eBike

E-bike Battery Basics & Terminology


Ebike Battery Basics

Cells are connected together to form a battery. Each cell has a distinct voltage, depending on its composition. This is around 1.2V for NiCad/NiMH, 2.0V for lead acid, and 3.7V for lithium cells.

Because typical ebikes and scooters are built to run on 24, 36, or 48 volts, many cells must be linked in series to create a ‘battery’ with the required net voltage. A reasonable 36V pack can be constructed from 10 lithium cells, 18 lead acid cells, or 30 NiMH batteries

 

Amp Hours

The voltage of a battery decreases over time as you draw current from
it. The voltage will drop gradually until the cells begin to flatline,
at which point it will no longer offer power. The capacity (Ah) of a battery
is directly proportional to how long it lasts. The capacity of a battery
is defined by the amount of energy it can deliver for an hour, or one
amp for an hour.
 
In general, the size and cost of a cell scale directly with its amp-hour
capacity. To a first approximation, twice the amp-hours would translate
to double the size, weight, and price.  With the exception of different
packing densities and production scales, this is generally true.  The
common ‘AA’ nickel-metal hydride batteries, for example, have a capacity
of 2 Ah, whereas ‘C’ cells pack 4 Ah, ‘D’ cells 8 Ah, big ‘F’ batteries
12-13Ah, and double-D cells 18-19Ah
 
 

Watt Hours

The most important figure when comparing how far a specific battery pack
will take you is not the amp-hour capacity, but rather the total energy
stored watt-hours.One watt-hour is equal to one thousandth of a
kilowatt hour, the unit of electricity used to quantify household
electrical usage.The watt-hours stored in a battery pack are estimated
by multiplying the actual amp-hours by the pack voltage.
If you want to travel farther, simply increase the voltage. This will
eliminate the need for a greater number of amp-hours. So a 24V 8Ah
battery can deliver 192 watt-hours, while a 36V 5.5 AH pack also has
roughly 192 watt-hours. Assuming that both batteries are of the same
chemistry, you may anticipate them to weigh about the same, cost roughly
the same, and provide approximately the same performance on properly
designed ebikes (i.e., ones designed for 24V and 48V)
 
The watt-hours commonly seen in retail ebikes is around 192.  This is generally the smallest battery size recommended  for an
ebike. Many of the store-bought ebikes have about this much capacity
since it keeps the battery cost down.  For people who want to ride longer distances or
commute reasonable lengths of 25 miles or more, we would recommend a minimum of 300 watt-hours.  This number can vary a lot with usage habits, and you can get a lot further using the bike as a pedal assist.  For educational purposes an
energy consumption of 8-9 watt-hrs / mile is typical on most ebikes.
 
 
 

Energy Density

The Energy Density in watt-hrs / kg is one of the most essential factors
to consider when comparing battery chemistries. This number indicates
how heavy a battery pack must be to cover a certain distance. For

  • Lead Acid it is 20-30 whrs/kg,
  • NiCad it is 35-40 whrs/kg,
  • NiMH is 50-60 whrs/kg,
  • Li-ion is ~110 whrs/kg,
  • Li-Polymer is up to 160 whrs / kg.
Knowing these principles makes it simple to estimate the weight of a backpack without consulting the manufacturer’s website.
 

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