Battery choice for halogen- Mtbr.com
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  1. #1
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    Battery choice for halogen

    Morning!

    I am repairing a battery pack for a friend. He has one of these...
    http://www.roadcyclinguk.com/reviews...set/55897.html

    All he wants is a new battery. The current one is a 6v Nimh. Can I give him a 7.2v Nimh? Or is that going to cause problems.

    The reason is, I can find more 7.2v packs than 6v ones. Dont fancy building one up as they are so cheap to just buy the pack!

    Thanks!

  2. #2
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    Overvolting was a cheap, commonplace strategy to improve both efficiency and brightness of halogens before LEDs took over. The only problem is bulb life is greatly sacrificed. I recall seeing some charts where the bulb life drastically decreased by 80%, for a mere 20% voltage/power increase, but brightness was nearly doubled. This might mean an overvolted MR16 bulb rated for 2000 hours will "only" run 400 hours. Many would consider this a very acceptable tradeoff for the increased brightness and improved lumens/watt

  3. #3
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    As desolder said overvolting is the way forward with halogens, I used to run my 12v mr16 bulbs from an 18.5v pack no worries.

    That set looks like it uses 6v mr11's. I'd look at 8.4v or even 9.6v if you need a new battery. You may need a new charger though.

  4. #4
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    Sweet, thanks peeps!

    I'll look for a 8.4v pack and give him some replacement bulbs too!!

  5. #5
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    Here's an interesting "lamp rerating" article I found on Wikipedia:
    http://en.wikipedia.org/wiki/Lamp_rerating

    Rerated life = (Va/Vd)^(-12 to -16) Life at design voltage
    Rerated luminous intensity = (Va/Vd)^(3.4 to 3.5) Luminous intensity at design voltage
    Rerated electrical power = (Va/Vd)^1.6 Electrical power at design voltage
    Vd= design voltage, Va=applied voltage

    So assuming a typical 2000hr rated lifespan, the new lifespan at 7.2V would then be 108 to 224. At 8.4V, the new lifespan will be only 9.2 to 35 hours (carry spares!!!)

    Assuming you start with a 10W, 200 lumen bulb, the new light output will be about 375 lumens at 7.2V, and 639 lumens at 8.4V. It will consume 13W at 7.2V, and 17W at 8.4V.

  6. #6
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    I overvolted a 12 volt halogen bulb at 14.8 volts. The first time I turned it on the 5 year old bulb blew, but I had no problems with the new bulb, and ran it for well over 100 hours over the following year.

    There's a graph somewhere on the forum that charts output and life as you overvolt; I'll see if I can find it later.

    You might want to check that the battery can handle the additional current. My Li-ion battery was underrated for the amps that were drawn at 14.8 volts and I needed to flick the switch on and off a few times to beat the battery's cut out. Not really a good thing to do, and I should have wired in a soft start switch to overcome this.

    I tried running 6 volt halogens at 14.8 volts and not surprisingly they blew straight away.

    Tim

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