The Magic Pencil Test

In Solar Batteries by African Energy

Batteries may look like simple boxes with two terminals, but they are actually complicated marvels of chemical, electrical and mechanical engineering.  And because you can’t see any of the processes happening inside, it can be difficult to understand how batteries work or how well they are constructed.  As a result, it is difficult for a typical user to verify claims made by battery manufacturers – the true quality can only be known with time and use.  So, one of the lessons there is to only buy batteries from manufacturers that you trust.

We were amused a few months ago to see a new battery introduced on the market with an amazing cycle life chart.  Amazing in the number of cycles that were promised, but also in how those cycles were extrapolated!  The manufacturer presented a hypothetical competitive comparison with other manufacturers via a cycle life vs. depth of discharge chart.  At 80% depth of discharge, the new battery performed in the average of the existing competitors.  Then at 50% depth of discharge it suddenly zoomed far ahead, and by 20% depth of discharge it was at least twice as good as most existing batteries.  This looks like an amazing feat, so we asked whether this battery had actually been tested at 50% depth of discharge – it turns out that it was only tested at 80%, where it performed in the average.  The manufacturers then performed the “magic pencil test” and extrapolated, or drew the line on the graph where they wished it would perform at lower depths of discharge.  Of course, it takes some time to test a battery at those depths of discharge, so the manufacturer has a year or more to trumpet their claim before they can be proven wrong.

The other interesting thing about this “magic pencil” is that it performed significantly better than the same battery made by the same manufacturer, but with a competitor’s label.  Magic indeed!