Rechargeable Batteries


PART A: What are rechargeable batteries?

A rechargeable battery is a battery that can be recharged and used many times. It is also known as a 'storage battery' as it has the ability to accumulate and store energy. The 3 most popular rechargeable batteries are:

 1) Nickel Cadmium (Ni-Cd)

 2) Nickel Metal Hydride (Ni-MH)

 3) Lithium Ion (Li-Ion)
How can the different batteries be compared?

 1) Energy density (how much energy a battery contains in comparison to its weight/volume)

Cell Type
Ni-Mh
Ni-Cd
Li-Ion
Gravimetric Density
55
50
90
Volumetric Density
180
140
210


  • Li-Ion Cells have a very high gravimetric density. 
  • Products can be made much lighter without compromising run time. 
  • When keeping battery weight the same, run time doubles for Li-Ion batteries are used. This is why Li-Ion has been displacing Ni-MH in many cellular phones and laptop computers.

2) Cell Voltage

 Ni-Cd and Ni-MH provides 1.25 cell voltage while Li-Ion provides 3.6 cell voltage.
3 Ni-Cd/Ni-MH cells are required to equal the voltage of 1 Li-Ion cell. However the first 2 kinds of cells have an advantage in that their discharge curve is extremely flat, almost close to an ideal battery. This means that Ni-Cd and Ni-MH cells are well suited for linear regulators, but Li-Ion batteries require switching converters to obtain good energy conversion efficiency in the power supply.

3) Peak Current

  • Maximum current that a battery can deliver is directly dependent on the internal
    equivalent series resistance (ESR) of the battery. 
  • The current flowing through the ESR will cause power dissipation within the battery that is equal to the ESR multiplied times the current squared 
  • Power = Current X Current X Resistance.
This can result in significant heating within the battery at high rates of discharge.

  • Ni-Cd and Ni-MH batteries have extremely low ESR values which means that ESR is almost never a limitation for peak discharge current in these cell types.
  • The Li-Ion battery will typically have a higher ESR (compared to Ni-Cd or Ni-MH), but
    will probably not be a problem in most applications.

4) Self Discharge
Self-discharge determines the "shelf life" of a battery.

  • In general, Li-Ion has the lowest self-discharge
  • Ni-Cd and Ni-MH are fairly comparable. Ni-Cd is typically a little better than Ni-MH, but this may even out as Ni-MH
    manufacturing technology matures.
Self-discharge is highly dependent on temperature, increasing
as the battery temperature is increased. Another characteristic is that the discharge rate is extremely non-linear. A battery which loses 30% in a month may lose 15 to 20% in the first few days.

Cell Type
Ni-MH
Ni-Cd
Li-Ion
Self discharge at 20 degrees Celsius (%./month)
20-30
15-20
5-10


5) Recharge Time

SLOW CHARGING
"Slow" charge is the charging current that can be safely applied to a battery
indefinitely without monitoring or charge termination. 


  • A Ni-Cd battery will easily tolerate c/10
  • Ni-MH cells are not as tolerant of constant charging, as most will not handle a sustained
    charging current greater than c/40.
  • Li-Ion cells will not tolerate trickle charging at all after they are fully charged. If current is forced into a fully-charged Li-Ion cell, the cell will be damaged. For this reason, Li-Ion cells are charged using constant-voltage (C-V) chargers, and not constant-current (C-C) chargers.
FAST CHARGING
"Fast" charge (1 hour recharge) needs more complex charging circuitry but gives a faster charging time. 

  • Ni-Cd or Ni-MH fast charger pumps current into the battery, and stops when it is full. 
  • Fast-charge systems designed to monitor cell temperature and voltage
  • They also have back up timers for cut off from high current
  • This is to prevent battery damage and user safety hazards

Cell Type
Ni-MH
Ni-Cd
Li-Ion
Slow Charge/h
12-36
4-10
Variable
Fast Charge/h
1
0.25-1
1.5

6) Cost

 Ni-Cd

  • Ni-Cd cell offers the best cost/performance value of any rechargeable battery
  • It is also a mature technology that is being produced in large volumes by many different manufacturers.
  • Continuing to improve in performance, due to competition of the Ni-MH battery. 
  • Modern Ni-Cd cells use foamed nicket substrate and feature volumetric/gravimetric energy density that is almost double the best Ni-Cd cells available a few years ago. 
  • Standard "AA" Ni-Cd cell rated at 600 mA/hr, but cells are now available with >900 mA/hr capacity. This added performance makes Ni-Cd an even better dollar value.
Ni-MH

  • The Ni-Cd battery contains Cadmium and is less environmentally friendly than Ni-MH. 
  • Alternate battery types may take over market share
  • Discharge voltage of the Ni-MH battery is considered identical to the Ni-Cd, so it can be used in most applications running on Ni-Cd.
  • 50 to 100% higher than Ni-Cd in price, the Ni-MH battery is displacing Ni-Cd in products such as cellular phones and laptop computers
Li-Ion
The Li-Ion battery is the most expensive of the types listed, presently appearing only in high end products where performance is the primary consideration.

7) Reliability

 NI-CD and NI-MH: HOW CELLS ARE DAMAGED

  • Sustained high-current overcharge
If a high charge rate is used, it is essential to terminate charge when the cell is full. If this is not done, the temperature and pressure within the cell will rise quickly as the charging current is dissipated as heat.
Both Ni-Cd and Ni-MH cells have internal vents which will open to allow gas to escape and prevent explosion of the cell. The gas that is lost can never be replaced, which means that the lost cell capacity which results from a severe overcharge is not recoverable.

  • Cell polarity reversal (during discharge)
Cell polarity reversal is a potential problem with any series-connected string of cells. As the battery is discharged, the cell that goes down to zero volts first will continue to have current forced through it by the other cells. When this occurs, the voltage across the fully-discharged cell is reversed.
A cell that has current forced through it with a reverse voltage across it will heat up very quickly and vent gas in a similar mode to that described for the sustained overcharge, with the same resultant damage.

LI-ION: HOW CELLS ARE DAMAGED
  • Large amount of energy in a small package
  • Internal resistance is high
Possible explosion may occur if the cell is accidentally shorted.The makers of Li-Ion cells handle the explosion threat by designing the case of the cell so that it will not explode easily. More important, the actual battery terminals are simply never allowed to reach the outside world.
The only manufacturer presently shipping Li-Ion batteries for consumer products does not sell single cells, only battery packs. Contained within the pack is circuitry that isolates the battery power leads from the outside world if excessive current is sensed, preventing battery damage and protecting the user.
Another easy way to destroy an Li-Ion battery is by discharging it too far. The Li-Ion cell should never be allowed to drop below about 2.4V, or an internal chemical reaction will occur where one of the battery electrodes can oxidize (corrode) through a process which can not be reversed by recharging. If this occurs, battery capacity will be lost. A similar process will occur if an Li-Ion cell is charged to too high of a voltage.

AGE-RELATED FAILURE MODES

Cell Type
Ni-Cd
After a period of time, the insulator within a Ni-Cd battery often develops holes which allow the cell to grow crystalline "shorts" that provide a conduction path between the positive and negative electrodes of the cell (which basically shorts out the cell).  If this happens, you may have to blow open this short with a high current pulse before the cell will again accept charge (a process sometimes referred to as "zapping").
A leaky Ni-Cd cell will always have a high self-discharge rate and will re-grow internal shorts if left on the shelf without some kind of trickle charge. The annoyance factor of batteries that go dead quickly often prompts users to throw away leaky Ni-Cd batteries, even though they may still be able to give nearly full A-hr capacity during discharge.
Ni-MH
The Ni-MH cell (according to the manufacturers) is not supposed to be prone to developing internal shorts like a Ni-Cd battery. User feedback (so far) on Ni-MH has been good, with no major problems reported.
Li-Ion
The Li-Ion battery got off to a weak start, as there were many failures in the first batteries shipped. However, the addition of the internal protection circuitry inside the battery (and increased knowledge about the failure modes) has improved this. At present, there are no known problems which appear significant enough to prevent Li-Ion from successfully penetrating the high-end consumer market.


7) Operating Temperature


  • Batteries are acutely sensitive to operating temperature with respect to their charging characteristics and A-hr capacity. Most well-designed chargers have temperature sensors to assure that the battery temperature is within the allowable "window" for charging
NI-CD/NI-MH

  • 0-50°C as the maximum operating limits for Ni-Cd and Ni-MH batteries
  • 10-40°C for fast charging of the batteries. 
  • Work best at temperatures close to 25°C, as their characteristics change very quickly when the temperature deviates from this "ideal" point.
At elevated temperatures, the cell experiences two undesirable effects:
a) The A-hr capacity of the cell reduces, meaning the cell will simply not deliver as much energy after being fully charged.
b) The cell gets reluctant to accept charge which makes it harder to fully charge the cell.

LI-ION

  • Safely charged at temperatures between 0-45°C.
  • Operating temperature range during discharge is specified as -20 to 60°C.
  • It is superior to Ni-Cd/Ni-MH in its performance over temperature.
The Li-Ion cell does not suffer a significant capacity loss at high temperatures, as the discharge curves at 20°C and 60°C are virtually identical. There is a progressive loss of capacity at low temperatures, with the 0°C delivered energy being about 90% of the 20°C amount, and at -20°C the cell delivers about 70% of the capacity that is delivered at 20°C.


PART B: How do they work?
Rechargeable batteries usually maintain a constant output until just before they go flat. They contain reactants that can be made again just by passing electricity through the products. Once the battery has gone flat, it can be connected to a recharger. This uses electrical energy to reverse the chemical reactions that happened in the battery while it was in use.
Car batteries are rechargeable batteries - they are constantly recharged while the car is moving, so the lights and horn will always work. Mobile phones, many MP3 players and other portable devices use rechargeable batteries. They must be recharged at regular intervals. It is usually recommended that such batteries should almost be flat before recharging. This allows the battery to be fully charged again.

The ability for reverse reaction, however, is not the sole characteristic of a rechargeable battery. It must also be able to undergo the reverse reaction both efficiently and safely multiple times. Some batteries can be recharged but because the chemical reactions are not completely reversed, they are only able to undergo the recharging process a few times and their performance is less efficient each time. Additionally, dangerous gases may build up and can lead to explosions or ignition either during or after recharging.

Main difference between normal batteries and rechargeable batteries is the way the battery discharges: (Alkaline batteries are normal ones)

The Charge/Discharge Curve
The measured terminal voltage of any battery will vary as it is charged and discharged
(see Figure 1).
The MPV (mid-point voltage) is the nominal voltage of the cell during charge or dis-
charge. The maximum and minimum voltage excursion from the nominal value is an
important design consideration: a "flatter" discharge curve means less voltage variation
that the design must tolerate.
When peak charged, the actual cell voltage will be higher than the MPV. When nearing
the EODV (end of discharge voltage) point, the cell voltage will be less than the MPV.
The EODV is sometimes referred to as the EOL (end of life) voltage by manufacturers.


PART C: What are its benefits?

Cost

  • Rechargeable batteries are advertised as being able to be reused more than 500 times, greatly reducing the cost of replacing the batteries. Some forms of rechargeable batteries will be more expensive than standard batteries, and the first-time cost of the charger itself must be taken into account. However, using the batteries repeatedly can save up to hundreds of dollars a year.

Performance

  • At one time, rechargeable batteries were believed to have a much lower performance than the standard alkaline batteries. However, technology has improved in this area, to the point where some rechargeable batteries are actually advertised to have a higher performance than normal batteries. Nickel-cadmium batteries have been known to perform well in very cold conditions and when excessively used, reducing the amount of times one would need to charge them.
PART D: What are its disadvantages?

Charging

  • Rechargeable batteries can be a hassle to recharge, especially if that forces any devices to be powerless while their batteries wait to recharge. Also, many such batteries have a high self-discharge rate, meaning they often need to be charged after just being stored. A second emergency set of batteries can solve the first problem, but the latter just complicates that solution.

Uses

  • Certain types of batteries, depending on their composition, are useful only with certain devices. Older hand-held devices will warn against using nickel-based batteries in them and recommend alkalines instead. Due to the cost of their materials, lithium ion batteries are used mainly in computers and cameras. The high number of different compositions can make it confusing for someone to decide which battery is best.

Hazards

  • Some rechargeable batteries, especially the nickel-cadmium variety, contain chemicals even more dangerous than those inside alkaline batteries. This makes them a very high environmental risk and one of the main reasons some countries place restrictions on the number of nickel-cadmium batteries that can be use

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