General

What does #CA discharge mean?

If the capacity of a battery is 7Ah (amps x hours), 6CA discharge means 6 x 7 x A = 42A discharge. If the battery is 12Ah and the max discharge current is 5CA, the max discharge current will be 5 x 12 x A = 60A.

What are the main specifications of rechargeable (secondary) batteries?

They include capacity (mA x hour), internal resistance(m Ohm), dimension, cycle life(times), maximum charge/discharge current (C), self discharge rate. There are other important specifications, e.g. under / over charge whether it will explode.

NiMh

General Notes

High self-discharge rate of over 30% per month in common nickel metal hydride batteries

There tends to be 2 grades of battery:- consumer where capacity may be higher but number of discharge cycles will be lower (big numbers are best), and industrial where capacity will be a bit less but discharge cycles of say 500 are common specs.

What kind of protections are needed for Ni-MH batteries, respectively?

In addition to the self-protections of the battery cells, a well designed charger and protection circuitry are also very important. For the protection circuitry, Ni-MH is much easier than li-ion. Often chargers include a polyswitch connected in series with the batteries and a thermoresistor with one-end connected to the ground power line and the other end to charger control. When short circuit happens, the polyswitch will be heated up and the resistance will increase rapidly to stop the current. When the temperature cools down, it will resume the supply of the power. The thermoresistor is used by the charger. When the charger find that the battery temperature increases (through the increase of the thermoresistor’s resistance), it will shut down the charging process. These two parts cost very little and are much cheaper than the li-ion protection circuit board.

What’s the maximum instantaneous discharge current of Ni-MH battery cells?

Typically the Ni-MH battery cell can be discharged at 5C~10C condition for several seconds. For example, an ED2000mA battery cell (capacity: 2Ah) can output 10A~20A current (2Ax5 ~ 2Ax10) if the discharge does not last for too long. This makes Ni-MH good for many high current applications, such as power hand tools and UPS.

What’s the maximum fast discharge current of Ni-MH battery cells?

Typically the Ni-MH battery can be discharged at 3C~5C condition for 12~20 minutes. For example, an ED1600AA battery cell (capacity: 1.6Ah) can continuously output 4.8A~8A current (1.6Ax3 ~ 1.6Ax5) for 12~20 minutes

Li-Ion

General Notes

3.0 to 4.2V (cell voltage typically specified as 3.7V)

Don’t allow the battery voltage to drop below 3.0V as it can damage the battery

Charge control IC’s are easily available for single batteries and dual in series batteries.

The preferred fast charge current is at the 1C rate, with an absolute maximum current at the 2C rate. For example, a 500 mAh battery pack has a preferred fast charge current of 500 mA.

Note that due to a large part of the charge cycle being constant voltage, with the charge current decreasing all the time, you can’t work out charge time by simply saying the charger will give say 2A charge current – it will only deliver that for the first part of the cycle. This means that going for a really high current charger may not be a real necessity as the battery will dictate how much current it will take.

Lithium-ion batteries can be formed into a wide variety of shapes and sizes so as to efficiently fill available space in the devices they power.

Li-ion batteries are lighter than other equivalent secondary batteries—often much lighter. The energy is stored through the movement of lithium ions. Lithium has the third smallest atomic mass of all the elements giving the battery a substantial saving in weight compared to batteries using much heavier metals. However, the bulk of the electrodes are effectively “housing” for the ions and add weight, and in addition “dead weight” from the electrolyte, current collectors, casing, electronics and conductivity additives reduce the charge per unit mass to little more than that of other rechargeable batteries.

Li-ion batteries do not suffer from the memory effect. They also have a low self-discharge rate of approximately 5% per month, compared with over 30% per month in common nickel metal hydride batteries (Low self-discharge NiMH batteries have much lower values; they can still hold 85% of their charge, after one year) and 10% per month in nickel cadmium batteries.

A unique drawback of the Li-ion battery is that its life span is dependent upon aging from time of manufacturing (shelf life) regardless of whether it was charged, and not just on the number of charge/discharge cycles. So an older battery will not last as long as a new battery due solely to its age, unlike other batteries. This drawback is not widely publicised.

At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 degrees Celsius or 77 degrees Fahrenheit will irreversibly lose approximately 20% capacity per year. However, a battery stored inside a poorly ventilated laptop may be subject to a prolonged exposure to much higher temperatures than 25 °C, which will significantly shorten its life. The capacity loss begins from the time the battery was manufactured, and occurs even when the battery is unused. Different storage temperatures produce different loss results: 6% loss at 0 °C (32 °F), 20% at 25 °C (77 °F), and 35% at 40 °C (104 °F). When stored at 40% – 60% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.

Under certain temperature conditions, the batteries have a tendency to become damaged and can sometimes never fully recharge again. In certain situations where the temperature is too cold (below the recommended battery temperature) the battery will still hold its charge but cannot be recharged as a result of the cold temperature. This is most common in smaller batteries such as cellular phones and handheld devices.

As batteries age, their internal resistance rises. This causes the voltage at the terminals to drop under load, reducing the maximum current that can be drawn from them. Eventually they reach a point at which the battery can no longer operate the equipment it is installed in for an adequate period. High drain applications such as powertools may require the battery to be able to supply a current of (15 h-1)C – 15/hours times C – the battery capacity in Ampere hours, whereas MP3 players may only require (0.1 h-1)C (discharging in 10 hours). With similar technology, the MP3 battery can tolerate a much higher internal resistance, so will have an effective life of many more cycles.

Li-ion batteries can even go into a state that is known as deep discharge. At this point, the battery may take a very long time to recharge. For example, a laptop battery that normally charges fully in 3 hours may take up to 42 hours to recharge. Or the deep discharge state may be so severe that the battery will never come back to life. Deep discharging only takes place when products with rechargeable batteries are left unused for extended periods of time (often 2 or more years) or when they are recharged so often that they can no longer hold a charge. This makes Li-ion batteries unsuitable for back-up applications where they may become completely discharged.

A stand-alone Li-ion cell must never be discharged below a certain voltage to avoid irreversible damage. Therefore Li-ion battery systems are equipped with a circuit that shuts down the system when the battery is discharged below the predefined threshold. It should therefore not be possible to deep discharge the battery in a properly designed system during normal use. This is also one of the reasons Li-ion cells are rarely sold as such to consumers, but only as finished batteries designed to fit a particular system.

When the voltage monitoring circuit is built inside the battery (a so-called “smart” battery) rather than the equipment, it continuously draws a small current from the battery even when the battery is not in use.  The battery must not be stored fully discharged for prolonged periods of time, to avoid damage due to deep discharge.

Battery Life Fuel Guage

On a lithium-ion cell, 3.8V/cell indicates a state-of-charge of about 50%. It must be noted that utilizing voltage as a fuel gauge function is inaccurate because cells made by different manufacturers produce a slightly different voltage profile. This is due to the electrochemistry of the electrodes and electrolyte. Temperature also affects the voltage. The higher the temperature, the lower the voltage will be.

Important Notes

Battery may swell during charging

For a battery thickness of more than 3mm, li-ion has more advantages than li-polymer, especially in price.

Why is a protection circuit board needed for li-ion batteries?

Lithium-ion battery operates between 3.0V and 4.2V. Outside this range, the capacity, life, and safety of the battery will degrade. When below 2.4V, the metal plates of the battery will be eroded, which may cause higher impedance, lower capacity and short circuit. When over 4.3V, the cycle life and capacity will be hurt. More over, lithium crystal will grow, which may eventually cause internal short circuit and explosion.

Why are there so many explosions been reported in the mobile phone industry?

When an adaptor (not a charger) is used to charge a lithium-ion battery pack, the safety of the pack is relied on the protection circuit board heavily. When the PCB fails to shut down a charge, explosion may occur. Although the chances for the PCB to fail is very low (e.g., 1 out of 1 million), 350 million new mobile phones a year can make many cases.

What the maximum discharge current of Li-ion battery?

About 1C for continuous discharge and 3C for instantaneous discharge. But these numbers can be changed by re-designing the battery.

What’s the cost structure and the key functions of the protection circuit board?

There are two ICs on the protection circuit board: the protection IC and the switch IC.  The key functions include over-current (include short circuit) protection, over-charge protection (limit the max voltage to about 4.25V), and over-discharge (limit the min voltage to about 3.0V) protection.

Guidelines for prolonging Li-ion battery life

Unlike Ni-Cd batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a long time, they should be brought to a charge level of around 40% – 60%. Lithium-ion batteries should not be frequently fully discharged and recharged (“deep-cycled”) like Ni-Cd batteries, but this may be necessary after about every 30th recharge to recalibrate any external electronic “fuel gauge” (e.g. State Of Charge meter). This prevents the fuel gauge from showing an incorrect battery charge.

Lithium-ion batteries should never be depleted to below their minimum voltage, 2.4v to 3.0v per cell.

Li-ion batteries should be kept cool. Ideally they are stored in a refrigerator. Aging will take its toll much faster at high temperatures. The high temperatures found in cars cause lithium-ion batteries to degrade rapidly.

Li-ion batteries should be bought only when needed, because the aging process begins as soon as the battery is manufactured.

When using a notebook computer running from fixed line power over extended periods, the battery could be removed, and stored in a cool place so that it is not affected by the heat produced by the computer.

Storage temperature and charge

Storing a Li-ion battery at the correct temperature and charge makes all the difference in maintaining its storage capacity. The following table shows the amount of permanent capacity loss that will occur after storage at a given charge level and temperature.

Permanent Capacity Loss versus Storage Conditions

Storage Temperature 40% Charge 100% Charge

0 °C (32 °F) 2% loss after 1 year 6% loss after 1 year

25 °C (77 °F) 4% loss after 1 year 20% loss after 1 year

40 °C (104 °F) 15% loss after 1 year 35% loss after 1 year

60 °C (140 °F) 25% loss after 1 year 40% loss after 3 months

It is significantly beneficial to avoid storing a lithium-ion battery at full charge. A Li-ion battery stored at 40% charge will last many times longer than one stored at 100% charge, particularly at higher temperatures.

Li-Polymer

What’s the difference between li-ion and li-polymer?

In terms technologies, their main difference is in battery packaging. Their positive and negative electrodes have similar chemical composition. Li-ion technology uses metal enclosure to limit the expansion of chemical materials over the battery’s life. Li-polymer uses polymer fibers to tie the chemical materials together. So it can use soft materials for enclosure, such as plastic or aluminum foil. For a thickness of 3mm or less, li-polymer has advantage in capacity. For a thickness more than 3mm, li-ion has more advantages, especially in price.