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…

4 thoughts on “Monthly Science: Five Months of Lithium Cell Wear

  1. I guess I should be amazed; one of the Sonicare handles just packed it in, with an apparent date code of 2008. These were Costco specials, two handles, one charger. Julie and I each have a set, so I’ve been swapping handles every month, so nothing is sitting idle too long.

    Cheapification is in full bore with these, with current reviews saying the batteries last 10-12 months. Costco isn’t selling the old style sets on-line anymore, and we have too many spare brushheads, so it’s Amazon Prime to the rescue with a single handle set. We’ll see how the new one goes with the monthly swapout schedule.

    1. Less than a year seems … feeble for NiCd/NiMH cells.

      Since replacing the cells in mid-2012, they declined from 21 to 12 days per charge, but that’s still fine by me. I run them until the thing buzzes the “low battery” alert: they’re getting the most exercise they can stand.

      On the other paw, I used known-good cells from a reputable source. You’d think Philips would do the same, but maybe Costco got a special build?

      1. It’s possible Costco got something special. Amazon reviews imply that there are several tiers of quality, fit and finish, not to mention some with lithium batteries. I already saw the cheapening process on the brush head (the original never leaked, while the newer Elites do unless one really dries the head.

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