Electronics, Lighting, & Gadgets - Need to ask the experts... one more time.

Hey guys;

I feel like a kid in class who just can't get the latest problem in algebra 1... or battery size + bulb wattage = burn time 101, that is. So if I can impose on y'all one more time...

I've got this battery/charger combo http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2462 from batteryspace.com. I have taken careful note of the first 6 full charge/discharge cycles with a 12V 20W halogen bulb.

1st 45 min.
2nd 1 hr. 10 min.
3rd 2 hr.
4th 2 hr.
5th 2 hr. 10 min.
6th 2 hr. 5 min.

So the burn times seem to be topping out at a little over 2 hrs. Some folks have suggested that this battery/bulb combination should be good for over 3 hrs. While that would be great, I wonder if it is a bit optimistic, since others have suggested that over-volting a given bulb will cause it to draw more power than the classic equation would indicate.

I got a responce on batteryspace's forum who's language led me to think the responded works there. The person was very nice, said the battery/bulb combo should be good for nearly 3.5 hrs, and if I continued to experience short burn times after careful break-in I should return the unit for inspection. But I don't really want to endure a round of return for inspection, only to have them declare the unit sound and ship it back (all at my expense) if what I am experiencing is simply real world performance.

So what say you? Anyone running this particular battery with a 20W bulb themselves? If not, could the experts weigh in before I contact batteryspace.com? Thanks for your time. Sorry for being so long winded.

DanO

PS: I will way that last night I bombed down a dark descent that had me riding my brakes when I was running twin CatEye EL500 LED's. I'm no longer blind. I CAN SEE!

Theoretically, you're right: it should burn for over 3 hours. But there's one thing you're forgetting, or may not be aware of. Any given battery will have a different capacity depending on how much current you are trying to draw from it. If you only draw 1 amp from it (about 14W) then it will have a longer life than if you try to draw 2 amps from it (about 28W). It's a shame that batteryspace doesn't provide "current derating" charts for its battery packs, because they would be very useful for sorting out questions like this.

Your battery pack is probably fine - chances are, you're just drawing too much current for it to keep up.

I did a bit more math, and found some more info that might ease your concerns. First of all, a 12V bulb drawing 20W would have a resistance of 0.6 ohms. Over-volting it to 14.4V means that it will draw 24W. To supply that much power, your 14.4V battery would have to output 1.6 amps.

I found the datasheet for the NiMH cell used in this pack here. (http://www.batteryspace.com/productimages/nimh-cell/MH-C5000.pdf) The datasheet claims a minimumbattery capacity of 5000mAh at a discharge rate of 0.2C, and only 4650mAh typical. The capacity at 0.5C is only 4500mAh typical. Your discharge rate is somewhere between these two. Based on this, I calculate an ideal run time of 2.8 hours until 100% discharge. However, there will be other losses due to other factors, such as connections (did you solder the connections or just crimp them?) and operating temperature.

I have a Jet Lites halogen system that runs a 12V 20W MR11 with a 4400mAh 14.8V Li-Ion battery. I haven't ever run it down yet, but the specs state a max runtime of 2hr 10min. Your 5000mAh should get you only a little more, so you're not short by much.

Thanks for the responce guys. I'm gonna bookmark this thread and try and get some of the math down. I'll most like just keep my battery and run it. Truth be told; 2 hrs covers the overwhelming majority of my night time riding anyway. And besides, I don't want to part with it... especially if it's just gonna check out fine.

I'm building another light. One with a Sylvania Tru-Aim MR16 10 degree spot. From what I've heard 'round here, it should really burn a hole in the night compared to the MR11 30 degree flood. And if the MR16 reflectors are really that much brighter, perhaps I'll pick up 10 and 15 watt bulbs to see if they are a reasonable alternative to extend run times when needed.

DanO

A 10 degree bulb will have a spot about 10x brighter than a 30 degree of the same wattage.
So if you go from a 20w 30 degree to a 10w 10 degree you should have a spot 5x the brightness, and double the runtime.
I used a bulb labelled "flood" and it was pretty much useless. I wouldnt run anything over 20 degrees on the road. Maybe for a MTB a flood pointed 6 feet in front would be useful for technical stuff.

Got another question... this time concerning bulbs. When I went to use a new bulb I noticed that the ceramic insulation surrounding the pins at the back of the bulb had a big void of air behind the glass. I took the bulb back and exchanged it. I thought I looked it over good in the store, but when I got back to work I looked again and it has some of the same voids in the ceramic. At this point I just wanna plug it in and see if it burns. Unless you guys who've been dealing with these little halogens for a while think this might pose a problem.

DanO

I did a bit more math, and found some more info that might ease your concerns. First of all, a 12V bulb drawing 20W would have a resistance of 0.6 ohms. Over-volting it to 14.4V means that it will draw 24W. To supply that much power, your 14.4V battery would have to output 1.6 amps.


You're on the right track, but I think you've got your equations a little mixed up.
V=IR
P=IV=V^2/R

Where V=voltage, I=current, R=Resistance and P=Power.

So a 20W, 12V bulb has a resistance of 7.2 ohms. With a 14.4V power supply, it would draw 2.0A and 28.8W. (That's assuming the resistance is constant; I suspect it would actually increase somewhat as the bulb got hotter with over-volting, but the conclusion doesn't change.)

Usually battery capacities are quoted at a 20-hour constant discharge rate, because that tends to give the best-looking capacity. The two-hour rate -- the "0.5C" -- is given as 4.5 Ah, or 2.25 A for two hours. If you're getting 2.0A for slightly over two hours, you're getting very close to the rated capacity -- which is generally not what happens with rechargeables! You're doing well.