# How to estimate number of hours remaining before battery dies?

Hi guys,

I am doing a battery life indicator for a device. A red led will light up when the voltage reaches below a certain limit, which is at least 2 days before the AA battery fully dies.

As such, I want to know how to determine the number of days left before a AA battery dies. I have attached the datasheet for a AA battery but it only shows the constant current performance chart. My device draws around 50ma current for about 10seconds, 4 times a day, when a motor rotates. And all along, it draws a current (ard 1ma I think) when it runs a timer.

So, how can I determine the voltage when the battery is abt 48hours before “dying”?

Find out the mAh of your battery brand/type and compute accordingly.

There is a wide range though for AA battery types. Anywhere from 400mAh to 3400mAh. http://en.wikipedia.org/wiki/AA_battery

and even then, what you'll come up with is a best guesstimate. Temperature, efficiency, losses will also come into play and affect battery life.

It's a bit like tightening a nut without a torque wrench.... do it up till it strips, then back it off a bit 8)

(ie, run the battery flat then you'll know....)

Judging by the chart for the remote, once you are down to 1V it's probably time to replace it.

There are so many factors (like the temperature) that you probably can't be much more specific than that.

(edit) Maybe 1.1V

Have you noticed how wrong your laptop or tablet gets this. They have vastly more engineering resource than you and they can't get it right. It is not something that ca be accurately predicted. There are too many variables.

Have you noticed how wrong your laptop or tablet gets this.

I think half the trouble is how they estimate the upcoming use. The might have a good handle on Ah left (but I wouldn't know if they do or not) but they seem to take the instantaneous consumption and assume you'll be doing that non-stop until it dies. So if it looks like you have a long time left, as soon as you do something intensive for a few moments, your time left drops drastically based on that heavy load.

Grumpy_Mike: Have you noticed how wrong your laptop or tablet gets this. They have vastly more engineering resource than you and they can't get it right. It is not something that ca be accurately predicted. There are too many variables.

Predicting "remaining life" is difficult.

OTOH knowing when it's "nearly dead" isn't. Most batteries go down to about 1.1V then drop off very quickly (think "cliff").

You need to put a small load on the battery for about 5-10 seconds then measure the voltage, eg. A transistor+resistor that lets 50mA pass. Switch the load on, wait, measure the voltage, switch the load off. If it's 1.15 volts or less, start flashing the warning LED. You could do this once per day.

I too want to be able to display something on my project before the battery completely dies off...

How does one "measure the voltage" programmatically within the sketch?

Is there some way to do this without adding a transistor?

Great ideas here... thanks!

DerekErb: I too want to be able to display something on my project before the battery completely dies off...

How does one "measure the voltage" programmatically within the sketch?

You could use the "Secret Voltmeter"

The secret voltmeter appears to work quite well on my Arduino Uno.

When I get the time I'll play around with dimming a LED as the "voltmeter" reduces...

Thanks!

JimboZA: I think half the trouble is how they estimate the upcoming use. The might have a good handle on Ah left (but I wouldn't know if they do or not) but they seem to take the instantaneous consumption and assume you'll be doing that non-stop until it dies. So if it looks like you have a long time left, as soon as you do something intensive for a few moments, your time left drops drastically based on that heavy load.

EXACTLY! That is also valid for cars. If my car tells my I have 50km worth of fuel left, I make that fuel last for at least 70km, because I will change the way I drive. Obviously the indicator will change as well, and increase the autonomy indication, which is a lot more than Windows does calculating remaining battery time.

Like it's been said, you can get at most a fair estimate, but not an actual precise prediction.

If you are trying to calculate remaining battery time on a device for which the power consumption doesn't change much over time and doesn't depend much on how humans use it, then you can get a very good estimate. In other words, if the consumption curve for the device is known and doesn't change much, calculating remaing battery time is pretty easy.

calculating remaining battery time accurately is pretty hard.

What you are attempting to do is to calculate the mAH charge remaining in the battery at any given time from when the battery last had a complete charge. To do this accurately requires that one reads the actual real time current consumption continuously and keeps an accumulated mAH consumed. This is then subtracted from the rated full mAH capacity of the battery. It can be done but it requires current sensing/measuring hardware and extra software.

Lefty

fungus: Predicting "remaining life" is difficult.

OTOH knowing when it's "nearly dead" isn't. Most batteries go down to about 1.1V then drop off very quickly (think "cliff").

Indeed - any you need to know the actual chemistry of the battery involved, AA cells might be Zinc/carbon, alkaline, NiMH or even NiCd or NiZn! Unless you know the chemistry you can't know which voltage represents the death-knell of the cell. For NiCd for instance its such a flat curve that in practice you can't tell, fortunately with NiMH you can fairly well. So you probably need to decide on a single type of cell to get a reasonably reliable indication.

It's like watching a download in Windows ...

Your download will complete in 3 minutes ... wait, 5 hours ... in 4 days ... 10 minutes ... 2 hours.

MarkT:

fungus: Predicting "remaining life" is difficult.

OTOH knowing when it's "nearly dead" isn't. Most batteries go down to about 1.1V then drop off very quickly (think "cliff").

Indeed - any you need to know the actual chemistry of the battery involved, AA cells might be Zinc/carbon, alkaline, NiMH or even NiCd or NiZn! Unless you know the chemistry you can't know which voltage represents the death-knell of the cell. For NiCd for instance its such a flat curve that in practice you can't tell, fortunately with NiMH you can fairly well. So you probably need to decide on a single type of cell to get a reasonably reliable indication.

It's been my experience that Nicad and MiMH have very similar flat discharge curves where the terminal voltage does not give you a good indication of remaining charge until it's almost at the end.

http://www.mpoweruk.com/performance.htm

Lefty

Remember all those milliamps are in relation to milliamp hours. So, the 1 ma timer draw = approximately 25 ma in a day. The 50 ma draw is only happening 40 seconds out of an 86,400 second day. A 2500 ma battery will last least a month. Your mileage may vary!

So, the 1 ma timer draw = approximately 25 ma in a day.

You mean…

25mA hours

Ya got me!

JimboZA:

So, the 1 ma timer draw = approximately 25 ma in a day.

You mean....

25mA hours

In Australia, it would be 24 hours in the day. YMMV.