12V from 9V battery or other easy alternative?

Hello, I need a 12V 0.2A(min) power supply for a project... I don't want to use any kind of strange or expensive battery because it's a gift for someone. I was planning on using a 9 volt battery, is there any easy way to build maybe a step-up transformer for it (needs to be very compact)? Or could I use AA batteries? I don't want to use 8 AAs though, I do know that's be 12 volts... Thanks, any help is appreciated. 

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If you want to use easy-to-find, primary (non-rechargable) batteries to power your thing, then I  recommend using 8 AAs.

The reason for this is that 9V batteries are kind of wimpy in terms of the current they can supply.  For your thing, a perfectly efficient DC-to-DC converter with an output of 12 V * 200 mA, would have power input at 9 V * 267 mA, and a fresh 9V battery can only supply that for... I dunno... maybe an hour.

BTW, the way you figure out how long the battery supply will last is by looking up the datasheet for the battery.  Then finding the capacity in A*hours, at the discharge rate you're expecting.  To do this I went here:
Then found this data sheet,
and looked at the bargraph titled "Milliampere-Hour Capacity"

Do the same thing for the datasheet for a AA battery
and you get a capacity of maybe... 1500 to 2000 mA*hour at your estimated discharge rate of 267mA.  Divide the rate into the capacity, and get
(1/267)*[1500 to 2000 mA] *hour = 5.61 to 7.49 hours.

So that's a battery life of roughly 6 hours, which to me sounds better than the estimate of maybe 1 hour from the little 9V battery plus power converter.

Not sure how much battery lifetime you want, but the examples above show how such estimates are calculated.
j.hawkins (author)  Jack A Lopez5 years ago
Who knows... The 9V alone might be enough to power it and I won't even need anything. I won't know till the mail man comes..
BradW75 j.hawkins9 months ago

So did the single 9V work?

I'm looking at running this light (http://www.attwoodmarine.com/store/product/3520) from a set of 8 AA batteries.

If I understand what I'm doing correctly, 2.4 watts at 12V should equal about 200 mA. So... Using Amazon AA NiMH batteries with a stated discharge of 2000MaH I should get roughly (1/200)*2000 = 10 hours of battery life?

I think the voltage per cell for NiMH is a little lower, like 1.2 V per cell versus 1.5 volts per cell for alkaline batteries.  So you would need a series stack of about 10 NiMH cells to get 10*1.2 = 12 volts, versus 8*1.5=12  for alkaline cells.

Your calculations for current and battery life look believable to me. 

I am guessing you want to make that light part of Halloween costume, or something like that, some application for which you want a battery that is not to bulky and heavy.

I mean, if you were planning on using it on your boat, it might just be easier to go with a 12V lead acid battery.

My kids wants to be a ski boat for Halloween.

I might use this in our canoe when I'm done. No battery on board.
j.hawkins (author)  Jack A Lopez5 years ago
Wow thanks for the in depth answer!! Well see the thing is the lifetime isn't really a problem for me.. All the battery will be doing is powering an electromagnet to unlock a lid on a nice stained wood box when I bring a magnet across the security window switch (I guess that's what to call it..) on the back. Is there an easy way to get away with fewer AA batteries maybe? Ian's still have better capacity but a good life? Like I sai it'll run for maybe 2-3 seconds at the very most at a time probly no more than 1-2 times a day if that even...
Hey thanks for the details. That helps. 

A quick and dirty way to get more voltage out of a 9V battery is to just use two of them in series for 18V.  I would not worry too much about the extra voltage hurting the solenoid, and since you only turn the thing on for a second or two.

Another idea I had was that of a small motor, geared down, and sort of pulling on a string, like a winch, and the string pulls on the latch bolt you're trying to move. Maybe there is a spring holding it in place.  I know you probably had your heart set on using a solenoid to move the bolt, and you should try building with the solenoid first since it looks that's what you've got the parts for.

I mean there is some amount of mechanical work to perform, for this task of moving the latch bolt.  There's probably work compressing a spring, and work against friction.

Using a solenoid, you're trying to sort of do all that work in one little burst, or impact.  It might be that trying to do things that way is making the electrical part more complicated.  I mean there are circuits you can build to sort of store up a bunch of electrical energy, and then release it all at once.  For example camera flash circuits do this. Using just one or two AA batteries, they can charge a capacitor up to like 300V, with the actual stored energy around 5 J.  I don't know if you have any broken cameras-with-a-flash in your junk box, but that might be something fun to play with.  A camera-flash capacitor charged to its full 300V or so, would be capable of hitting your solenoid pretty hard, maybe too hard.

Anyway, I guess what I was saying is that there might be a trade-off between mechanical complexity and electrical complexity.   

A geared down motor driving a winch is mechanically complex, but simple to drive electrically, maybe just 2 AAs and a switch. 

In contrast the solenoid is mechanically simple, but electrically complicated.  It seems to want big voltage and current delivered in a short burst, and that might be leading to stacking battery cells to get more voltage, or complicated boost circuits, etc.

Anyway, I'm sure you'll figure something out.

Oops... I did something dumb there.  The estimated discharge rate should only be 200 mA, because there's no power converter involved.  The lifetime estimate should be a little longer:
(1/200)*[ approx. 2000 mA] *hour = 10 hours

Of course, I keep using the word "estimate" here, because this is guesswork.  For your prototype there, you should actually hook it up to 8 AAs, and test how long it runs.

The other thing I was going to mention, is that if you really did want to go with a DC-to-DC boost converter, one thing about those is that they can be made to work over a wide range of input voltage, so that they can really squeeze the the last few joules out of a battery, a la joule thief.  But of the power converter adds expense and complexity.  Building it might be above your skill level, and as for buying such a boost converter, it might be difficult to find. But if there was anyone out there who would have it, it might be these guys:
They really know their stuff.
iceng5 years ago
this should do it  .  .  .  A
j.hawkins (author)  iceng5 years ago
I'm sorry.. I don't completely understand the schematic. I'm kinda a noob :P. but thanks for the quick reply!
frollard5 years ago
My default answer as always is that 9v batteries are terrible and should never be used unless avoidable.

Good on ya for considering AA or AAA -- what exactly is your circuit? Is there a way to make it function on less voltage?

12v 200ma is 2.4 watts. A 9v battery short circuited would have a hard time putting out that amount of power. 3 AAA batteries can source about a watt...

a boost converter can be had for cheap (10-20 dollars) that is 90+% efficient at getting say, 4AA 6v up to 12 volt, but again it's moot to consider if the circuit can be redesigned for less voltage.