[subego]

I need 13-18vdc.

The seemingly simplest way to do this is gang three 6v lantern batteries in a series. The problem is that's going to give me about 19.5-20v to start with.

How do I drop it to 18v? A resistor? What size? I've got a Radio Shack across the street, so I should have most basic circuit parts available.

I'd normally figure this out myself, but I'm hugely pressed for time and would appreciate the help.

Thanks!

[subego]

Just talked to the mfr., and he says it's rated for 25v, he just didn't want to list that, or people would try and put in fully charged 24v batteries, which could be as high as 28v and would blow the circuit.

Seems like a false alarm... Thanks anyway!

[chabig]

OK. You got me. What requires 13-18 VDC? Is it going to get regulated to +12?

[bstone]

V=IR

[subego]

Originally Posted by chabig 

OK. You got me. What requires 13-18 VDC? Is it going to get regulated to +12?

Yup.

It's a cage for a HDSLR. It powers the camera (6v) and accessories (12v). Much nicer than trying to run a mess of separate battery packs, however my big 12v batteries can't feed it enough voltage. It really wants at least 14v

[subego]

Also, it's taking the 20v no problem.

[Railroader]

Batteries for RC cars/trucks are available in 4S LiPo configurations and output 14.4 volts (16.8v fully charged) and they are rechargeable. Go to Tower Hobbies - Best Source for Radio Control ( R/C or RC ) Cars, Trucks, Airplanes, Boats and Helicopters or RC Batteries, Traxxas lipo battery, Hyperion lipo charger, E-Revo, Rustler VXL, Slash Lipo, E-Maxx, NIMH rc battery search for 4s battery.

[ghporter]

A ready-made regulator is a much better choice than rigging one yourself. The first choice most people make is to use a "dropping resistor," which, while workable, is wasteful and can cause some rather spectacular failures. A modern regulator, on the other hand, will manage the voltage and current appropriately while not generating excessive heat.

Further, a rechargeable battery system, such as what Railroader points out, is superior to using disposable batteries for several reasons, not the least of which is reliability. Alkaline batteries tend to "ramp down" their voltage as they are exhausted, with a final "usable" voltage often being near 50% of rated voltage. Heavy current draws on alkaline batteries generate significant amounts of heat from the electrochemical reaction within the battery, causing a potentially hazardous situation where the battery could burst or even explode with significantly dangerous consequences. Rechargeable batteries, especially modern chemistry units, are built to avoid these problems, to provide reliable and consistent voltage and current levels, and are often significantly lighter and smaller than traditional alkaline batteries.

Figuring out that you can do what you need with an arbitrary DC source is cool. If you plan to continue doing this stuff, go with something that's going to be more reliable and safer, and that will save you some money along the way.

[subego]

This was a super-emergency type situation. I'd never want to do something like that long term.

As it happened, I got the lantern battery rig to work, and my setup drained it to uselessness in about 5 minutes.

What I ended up doing was ganging my 12v rechargeables, but I drained them to 12.2v beforehand.

That ended up sort of working. I had plenty of power, (after a 6 hour shoot, I was at about 22v), but my monitor kept on rebooting every 30 seconds or so. Half the time I'd end up framing the shot on the video assist. I'm assuming that was because the voltage was too high, but who knows?

Of course, it wasn't happening when I tested it yesterday.

[subego]

Originally Posted by Railroader 

Batteries for RC cars/trucks are available in 4S LiPo configurations and output 14.4 volts (16.8v fully charged) and they are rechargeable. Go to Tower Hobbies - Best Source for Radio Control ( R/C or RC ) Cars, Trucks, Airplanes, Boats and Helicopters or RC Batteries, Traxxas lipo battery, Hyperion lipo charger, E-Revo, Rustler VXL, Slash Lipo, E-Maxx, NIMH rc battery search for 4s battery.

I'll definitely look into that. Thanks!

I'm hoping I can find a lead acid solution though. I'm really tough on my batteries, and the lead acid ones seem to shrug that off the best. Not to mention I've already got a bunch.

I've got at least a week now to come up with alternatives.

One thing I was thinking of was a step-up transformer. Anyone have any experience? These are 18Ah batteries, I've got three of them, and I'm running only 20-30 watts. I've got juice to spare, so I can lose some efficiency without too much problem.

[ghporter]

DC does not "step up" the way AC does. On the other hand, there are DC to DC converters that will do some good stuff for you. They're kind of specialized, but a converter that takes a nominal 12VDC input a la automotive battery and puts out 20VDC should be findable. You need to know the maximum and avereage (a SWAG at least) current draw for your application. The rest is up to you and Google.

[subego]

Good deal! Thank you! When I did a quick search yesterday for a step-up transformer, it seemed pretty dead end. It sounds like I didn't have the right search terms, and now I do.

I'm happy I have time now to do some proper research. I generally prefer that anyway, but was way too up against it to let my preferences rule the day.

[subego]

Like this, say...

75 Watt Automobile 12 volt input DC/DC converter with high reliability and high efficiency

Thanks again Glenn!

[subego]

Originally Posted by subego 

I'm hoping I can find a lead acid solution though. I'm really tough on my batteries, and the lead acid ones seem to shrug that off the best.

Except when my "smart charger" has a critical intelligence failure.



Hmmm... why is it smelling like sulfur in my apartment?