1. The Different Li batteries

The biggest difference of Li-ion and Li-polymer battery is the different state of their electrolytes (Not their appearance, Li-ion battery could also be made into Li-polymer batteries’ form, and this was actually many “Li-polymer” battery manufacturers are doing.). The Li-ion battery used the liquid electrolyte while the Li-polymer battery used the colloid electrolyte. And definitely the Li in the liquid state is much more active than the Li in the colloid electrolyte, and the more active the more dangerous. Another problem is the Li in the liquid state is likely to gather at special points in the battery (because of the electric field) to form very small metal pieces. Then these small metal pieces could pierce the film which divide the anode and the cathode material and caused disaster, this is unavoidable (And this is the real reason of the Sony and the other Li-ion batteries’ explosion, but they lie to the public as the metal pieces were been brought into the battery during the manufacturing process, which can be avoid. )
Both Li-ion and Li-polymer battery are evolved as LiCoO2 – LiMn2O4 – LiFePO4 - Li4Ti5O12.  Each of them has their own advantages and disadvantages.
LiCoO2 battery is the first one which has been introduced to the market, it has the highest Capacity/weight performance, but it is also the most active one, so mostly it was used in the small capacity batteries (A simple logic is even if it exploded, it will not arouse very big trouble), and it has a cycle life of about 500 cycles.
LiMn2O4 battery. It is much safer than the LiCoO2 battery, but its cycle life performance is poor, only about 300 cycles, and its capacity/weight performance is also poorer than the LiCoO2 battery, about 20% heavier than the LiCoO2 battery which has the same capacity. For the big capacity batteries, it is a much better choice. And we adopted this battery in (in order to reach their life span requirement, we used a tri-material, not the pure LiMn2O4).
LiFePO4, it is quite attractive because while maintain a high safety performance, it has a life span performance of about 1000 cycles, it also has a very good high temperature performance (we didn’t do the experiment but get this information from a partner) but its capacity/weight performance is poorer than the LiCoO2 battery (a little better than the LiMn2O4 battery), and its nominal voltage is about 3.2V, lower than the two kinds (3.7V) above. And I wish we can get this battery at a much cheaper price in the next year, at present its price is about 2 times of the LiMn2O4 one.
Li4Ti5O12, it is still under-research, but we can predict it will be the last Li battery before the fuel battery been released to the market, and our strategy is to control this material’s supply, in that case we could control the cell manufacturer.

Battery technology is advancing rapidly giving customers a bewildering number of choices. This short fact sheet aims to identify the selection criteria and evaluate the battery types against these criteria.

Battery chemistry

There are three main battery chemistries used to power electric bikes today. The mainstay for many years was the conventional lead acid battery, the modern version is sealed, hence Sealed Lead Acid (SLA) battery, although cheap these are still both heavy and inefficient.
Nickel Metal Hydride (NiMH) batteries offer the same performance as SLA at a fraction of the weight. They are also smaller and therefore easier to fit onto the bike. Lithium based batteries offer another step change in terms of performance being both lighter and more compact than NiMH batteries.
Lithium batteries come in a number of variants; Li-Ion is a (flammable) liquid electrolyte and proves to be the least stable, while Lithium Polymer (Li-Po) provides good performance in a stable substance. We expect to see other Lithium variants enter the market in the coming years.

Key battery selection criteria

Distance

The capacity of a battery is measured in Watt/hours where:
Watt/hours (Wh) = Battery Amp/hour (Ah) x Battery voltage. (v)
For example a 7Ah 37v battery has a capacity of 259Wh.
Unlike a car it is not possible to accurately state the number of miles a battery charge will last. It will depend upon many factors including battery capacity, terrain, amount of assistance (peddling) given by the rider, wind resistance, efficiency of electric motor and weight of rider.
As a rule of thumb an efficient electric motor like the nano needs approximately 12Wh per mile range. Hence a 259Wh battery will give approximately 22 miles.
The size of battery will depend your individual requirements. For example we use a small 2Ah Lithium battery around town while a 7 or 8Ah battery is used for traveling further afield.

24v or 36v

24v started as the norm based around 2 x 12v SLA units although as SLA batteries are being replaced, the trend is towards greater efficiency with higher-voltage motors to keep the peak current demand lower (10-15Amps). In the above table notice when compared to a 37v battery the 24v battery delivers a lower mileage range for the same Amp/hour rating. As 24v systems need components of a higher specification to handle the higher current it is both more efficient and cheaper to provide the higher voltage than current.

Weight

Although the electric motor is doing most of the work it is still more efficient to travel with a light battery, to say nothing of making the bike easier to handle. The following table compares the three main technologies with Lithium Polymer offering the greatest range for a given weight.

Size

Space is a precious commodity on most bikes so the smaller the volume occupied by the battery the better. The following graph/table should be used as a guide since these are just looking at battery sizes. Most batteries will be housed in some form of case which will increase the space required.

Battery characteristics

The following table summarises the characteristics of the different battery technologies

Cost

Cost can be considered from different perspectives:

The following table summarisies these costs based upon:

Current purchase price

SLA, 36v, 9Ah ￡45. This battery produces 324Wh, hence 100Wh costs ￡14
NiMh, 36v, 9Ah ￡195. This battery produces 324Wh, hence 100Wh costs ￡60
LiPo, 37v, 7Ah ￡225. This battery produces 259Wh, hence 100Wh costs ￡87

Recharge costs

A green electricity supplier quoted 10.4p per KWh and this is used as the basis for calculating recharge costs.

Recharge efficiency

The recharge efficiency of the battery chemistries are different:
SLA and LiPo both require 100Wh to generate 100 Wh
NiMh requires 140 Wh to generate 100 Wh

Recharge cycles

SLA 200 cycles
NiHm 400 cycles
LiPo 600 cycles

Battery options

The following table summarises the key facts for our popular batteries.