Good Resources

Battery University

Lithium Cell Voltage

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

Series battery packs:

2 cells in series: 6.0 to 8.4V (7.4V typ)

3 cells in series: 9.0 to 12.6V (11.1V typ)

4 cells in series: 12.0 to 16.8V (14.8V typ)

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

Maximum discharge current

Lithium batteries will often have a specified maximum discharge current of say 2C, which means 2x their mAh rating.  For example a 120mAh battery with a 2C max discharge current would only allow you to draw up to 240mA continuous operating current.  This means for applications where you want high current but limited operating time you may need to select a larger battery than you'd ideally like so that you can obtain the continuous discharge current you need.  For applications such as UPS this may make lithium a non perfect choice over other battery chemistries.

General Notes

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 power tools 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

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 re-calibrate 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.

Temperature

Maximum generic lithium battery charge temperature: +45ºC

Maximum generic lithium battery discharge and storage temperature: +60ºC

The max temperature is not a safety limitation, as the batteries are tested up to +130ºC as part of their UL testing to ensure there is no thermal runaway or fire.  The limitation is due to the drop in performance at higher temperatures.  Some applications call for +70ºC operating temperature and it is possible to get a manufacturer to agree to the higher temperature usage of a 60ºC cell, but probably with guarantees on datasheet performance removed.  You also need to consider the usage and any self heating of the battery if the discharge current is high.  The same increase in temperature can be applied for charging also, in that you could probably get an agreement to charge a +45ºC specified cell at +55ºC, but again with performance drops.  Due to the self heating of charging you can't charge at the higher +60 / +70ºC temperatures.  Also bear in mind that lithium battery self discharge is much higher above +30ºC.

Lithium Battery Pack Standards

Lithium batteries need to be tested to UN38.3 in the EU.  Typically when buying single lithium batteries the battery will already have been tested to this.  However if your application needs a specially made lithium battery pack you will need to have this testing done if its not been done by the manufacturer.  Even if a custom battery pack is made from UN38.3 tested batteries, the act of assembling them into a new pack with a new protection board etc will require a re-test.  This can seem a bit unfair as you could argue that your new pack is safer due to the addition of a new additional protection board, but that's the requirement.

Testing to UN38.3 costs around €4000 from many companies but there are some who can do it for around GBP 1500.00 (PMBL as of Dec 2013) by using a far east test house and not marking up significantly.  The test house will also require a number of sample battery packs to be sent to them for destructive testing so this cost needs factoring in too.

Dangerous Goods Above 100Wh!

To be classed as non dangerous goods, class 9 a lithium battery needs to be <= 100Wh (e.g. a 11.1V 9Ah battery).  Above 100Wh the battery is classed as dangerous goods and if you need to air freight the battery you need to find a carrier willing to take it (you can't send if via standard air freight services).  This is regardless of UN38.3 testing. 

Note that this limit only applies to a single battery back.  You can ship multiple UN38.3 tested 99Wh batteries on standard services!  If your product needs more capacity and lithium is a must compared to other battery technologies then you could consider using multiple removable battery packs, each < 100Wh, as a way to avoid each individual battery pack being classed as dangerous goods.  A good example of this is the transport of lithium batteries in that a lorry is allowed to carry crates full of laptops each with its own battery even though the combined capacity of the batteries is far above 100Wh. Think also of a plane with many of its passengers carrying laptops each with high capacity lithium batteries.  These examples are not quite as simple as they seem due to equipment using lithium batteries being classified slightly differently to simply shipping boxes of lithium batteries and you will need to speak to your carrier about the requirements, but on a simplistic level if you want to air freight your batteries on normal services then you need to size the individual battery packs < 100Wh.

Using Multiple Lithium Battery Packs

For example, say your equipment uses 2 x 99Wh removable battery packs as a way of avoiding the dangerous good classification.  You can discharge the battery packs together, but when charging each battery pack must be charged individually.

Charge Thermistor

Thermistors are used for charging to protect against overcharging and over temperature.  Lithium batteries can only be charged safely within a specified temperature range.  This isn't just an ambient temperature range, if the battery has been discharged fast causing it to heat up and is then put on charge, the thermistor needs to protect the charger from being able to operate until the battery pack has cooled sufficiently.  The thermistor also adds a level of protection should the battery get too hot during charging.

Li-Polymer vs Li-Ion

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.

Series and parallel connection of Lithium batteries

Series connection is common where you need a higher overall battery voltage.

Parallel connection is also fine.  It's generally a good idea to try and limit to 4 cells in parallel, but it is possible to go up to 10 cells or more in parallel.  

Series and parallel connection is also fine where you need higher voltage and higher capacity, as long as you parallel the same number of cells for each series section. It's important that the cells are good quality so that the characteristics of the cells are the same and using a specialist battery company to build up these types of battery packs is highly advisable if you are not an expert yourself due to the inherent dangers of lithium cells if not properly used.

Building Block cells used to assemble larger parallel cell battery packs:

18650 is a standard building block cell.

Charging Lithium Batteries

Charge control IC’s are widely available for single batteries and in series connected batteries.

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

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 will only help during the first phase of the charging as the battery will dictate how much current it will take during the 2nd phase.

Note though, that you do not have to charge at the 1C rate. It is fine to charge at a much lower current, all that happens is that you lengthen the constant current phase of the charge cycle.  Whilst lithium cells are somewhat scary in that misuse can be dangerous, they are also quite simplistic in that to recharge then you simply put in the same amount of Ah you take out. The issue isn't putting it in fast enough, it is to avoid putting it in too fast and being very careful not to put too much in (overcharge).  Take an example of a 7.4V 24.8Ah lithium battery pack (16 cells connected in 2 series, 8 parallel):

To fully recharge the pack you need to put in up to 24.8Ah.

Say you have a max charge time of 8 hours to do this in. 24.8Ah / 24 hours = a 3.1A charge current.
To allow for the constant voltage phase you could add a bit more charge current to be sure you we're done, or maybe not if you would not expect the battery to be right down at its minimum capacity.

In this example you might opt for say a 3A or 3.5A charger IC, rather than a 24.8A charge solution at the 1C rate!  That means you would be charging at a C/8.3 or C/7.1 rate)

The constant voltage cut off point is often 4.2V.  You can improve the working life of a battery by setting your charge solution to switch to constant voltage at a slightly lower voltage (e.g. 4.1V) if maximum capacity isn't the primary concern.