kelseymh (author)  NYDouc11 year ago

You need to know how much current ("amps") the pump motor draws. If the motor is visible to you, there should be a data plate that shows the voltage and amperage required. Any adapter which is rated for _more_ than the motor amperage should work.

dbucher12 years ago
So if I were to convert a Hot Wheels Criss Cross Crash set that uses 4 D batteries, what adapter and parts should I buy?
Id like to know this as well!
4x D = 6volts.
Thanks for the quick reply!

Actually I have 3 led strips I want to convert, all using 3 AA batteries each.

The easy way would be to use three separate 4.5v adapters.
The ideal would be to connect them all together somehow and use only 1 adapter, maybe a 13.5v?

Still figuring out the latter. :)
Hi, I'm trying to do something very similar with a 3 aa battery led strip.

A question about the adapter you used. If your adapter, was rated 4.5v but 2A instead of 700mA. It would still work right?

Not very efficiently but the device would still only draw as much it needs?
kelseymh (author)  dmalamatenios1 year ago
Oh, yes. So long as you (a) have the right voltage, and (b) the current rating is higher than what the device needs, you'll be fine. If the current rating is too low, the wall wart will overheat. What you describe should be fine.
cliffypop2 years ago
Many thanks for the helpful reply! The batteries are in series as they're connected in a daisy-chain. Also, the housing has "12v" printed in large characters. I tried to find any info about calculating wattage from volts and Ah without success. I found an equation for watts=volts*amps, but that's not the same ampere-hours.
kelseymh (author)  cliffypop2 years ago
Right. You can't calculate watts from volts and coulombs (== ampere*second). Watts are units of power, which is energy used per unit time (watt = joule/s). (And, since W = V*A, you have V*A = J/s, or J = V*A*s = volts * Ah/3600). To convert energy to power you need to know how fast you're discharging the batteries.

When you used the 1A and 2A wall warts, did they get hot when you were trying to use the vacuum? If so, the vacuum was trying to draw more current than those adapters were rated, and you'll need a higher amperage adapter.
cliffypop2 years ago
I have a (no-longer-very) rechargeable vacuum I'd love to convert to a wall wart but have been failing with various adapters I've tried. The vacuum uses ten 1.2v (1300mAh) batteries. So that equals 12v total. I thought a 12v adapter at 1.5Ah would do it, but the unit ran very low and kind of pulsed off and on every second or so in a very about-to-die way. I tried a 12v, 2Ah adapter with similar results. The batteries run in series so I know I need 12v. Do I have to multiply the mAh rating by the number of batteries too? That would be 13Ah but that doesn't make sense since mAh represents capacity and not amount of power which is voltage. Any help is much appreciated!
kelseymh (author)  cliffypop2 years ago
You need to know whether the vacuum was set up with the batteries wired in series (which would be 12V), all in parallel (which would be just 1.2V), or some combination of parallel and series. If you've already opened up the unit, then you "should" (depending on how it was built) be able to see the outside of the battery holder and trace how the terminals are wired together.

Also, mAh represents the total charge stored (ampere = coulomb/second). If you multiply by voltage you get total energy (joule = ampere*volt). That doesn't provide any information on the way the batteries are wired, just their total output.
HappeeDad3 years ago
Alright, so I'm converting my Dremel to use AC.  The Dremel currently runs on rechargeable batteries that connect in the configuration shown in the attached picture.

The first set of 3 AA batteries connect to 1 and 2.  The second set of 3 AA batteries connect to 3 and 4.  I did a continuity test, and 1 and 4 are tied together and 2 and 3 are tied together.

So my question is, how do I wire this guy?  I tried wiring it with positive from the adapter socket connected to 2 (current feed), then tying 1 and 2 together, tying 3 and 4 together, and then having negative from the adapter socket connected to 3 (current return).

I then plugged in a 7.2V 1000 mA AC adapter, and the light on the goes off when I plug in the guy.

- Darin
kelseymh (author)  HappeeDad3 years ago
There must be a switch or something in that circuit somewhere -- otherwise you're just running current around and around, draining the batteries. Because of the way the two sets of batteries are chained in series, you need to provide that same chaining by hand, but without leading to a short circuit.

What other people have done in this case is to build "fake batteries" -- a wooden dowel with a metal contact at each end. Those contacts are wired to the AC adapter. For example, http://www.instructables.com/id/How-to-Make-AC-Powered-Batteries-for-your-DC-Pow/
Ok... so I jumped back on this project, and I started-out by re-checking the continuity between connections. Come to find out I was initially wrong. The attached diagram is the correct layout (which makes more sense with the batteries in serial).

So given the new diagram... anyone have any ideas?

And as I initially stated, the positive terminal from the adapter socket is connected to 2 (current feed), 2 and 1 are tied together, 4 and 3 are tied together, and the negative terminal from the adapter socket is connected to 3 (current return).

Should I try connecting the negative terminal of the socket adapter to 2 and positive terminal to 3? I just don't want to break anything.

- Darin
Ok... got this working. Wasn't thinking straight on this, I just needed to hook up the positive to 2 and the negative to 3... everything worked from there.

- Darin
Sorry, picture should be numbered as follows:
It should be noted that each set of rechargeable batteries was showing as approximately 3.6V. Which is why I'm using a 7.2V AC adapter.
HappyChef3 years ago
This is a great idea! Thanks!

Is it possible to do something like this for said battery-powered device but modify the parts & plan so that the device uses rechargeable batteries and the wall-wart charges them within the device? This would give you the flexibility of portability with rechargeable batteries as well as long-term use plugged in a wall outlet.
kelseymh (author)  HappyChef3 years ago
I thought about that, but the recharging circuit is non-trivial. If you have a device with a lot of empty space inside, you might be able to fit it in.
melw3653 years ago
I am trying to change 3 sets of branches that have led lights on them from battery powered to direct electric. Each set is powered by 3 AA batteries (total of 6 batteries). I would like to tie all the wires together and I believe attach to a transformer. Any advice .
melw365 melw3653 years ago
Yes, you are right. A total 9 batteries.
kelseymh (author)  melw3653 years ago
I think you mean a total of 9 AA's (3 each on 3 lines). Tie them together in parallel (connect the three + wires together, and connect that to the positive contact on the wall-wart plug, and similarly with the three - wires).
redtux7773 years ago
I have 4 AA batteries in series in golden tee golf game I have. The batteries last 3 to 4 hours.
I am trying to do the math on that. Does that mean I would need 6v .5A power cord?
kelseymh (author)  redtux7773 years ago
That sounds about right, based on typical AA capacities. Just to give yourself some headroom, you might consider a higher-current supply (say 6V @ .75A or 1A).

My apologies for the delayed response.
viky_vandna3 years ago
Hi I have drum player with 6 1.5v AA batteries thought of converting to AC adapter when opened found that it actually has three wire black and red at each end but found one more green at middle and it seems that it supplies 4.5 to one component and 9 / 4.5 v to speaker. So is there any way to split the power supply to these three wires???.
I checked that when I only place 3 batteries in slot 1,2 and 3 (please see the picture) I get lights on panels but no sound.
Drum Power Supply.png
kelseymh (author)  viky_vandna3 years ago
Blech! If you wanted to do that with a wall-wart, you'd have to build a small circuit to split the voltage. I think you could probably do it with just a pair of big (10 meg or more) resistors: the green wire would attach to the middle between the two resistors.
viv_virus065 years ago
i'm planning to run my remote controlled helicopter on ac adapter. i know that it use 9.0V but i don't know what mA should i buy. can you help me?
kelseymh (author)  viv_virus065 years ago
Do you mean you want to run the control box from an adapter? You obviously can't connect the helicopter itself to an AC adapter; it won't be able to fly :-) Did you read the manual? Usually toward the back there's a specifications sheet that should tell you the voltage and current usage.
i'm connecting the ac adapter to the remote, or the controls. it requires 6 1.5 volts batteries. the box only specify the voltage requirement and the mAh. i cant find the mA requirement. can i use any 9v adapter without taking note of the current usage requirement?
kelseymh (author)  viv_virus065 years ago
Hmmm....yeah, that's not enough information to solve the problem. In principle, you could measure the current draw with a multimeter and four-lead bridge, but it's not important. Choose an adapter with a high current rating. The remote will only draw as much current as it needs; as long as the adapter can supply at least that much current, it's fine.
thank you very much! you helped me a lot.
anyway, how can i set the polarity of the adapter to its correct setting?
jayhawk675 years ago
i have a fountain that requires 2 double a batteries what adpater should i use?
kelseymh (author)  jayhawk675 years ago
Each battery is 1.5V.  If they're connected in series, you'd need a 3V adapter.  If they're connected in parallel (unusual), you'd need a 1.5V adapter.
ty very much
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