Reducing 18 volt DC to 12 volt DC

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They will not only be too bright, they will only live too short. Guess you would also do with 1.5 times overload. You'll have to bring back the voltage to the 12V required by the bulbs or use 18V bulbs. Guess you will have 18V bulbs when you have 18V flashlight. Bringing down the voltage is not that difficult but the easy way spills a lot of power. That's either a series resistor/bulb or a 12V lineair regulator. Both will produce a lot of heat, amount depending on the wattage of the bulbs. The best way is a switching regulator but I doubt a newbie in electronics can handle such a circuit. You can nevertheless google on it. Lineair has a lot of examples, Micrel is another. There are much more. If you like experimenting try the "2 transistor Black regulator."

petrus bitbyter

Use an auto battery to run the lights and charge it off the 18 V supply.

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Thought of using 12 volt car battery. Any better way to charge? From my truck? Cigarette lighter?

Cheapest is a resistor from the supply, but ideally you would use a proper charging circuit. This will prevent overcharging and drying out the battery.

Have you tried a truck supply store? Someone must have done this before.

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is possible. Give them a call and see what they have.

Am I correct in assuming that what you are trying to do is to run 12V lamps (4) from the fully charged (spare) Dewalt 18V power tool batteries?

If I have this correct then you must use some form of voltage step-down. Either a linear or switch mode PSU which accepts 18V dc input and ouputs 12V dc at a suitable current determined by the total wattage of lamps you are trying to drive.

Other than that, as long as your truck has a 12V battery which is charged by the vehicle alternator, why not wire in a separate feed (with fuse) just to run the lamps? That is simpler and cheaper than any other solution.

While you could run the lights off 18V batteries, I think that would prove to be unsaticfactory. You'd be better off to use the xisting 12V auto system. I think you have a couple of issues to address, so here are some things to consider:

1) Wire it up to turn the trailer lights on by a separate switch that does not require the parking lights to be on. 2) Include a timer in the separate switch circuit so that the lights will turn off automatically after X amount of time.

Depending on how you use the cargo trailer, that may be all you need to do. You can build a simple circuit to turn the lights off say 2 minutes after you turn them on, and you can make the timing adjustable. I'm envisioning going into the trailer to get something periodically, but with a long time between entries to the trailer. So with the lights off the vast majority of the time, you shouldn't run the battery down.

Ed

If you put pairs of lights in series, across the 18 volts, you'd get 9 volts per lamp, which might be bright enough... try it. The lower current will extend battery life, about double what you're seeing now.

If you put two batteries in series, that's 36 volts, which is right to power *three* 12 volt bulbs wired in series.

But little batteries like this won't last long with a heap of lights attached. Why not use a car battery?

John

Just to add another thought on this subject, you may also run into an issue with the type of battery chemistry versus the loading. Not all batteries are created equal with regards to their discharge patterns and the type of load they are designed to work with. Some batteries are capable of providing a constant discharge, such as alkaline, where as others are better at short bursts, such as lithium (think digital camera. If you are using the wrong type of battery for an application, you will likely find that it doesn't perform as well or as long as you would expect for its amp-hour rating.

add another thought on this subject, you may also run into an

no resistor but you're turning the same amount of energy into heat inside the diodes,

It'd be better to use three 6V lamps (try a motorcycle spares place)

Or maybe a PWM circuit based on a 555 and a mosfet.

but really a 12V lead-acid battery is the best solution as they're cheaper than those tool powerpacks.

Bye. Jasen

Don't use a CAR battery! Use a golfcart, marine, or RV battery - the deep-cycle kind. Car batteries are designed to be on a full float charge while the motor's running, and provide 600 amps for as long as it takes to start the car, and that's it. If you draw a light load for a long time, the cells deteriorate.

A deep-cycle (marine, RV, golf cart, etc) battery is almost the opposite. You start with it fully charged, and it can supply a moderate load seemingly forever, until it reaches "end of charge", and needs to be recharged again - they'll do this happily for years, but don't try to start a car engine with one. :-)

Get a switch or plug; when you park, disconnect the trailer lights from the truck and use the RV battery in the trailer. When you're done, start the truck, and plug in the trailer battery to charge alongside the truck battery - when you get back, the trailer battery will be recharged.

Or, you could go to an RV place and get a battery isolator, so that that would be fully automatic.

Have Fun! Rich