How powerful a motor do you need?
Some of that depends on the terrain
you ride. If you climb a lot of steep
hills, a motor rated at 500 watts can
really help flatten the hills. If you have
fewer hills, a 250–350-watt motor is
often sufficient. Higher-wattage motors
provide more power and are also more
expensive. They also use more battery
power. If you like to have extra power
on those occasions when you need
it and you don’t mind paying a little
more for it, you can have an advantage
by usually using it with lower levels of
assist, which will use far less of your
One of the last considerations you’ll have is where the battery is mounted. The ones
integrated into the frame are the most stealthy and usually the most expensive. They
can still be removed for charging or replacement.
Another example of a finished battery
system. Lithium batteries and the battery
management system (BMS) are housed
in a lockable, removable case that can be
taken off the bike for charging or storage.
Typical off-the-shelf, sealed lead-acid battery. These were the types originally used in
bikes 15 years ago, and they’ve been replaced in all but the cheapest systems now.
In the United States you can have up
to a 750-watt power-system (less than
1 horsepower) motor. All OEM systems
will be electronically limited to only
provide assist or throttle power to under
20 mph, per the law. You can ride the
bike as fast as your legs will push you,
but you’ll only be getting assist to 20
mph. For commuting, this is more than
fast enough, and it can be so much fun
to blast away from stoplights. Doing
that speed up a hill seems even faster
when you’re not sweating doing it. The
more power you have available, the less
you and the motor will have to work.
It will also give you more available for
those times you’re in traffic and need
just a little more speed to safely make it
through a situation.
Most bikes will have two to four
levels of assist, and many can also be
driven via a throttle for those times you
want to let the bike do all the work. The
higher the level of assistance, the more
of the battery it will use.
WHAT KIND OF BATTERY
SHOULD YOU LOOK FOR?
Lead-acid batteries were the most
common in early electric bicycles.
They provide some power and are the
least expensive, but they’re also the
heaviest and live for a smaller number
of charge cycles. Lead-acid batteries
aren’t designed for high-amp-draw
applications, and this affects their real-world ability to deliver rated capacities.
Peukert’s law says that as the rate of
a battery’s discharge increases, the
capacity of that battery decreases. The
fully detailed nerdiness of this is fodder
for another article. Suffice it to say that
lead-acid batteries for electric bicycles
should be avoided.