The proper way to nurture mobile device batteries is one of the most contested and and confusing topics in tech. That's because rechargeable battery technology has changed a lot over the past decade, and each of the four popular battery types--NiCd, NiMH, Li-Ion, Lithium polymer--differ slightly when it comes to how to extend its useful lifespan. For Lithium-ion polymer batteries, the kind that are in your thin and light laptops and smartphones, battery experts say that the ideal way to use them is to keep them charged to 80 percent, and only let them drain regularly to 40 percent. Why not go for a full charge? At 100%, the max voltage level puts the battery under some stress, which results in fewer overall discharge cycles (times you can recharge it). The 40-80% rule is easier said than done, but one thing you should not do is keep laptops permanently plugged into an outlet. That is, if you ever plan on using it as a laptop.