Used lithium-ion batteries sold as new?

October 3, 2012 1:27 AM

I suspect that unscrupulous dealers buy used Lithium-ion batteries, reset or replace the circuitry so that the capacity and charge cycle count are as-new, and then resell the batteries as new. How can I collect evidence for or against this hypothesis?
posted by beniamino to technology (6 answers total)

Can you be more specific about the device? Many devices I know of, the capacity is measured by the device itself monitoring battery voltage (under load) over time. If that is the case here, then it's strong evidence against the hypothesis, because the battery and its circuitry gets no say in the measurement - the battery is measured as delivering power only if it is, in fact, delivering power.
posted by anonymisc at 1:49 AM on October 3, 2012

The latest example is an iPhone 3GS. I bought a 'new' battery which initially reported its max capacity as 1118mAh. After 10 days, the max capacity is down to 988mAh. This suggests to me that the battery came with a fake high starting value, and as the actual capacity has been measured in the course of charge, it has been revised down to the real value.
posted by beniamino at 2:26 AM on October 3, 2012

The battery may need to be re-calibrated. Let it run down until the device shuts down, then charge and check the capacity again. Also, a lot of devices use 'fuel gauge learning' over many charge cycles to gradually improve their estimate of capacity, because simply checking voltage or doing 'coulomb counting' are not terribly accurate. Doing a full discharge-recharge every few weeks should result in an increasingly accurate measurement, as the circuit builds a better model. You can also expect a 20% loss of capacity for every year after manufacture.
posted by pipeski at 2:50 AM on October 3, 2012

I bought a 'new' battery which initially reported its max capacity as 1118mAh. After 10 days, the max capacity is down to 988mAh.

Anecdotally, I've seen similar results with MacBook Pro batteries directly from Apple, and I very much doubt Apple is installing used batteries with doctored capacity figures in both new machines and when batteries are replaced under warranty. The full charge capacity will pretty much always fluctuate a bit based on temperature/load/moonphase (temperature can matter a lot with some Li-ion batteries in particular), though it will gradually decrease over time. I don't know the gory details of how the measurements are figured in the battery circuitry, but my understanding is the reported max capacity figures are estimated based on modeled data of the battery's lifecycle behavior, and that the estimates are revised over time based on measured performance. The only way to determine the current capacity for real is to completely discharge the battery under a constant load and temperature, which isn't going to happen in real-world use.

Assuming the iPhone 3GS replacement battery you're talking about here is a third-party aftermarket product, it's likely that this whole process is handled substantially there differently from Apple's stock batteries. As I said, the figures seem to fluctuate a bit anyway, but at least Apple would have been able to tune the starting parameters to the iPhone's typical power draw and the battery chemistry. A third-party battery wouldn't determine these values the same way as an Apple-supplied battery, so it wouldn't be unreasonable to see some differences there.

Also of note, a quick Google search reports that the stock iPhone 3GS battery was apparently rated for 1219 mAh. What was the figure on the packaging for your replacement?

Far more likely than unscrupulous dealers reselling used batteries as new after messing with the circuitry would be dealers selling poor-quality batteries (just how good can a $5 iPhone 3GS battery with free worldwide shipping be? Note that I have no experience with this particular product) or selling mislabeled batteries (see these people complaining about an eBay seller) with overstated capacity figures.
posted by zachlipton at 3:47 AM on October 3, 2012

I learned a lot about how lithium-ion batteries work from reading this post. It's about the kindle fire, but it will explain what's going on with your iPhone.

Basically, the device has no way to directly read the charge state of the battery, so it guesses. That guess is refined over several charge cycles, and can become out of whack with the real answer under several conditions.
posted by zug at 8:41 AM on October 3, 2012

I think people are doing this.
Amazon battery reviews
posted by couchdive at 1:21 PM on October 3, 2012