# Accurately Calculating Battery Life!

1. Jun 18, 2009

### big_tobacco

Hello

Could you tell me is there a way of accurately measuring battery life? Basically I have a listening device, it requires 2 x AA batteries, say 2400mAh each (so....4800mAh in total).

I am aware you can ‘estimate’ battery life by determining the device’s current consumption and then calculating against the battery(s) mAH rating, however this assumes the supply voltage remains constant.

I have measured the device as having an on-load voltage of 3.04V, and an off-load voltage of 3.08V.

Also I have found the minimum total voltage required to power the listening device is 1.35 V. (0.67 V per battery)

With this information how could I find out how long the device will run for by cross-checking it with a manufacturer's ‘Discharge Characteristics chart’.

I would be most appreciative if anyone could help.

Kind regards

Lee

2. Jun 18, 2009

### Staff: Mentor

Welcome to the PF. What do you mean by on-load and off-load? If you mean a changing load on the battery supply voltage, those should be in currents, no?

You estimate battery life using the discharge curves you mention. If the load current is varying, you need to go back and forth between the different load curves, ratioed by whatever the duty cycle of the load is.

Also, the batteries are considered used up typically when their voltage falls to something like 85% of their starting voltage (varies by type of battery), so you are not going to get much extra life between there and the low 0.67V per cell that you mention in your post.

Can you post a link to the discharge curves for the battery that you want to use, and say more about the current consumption of the load and the duty cycle and frequency of the changing load current?

Last edited: Jun 18, 2009
3. Jun 18, 2009

### chroot

Staff Emeritus
I think he means the open-circuit voltage is 3.08V. When he connects his load, the voltage goes down to 3.04V.

- Warren

4. Jun 18, 2009

### negitron

You can't. Any discharge curves you obtain from the manufacture will merely represent a sample, at best. They will not necessarily reflect individual cell characteristics accurately.

5. Jun 18, 2009

### DaleSwanson

Just to point out, when you connect batteries in serial the voltage adds but the Ah remains the same. When they are in parallel the voltage remains and the Ah adds. So your two AA batteries are 2400mAh at 3v.

6. Jun 18, 2009

### skeptic2

Agreed. For awhile I was a product engineer at a company that made consumer products with the batteries installed at the factory. I was supposed to write a spec. for testing the battery because customers were complaining the batteries were DOA. I called the battery manufacturer up to get their spec. and was surprised to find out there was none.

One of the problems is that batteries deteriorate very rapidly when exposed to temperatures over 40 C. That's an uncontrolled variable during shipping, both from the manufacturer to the factory and from the factory to the store. How hot do you think semi-trailers get inside during shipping in summer? In short, lifetimes can vary tremendously from battery to battery.

7. Jun 19, 2009

### big_tobacco

Fantastic, to be honest everyone has given me some very useful ideas - I shall definately keep you posted!