Monthly Science: Five Months of Lithium Cell Wear

I’ve marched the four STK NP-BX1 lithium batteries through the Sony HDR-AS30V camera in constant rotation since last November. The A battery drained 35 minutes into an ordinary ride on a pleasant day, so charging and measuring the entire set seemed in order:

Sony NP-BX1 - STK ABCD - 2015-11-03 vs 2016-03-24
Sony NP-BX1 – STK ABCD – 2015-11-03 vs 2016-03-24

The dotted curves come from early November 2015, when the batteries were fresh & new, and the solid curves represent their current performance.

It’s been a mild winter, so we’ve done perhaps 75 rides during the last 150-ish days. That means each battery has experienced under 20 discharge cycles, which ought not make much difference.

The B battery started out weak and hasn’t gotten any better; I routinely change that one halfway into our longer rides.

The A battery started marginally weaker than C and D, but has definitely lost its edge: the voltage depression at the knee of the curve might account for the early shutdown.

Figuring that the camera dissipates 2.2 W, a battery that fails after 35 minutes has a capacity of 1.3 W·h. That suggests a cutoff voltage around 3.8 V, which makes absolutely no sense whatsoever, because the C and D batteries deliver at least 75 minutes = 2.8 W·h along similar voltage curves.

The B battery goes in the recycle heap and we’ll see how the A battery behaves on another ride…