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Can anyone do me a favour and let me know what the run time and bulb power will be for the following? I can't remember the formulas
I have a lumi 35W mid halogen bulb on the bars. I was running it with 12x AA rechagables (12 x 1.2v = 14.4v 2000mAh battery pack) The light was OK but not brilliant (compared to the HID I had before)
I used it last night with 14 x AA rechargables(14 x 1.2v = 16.8v at 2000mAh battery pack). The light was good, and a whiter colour temp with 16.8v pack than with 14.4v pack.
Q1 - The bulb rating of 35W is measured at 12v right? what power will it be at 16.8v?
Q2 - How long a run time will I get at 16.8v from a 2000mAh pack? 20-30 min at a guess?
Power = VI - but you have no way of knowing what resistance your bulb is when it's at its new temp without measuring it, which means its hard to tell what current it's drawing.
Do you have a digital multimeter?
Its also now obviously 14% overvolted, which means its lifespan will be notably reduced.
got a cheapy multimeter, just set to the Ohm symbol and connect acros + and - while the light is running?
Better make that a very cheap multimeter as it's likely to go pop if you do that.
As for formulae:-
V=IxR, I=V/R, R=VxI, P=VxI, P=V(squared)/R, and P=I(squared)xR.
From that lot you should be able to work out your current draw at different voltages(if you haven't fallen asleep at the back of the class yet), wouldn't worry too much about different temp/resistance values as amp/hour values for your batteries are a bit hit or miss in the real world anyway.
I always used the assumption that resistance stays pretty similar, and calculated it as if for a 12v battery:-
If you do that:
12v 35w on a 2000mah battery would run for 40 minutes.
wattage at 16.8v = 49W (again running for roughly 40 minutes).
You're overvolting by 40% though, so who knows how long the bulb will last for / whether it'll melt. Whatever you do, don't leave it on when you stop.
Joe
got a cheapy multimeter, just set to the Ohm symbol and connect acros + and - while the light is running?
er, NO!
Resistance isnt that similar, but I suppose we could use it to make up numbers since as you say the battery capacity is unliekly to be what is stated. Also bear in mind that the quantity of halogen is picked for a certain element temp, exceeding that reduces the life of the bulb - generally accepted values are 5% overvolt=50% reduction in life. Fair point about the 40% overvolt, I'd totally ignored the fact that it was a 12v bulb and worked from his 14.4v value - that'll teach me to read!).
Easiest way to measure bulb resistance at any temp is to stick your meter in current mode and run it in series with the battery. Knowing this current will tell you as close as damnit your runtime. All the equations in the world wont help you without knowing one the key variable!
As mentioned, usign the ohm measurement with power applied is likely to hurt the meter (although my expensive fluke ones have survived students doing this) as to measure resistance the meter outputs a small voltage and measures current (or voltage drop across a known accurate resistance in series IIRC). If you apply power while it is doing this you're essentially back-feeding it with far too much power and it may release its magic smoke.
I always used the assumption that resistance stays pretty similar
It doesn't though... and unfortunately your calcs actually assume the resistance goes up in proportion to the voltage - ie current is constant!
There's a good graph and explanation at [url] http://www.myra-simon.com/bike/lights.html#overvolting [/url] - unfortunately it only goes to 20% overvolt, but you can extrapolate. At 40% overvolt you'll be using ~60% more power, ie 56W. Your battery is 33.6Wh, so in theory you'll get ~35 minutes runtime, though given other losses running that high a power (such as that the batteries won't give their rated capacity at that current draw) it will probably be under 30.
Regarding bulb life, you have to bear in mind that your battery will be ~18V when fully charged (I'm derating for the high current) which will really knacker your bulb life - if I were you I'd carry a spare bulb when running that much overvoltage!
Cheers for all the info. Sounds like it may be about to got pop soon, but that'll just make the DH sections more interesting 😉
Only used it for DH sections at the moment to save the battery, as I'm riding off the helmet light for uphills and the night rides only last about 1.5 hrs usually.
It does get very hot, but only for 2-4 mins for a DH. I'll try it for a few more rides and see if it blows up 😉
really a temp measure since my HID died, and until I can afford a homebrew LED setup.