Battery Self Discharge Rates

Battery Self discharge rates:

  • Bad news: all batteries have what is called a
    self discharge rate.
  • Good news: if you have a rechargeable battery
    all you need to do is recharge your battery!

Let me explain in greater detail the concept of battery self discharge.

All batteries will self discharge over a period of time naturally whether the battery is used or not. This means that the battery capacity will go from 100% down to 0% over a time period regardless if the battery is being used or not. Again recharging the battery corrects this naturally occurring reality.

Battery self discharge rate varies based on the chemistry type used and the temperature the battery is at: higher temperatures increase the self-discharge rate. This explains why batteries left inside cars on hot days must be recharged more frequently!

Battery self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery. Battery self-discharges does decrease the overall shelf-life of batteries and causes them to initially have less than a full charge when actually put to use.

Here are average self discharge rates for the following chemistries:

    

NiCd

  

NiMH

  

Lead Acid

  

Li-ion

  

Li-ion polymer

  

Reusable Alkaline

Self-discharge / Month (room temperature)

20%

30%

5%

10%

~10%

0.3%

 The rate at which batteries self discharge depends on the type of battery, state of charge, charging current, ambient temperature and other factors.  Lithium based batteries suffer the least amount of self-discharge (around 2–3% discharge per month), while nickel-based batteries are more seriously affected by the self-discharge rate.

To reduce the rate of battery self discharge during storage then store the battery at lower temperatures to reduce the rate of self-discharge and preserves the initial energy stored in the battery.

What Battery Chemistry Type is Better To Use With Power Tools?

There are three types of battery chemistries currently in use with power tools. They are NiCD, NiMH, and Li-Ion chemistries. The decision to choose one of the specific battery chemistry for your power tool depends on your application, the frequency of use, and the amount of power you need. Here is a brief breadown of the top benefits of each type:

Benefits

NiCd

NiMH

Li-ion

Life Span

Avg 5-10 years

Avg 3-5 years

Avg 1-3 years

Weight

Heavy

Medium

Light

Capacity – The higher the capacity rating, the longer your battery can last between charges (this is runtime).

Average runtime per charge

Highest -longest lasting runtime per charge

Good runtime per charge

Power per Charge

Average

Excellent

Good

Charge Time

Depends on Capacity

Depends on Capacity

Depends on Capacity

Environmentally Friendly

Ok

Good

Excellent

Economical

Lowest Cost

Average Cost

Highest Cost

Best Suited For

Home Owners

Contractors / Companies

Homeowners / Contractors / Companies

 Here is a more detailed account of the 3 chemistry types…

The Nickel Cadmium (NiCd) battery is a popular choice for two-way radios, emergency medical equipment and power tools.

NiCd batteries have both their advantages and disadvantages:

Advantages of NiCD

  • Fast charge.
  • High number of charge/discharge cycles — if properly maintained, the NiCd provides about 1200 charge/discharge cycles.
  • Good load performance in low temperatures.
  • Long shelf life.
  • Simple storage and transportation — no special conditions exist for most airfreight
    companies.
  • Forgiving if abused — the NiCd is is a rugged rechargeable battery.
  • Economically priced
  • Available in a wide range of sizes and performance options.

Disadvantages of NiCD

  • Low energy density — compared with NiMH.
  • Memory effect.
  • Environmentally unfriendly .
  • Has relatively high self-discharge.
  • Battery failure typical between 5-10 years based on usage.

The Nickel-Metal Hydride (NiMH) battery offers high energy density and the use of this chemistry is vastly more environmentally friendly than its counterpart NiCD batteries.

NiMH batteries have both their advantages and disadvantages:

 Advantages of NiMH

  • 30– 40 percent higher capacity over a standard NiCd.
  • Less prone to memory effect than NiCd. Periodic exercise cycles are required less often.
  • Simple storage and transportation — no transportation regulatory control.
  • Environmentally friendly — contains only mild toxins; profitable for recycling

 Disadvantages of NiMH

  • Limited service life — if repeatedly deep cycled, especially at high load currents, the performance starts to deteriorate after 200 to 300 cycles.
  • Limited discharge current — although a NiMH battery is capable of delivering high discharge currents, repeated discharges with high load currents reduces the battery’s cycle life.
  • More complex charge algorithm needed
  • High self-discharge
  • Performance degrades if stored at elevated temperatures — the NiMH should be stored in a cool place and at a state-of-charge of about 40 percent.
  • About 20 percent more expensive than NiCd — NiMH batteries designed for high current draw are more expensive than NiCd.
  • Battery failure typical between 3-5 years based on usage.

The Lithium Ion battery is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight.

Advantages of Li-ion

  • High energy density — potential for yet higher capacities.
  • Relatively low self-discharge — self-discharge is less than half that of NiCd and NiMH.
  • Low Maintenance — no periodic discharge is needed; no memory.
  • Lightweight compared to NiCd and NiMH.
  • Environmentally freindly.

Disadvantages of Li-ion

  • Requires protection circuit — protection circuit limits voltage and current. Battery is safe if not provoked.
  • Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect.
  • Moderate discharge current.
  • Shipment of larger quantities of Li-ion batteries may be subject to regulatory control.
  • Expensive to manufacture — about 40 percent higher in cost than NiCd.
  • Capacity deterioration is noticeable after one year, whether the battery is in use or not.
  • Battery failure typical between 1-3 years based on usage.

 

 Battery Chemistries by the numbers:

 

 

NiCd

NiMH

Li-ion

Gravimetric Energy Density(Wh/kg)

45-80

60-120

110-160

Internal Resistance 
  (includes peripheral circuits) in mΩ

100
  to 2001

  6V pack

200
  to 3001

  6V pack

150
  to 2501

  7.2V pack

Cycle   Life (to 80% of initial capacity)

15002

300
  to 5002,3

500
  to 10003

Fast Charge Time

1h
  typical

2-4h

2-4h

Overcharge Tolerance

moderate

low

very
  low

Self-discharge / Month (room temperature)

20%4

30%4

10%5

Cell Voltage(nominal)

1.25V6

1.25V6

3.6V

Load Current

  – peak

  – best result

  20C

  1C

  5C

  0.5C or lower

  >2C

  1C or lower

Operating
  Temperature(discharge only)

-40
  to

  60°C

-20
  to

  60°C

-20
  to

  60°C

Maintenance
  Requirement

30
  to 60 days

60
  to 90 days

not
  req.

Typical
  Battery Cost

  (US$, reference only)

$50

  (7.2V)

$60

  (7.2V)

$100

  (7.2V)

Cost
  per Cycle(US$)11

$0.04

$0.12

$0.14

Commercial
  use since

1950

1990

1991

 

1: Characteristics of commonly used rechargeable batteries

  1. Internal resistance of a battery pack varies with cell rating, type of protection
    circuit and number of cells. Protection circuit of Li‑ion and
    Li-polymer adds about 100mΩ.
  2. Cycle life is based on battery receiving regular maintenance. Failing to apply
    periodic full discharge cycles may reduce the cycle life by a factor of three.
  3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles
    than deep discharges.
  4. The discharge is highest immediately after charge, then tapers off. The NiCd
    capacity decreases 10% in the first 24h, then declines to about 10% every 30
    days thereafter. Self-discharge increases with higher temperature.
  5. Internal protection circuits typically consume 3% of the stored energy per month.
  6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference
    between the cells; it is simply a method of rating.
  7. Capable of high current pulses.
  8. Applies to discharge only; charge temperature range is more confined.
  9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
  10. Cost of battery for commercially available portable devices.
  11. Derived from the battery price divided by cycle life. Does not include the cost of
    electricity and chargers.

Data taken from Battery University all rights reserved

What Battery Chemistry Will Be the Best for My Device?

The battery chemistry that will be best for your device depends on the device and the application. Many devices will simply not allow you swap chemistry types. The big exception to this rule is power tools where you can use NICD, NIMH, or Li-ion battery chemistry types.

Let’s take a more detailed look at: NiCd, NiMH, Li-ion, Li-Po, and Reusable Alkaline batteries.

By the numbers we get the following:

 

NiCd

NiMH

Lead
  Acid

Li-ion

Li-ion
  polymer

Reusable

  Alkaline

Gravimetric Energy Density(Wh/kg)

45-80

60-120

30-50

110-160

100-130

80
  (initial)

Internal Resistance 
(includes peripheral circuits) in mΩ

100 to 2001

  6V pack

200 to 3001

  6V pack

<100

12V pack

150 to 2501

  7.2V pack

200 to 3001

  7.2V pack

200 to 20001

  6V pack

Cycle   Life (to 80% of initial capacity)

15002

300 to 5002,3

200 to 3002

500 to 10003

300 to 500

50(to 50%)

Fast Charge Time

1h

2-4h

8-16h

2-4h

2-4h

2-3h

Overcharge Tolerance

moderate

low

high

very
  low

low

moderate

Self-discharge / Month (room temperature)

20%4

30%4

5%

10%5

~10%5

0.3%

Cell Voltage(nominal)

1.25V6

1.25V6

2V

3.6V

3.6V

1.5V

Load Current

  – peak

  – best result

  20C

  1C

  5C

  0.5C or lower

  5C7

  0.2C

  >2C

  1C or lower

  >2C

  1C or lower

  0.5C

  0.2C or lower

Operating Temperature(discharge only)

-40 to 60°C

-20 to 60°C

-20 to 60°C

-20 to 60°C

0 to 60°C

0 to 65°C

Maintenance Requirement

30 to 60 days

60 to 90 days

3 to 6 mos9

not req.

not req.

not req.

Typical Battery Cost

  (US$, reference only)

$50

  (7.2V)

$60

  (7.2V)

$25

  (6V)

$100

  (7.2V)

$100

  (7.2V)

$5

  (9V)

Cost per Cycle(US$)11

$0.04

$0.12

$0.10

$0.14

$0.29

$0.10-0.50

Commercial use since

1950

1990

1970

1991

1999

1992

 

 

1: Characteristics of commonly used rechargeable batteries

  1. Internal resistance of a battery pack varies with cell rating, type of protection
    circuit and number of cells. Protection circuit of Li‑ion and
    Li-polymer adds about 100mΩ.
  2. Cycle life is based on battery receiving regular maintenance. Failing to apply
    periodic full discharge cycles may reduce the cycle life by a factor of three.
  3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles
    than deep discharges.
  4. The discharge is highest immediately after charge, then tapers off. The NiCd
    capacity decreases 10% in the first 24h, then declines to about 10% every 30
    days thereafter. Self-discharge increases with higher temperature.
  5. Internal protection circuits typically consume 3% of the stored energy per month.
  6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference
    between the cells; it is simply a method of rating.
  7. Capable of high current pulses.
  8. Applies to discharge only; charge temperature range is more confined.
  9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
  10. Cost of battery for commercially available portable devices.
  11. Derived from the battery price divided by cycle life. Does not include the cost of
    electricity and chargers.

Data taken from Battery University all rights reserved