If you are thinking about building an electric bike you will obviously need a battery pack. Many people get confused when they started shopping for batteries because there are so many different types and various specifications for each brand of battery. Hopefully I can help clear up some of the mystery for you to make you decision easier. The first thing to consider is what voltage battery pack you need. The top speed of your bike depends directly on the voltage of the battery pack, the higher the voltage the faster the bike. If you want a motor just to assist you while you pedal your bike continuously you will probably only need a 36 to 48 volt battery pack. On the other hand if you want to use the bike more like a motorcycle and cruise around at high-speed without pedaling something in the 72 to 100 volt category would provide much better performance. You also need to make sure your motor and controller are rated for the voltage of battery pack you are using. Next thing to consider is the battery capacity which is how long a battery will last on a single charge. It is measured by how many amp-hours (Ah) it can discharge before it is empty. If a battery can discharge 1 amp continuously for 1 hour it has a capacity of 1 Ah. The more Ah of capacity a battery has the larger its physical size will be if you are comparing the same chemistries. A 10Ah battery will be twice the size of a 5Ah battery if they are the same type and be able to get your bike twice as far if riding at the same speed. Multiple batteries can be connected in parallel (all the positive terminals connected together and all the negative terminals connected together) to increase the capacity or Ah to your desired level. Last thing to consider is 'C' rating or discharge rate of the battery. The 'C' rating of a battery is a number that represents the safe continuous discharge rate (in amps) of the cell. A battery rated at 1C can discharge its rated Ah capacity in current without damage. A 2C rated battery can put out twice the number of its Ah rating in amps. If a 10Ah battery is rated at 1C, its maximum safe discharge rate is 10 amps, if the same 10Ah battery has a rating of 3C, it can safely discharge 30 amps. Likewise if that 10Ah battery is rated at.5C it can only put out 5 amps safely. Hope this helps clear up some of the mystery of what to look for when shopping for batteries for your electric bike.