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  1. #1
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    Using a multi-meter to check output of a battery?

    Hi all
    I'm hoping to finish my helmet light soon and want to check my battery amperage to help work out my run times.
    Is there a way of doing this with a normal multi-meter?
    I've not got any instructions for it and can't find any online

    This is the one I've got:
    Attached Thumbnails Attached Thumbnails Using a multi-meter to check output of a battery?-white-gold-wg-026.gif  


  2. #2
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    A meter won't do anything useful on its own.

    If you want runtime, then you need to do some calculations. Figure what the average current draw of your driver/LED setup is from the typical battery voltage you have.

    With the battery amp.hours you can then divide that by your current draw (above) and that's your hours of runtime.

    Obviously there's a lot of variables here, especially if you are using a multi-level driver... You could measure the current draw of the driver at the various output levels. Then you could calculate runtime for each level.

    So, the only use of your meter is to measure the battery voltage and driver/LED input current. i.e. the meter can't directly measure your 'battery capacity'.

    cheers,
    george.

  3. #3
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    Hi George
    Thanks for the in-depth reply I understand the different runtimes etc but I was wondering if I can check the output of my battery as I can't remember what the capacity is. I'm not sure if it's a 2.4/2.8ah etc so just wanted to get a ball-park figure for run times.
    Will be an LFlex running 2 x XML form a 7.4v Battery..
    Cheers
    Steve

  4. #4
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    You can use the meter to measure the voltage and current output of your battery while the light is running. But that won't help you with runtime if you don't know the actual amp-hour rating of the pack. The only way to determine the total capacity of the battery in amp-hours (ah), is to actually run the battery through a cycle while taking measurements throughout the whole cycle. You can't measure the amp-hour rating of a battery with any kind of instrument directly.

    However, at that point you'll have the answer to your ultimate question, how long will the light run. The meter isn't really needed unless you just want to know the amp-hour capacity of the battery. Even so, it'll be really hard to do with that meter. You need a meter that'll collect / record data during the whole discharge cycle. It's not completely impossible with that meter, you could do something like read the voltage and current every 5 minutes and put the data into a spreadsheet. Then with the right math, you can get a fairly close approximation of amp-hours. Something like a Watts-Up meter will collect data during the full discharge cycle and calculate amp-hours automatically for you.

  5. #5
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    As above, if you don't know the capacity of the cells, then you will need to run the light and determine how long it runs at a particular current draw down to a particular cutoff voltage.

    You can't 'measure' battery capacity with a meter. What your are trying to do is analogous to determining how far your car can travel if you know its MPG but not how big the fuel tank is...

    cheers,
    george.

  6. #6
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    deesta, What sort of charger do you have? If you have a hobby type charger then most can do a capacity check of sorts. They can drain the battery and tell you what it took out then charge it back up and tell you what it put back in.

  7. #7
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    YB: I've only got the standard charger for the battery..

    I guess I'll just have to build the light up, run it and check the times.

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