# Measuring state of charge of a non rechargeable CR123A lithium battery

Is there a method (voltage, resistance, other...) of measuring leftover charge in a non rechargeable lithium battery?

Background:

I'm trying to get a general overview of power consumption for few battery powered devices which have been running in different conditions for a few days. The devices have a battery level percentage indicator but I don't know how its implemented and would like to do a side measurement.

There are suggestions to measure the state of charge by checking the voltage level of the battery. The problem there are no characteristics for the low power my device is using (5 miliampSeconds every 5-10 minutes).

• The capacity is often important because the best guess you have comes from the ratio of percentage of capacity used to open circuit voltage or voltage under defined load. Have a look at what the datasheet of your battery provides you with – PlasmaHH May 31 '16 at 11:45
• For batteries, everyone in the field uses Ah (Amp Hours) or Wh (Watt hours) or even Joules when in a weird mood. Specifying battery capacity in F (Farad) is just for people that want to sound interesting. – Bimpelrekkie May 31 '16 at 11:59
• I think I'm gonna need another question for this :) – TheMeaningfulEngineer May 31 '16 at 12:06
• Determining the leftover charge is difficult without discharging the battery completely. The chemical processes in the battery restore the voltage after some time (the battery recovers), also temperature is a factor. – Bimpelrekkie May 31 '16 at 12:17
• @FakeMoustache This is a difficulty related to lithium batteries? I've seen some battery charge testers being marketed. – TheMeaningfulEngineer May 31 '16 at 12:27

Standard CR123A comes in variety of brands and chemistries. Their standard loaded voltage is 2.5 Volt at 700mA load current. This data can be easily obtained from internet.

Now the easy way to tell how usable the battery is (not actually gauging left over charge) by loading it with a 3.3 ohm resistor while measuring its terminal voltage. If the voltage drops below 1.5Volt, you can assume the battery can be send for recycling.

• Why not just measure voltage? That's how all devices and cars measure how much charge is left. Shorting it with a resistor just makes things more difficult and discharges the battery furthermore. – Giedrius May 31 '16 at 16:03
• Its not shorting, its called loading. Internal resistance of the battery won't show up until you load the battery. – soosai steven May 31 '16 at 21:49
• Yes, my mistake. But there is no reason to measure internal resistances. It would be useful though when you are trying to quess max curren of the battery – Giedrius Jun 1 '16 at 1:08
• True.... That's why I mentioned in my answer this method is only to decide on the usefulness of the battery and not to gauge the remaining charge in the battery. – soosai steven Jun 1 '16 at 1:10

The method I came up so far is to completely drain one battery with a constant 5 miliamp current and draw a Voltage-Time characteristic from that.

After that I could check if the voltage drops are measurable and if they are, assume that all the other batteries from the same vendor in similar conditions would behave the same.

• You should check this first: powerstream.com/cr123a-tests.htm – Giedrius May 31 '16 at 16:16
• One more thing - the batteries stanard (rated) discharge is 10mA, not 5mA so you should measure with 10mA. – Giedrius May 31 '16 at 16:20
• @Giedrius The link you provided is addressed with problem there are no characteristics for the low power my device is using. Also, I want to know how the battery is behaving when being used by the exact device, hence the 5mA. – TheMeaningfulEngineer May 31 '16 at 16:24

All devices get "leftover" battery charge percentage by simply measuring the voltage. The thing is that batteries when fully charged have a higher voltage and when fully discharged - lower.

For example a 12v battery: charged - more than 12.6V, fully discharged 11.6V - 11.8V.

A 3.7V battery: (fully) charged - 4.2V, fully discharged - 2.6V - 2.8V.

You need to test the voltage. Here is short information from wikipedia: As we can see its nominal voltage is 3.6v so basically it should behave the same (or very similarly) to a 3.7v battery (18650 for example). Fully charged at 4.2v and fully discharged at about 2.5V-2.6V.

However not all bateries are the same. Some can give off current better when they are discharged (hence last more) and some fail to give even 20% of their 'advertised' current when they are near discharge point. You should check this out for more information: http://www.powerstream.com/cr123a-tests.htm But basically - 2.5V is the '0%' point of the battery.