# Battery charge indicator LEDs

Many laptop batteries have a button on the bottom that will light up 1-4 LEDs to indicate how much power is remaining.

I'd love to build something like that into my own device, which is powered by 2 18650 LIon batteries in series.

Is there a simple way to build a circuit that will light up different LEDs depending on the voltage of the battery pack? Or a ready made IC that does something like this I could easily use?

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TI/Benchmarq makes battery fuel gauge devices, a typical one for two Li-Ion cells is the BQ28400. You will need to interface it to a suitable MCU that drives the LEDs. This technique should give an accurate indication of battery life, which you won't get simply by using the voltage.

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One simple way to do this is to use a series of comparators that power LEDs. You will need a stable voltage to compare against so you would need to place a voltage regulator on the batteries with a voltage out that is lower then what you plan on the batter being.

You then can use some voltage dividers off of the battery to get different voltages to compare the battery to. Since you have already stepped down the battery voltage with the voltage regulator you will also need to use a voltage divider on the battery to get it in the range that you are comparing against.

Basically if you have a battery that ranges between 6v and 10v, you will want a voltage regulator around 5v. You can then set the comparator voltages to be 3v, 3.5v, 4v, 4.5v and then use a resistor divider to divide the batter voltage by 2. So your new range is 3v - 5v and if it is above the 3, 3.5, 4, 4.5 then the LED associated with that comparator will turn on.

NOTE: 6-10v is just a random range I picked to illustrate my example, don't expect for your batteries to be like that.

The biggest problem with this method is the poor accuracy you get. Batteries don't discharge with a linear voltage drop, so it can be hard to perfect what is considered 25% charge, or 50% charge, or what ever values it is you are looking for.