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Batterys

Fundamentals of battery technology

A battery is a device for storing electrical energy in the form of chemicals and for re-converting these chemicals into direct-current electricity. There are essentially two types of battery - primary and secondary. Primary batteries are discharged once and discarded. Secondary batteries are commonly known as rechargeable batteries. Their energy storage capacity can be revitalised many times. Rechargeable batteries have become important in contemporary society as concern over energy usage increases and portable electronic devices such as laptops and mobile phones are widely used.

There are a number of performance parameters which govern rechargeable battery technology:

  1. Energy density - the amount of energy that can be stored by the battery in relation to its weight. This is measured in Watt hours per Kilogram (Wh/Kg).
  2. Cycle count - the number of charge/discharge cycles that a battery can deliver within its service life. A battery?s service life is its useful life. This is generally considered to be up until its capacity has diminished to 80%.
  3. Shelf-life - the self discharge rate of a battery when not in use. This is usually measured in % per month.
  4. Environmental concerns - the toxicity of materials used in the battery and their disposability and recycleability.
  5. Operating conditions - atmospheric conditions at which the battery operates. Some batteries perform poorly in extreme temperature conditions.
  6. Cost - some materials used in battery manufacture are more costly than others. This naturally affects the price of the product. Also, newer technologies incur a higher price due to the cost of research and development.

Clearly, the perfect rechargeable battery would have a high energy density active over a large number of cycles. It, would have a long shelf-life, be made from inexpensive, environmentally friendly materials and would operate within a wide temperature range. In truth, this is an ideal. There is yet no such thing as the perfect rechargeable battery. The best option depends upon the use for which it is intended. There is always compromise involved in the decision making process.

Naturally, rechargeable batteries have been central to the development of High Power Cycle Lights.Lumicycle utilises two types of rechargeable battery in its lighting systems. These are NICKEL METAL HYDRIDE (NiMH) and LITHIUM ION (Li-Ion). They are high performance, technologically advanced batteries and are chosen to suit the specific requirements of cycling at night.

For many years, Nickel-Cadmium (Ni-Cad) was the only suitable rechargeable battery for portable electronic devises. Then in the early 1990s, NiMH and Li-Ion emerged for implementation in the commercial arena,[1] offering significant performance advances. Ni-Cads are still in use. However, they are highly toxic to the environment and will be banned in the European Union in the near future. NiMH and Li-Ion have become the dominant and most developed technologies. Whilst NiMH is nearing developmental maturity, Li-Ion appears to offer greater potential for future advancement. For this reason, Li-Ion has become the most talked about battery technology in recent years. Increasing numbers ofLumicyclecustomers are moving to Li-Ion rechargeable batteries but many still prefer the NiMH technology. For this reason, we supply both. Each has its own distinctive advantages and limitations which must be considered within the demands of each rider?s profile.

Nickel Metal Hydride rechargeable batteries

Advantages

  1. 30-40% higher energy density than standard Ni-Cad rechargeables.
  2. Smaller and lighter in weight than Ni-Cads.
  3. Less prone to memory effect than Ni-Cads.
  4. Less environmentally damaging than Ni-Cads - no Cadnium.
  5. Less maintenance than Ni-Cad - fewer exercise cycles are required ? once every 3 months versus once every month for Ni-Cads.
  6. Cheaper than Li-Ion.

Limitations

  1. High self-discharge - typically 50% higher than Ni-Cad.
  2. Performance degrades if stored at elevated temperatures - NiMH should be stored in a cool place at 100% state-of-charge.

Lithium Ion rechargeable batteries

Advantages

  1. Even higher energy density than NiMH - up to 40% more.
  2. Does not need prolonged priming when new. One regular charge is all that's required.
  3. Low self-discharge - self-discharge is less than half that of nickel-based batteries.
  4. Low Maintenance - no periodic discharge is needed and there is no memory effect.
  5. Better voltage conditions under high strain. This makes Li-Ions a better choice for riders using high wattage bulbs.
  6. Environmentally friendly - even more so than NiMH.

Limitations

  1. Subject to aging, even if not in use - storing the battery in a cool place and at 40% charge reduces the aging effect.
  2. Cost - greater than NiMH.
  3. Not fully mature - metals and chemicals are changing on a continuing basis.

For more information see the Wikipedia Lithium Ion article

Performance comparison

Below is a table comparing typical key performance statistics for NiMH and Li-Ion batteries:

NiMH

Li-Ion

Energy Density - Wh/Kg

60-120

110-185

Cycle Life ? to 80% initial capacity

300

300

Fast charge time - hrs

2-6 hours

2-6 hours

Self discharge/month at room temp.

30%

10%

Single cell voltage (nominal)

1.2v

3.7v

Operating temperature - centigrade

-20-+60

-20-+60

Maintenance required

60-90 days

Minimal

Environmental concerns

Relatively Low toxicity

Low toxicity

The bottom line ?Lithium Ion rechargeable batteries generally offer greater performance than NiMH. However, this comes at a greater cost. The correct choice will largely depend upon the importance of weight and price to the rider.



[1]Research for rechargeable NiMH and Li-Ion batteries developed during the 1970s and 1980s but the technology was not safe and robust enough for commercial adoption until the early 1990s.