Help with custom battery testing circuitry

[dethlore]

I want to build a battery tester that will light certain LEDs when a certain amount of voltage is present. For example, if I were to power up the circuit with an 18650 at 3.7v fully charged, it would light all the LEDs from 0 to 4v.
I need some extra minds to help me brainstorm.

Thanks!

 

[Jacob32123]

I would use 3 MOSFETS each with a different voltage divider going across the input. I will see if I can make a computer based prototype for you

 

[Benm]

One chip comes to mind for something like that: the LM3914. This can drive up to 10 leds based on an input voltage range, say from 3.0 to 4.2 volts. Implementation is dead simple as the datasheet provides excellent examples.

I have my doubts about the working mechanism though: With lithium batteries the output voltage isn't that good an indicator of charge state, even at a constant load. This discharge curve is usually very flat, and factors like temperature also have an effect on the cell voltage.

In things like laptops the remaining charge is actually calculated rather than measured - the device keeps track of current consumption and caculates the remaining charge. The total charge is reset when you fully drain the batteries and the laptop can actually calculate how much current was used before the battery died.

 

[Jacob32123]

One chip comes to mind for something like that: the LM3914. This can drive up to 10 leds based on an input voltage range, say from 3.0 to 4.2 volts. Implementation is dead simple as the datasheet provides excellent examples.

I have my doubts about the working mechanism though: With lithium batteries the output voltage isn't that good an indicator of charge state, even at a constant load. This discharge curve is usually very flat, and factors like temperature also have an effect on the cell voltage.

In things like laptops the remaining charge is actually calculated rather than measured - the device keeps track of current consumption and caculates the remaining charge. The total charge is reset when you fully drain the batteries and the laptop can actually calculate how much current was used before the battery died.

I forgot about that. Even simpler

 

[dethlore]

One chip comes to mind for something like that: the LM3914. This can drive up to 10 leds based on an input voltage range, say from 3.0 to 4.2 volts. Implementation is dead simple as the datasheet provides excellent examples.

I have my doubts about the working mechanism though: With lithium batteries the output voltage isn't that good an indicator of charge state, even at a constant load. This discharge curve is usually very flat, and factors like temperature also have an effect on the cell voltage.

In things like laptops the remaining charge is actually calculated rather than measured - the device keeps track of current consumption and caculates the remaining charge. The total charge is reset when you fully drain the batteries and the laptop can actually calculate how much current was used before the battery died.

When using a multimeter to check voltage on a battery, I thought that was a good indicator of general charge on the battery. ie 18650 3.7v battery reads out at 4.2v means it's fully charged and 2.5v means it's fully drained?