How to get 12v from 48v battery to run accessories?

[Mark42]

Is there a way to get 12v from the 48V battery to run accessories without using a golf cart converter? Mainly its 12v LED light bulbs (automotive style) that I want to power and not use a separate battery. I know this can be done with the proper resistor, but I have no idea how to determine what the value should be. Want to run one or two 1156 multi led bulb tail light bulbs. Each bulb pulls less than 50ma and runs on 9 to 14 volts.

Any help is greatly appreciated.

 

[Ypedal]

How much wattage ( amps ) do you need the 12v supply to be ?

DC to DC converter is what you need but it has to be capable of the power output .

 

[Mark42]

I just updated the post. The bulbs are 50ma each and 9 to 14 volts. How do I calculate the proper resistor?

The dc to dc converters waste power and from what I read still leach power even when there is no load. Would like to avoid them. A few years back I did this to put led indicators in a car dash panel. Wish I remembered how I did that. But it was done with resistors to get low volt led's to work on 12 volts.

 

[Mark42]

Ok, I found what I needed. Everything is available on the internet! LOL! Found a resistor calculator. For a 48v source to be dropped to 12 volts and supply 50ma, a 720 ohm 1.8 watt resistor is needed. A lot more efficient that using a golf card voltage converter.

This is the calculator I used:

http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

 

[ax57ax57]

Although a resistor might be more efficient that an over-sized golf cart converter, it certainly won't be more efficient than a small DC to DC converter. That resistor is going to drop 3X the voltage that your led bulbs will use, and waste that energy as heat. A small, low-current switch is all that you need to keep a DC-DC converter from wasting any power when you're not using lights, and even if you did leave it on all the time the idle current that they consume is very small.

 

[Mark42]

Ok, its coming back to me now....

So the calculator says 12 v @ 50ma needs a 720 ohm 1.8 watt resistor. But that is not a common size. A common size is a 680 ohm 5 watt resistor. Dropping to 680 ohms raises the voltage to 14 v, which is still within the LED bulbs spec, and raising the wattage to 5 watts gives the resistor much better protection against heat failure. The resistors are +- 5%. A box of 10 axial cement resistors in this size is about $5.

Bottom line is a 50 ma draw (0.05 amp) is not going to make much heat at all and this is efficient compared to converters, especially when only one or two bulbs are needed.

Thanks for letting me talk this out! LOL!

 

[bowlofsalad]

The dc to dc converters waste power and from what I read still leach power even when there is no load.

Nothing is going to be 100% efficient. By all means, achieve the highest degree of efficiency that pleases you, but if you are worried about leaching power you can install a switch (I assume you would either way) before the converter and make that statement irrelevant.

http://www.ebay.com/itm/DC-24V-36V-48V-60V-20V-60V-to-12V-5A-DC-DC-Converter-Step-Down-Buck-Module-Car-/251382888623? Lots of options for converters and switches.

 

[Ypedal]

I have no clue how precise those resistors will be but just throwing this out there as a thought.. what happens with the voltage swing from a fully charged 48v battery to near LVC ?

 

[teklektik]

The dc to dc converters waste power and from what I read still leach power even when there is no load.

If you mean they are not 100% efficient, this is true.
Other than that - your technical references are overstating the case for common switching converters. What are the links to these sources?

Dropping to 680 ohms raises the voltage to 14 v, which is still within the LED bulbs spec, and raising the wattage to 5 watts gives the resistor much better protection against heat failure. The resistors are +- 5%. A box of 10 axial cement resistors in this size is about $5.

Bottom line is a 50 ma draw (0.05 amp) is not going to make much heat at all and this is efficient compared to converters, ...

My goodness...

Let's take a look at your design:

• 48V-14V / 0.68K = 50ma
  50ma x 48V = 2400mw total dissipation
  50ma x 14V = 700mw dissipated in LEDs

Your 'efficient' design consumes 2400mw and yields a whooping 700mw of usable power:

• 700mw / 2400mw = 29% efficiency

BTW - dumping 1.7W of waste heat with limited surface area will cause the resistors to become very hot - be sure to mount them away from wiring, plastic, etc.

Or - buy a DC/DC converter with efficiency in the 80s or 90s for $10-$15 - like this one from Lyen that will develop only a small fraction of the waste heat of the resistor approach.

(And frankly - who cares about efficiency for 50ma compared to the draw of the motor?)

 

[Mark42]

Hey! Thats a nice little converter. Thanks for posting, because I didn't know what to search for. Now I see that there are many small converters that handle 50v or 40v to 12v out there! Most are less than 2" square and only cost $5. Ebay has lots of them. I'm going to do some browsing and find one that meets my needs, which is basically a 50ma headlight and two 20ma taillights so its less than 1 amp. Even those little 3 amp converters should run fairly cool. And its really only for night time driving so it won't see much use.

Thanks for your input! I appreciate you steering me in the right direction! You've been a big help!

 

[friendly1uk]

A pair of 24v lamps in series is 48v and I guess the 24v lamp will take close to 30v without issue. However, It might just be a 12v lamp with a resistor inside. A 24v lamp would likely have at least 6 leds, or multiples of 6 as they are generally 3.7v each. 3.7v x 6 = 22.2v + a resistor to take the extra voltage away.

Your headlight won't be a headlight you realise. 0.6w is barely a marker lamp. I have a 5w rear and 30w front. You likely want at least 3w as a headlamp, and an entire unit would be like $10 and take pack voltage.

 

[Mark42]

The led headlight bulb has 15 leds and is rated at 70 lumens. I know its not much, but it will let oncoming cars see me.

 

[bowlofsalad]

It probably sounds counter productive, but it might make things a bit easier and more ideal to get new lights. Not only do many lights include appropriately sized converters built in that will accept a wide range of voltage, but there are lights that will give you much better(brighter) all around visibility. I assume since you are asking in an electric bicycle section of the forum that you intend to travel at 20-30mph, I'd say at least 10w of total forward illumination is a good minimum for todays LEDs.

http://www.dx.com/p/exled-electric-cars-motorcycle-cree-xm-l-u2-led-headlights-modification-lens-strong-light-302629

http://www.ebay.com/itm/Pair-LED-CREE-Day-Spot-Light-Motorcycle-Car-Truck-Bicycles-Boat-Off-Road-12V-24V-/251550046867?

Pros and cons to every route and option, but it's nice to have lots of options.

 

[Mark42]

..... I assume since you are asking in an electric bicycle section of the forum that you intend to travel at 20-30mph, I'd say at least 10w of total forward illumination is a good minimum for todays LEDs.....

Actually, I am a slow poke. Strava and Map My Ride apps show that I typically travel at 9 to 10 mph on 14-20 mile cruise on my road bike, and top out at 18 mph on long downhill runs (which scares the heck out of me). Been riding for many years, and I am happy to go 10-12 mph. I have a popular 3 led light on my road bike that uses 3 AAA batteries and that is plenty of light for the occasional night time rides. I am more interested in distance than speed. And from what I have read on these forums, slower = longer distance, faster = shorter distance.

Thanks for the links to those lights, will definitely check them out.

I found a few DC/DC converters that take 63 volts max input and will output 12V at 3 Amps and they are only $5 or less from China. Many on ebay, aliexpress, amazon, etc. Some are only $3 shipping included. Some even come in their own extruded aluminum case. These are all less than 2x2x2". Thought I would order a few different ones and see what works, what lasts, etc. They are cheap enough to play around with.

Last time I worked with LED lights was in the late 80's when I put them in the dash of 40 Ford hot rod I was building. There was no internet then, and only radioshack. I had to go with what I had available at the time, and that was resisters on the LED's to drop 12 v to 1.5v for the indicator leds.

Thanks for your help.

 

[Mark42]

A few little things came from China today..... This one seems to be the best of the ones I ordered....

http://www.ebay.com/itm/321471215003?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Have been running the headlight (9 leds) and two tail lights (one led each light) and although things are feeling not so hot this circuit seems to be working just fine. Although you have to admit that with a huge heat sink, you know its going to get warm... you have been warned.... LOL!!!!

 

[Mark42]

I'm testing 3 different dc-dc converters. The one listed above is currently in use. The led bulb at the top of the thread (42 mA) throws plenty of light for low speed night riding in the classic chrome bullet housing. Also used 1 BA9 LED in each tail light (19 mA each). Total is 80 mA, far less than the 3 A output of the dc converter. Probably why the converter runs so cool, even though its listed as 50v max input. I'll run it till it fails then switch to another. The other two converters can handle up to 55 v for one and 60 v for the other. The one in use now has the beefiest heat sink.
Will keep you posted.

 

[Mark42]

Here is how the dc-dc converter fit in the side bags. A bit tight, but it fits just fine. This one is rated up to 50 volts. But right now its getting about 56 volts and it still puts out the required 12.4 Volts. Notice all screws/hardware are marine grade stainless. I'm a boat guy. I hate rust.

I am such a deutchbag..Should have arranged the hardware so its nice and neet... Why make things ugly??? LOL??????



And I added this switch so the power can be turned on to the auxiliary lighting. All I have to do is reach back and touch the bag housing and find the switch and then I know the power is on. Oh well.... Rather low tech. The switch is 100 % plastic, so there is nothing to rust if its left outside, unless water gets inside. So I packed the ball joint with marine grease. Now have to find a soft black putty to seal where the wires pass through the side bags.



And that red plastic piece is there to hide a very ugly looking hinge casting. China.... go figure.

 

[Mark42]

Wow! things are working well. The cheap piece of gunk from China is working very well, even though the battery is overriding its voltage . LOL!!!!
The headlight and tail lights are all on and running fine. I now have about 40 hours on the dc-dc converter and it runs very cool. What can I say? This stuff works as advertised.

BTW, the stock converter does get hat after a long ride. About 4 hours. Big aluminum heat fins should help. I'm thinking of cutting some big fins from the sheet metal I have. Maybe 1/8" would work.

 

[Mark42]

The cheap DC-DC converter is still working fine even most of the time its being over-volted by as much as 6 volts. (its rated 50 volts max, but gets 56 when battery is fully charged). The other day I charged the battery and then put on the lights and just let it sit all day in the garage. Although I was too busy with yardwork to measure the voltage, I can say that the big heat sink on the converter was only mildly warm. And because it was parked in the hot garage, there was no air flow into the saddle bag due to riding. So, I'm guessing a little over voltage (about 10%) is not a big issue as long as the watts are under the units capacity.

I would like to hear any opinions from the real electronics wizards on the boards. Thanks!