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# the perfect circuit to indicate battery level

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#### nguyenvanthien

##### Member level 4
hi all,
I studied lots of circuit's forms to 3.7V battery indicator. However, they always have disadvantage: waste power.
So, I'd like save power by way: voltage <3.2 : only red led on; >4.2V: only green led on; from 3.2 to 4.2V; circuit doesn't operate > ultra low current- ultra low power. Give me any idea! thanks

Hi,

Instead of a LED use LCD to safe power.

Let's say a 7 segment LCD. Thrn you can use a complete chip to display the volt in the display.

Or - for even less power - you just can control single segments and treat them like LEDs.

Maybe on a single digit you may use only the three horizontal segments "a", "g" and "d".

****
But if you want to use LEDs, I recomend to pulse them: maybe 0.1 s every 3 seconds...then you have only 1/30 average LED current.

Klaus

Hi,

Instead of a LED use LCD to safe power.

Let's say a 7 segment LCD. Thrn you can use a complete chip to display the volt in the display.

Or - for even less power - you just can control single segments and treat them like LEDs.

Maybe on a single digit you may use only the three horizontal segments "a", "g" and "d".

****
But if you want to use LEDs, I recomend to pulse them: maybe 0.1 s every 3 seconds...then you have only 1/30 average LED current.

Klaus

I have just thought to current controlled pulse. However, It needs at least IC to generate pulse and supply voltage in range 3-4.2V. So, is it reasonable?

Hi,

So, is it reasonable?
We can´t answer this. You need to decide.

You asked for a "perfect" circuit.
* If you want it to be cheap, then you have to say so.
* If you want it to be simple you have to say so.
* if you focus on some other feature, then you have to say so.
* But you only talked about "power". --> Therefore I gave recommendations to save power.

You surely know a "perfect" thing is not necessarily the "cheapest" or the "simplest" thing.

Best is if you give values.
We don´t know if 10mA is low enough for you, or 1mA, or 0.1mA, or 10uA or.... for sure you now like to say "zero uA" ;-)
--> expect that the lowest current means the highest effort

Klaus

What's your objection, the voltage divider power? The comparator power? Neither of these need to be particularly high power and I suspect you can get them to within the same order of magnitude as the self-discharge rate of the battery.

On a quick look Linear's line of micropower comparators come up that need just 300nA.

It would be possible to switch the comparator circuit in and out. But this just means that some other circuit needs to be continuously running to keep track of time.

As I mentioned at initial thread, I just want to sole Led to indicate. (If you use comparator circuit- Lm338,lm339 for example, when full voltage battery, all led will be on- waste current whiich compared only Led ON)??? I haven't find out way to display voltage level to 3 state: <3.2; 3.2-4; >4VDC. Current consumpated about 6-8mA.

This battery monitor has 3 led's. Posted by someone at this forum. Adjusted for a range of 2 to 5V. Each led turns off as the next higher led lights.

As long as the supply is below 4.2 V it has less than 10mA taken from it. You can further adjust values to conserve battery power. Or, take this concept of operation and improve on it.

nguyenvanthien

### nguyenvanthien

Points: 2
Hi

Use a quad package low power op amp or comparator, and make a window detector, three-stages for LEDs to light up for. Window detectors tend to need level translation at the output which is why I said a quad package of op amps or comparators.

This battery monitor has 3 led's. Posted by someone at this forum. Adjusted for a range of 2 to 5V. Each led turns off as the next higher led lights.

As long as the supply is below 4.2 V it has less than 10mA taken from it. You can further adjust values to conserve battery power. Or, take this concept of operation and improve on it.

Thank for giving me this circuit. I'm calculating R, BJT and simulating it to suitable. If any problem, I hope to receive your instructions.

Hi,

I just realized that I have a different idea from "perfect" and "ultra low current" :-(

I really thought you wanted it below 100uA, maybe below 10uA, but now I see you are satisfied with 20mA.
If you want to go on with LEDs I recommend to use low current LEDs. You can run them with 1mA or less.

Klaus

An old-fashioned D'Arsonval meter uses miniscule current. Add a resistor inline to measure voltage. Radio Shack once carried a 1mA type (#22-410). There are even more sensitive types (50 uA, 100 uA, etc.)

Just like the body mass is rarely a reliable indictor of health, the battery voltage is not a good indicator for the battery level. However, if you just want an indicator to tell that it is time to replace the battery, then I guess it is fine to use the voltage.

Hi all,
My purpose is show captivity of battery in external backup battery. So, saving current is very important. If I use circuit which Bradtherad posted, is it best way?

How often will the battery be looked at? Do you need to have it lighting an led 24 hours a day?

An alternative is to press a button momentarily anytime I wish to take a reading. Then an indicator consisting of several leds in a bargraph would be as good as I need. No harm if it consumes a larger amount of mA, since it only operates briefly.

Yes, its kind of like asking what is the perfect thing to have for breakfast every day.
Or what is the perfect type of music to listen to.

Everyone has a different idea of what a "perfect" means.

FvM

### FvM

Points: 2
I appreciate the perfect breakfast comparison. Regarding this specific questions, I'm completely missing a requirements specification.

All we know about the application is "external backup battery". However, in which operation points do you want to monitor battery level? Charge? Discharge? If so, at constant or varying currents?

Look at the charge/discharge curves of LiIon batteries and realize how ambiguous battery voltage information is. Using pulse discharge is in fact a suitable means to reveal the state of a battery with unknown history. Another possible way would be to measure voltage, actual current and temperature for level estimation.

The "perfect" method implemented in notebooks and smart phones is to balance charge and discharge, requiring a permanently powered microcontroller or dedicated battery management chip.

My circuit is to display discharge. Li ion battery is discharged to another circuit.
So:
Firstly, minimum current is necessary.
Secondly, It is really better if indicator circuit is only operates when Li ion is connected to discharge.

Hi,

please understand: We need values, no text. Decide what value you try to reach. And decide what is the max. value you can live with.
* With values we can calculate.
* When we have values we can say this is possible or not.
* With values we can decide if one solution is possible or we need to look for another solution.

There is at least the factor 1000 in the current consumption of different solutions.

Klaus

Hi,

please understand: We need values, no text. Decide what value you try to reach. And decide what is the max. value you can live with.
* With values we can calculate.
* When we have values we can say this is possible or not.
* With values we can decide if one solution is possible or we need to look for another solution.

There is at least the factor 1000 in the current consumption of different solutions.

Klaus

Thank for your opinion. Because I design this circuit in first time, I can only give a range for specifications.
- supply voltage: 3-4.2V ( voltage of Li ion).
- current when indicator: 8-10mA.
- currenr when no indicator ( battery hasn't load): less than 0.3mA.

I am still confused about what you want. Some possibilities:

1. You want to show the approx capacity left in the battery; how many hours (or days, suit yourself) it is going to live under specified load.

2. You want to tell the user that it is time to replace the battery now; the system cannot run at this voltage.

3. Or just a battery voltage indicator as mentioned in your original post. I presume this voltage is measured under load. I also guess the battery you are using is equipped with standard protection circuit.

4. You do mention that 3.2 to 4.2 the circuit does not operate; not clear to me, why so?

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