The 3 most important factors in choosing a battery:

1. Trusted brand.  18650 Lithium Ion cells are by far the most popular cells found in ebike batteries.  This is because the energy density is very high and they are being manufactured in huge quantities.  They are being used everywhere including electric cars like Teslas.  There are many cheap lithium ion 18650 cells that don't last and can be very dangerous especially in the quantities found in ebike battery packs.  It is not uncommon for an ebike battery to have over 50 18650 cells. All it takes is one bad cell to destroy a pack and create a fire hazard.  This is why the manufacturer of the cells is extremely important. 
Close up of small Lithium Ion Batteries
Brands like Samsung, Panasonic, Sanyo, LG are all excellent brands with excellent process and quality control.  You can't go wrong with any of these brands.  However, know your source.  There are knockoffs with the brand names printed on the cases so buy from someone you trust or that have a good reputation and others trust.
2. Power & Capacity.  How many volts?  How many Amps?  How many amp hours?  Without getting into how cells are made into battery packs if you know the answer to the first 3 questions you can calculate watts and watt hours which are the 2 most useful numbers.  We use 52 volt packs everywhere we can and only sell motors that can support them.  52 volts is like 48 volts after morning coffee.  It packs a little more punch and gets a little more speed out of your motor.  However, if you are not using the speed or power the extra volts transfer directly into increased range.  How does that work? It is simple math.  Volts x amps = watts.  Watts equals power.  Volts x amp hours = watt hours.  Watt hours = capacity or range.  Think of watt hours as how much gas you have in the tank and think of watts as how much power it can put out.  If you have a 2000 watt battery supplying a 750 watt motor but you are only asking it for 250 watts on your ride you have plenty of reserve power and your battery and motor will run cool.  If that battery happens to have 750 watt hours of capacity it will be able to power the bike for 3 hours at 250 watts.  3 hours x 250 watts = 750 watt hours.  If you were going 20 miles an hour that would mean your range was 60 miles.  See!  Simple math.

3. How big?  We always ask our customers 3 questions.  What is your average ride in miles?  What is your longest ride?  How many miles per week do you ride?  This gives us a pretty good feel for the best battery options.  If your longest ride is fairly frequent then you should pick a battery with that much range.  If however, the long rides are infrequent you many want to think twice about adding that weight to your bike for all of your shorter rides as well.  Sometimes it makes sense to get a smaller battery that will handle 90% or more of your rides and then get a second one to throw in your pack when you take longer rides or just want to know you have a spare tank.  Our most popular battery is the 52v 14ah Panasonic Super Shark.  We call this our 50 mile battery.  It is 728 watt hours and mounts on the down tube on water bottle mounts.  It fits in most bike frames but if it won't it can sometimes be put on the bottom of the down tube or on the rear rack.  A step up for those that need even more range is the 52v 17.5ah Panasonic Jumbo Shark.  We call this our 70 mile battery.  It is 910 watt hours and mounts just like the Super Shark.  For those that want to keep it light and most of their rides are under 15 miles, we have our 52v Samsung Mini-cube battery that fits in a saddle bag under the seat.
52v 17.5ah Panasonic Jumbo Shark
52v Samsung Mini-cube
Mini Cube Battery

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