12v from a 9v

[Tsukai]

I'd like to power a 12v portable device using common batteries in a small package. The device only draws 500ma at most. I'm thinking 9v cells would be good enough for an hour or two.

I figure two things I could do:
-Put two 9v cells in series and use a 12v voltage regulator.
-Put one 9v cell on a 5v regulator and chain that to a 12v DC-DC converter.

I'm mainly curious about the second one. The DC-DC 12v up-converters I found on digikey had 5.5v max input voltage so a 9v cell > 5v reg > 12v converter seemed reasonable. I'm wondering what someone more experienced thinks of this.

 

[chopsuwe]

Solution 1 would work but won't last long. I think 1-2 hours is all you would get out a pair of those of batteries.

Solution 2. Running it from 9 down to 5 up to 12 is kind of wastefull! The converter you found on digikey will probably be happy running on 4.5V which is three AA batteries in series, have a look at the datashet for it ( google: "$partnumber datasheet"). I would recommed this method as it is the most efficient and good learning experience, even if these converters do need a but of messing with sometimes.

 

[gee]

9V->5V->12V would be extremely inefficient - you're losing almost half your efficiency in the 9V->5V conversion, and the 5V->12V converter might only be 80-90%.

Check the specifications on those DC/DC converters - you might find one with a wide enough input range to accept 9V. If you're using rechargeables, you'll want to make sure the DC/DC can handle up to 12 volts in in order to handle any top charge on the battery.

 

[Tsukai]

thanks, I'll recheck but probably go with my first idea.

 

[mikeblas]

An alkaline 9V battery is going to be good for only 600 or 700 mAh, so I don't think you're going to get the endurance you require. Especially after you factor in the ineffency of your voltage-matching solution.

See this PDF for a comparison.

You can get some custom rechargables that would do the job. You'd need 8 or 9 regular AA-size nicads to get to 12 volts, and that will be quite bulky.

.B ekiM

 

[theshadow27]

You can get camera batteries just about anywhere. They sell 12v ones in most stores, and you could probably fit 6 in parallel in the space that two 9v’s take up. Another option is 3v camera batteries.

Without knowing what you’re trying to do (i.e. what device you are running) its hard to determine what type of batteries to use. Many things designed for computers/car (12v system) can actually run off much less with just a little modification.

Batteries are unquestionably the most inefficient aspect of modern electronic technology, and therefore they require the most attention in design.

 

[mikeblas]

They sell 12v ones in most stores, and you could probably fit 6 in parallel in the space that two 9v’s take up.

I've never seen such a battery. Can you provide a part number or NEMA code?

.B ekiM

 

[joecool234]

I've never seen such a battery. Can you provide a part number or NEMA code?

.B ekiM

AA...1.5 volts each. 4 will give you the voltage necessary to power the DC-DC converter.

 

[mikeblas]

AA...1.5 volts each.

I'm confused. In your previous post, you said "they sell 12v ones in stores". You were first talking about twelve-volt batteries, and now you're talking about 1.5-volt batteries.

.B ekiM

 

[theshadow27]

I'm confused. In your previous post, you said "they sell 12v ones in stores". You were first talking about twelve-volt batteries, and now you're talking about 1.5-volt batteries.

.B ekiM

no it was in MY post that i was talking about 12v batts. comon now

http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_009_014_002_000&Page=1

rad shack:

Cat # 23-154
Cat # 23-144
Cat # 23-279

and the 6v batteries. note that they are designed for cameras and thus are inherently low current:

more rad shack
Cat # 23-266
Cat # 960-1190
Cat # 23-471 <-square alkaline
Cat # 23-469

just so you dont think im pulling this outta my ass, here is a 6v batt at cvs (7K67B)

http://www.cvs.com/CVSApp/cvs/gateway/detail?prodid=363432&previousURI=/CVSApp/cvs/gateway/search?page=2^Query=battery^ActiveCat=65^startLink=1

glad to enlighten you

 

[mikeblas]

Thanks for the specifics.

RS #23-154 and #23-144 are NEDA 1811A. That battery only offers 40 mAh, which means he gets a runtime of less than five minutes if he's drawing 500mA.

.B ekiM

 

[theshadow27]

RS #23-154 and #23-144 are NEDA 1811A. That battery only offers 40 mAh

hmm...

...you could probably fit 6 in parallel in the space that two 9v’s take up...

 

[mikeblas]

With six in parallel, you get 240 mAh, assuming perfect efficiency. That boosts your runtime to just less than 30 minutes at the 500 mA drain Tsukai specified, and doesn't meet his "hour or two" goal.

I think nicads are an interesting alternative. Lithium 9v batteries (like Energizer's) offer interesting numbers, but I've never used them in practice. Depending on what "portable" and "small" mean, a lantern battery might be applicable; many can source 10 Ah or more, and that will let the device run for up to 20 hours!

.B ekiM

 

[Othersider]

http://www.allelectronics.com/cgi-bin/category.cgi?category=160&item=BAT-12&type=store

Or just stack 4 x 3V coin cells like CR2025.

Of course I'd be lazy and just use 8 x AAA unless there's a really good reason for compactness.

 

[Tsukai]

The device is just a 4" lcd and a receiver for a cheap wireless video camera.

I don't really have a good reason for compactness other than maybe then I'd use it more. I was thinking using 9v because I'd only need 2 batteries, and secondly they can be bought anywhere. I've found that in the past I was deterred from using various devices I have simply because I have to charge or take care of a relatively uncommon battery, like 12v gel cell, or making sure a bunch of nimh AA are charged, etc. Being lazy basically.

Using just one or two cells which are very common just seems easier.

 

[mikeblas]

http://www.allelectronics.com/cgi-bin/category.cgi?category=160&item=BAT-12&type=store

The Panasonic L1028 battery is just another 1811A battery. These are good for about 40 mAh. Taking 500 mAh from this battery (assuming it can even source that much!) means the battery will last less than five minutes.

Or just stack 4 x 3V coin cells like CR2025.

These aren't much better. The CR2025 is intended for low-power applications. 500 mA at 12 volts isn't "low power". (Maybe it was in, say, 1975 or so.)

A CR2025 sources about 170 mAh. You've got a run time of 20 minutes.

Of course I'd be lazy and just use 8 x AAA unless there's a really good reason for compactness.

Now, you're getting somwhere! You should expect a AAA alkaline battery to give more than 1200 mAh. That leaves you with a runtime of nearly 2.5 hours.

There are latern batteries that would deliver 50 to 100 hours of runtime, but weigh more than a pound. Maybe a DC-to-DC converter is the wrong way to go; perhaps it's more appropriate to modify the circut to run directly from the battery power instead of wasting a few dozen percent of the battery's capacity in the inefficiency of the chopper.

Powering an unknown device with a battery that hast to be smaller than an unknown size is tough guessing game.

.B ekiM

 

[Othersider]

The device is just a 4" lcd and a receiver for a cheap wireless video camera.

If it's the kind of receiver I think it is, 8 AAAs should fit very neatly under it.