MR16 with 9.6V OR 12V Ni-cd

Hello, I bought one of these 50W bulb for making my own bike light. Since I only have a 9.6V 2000mAh Ni-cd cell avaliable, I feed the 12V 50W bulb with it and the run time is under 10mins. My smart charger only support a max. of 9.6V battery pack. If replacing with a 12V Ni-cd battery won't do much help with the runtime, I would not like to spend a huge sum of money buying a new charger and battery.

Is it ok to just power it with 9.6V?
Will the under voltage damage the bulb?
Will run time significantly longer with 12V?

Any inputs would be great!

Best regards,
Billy

The run time of a system depends upon the current being drawn by the load (the lamp in this case), and the capacity of the battery.

A 50W 12V lamp should draw 50W/12V = 4.167A when supplied with 12V. Supplied with 9.6V, I would expect the draw to be something on the order of 3.5A. So the bulb would drain your battery is about 1/2 hour.

But you have another issue to deal with. You are operating the bulb at well below its design voltage. Light output of a halogen lamp is _eponential_ with voltage, in fact it is proportional to voltage^3.56, meaning that if you operate the bulb at 9.6V/12V, you can expect to get only about 45% of the rated light. And as the battery drains, this light output will plummet.

My guess is that you still had significant power available from the battery, but that huge bulb operating below its rated voltage probably means that the bulb dims out and looks drained very quickly.

If you go to a full 12V, then you will get much more light, and it will be much whiter. However rather than going to the expense of a new battery and a new charger, you might consider going with a 6V 20W lamp, and using something like Willie Hunt's LVR system to take the 9.6V and regulate it down to 6V. This will likely give you _more_ light than your current system, and will do it for about a full hour.

See http://www.cs.indiana.edu/hyplan/willie/lvr.html

-Jon

Thanks, Jon
Suprisingly I got only 10mins of light before it dims out
Maybe I should try a lower wattage bulb since my skills are limited and I don't know how to construct an advanced system like voltage regulation.
Do you think that it is the under voltage that cause the reduction of runtime? or the power required by the lamp is just too high. From your calculation, the current is over 3A, but still the 10mins light is somewhat disappointing from a 2000mAh battery.
If the current running with 12V is even higher, then the runtime with 12V 2000mAh seems no better than 9.6V----> Am I right?

the output now is good to me but is there anything I could do to increase the runtime? i.e. over 40mins?

All the best,
Billy

My feeling is that the actual run time is right now longer than 10min, but you can't tell because the light dims down into the IR, so you don't see it. When you lower the voltage on a halogen lamp, it gets dimmer, less efficient, and more red. And you are operating at a much lower voltage than the lamp is designed for. Of course you might also have a problem with the battery; you will see poor battery run time is a single cell in the battery is defective.

Going at 12V, you would expect the run time to be shorter, since more current would be flowing. However you will get much more light out of the
bulb more efficiently, and the lamp will run at full brightness for most of the discharge period.

I believe that if you go to 12V, and use a 12V, 20W bulb, then you will get a much more useable light for a greater run time.

Please take a look at the LVR link that I sent. Building a regulated system is actually quite simple. You essentially splice the LVR in series with the lamp, and with little else you will have a functioning _regulated_ lighting system. Even if you don't want a regulated system, the page describes all of the issues associated with changing the voltage applied to a halogen lamp.

With regards to run time, even at 12V, you are not going to get a very long run time. The current required by the lamp is 4A (4000mA), and your batteries are only 2Ah (2000mAh). This means that _at best_ you can only expect 2 A h / 4A = 1/2 hour of run time. If you want a longer run time you will need higher capacity batteries (meaning more Ah) or lower power lamps (meaning fewer Watts).

As you noted, the output with the current system is acceptable to you...but you are using the lamp in a very inefficient fashion. Running a 20W lamp properly will produce more light and consume less power than your grossly underdriven 50W lamp.

Regards,
Jonathan

The temporary solution now would be lower wattage(20W) and higher capacity battery (2400mAh).
Hope it will reach half an hour

As soon as I save up enough $$$, I'll upgrade the system. (I think I will try LVR, seems good to regulate to 12V using 9.6V battery )
Have a nice day!
Billy

Actually, it ISN"T a good way to get to 12V from 9.6 volts. As I understand it, the LRV system will only take voltage DOWN, so if you wanted a regulated 12 volt output, you would need to use two of those 9.6 volt packs for input. THEN it would regulate it at 12 volts, and you could still use your current charger as well.

Spud

two 9.6V Ni-cd!!!
wow that will take up a lot of space...
don't know whether my bike can accomdate it or not
yeah, maybe 2 8.4V packs?

I'm glad to have a easy solution
Thanks Spud and Jonathan

Yeah, two 8.4 volt packs would work also. I think anything above the desired output voltage would work (for me the desired output voltage would be around 14 or 14.5 to overdrive the heck out of it and make it more efficient and brighter) so the 16.8 volts would work.

Spud

Spud is exactly right, the LVR is a down regulator, so your battery voltage needs to be higher than the lamp voltage, that was why I suggested using a 6V lamp with your 9.6V battery. But the idea of using _two_ battery packs is a very good suggestion, since you won't need to invest in a new charger.

If you use 2 8.4V packs, _connected in series_, then you will have enough 'headroom' to 'overdrive' the MR-16 lamp. The LVR will let you precisely regulate the amount of overdrive, so that you can safely push the lamp to maximum efficiency. 16.8V will burn out a 12V lamp _very_ quickly; probably way too quickly for reasonable use. But 14V is a good bet, and the LVR will let you run the lamp at 14V from the 16.8V battery.

If you use a pair of 8.4V packs, rated at 2000mAh, then your energy storage is about 32 watt-hours. Use a 20W lamp, but run it at 14V, and it will consume about 25W. So you would expect a run time of a bit more than an hour. The light output will be a bit more than your current system.

If you can't get the LVR, then run with a 7.2V and a 6V pack. Make certain that the cells are the same capacity (mAh rating), and make sure that the packs are connected in series. This 13.2V 'battery' would do a good job of driving your 12V lamps.

-Jon