My thought is are they from different lots and are the batteries from different manufacturers and different lots? You don’t want to have 5 devices and them all from the same batch and all fail in the same way. A company not concerned by safety probably wouldn’t consider any of this. I also bet their SOP didn’t include changing batteries every time, so you have to rule out that these spares probably weren’t getting charged up or have their batteries replaced either.
My thought is are they from different lots and are the batteries from different manufacturers and different lots? You don’t want to have 5 devices and them all from the same batch and all fail in the same way. A company not concerned by safety probably wouldn’t consider any of this. I also bet their SOP didn’t include changing batteries every time, so you have to rule out that these spares probably weren’t getting charged up or have their batteries replaced either.