Really simple battery charge indictator

Discussion in 'General Electronics Chat' started by james211, Oct 14, 2013.

  1. james211

    Thread Starter Member

    May 29, 2012
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    Does anyone know of a really simple method for showing battery charge? Even as simple as a red, yellow, green led? Or if there is a simple LED of some sort that would show it as a percentage.

    I built a battery powered USB hub and I need to know how much charge the battery has. If I have a battery die while transferring data from one or two hard drives it could make for a bit of a mess.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I can only think of building some kind of comparator circuit.

    I have only done comparators using op-amps, but if you have op-amp, you need an external power supply to power it. Which, in turn, means that you don't really need a battery in the first place.
     
  3. james211

    Thread Starter Member

    May 29, 2012
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  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Comparators sense voltage levels, battery charge is usually not accurately indicated by voltage alone.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    What kind of batteries, and how low can they go before you have trouble? Is the load constant or can it be one drive running or two running at the same time.
     
  6. doug08

    Member

    Jan 30, 2011
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    is a low DC voltage indicator. I used this circuit in a few of my devices. The LED will illuminate when the battery level drops to a low level that you set depending on the type of battery being used. The circuit is SUPER simple!

    https://www.youtube.com/watch?v=Z0cOO0TNjNA
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Voltage can be a pretty good indicator though, especially if the appliance uses a fairly constant amount of current, which a lot do.

    3 light and 1 light battery level indicators are common in appliances and usually just sense battery voltage.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If done at a constant load and corrected for temperature, voltage can be a crude method of estimating charge but it is not very accurate.

    http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

     
  9. wayneh

    Expert

    Sep 9, 2010
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    You may like an LM3914 circuit. It's just a bunch (10 I think) of purpose-built comparators for making a bargraph or dot display of a voltage range, which you can control. So if you can define what the top and bottom voltage should be, the single IC will do the rest. Much easier than making your own.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Usually a simple window comparator made with a couple of op-amps, or you can be really flash and use an LM3914 which has enough comparators for 10 LEDs, it also has a dot/bar mode select pin.

    Some really cheap wall-wart battery chargers just use a LED & zener with a current limiting resistor - nothing happens until the terminal voltage exceeds Vz, then the LED comes on dim and brightens up as the battery gets to full charge.
     
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