[sheblindedme (OP)]

I'm a nubie to this forum so please forgive me if this question is in any way out of place.  I'm curious to know if there is a simple and compact method/device one might use to step-up DC voltage output? An example might be where one has an old but treasured 6-D Maglite flashlight which she would like to upgrade using an HID system (i.e. HID bulb with ballast circuit) she happens to have handy.  But whereas the battery output of the Maglite's 6 "D" cells puts out 9 volts or better, this particular HID system requires 12 volts or better to run.  (The alternative, modding the Maglite body -somehow splicing in an extension that would allow it to accomodate 2 more "D" cells- just not um... no!)

[wolfekeeper]

You could try fitting more small cells in series to make up to that voltage. You may lose overall capacity if you can't get as much battery packed in though.

Another thing is the so-called joule thief type circuit. That's a tiny step up switch mode power supply using little more than a small bead ferrite and a transistor. I don't know of any off-the-shelf design that would give you voltage you need though, but if you know somebody good at electronics, they might be able to knock you up one that does what you need. That way you could run the torch off a single cell or a few, although it wouldn't last as long.

[sheblindedme (OP)]

Thanks for the response.   I think I'll have to settle with the idea of removing a "D"  cells,  altering the dimensions of the battery compartment and try your suggestion of fitting in smaller cells to increase their number.   

[CliffordK]

Similar topics have been discussed on TNS in the past.

http://www.thenakedscientists.com/forum/index.php?topic=45893.0
http://www.thenakedscientists.com/forum/index.php?topic=16892.0
http://www.thenakedscientists.com/forum/index.php?topic=44559.0
http://www.thenakedscientists.com/forum/index.php?topic=44718.0

Anyway, what I would suggest is looking for (or making) a DC-DC Boost Converter. 

See if you can get one the size of a D cell.

Then pull one of your D Cells. (6 down to 5 cells, (7.5V)), then boost it up to your desired 12V.

You will have to rewire the connections somewhat.  Perhaps build a D-Cell shaped block to hold the rest of the batteries in place. 

Also consider the ease of loading and unloading the batteries.
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I have seen smaller mini-mag lights with battery cartridges, 3 or 4 AA (or was it AAA) batteries into the space of a C or D battery.  You could squeeze several of those into your mag light.

[wolfekeeper]

Thanks for the response.   I think I'll have to settle with the idea of removing a "D"  cells,  altering the dimensions of the battery compartment and try your suggestion of fitting in smaller cells to increase their number.   

Many D cells are actually only C cells manufactured inside an extender to make them physically fit anyway, so with care you may find you end up with a longer life flashlight!

[CliffordK]

So,
Could you fit 7 C cells in the space of 6 D cells?
Perhaps make a tube to hold them in place.  That should give you about 10.5V.  Is that enough?

[Lmnre]

Many D cells are actually only C cells manufactured inside an extender to make them physically fit

My experience does not support this claim, which also indicates consumer fraud. D cells are twice the volume and weight of C cells, and they perform accordingly.

The only circumstances that I know of batteries-within-batteries are some square lantern cells that consist of 4 F cells wired in series, and alkaline 9-volt cells that seem to consist of 6 AAAA cells wired in series. Regular 9-volt dry cells are not built this way, but consist of a stack of 6 squat 1.5-volt cells.

Also, most cells actually operate at less than the nominal 1.5 volt figure for most of their usable life. It's simply the nature of the science involved. The average operating value of 1.2 volts is more realistic. You might notice that most applications don't specify the actual voltage required, but simply designate the number and size of batteries required (2 D cells, 6 AA cells, etc). If you actually need 12 volts supplied, then you'll need 10 regular-sized cells (D, C, AA, etc).

[CliffordK]

What about using Lithium Batteries, with 3+ volts per cell?

http://www.ebay.com/itm/D-Size-Lithium-Cell-for-ALLEN-BRADLEY-1771-BL-/130327495369

Of course, one could quickly have a very expensive flashlight.

Take apart a power supply from a computer, and you'll find batteries that are more or less C sized.

[wolfekeeper]

Many D cells are actually only C cells manufactured inside an extender to make them physically fit

My experience does not support this claim, which also indicates consumer fraud.

Not sure. I think a lot of nicads used to be sold with a particular capacity, but the industrial versions were much bigger capacity, but correspondingly more expensive. The consumer versions were often smaller cells fitted in a plastic extender. Because they were sold with a printed Ah capacity, I doubt that this would be considered fraudulent.

It was more normally done with D cells because otherwise they would be particularly expensive.

[Lab Rat]

Could a step-up transformer work?

[wolfekeeper]

Not at DC frequencies, unless you run it through an inverter, and then rectify it afterwards.