Electronics, Lighting, & Gadgets - Lights, batteries, amps

I'm building a new light set up with two MR16 50W (12 volts). For testing I made a battery pack using eight standard D batteries. That gives me the 12 volts not sure if I'm getting the Aprox 8.5 amps I need to run the lights at their full capacity.

With one light on, looks pretty bright. Two lights on and they dim a lot.

The pack has the 8 batteries on serial to get the 12 volts. How can I calculate the max amps that I could squeeze out of the pack? With 8 batteries per light, I'm I running the light at it's fullest?

I could only think about connecting one line to my car's battery (lots of amps there) and compare if it's brighter than the light with the eight Ds.

Also, if going the rechargeable way, which batteries should be used? How many in the pack?

With the Ds I thing I'm gonna end up with 2 packs of 8 D. Serial in the packs, parallel among packs. But I'm not sure.

please help...

If these are regular alkaline D cells you aren't going to get 8 Amps out of them and even at 4 Amps they'll be discharged much quicker than you'd expect based on their nominal capacity in mA-hrs. NiMH cells can put out much more current and would be a better choice. They'd run longer for a single charge and can be recharged hundreds of times. Be sure you're getting true 'D' cells though - many of the cells in that size are actually just smaller cells with a plastic case to make them D-cell sized. A real D-cell NiMH cell should be rated at least for 8000 mA-hrs. Their voltage is about 1.25V/cell, so you could use up to 10 in series to give you the equivalent voltage as a 12V car battery. With a capacity of around 8000 mA-hrs and a current drain of 8A you'd still only get about an hour's worth of good light.

I'd also look at lithium ion battery packs for lighter weight - they are also good at high current applications.

Why do you need 50W? Try 5W and see if that is enough.

The car battery should give you full brightness, since it can easily put out 8 amps without significant voltage sag. The D cells can't. The lights are dimming because the current you are putting out, whatever it is, is causing the voltage at teh light to sag. The D cell string has too much internal resistance.

These are the principles. I think if you need one hour of light, go with the string of 10 8 amp NiMH. To get more run-time, add another complete string in parallel per additional hour of run time. There is an issue of balancing teh load among the two or more strings. If it isn't balanced you might not get full capacity.

One way to improve balance is diversity. For a 2 hour battery, wire the 20 cells in 10 parallel pairs, then connect the pairs in series. Same can be done for 3 hour triplets or 4 hour quartets. It's not as good as an electronic balancing system, but better than no balancing. Matched cells would be even better, but not feasible IMO.

I sometimes use a 50 watt headlamp. That much light is not necessary in street-lit traffic situations.

But it's great to have when you're riding where there are no lights or in a terrible downpour, ect. Times and places where you need a lot of light.

I think to get enough usable light for reasonable amounts of time (say 1 hour plus) you're going to need far more of those D cells than you really want to carry. Not to mention the headache of wiring them together, keeping them from jostling around, and so on.

Why not get yourself one of those small 9 amp/hour batteries that power those little electric cars for children. They cost about $50.

I use a 14 amp/hour ATV battery. It's great for having that extra light when I need it and for inflating tires on the road. But it's heavy.

Why do you need 50W? Try 5W and see if that is enough.

that's not enough... not in halogen at least. 5w in leds maybe. But I want cheap bulbs and few things beat the good old mr16.

back to my rig...

I just teamed up with the local electronics techie. We measured the actual amperage/wattage at different voltages... at 12v, one lamp draws 4.24amps and uses ~ 50watts. Increasing the voltage to 13.8 (car's standard) puts the wattage little over 61w and the amps only increase a few mili, still under 5amps... lot brighter.

The 8 D cells pack is definitely not putting enough amps to operate at full brightness. Still pretty darn bright tho, but not full brightness.

Now I wonder how many continuous amps could I get from 4 18650 cells in a pack.

Halogens should not be run overvoltage or undervoltage. At least in automotive headlamp bulbs, both changes reduce reliability. Over voltage reduces life because it's too hot, and undervoltage reduces life because it slows the halogen cycle, and the tungsten filament will thin and fail too quickly.

Where did the 8 amp requirement come from?

Halogens should not be run overvoltage or undervoltage. At least in automotive headlamp bulbs, both changes reduce reliability. Over voltage reduces life because it's too hot, and undervoltage reduces life because it slows the halogen cycle, and the tungsten filament will thin and fail too quickly.

Where did the 8 amp requirement come from?

Overvolting halogens is an easy way to get an awful lot more efficiency out of them.
14.4V will give almost double the lumen output compared to 12V, at around 50% greater lumens per watt.
These are comparable to HID/LED efficiencies.
http://nordicgroup.us/s78/wattslumens.html
Bulb life does go down.

I doubt you actually need 100W of light if you have a good beam angle and the right power source.

Overvolting halogens is an easy way to get an awful lot more efficiency out of them.
14.4V will give almost double the lumen output compared to 12V, at around 50% greater lumens per watt.
These are comparable to HID/LED efficiencies.
http://nordicgroup.us/s78/wattslumens.html
Bulb life does go down.

I doubt you actually need 100W of light if you have a good beam angle and the right power source.

great link... good info

100 watts of power... something I don't need but I really really want :D

If you don't mind having a huge battery pack to run that setup, it'll be brighter than any bike lights I've ever seen. I think it will require a massive battery, though, to get more than 20 or 30 minutes out of a charge - not to mention those bulbs will get really hot. You probably know this already, though.

I'm running dual MR16s now - a 20W and a 10W. The 20W is uber bright: 1500 lumens (14.4 volts). I've run a 20W and 35W side by side, and frankly, couldn't tell the difference. I've never tried a 50W, though.

Something else I've observed with a dual light setup is that lumens don't really add up neatly - at least they don't appear to. When I have two 10W lights on at once, it's not nearly as bright as one 20W bulb. So, if you're going to burn 100 watts either way, I believe a 100W MR16 will be significantly brighter than two 50W MR16s.

Overvolting halogens is an easy way to get an awful lot more efficiency out of them.+1. Many bike light manufacturers did this routinely in their halogen lights.


Bulb life does go down.I've seen the 20% overvolting as a rule of thumb, an acceptable trade-off between increased efficiency and reduced bulb life. Putting 14,4V through a nominal 12V bulb is exactly that.

--J

I've run a pair of 50 watt MR16s in my old metal NightSun. Remember to run 14 of even 12 gage wiring to reduce voltage drip and not burn out the wires. I traded up to a PAR 36 100 watt light. I have not had a car cut in front of me or pull out of a driveway since I rigged it. Pity it takes a heavy SLA from (((-r-Us. If you go to over-volting please carry a spare bulb or lighting system.