Don't let your battery fully discharge. If you let your battery die frequently, you are putting extra strain on the battery. Most companies recommend discharging the battery once in li ion stock every charges or so.
In case you are using new nickel-metal hydride (NiMH) battery, remember you should not charge the battery for too long or it will overcharge it and end up shortening the life of the battery. However, it is important to charge and discharge the battery three times just like the Lithium mines Ontario-ion battery to ensure that it has a long a life.
In case you some how do not use the battery for 3 months or more, the above described procedure should be followed before you restart using the battery as normal.
When it's time to buy a new lithium ion battery, be sure you're buying one that's new (as in recently produced). Just like fruit has a shelf-life, so do batteries and just like you want to buy fresh produce, you also want to invest in "fresh" Lithium battery stocks ion cells. Check the date-code on the battery or packaging to ensure you're not buying a pack that's been degrading for a year on some shelf.
Agitate the water gently, until the light tells you it's done. UV purifiers don't work in murky water, so if that's the only water you have, you'll have to pre-filter it until it's not cloudy. You also need to make sure you don't drop a UV device.
So, it's easy to see how a lithium 18650 battery, and Li-ion cells in general, have found there way into the homes (and hands) of many modern consumers.