They tend to go bad pretty fast when they are below 30% charge. I think below 20% for too often is flirting with death. If they get that low or lower, charge them up right away. Doesn't have to be full charge, but put a decent charge back on them. And they get worse the colder the climate they're in. 45-80% charge is kind of the sweet spot for Li-ion longevity. But the chemistry just does not like cold. And neither do their circuit boards. In colder climates I would always prefer to keep them fully charged as possible. However, I live in the midwest tundra, so you might have different concerns. I keep a good stash of primaries around. And keep the Lithium-Ions I use warm and charged. And if I take them out on a trip, I understand their possible limitations. I don't know if you're familiar with lead acid car batteries and what they do compared to these, but they are a lot more robust. However.. they are normally considered 12 volt batteries right? Well they are actually 14.4 volt batteries. If you ever put a battery tester on a car battery and it actually reads 12 volts, it's basically done.