I've recently had to purchase a new battery for my laptop as it was down to less than 10 mins of life but after 4 years good use. I always tried to keep a mixture of mains and battery as its used on the road frequently. Booklet with new battery stated that it should be removed if connecting laptop to mains for long periods. Great but you can't use standby. I always thought that charging and discharging to the full extent was best but it seems somewhere between the 2 is optimal. There is no definitive answer and varies by manufacturer. Batteries do have a lifespan but not only is it related to charge and discharge cycles but the temperature they are kept at. Best advice is to stick with the manufacturers guidelines within reason for your needs without going to extremes.