1. Lithium batteries do not have a memory effect. You can charge them at any point in the discharge cycle and it only counts as a partial cycle. In fact it's best if you do not discharge the battery below 20% of remaining capacity. (About 3.55V per cell, or 46.15V for a 48V pack when not under load.)
2. The bast way to get a long life out of an e-bike battery is to get a larger pack than you really need. If you can keep the pack between 80% and 20% charged most of the time it should lead to a long life for that battery. It can be programmed with a number of different batteries, charge rates, and partial charges. It can also charge 2-3X faster than the stock chargers, so it is great for road trips.
3. These batteries work best at room temperature. If they are cold, they will not give as much speed and range, but will not be damaged. Do not charge below freezing, discharging is OK to about -20C.
4. If storing the battery for a long period, it is best left at half charge, in a cool place. (between 47-49 volts is good for a 48V pack). Put the battery on the charger for 10 minutes every couple of months. Some lithium batteries were badly designed, and would discharge themselves over a couple of months to the point where they were below a critical voltage, (about 2.5V per cell or 32.5 volts for a 48V pack) This causes permanent damage, and it can actually be dangerous to try recharging if it has got down really low. This problem has now been solved in most cases.