Most batteries just display a  basic SOC – State of charge. This only answers the question:

 How full is my Battery?

This is usually expressed in percentages with 3-10 LEDs or bars in a display.

Customers tend to trust this, since they are used to a car fuel tank going from full to empty.

But Beware!  This is the oldest trick in the battery business. Li-Ion batteries unlike car fuel tanks age in a slow (or quick) linear decline. The fuel tank gets smaller every day. So the user is misled into believing that the 100% full battery is fine.


The Key information for  Maintenance and critical application users is:

How big is my fuel tank now?

This is called SOH – State of Health. When your fuel tank has shrunk from a 60 Litre to a 30 Litre this would show 50% full. A Li-ion battery below the 60-70% SOH is considered unreliable and potentially due to fail, which in Health Care applications means it should not be used.


Finally there is  SOF. State of Function combines SOC and SOH info to answer the questions:

How big is my fuel tank and How full is it?


This is ideal for the Nurse:  SOF should be related to the load being used, and adapted to variations/ spikes in the PC /monitor power draw.

This is also called Remaining Run time

It answers the question: How long will it work now?


The Circadian LCD display shows  SOF. The Circadian batteries show both SOC and uniquely SOH. So all levels of information are available.


Final Question: Why would a Hospital even consider entrusting the IT system functionality to a simple State of Charge info?