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Cordless power tool batteries: NiCd vs NiMh vs LiIon


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Wednesday, May 23 2007, 00:00:13 #40074     Cordless power tool batteries: NiCd vs NiMh vs LiIon


NiCd

The most common power tool battery chemistry is NiCd or Nickel-Cadmium. NiCd cells use nickel hydroxide (NiOH) and metallic cadmium (Cd) as the active chemicals. The principle advantages of NiCd over other rechargeable types is good charging efficency, small variation in terminal voltage during discharge, low internal resistance, and non-critical charging conditions.

Note that the abbreviation NiCad is a registered trademark of SAFT Corporation and should not be used to refer generically to nickel-cadmium batteries, although this brand-name is commonly used to describe all nickel-cadmium batteries.

Nickel-cadmium cells have a nominal cell potential of 1.2 V. Unlike common primary cells, a NiCd cell's terminal voltage only changes a little as it discharges.

NiCd batteries, when not used regularly, tend to develop dendrites which are thin, conductive crystals which may penetrate the separator membrane between electrodes. This leads to internal short circuits and premature failure, long before the 800Ц1000 charge/discharge cycle life claimed by most vendors. Sometimes, applying a brief, high-current charging pulse to individual cells can clear these dendrites, but they will typically reform within a few days or even hours. Cells in this state have reached the end of their useful life and should be replaced. Many battery guides, circulating on the Internet and online auctions, promise to restore dead cells using the above principle, but achieve very short-term results at best.

http://www.repairfaq.org/ELE/F_NiCd_Battery.html

NiMh

A nickel-metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but using a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd batteries, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size NiCd. However, compared to the lithium-ion battery, the volumetric energy density is lower and self-discharge is higher.

Li-Ion

Lithium-ion batteries (sometimes abbreviated Li-ion batteries) are a type of rechargeable battery in which a lithium ion moves between the anode and cathode. The lithium ion moves from the anode to the cathode during discharge and from the cathode to the anode when charging.


The three primary functional components of a lithium ion battery are the anode, cathode, and electrolyte, for which a variety of materials may be used. Commercially, the most popular material for the anode is graphite, although materials such as TiS2 were originally used.[3] However, the cathode is generally one of three materials: a layered oxide, such as cobalt oxide, a polyanion, such as lithium iron phosphate, or a spinel, such as manganese oxide. Depending on the choice of material for the anode, cathode, and electrolyte, the voltage, capacity, life, and safety of a lithium ion battery can change dramatically. Lithium ion batteries are not to be confused with lithium batteries, the key difference being that lithium batteries are primary batteries containing metallic lithium while lithium-ion batteries are secondary batteries containing an intercalation anode material.

A123 Systems Li-Ion (LiFePO4)


Code:
Chemistry Type             Ni-Cd      Ni-MH       Li-Ion     LiFePO4
Nominal Voltage (V)        1.2        1.2         3.6        3.3
Power/weight (W/kg)        150        250-1000    1800       1350
Cycle Life (Times)         2000+      500-1000    1200       1500
Shelf Life (Months)        48         36          24-36      72
Safety                     High       High        Low        High
Cost                       Low        Medium      High       High
Self-Discharge (%/month)   25-30      30-35       5-10       5-10
Memory Effect              Yes*       No          No         No
Charge/discharge eff %     70-90      66          99.9       99.9


*With power tool application, due to uneven discharge rates and high currents, memory effect does not affect NiCd. Memory effect in NiCd is only noticeable with applications such as cordless phones, where the cells are being charged repetitively and never fully drained.

**Li-Ion batteries made by A123 Systems (used in DeWalt tools) are LiFePO4 and are very safe. Puncturing a standard Li-Ion battery will cause it to burst into flames and explode, where an A123 cell will just heat up and vent without flames or explosion.


Link on YouTube


Here are the pros and cons of different battery technologies:

NiCd

Pros:
+ charging with very high currents does little or no damage to the battery allowing very fast charging (15 minutes)
+ can provide very high currents, suitable for high-drain application such as power tools
+ work well in cold weather
+ have long shelf life
+ can withstand 400-1000 cycles with minumum capacity loss
+ relatively safe
+ can be stored discharged

Cons:
- has high self-discharge rate (20% or higher per month) 15-20% the first 24 hours, 7-10% the next day
- has low power-to-weight ratio compared to NiMH or LiIon
- very toxic inside and has to be recycled
- can develop internal shorts (dendrites) over time

NiMH

Pros:
+ high energy density compared to NiCD, up to 3600 mAh per cell compared to 2400 in NiCd
+ cheaper than Li-Ion
+ high shelf life
+ can withstand 500 cycles with minumum capacity loss

Cons:
- has high self-discharge rate (30% or higher per month)
- cannot provide as much current as NiCd
- cannot be charged fast without shortening cell life
- freeze at 4 degrees C and must be warmed up to activate catalyst
- must be stored charged
- half the charge/discharge cycles compared to NiCd

Standard Li-Ion

Pros:
+ low self-discharge rate (5% per month)
+ very high energy density (25-33% higher than A123 Systems LiFePO4 Li-Ion aka DeWalt NANO)
+ can be charged fast without damaging the cells
+ can withstand ~1000 cycles with minumum capacity loss

Cons:
- unsafe: can burst into flames and/or explode if shorted or damaged.
- a little expensive compared to NiCd or NiMH
- perform poorly in low temperatures (below 0 C / 32 F)
- toxic and must be recycled
- cannot provide as much current as NiCd
- lower number of cycles compared to A123 Systems LiFePO4 Li-Ion aka DeWalt NANO

A123 Systems LiFePO4 Li-Ion aka DeWalt NANO

Pros:
+ very safe compared to traditional Li-Ion batteries
+ low self-discharge rate (5% per month)
+ high energy density (lower than standard Li-Ion but it's a price of safety and high discharge current)
+ can be charged fast without damaging the cells
+ can withstand up to 2000 cycles with minumum capacity loss
+ perform fairly well in cold weather

Cons:
- very expensive compared to NiCd, NiMH or Li-Ion - up to $16 per cell.
- toxic and must be recycled
- lower energy density compared to traditional Li-Ion.
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