Overcharge and undercharge protections only prevent the batteries from being discharged / charged below / above dangerous levels where it becomes a potential fire hazard. They do not protect you from shortening your battery's life expectancy. To get the most out of a battery, the charge level should be kept between 20% to 80%, and it should only be charged to higher levels or discharged to lower levels if really needed. Leaving it on a very low charge or 100% charge for extended periods of time (e.g. a couple of days) isn't good for the chemistry. Of course, the damage is tiny when it happens just one time, but if you do it repeatedly, the battery will age faster.