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  1. #1
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    Ok...please explain the battery thing and how to test them

    Alright...I am turning into a light nerd. For the batteries, how do I tell what I have in terms of output, etc.? I see the 4400mah ratings, but this doesn't mean much to me. How do I check my cheap-o Chinese batteries to see what I ended up with? What is the issue with the cheap-o Chinese chargers that come with them? They get really warm during charging. Any simple answers/advice would be appreciated. What kind of meter do I need to get?
    Geologist by trade...bicycle mechanic (former) by the grace of God!

    2012 Specialized Stumpy EVO 29 HT

  2. #2
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    To test the capacity you have to measure the current draw from the battery and then measure the amount of time the light stays on. So if you have a light that draws 2.0 amps (i.e 2000 mA) from the battery and the light stays on for 2 hours then you have a 2h* 2000 mA= 4000 mAh battery. A complicating factor with this method is that the current drawn may be, or probably will be, affected by the voltage of the battery pack. As the energy is withdrawn from the battery pack, the voltage drops. This voltage drops then causes the led driver to draw less current in some cases. So the current draw would have to be an average current over the whole discharge cycle. You're basically trying to measure the total area under the current-vs-time curve.

    An easier way is to buy a hobby charger meant for RC cars and planes. Those can measure capacity by discharging the battery pack in a similar manner. They do the calculation for you. I think they may also measure the capacity by calculating the charge put in the battery during a charge cycle. I don't own one, but this is what I gather from reading on the web.

    Edit: sorry for the bad grammar, I hardly slept last night.

  3. #3
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    While what varider said is true, here's the thing; Knowing the milliamphour ( mAh ) rating of a battery, while nice to know, isn't going to tell you a whole lot unless you know how that information translates over into the real world of bike lighting. One way it can help you is if you own a slew of different batteries that are listed at different capacities. That being the case you would then know the difference by using the batteries and comparing them to each other with your lamps. This means you have to do a " run time" test using one of your lamps and then use that information as a base.

    If you have a couple of the cheaper "supplied" batteries that came with your Chinese lamps I highly suggest doing a run time test with them before you start doing rides beyond two hours. Sometimes the supplied batteries are near decent and sometimes they are total ***t. A battery that list as (i.e.) ...4400mah and turns out to be say 3600mah is actually not too bad ( when it comes to cheap batteries ). On the other hand if it turns out to be around 3000mah, that would be a crap battery.

    I know it's a PITA to do run time tests; first you have set everything up...lamp, battery, fan...and then you have to wait while the lamp is running down the battery and be watching so you can make *notes if your lamp has voltage indicators. ( *This can be made easier if you have a Video camera that can record the results. However the camera needs to be able to keep running. If the camera is running off a battery it might not last as long as the lamp battery. ) That said if you use the same battery with different lamps the results will change because different lamps draw different amounts of current based on their design.

    I should also note here that the voltage indicators on lamps will vary as well as to, "How they relate to actual run time". That is why doing the run time test is your most useful friend.

    While it's nice to know how good your battery is ( or isn't ), or how much current your lamp draws, or how long it takes for your different voltage indicators to do their thing....without a run time test you won't know "exactly" how all those things relate to each other.

    Personally I don't know the "actual" capacities of all my batteries. The ones I own that were built with better cells I trust will work but after three years I need to test those so I know whether or not I need new batteries. I rarely ride more than two hours at night and when I ride I generally use the middle power levels. I never worry about running out of battery power because I also carry a single cell ( 18650 ) XM-L torch for back-up. Used alone it lights up the trail very nicely and will run on medium ( ~250 lumen ) for about three hours with a good quality cell. Now for the single cell torch I make sure I am using the very best of cells ( Panasonic ) because that is my fail-safe. All said, I've never had a ride where one of my lamps have run out of power but if it did I would have a back up ready and waiting.

  4. #4
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    Cat is on the money here. I'd advise doing a run-time test as well. IMO that's really the number that matters.

    How the current draw from a battery occurs depends a lot on the driver type the light uses. If the driver is a linear type (pretty common in cheap lights) then the current will draw start high and will drop as battery voltage drops. If the driver is a switching type then the current will start low and rise as the voltage drops. In either case it is difficult to accurately determine the capacity of the battery as the change in current is not linear over time.

    The hobby chargers that can do a discharge/charge cycle to calculate battery capacity will almost always show a greater capacity than the real world run-time number gives. This is because the hobby charger discharge is typically a 1A load at most. A dual XML lamp, if driven well, will draw ~3A. The discharge characteristics between 1A draw and 3A draw are different enough to affect the numbers.

  5. #5
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    Run time test on high to see how long you can ride for. Use a fan and a time lapse camera.

    Check the current draw on all modes to estimate real life runtimes and actual battery capacity. You might want to repeat yearly because as the batteries age they lose capacity.

    I recently ran my XPE magicshine on low with 4 batteries (its just a backup so only used on low)... got 6 hrs for the 2x18650 it came with and 38hrs from my new Hunklee battery.


    If you don't want to use your light you could wire up a LM317 constant current regulator at 1A and a few load resistors.
    Constant current source with LM317
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

  6. #6
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    go to candlepowerforums and poke around for battery and light reviews, they show charts or batteries being drained... and all the info needed to run tests on both batteries and stuff that drains them. this web site, that web site, and batteryuniversity.com will edumacate you


    what cat-man-do said about MAH rating. yeah MAH numbers mean a lot but doesn't tell the full story of how the battery behaves. One example:

    pick a NCR3400mah lithium and a NCR2400mah lithium, and power a cree XML light with it (i say NCR basically meaning known top notch batteries)

    for some MAH battery and situations both will power the light on high for a period of time, then voltage drop off. the 3400 might volt drop off sooner than the 2400 battery. later, the 2400 quits sooner (drops below ~2.9 volts) and the 3400 will keep cranking along longer. but you got more 'runtime on initial bright level' with the 2400.

    batteries are rated by capacity but also how much power they can safely dump over time and the curve they have doing so varies.

  7. #7
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    Thanks for all the info; I have done runtime tests on my lights and this seems to be the most straightforward thing for me to do. The cheap-o batteries I have both will run the lights on high for over 2 hours so that is more than enough for me.
    Geologist by trade...bicycle mechanic (former) by the grace of God!

    2012 Specialized Stumpy EVO 29 HT

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