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

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.

Comments

[cliffypop]

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]

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.

[cliffypop]

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]

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.

[dbucher1]

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?

[dwerg85]

4x D = 6volts.

[HappeeDad]

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.


Thanks,
- Darin

[kelseymh]

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/

[HappeeDad]

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.


Thanks,
- Darin

[HappeeDad]

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

[HappeeDad]

Sorry, picture should be numbered as follows:

[HappeeDad]

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.

[HappyChef]

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]

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.

[melw365]

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 .
Thanks

[melw365]

Yes, you are right. A total 9 batteries.
Sorry
Mel

[kelseymh]

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).

[redtux777]

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]

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_vandna]

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.

[kelseymh]

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_virus06]

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]

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.

[viv_virus06]

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]

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.

[viv_virus06]

thank you very much! you helped me a lot.

[viv_virus06]

anyway, how can i set the polarity of the adapter to its correct setting?

[jayhawk67]

i have a fountain that requires 2 double a batteries what adpater should i use?

[kelseymh]

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.

[jayhawk67]

ty very much

[Evil6]

I've done roughly the same thing with ac adapters from a thrift store and dowels cut the length of the batteries used. Using a screw to hold the wire to the dowel and make the connection to the two terminals you don't void the warranty, just route the wires under the battery plate. On my second child and many batteries later you tend to get desperate! Great Ible BTW!

[xrobevansx]

CONGRATS ON THE BABY!! By the way look at my instructable or kill your friends. Your choice.

http://www.instructables.com/id/A-More-%22Baby-Friendly%22-Doorbell/

PS- great instructable! I am going to give it a go on a few things.

[kelseymh]

Thanks :-) Ironically, we haven't had your doorbell problems -- ours is already "broken" and we just get a dull "dong-k" rather than a proper chime. Perhaps the previous tenants already implemented your hack ... :-/. As for my Instructable, note that the components cost about $35 bucks. I tried finding cheaper sources than RS, but the single-voltage adapter was a dead end :-(

[Goodhart]

Ah, a doorbell like the one at the Huckstables' house LOL

[Goodhart]

VERY well put together.

This is a very good ible for any toy that is not actually touched or played with by the baby. I personally get a bit skiddish when having AC main current anywhere near a baby if there is any possibility of contact through fault or ground fault.

Of course, a simple ground fault circuit in the wall plate might be overkill, but I would probably do it if I were doing this....but that is just my paranoia showing, maybe :-)

[kelseymh]

Not paranoia at all! As I wrote in my very first I'ble, "babies have a terrific knack to find whatever cliff is around them to fall off!"

For that reason, I haven't installed an adapter on the other mobile (the one on the side of my TV chair, featured in [Silence of The Toys]). This one is on her crib, and I've tied off the adapter cord at four different places in such a way that the plug cannot actually be pulled out of the jack.

[Goodhart]

It is amazing how fast they learn to climb, sometimes to rather scary heights too :-) I know how much I got into when I became just a little older and could hold a screwdriver and almost remove a plug from the socket. The experience had me fearful of electricity even today (funny that I picked electronics as a hobby though LOL ) . I'd love to be as skilled with hardware as I am with a p/c board and solder :-)
Oh, and sorry for cluttering up you comment section with my ramblings.

[mman1506]

when i was little i tried to stick a fork in a socket to see if it was working (before i knew that it has more volt tthan a 9v batterry

[Goodhart]

THAT could have ended up worse then it did. Thankfully you did not cross between both contacts. The shorting out of 110 vAC can melt through screwdriver handles (blush). Don't try this at home....

[mman1506]

when i was about 6 i was playing with an old outlet timer in the garage and touched a contact (no i didnt take it apart) and blzzzz my arm got a shock,it kinda feels like an overly strong massager when it goes through the musle,my arm was sore for a few days later and i think possibly it may contrubuit to my right arm not being able to write very well