How to test battery health

Posted on January 7th, 2019 by Rob Marshall

How to test battery health


Several GEM members have called our technical helpline recently, asking why a battery that has a healthy voltage can seem to be unable start an engine from cold. A voltage test does not tell the whole story and this is why:


State of Charge vs. State of Health

Taking a voltage reading will tell you only the battery’s state of charge. The battery might be charged fully but this does not mean that it can deliver sufficient power to start the engine, particularly on a freezing morning, when low temperatures conspire against the battery, robbing it of some capacity and making the engine harder to turn over.


This is because various chemical reactions that take place over time, between the acid and the active lead-based materials inside the battery, cause its capacity to reduce. This cannot be assessed by voltage reading. Instead, the State of Health is checked by measuring current, i.e. amps.


Old fashioned ways of doing this, such as using a drop tester that creates a very intense discharge can be dangerous. Modern methods of assessing the State of Health can be performed with a special electronic tool that measures the battery’s internal resistance, and uses the figure to calculate an accurate maximum amperage output, which can be compared against the CCA figure that is marked on the battery label.


While assessing the battery State of Health seems complicated and within the realms of professionals only, an increasing number of DIY smart battery chargers are available that can perform this task for you – all you have to do is connect one to your battery and follow the on-screen instructions.


For example, we recently tested the RING RSC612 smart charger (which costs around £100) and, compared the State of Health reading (pictured) of several batteries against the figures from a professional garage tool. The results were within an impressive 3% tolerance.