What is the difference between the useful life and reliability of the battery? How is reliability expressed?

Reliability is the capacity of an object to conserve over time and within established limits the values of all parameters characterizing its ability to perform the functions required of it in set modes and conditions of use, maintenance, storage and transportation.

Measures of battery reliability are the number of charge-discharge cycles of the battery while maintaining its characteristics.

The charge voltage and the number of charge-discharge cycles relationship for lithium-ion battery cycles.

Lithium-ion (Li-ion) batteries normally have a lifespan of between 300 and 500 cycles. In normal use it can be expected that after 300 charge-discharge cycles (approximately one year of operation) its capacity drops to 80%.

Raising or lowering the ambient temperature during operation may lead to a drop of 70% or more for a similar number of charge-discharge cycles.

The nominal capacity of the battery is the amount of electrical energy which the battery theoretically should have in the charged state. The amount of energy is determined by the direct current discharge of the battery for a measured period of time until it reaches a predetermined threshold voltage. It is measured in ampere-hours (A * h) or milliampere-hours (mA * h). Its value is indicated on the battery label or encrypted in the type designation.

In practice this value ranges from 80 to 110% of the nominal value and depends on many factors: the manufacturing firm, storage conditions and time, the technology for its introduction into service,  the service technology while in operation, the chargers used, the conditions and duration of operation etc.

Theoretically a battery with a nominal capacity of 600 mAh can give a current of 600mA for one hour, 60 mA for 10 hours or 6mA for 100 hours. In practice, at high discharge currents, nominal capacity is never reached and it is exceeded at low currents.

As an example shown below are the typical discharge characteristics of Li-ion elements at different discharge currents.

The life cycle of a lithium-ion battery when it is charged and discharged at currents of 1C, 2C and 3C.

Several recommendations which will help extend the life of lithium-ion batteries.

Always try to put the battery on charge straightaway after its purchase if it is completely or almost completely discharged. At the same time avoid deep discharge-charge cycles. The table shows how many charge-discharge cycles will follow before losing 50% of capacity in relation to the depth of battery discharge.
Depth of discharge Number of cycles (Li- cobalt)

100%

300 – 500

50%

1 200 – 1 500

25%

2 000 – 2 500

10%

3 700 – 4 700

Do not charge batteries at low temperatures. They will lose capacity.

Do not store the battery in temperatures above 30 degrees Celsius. The table shows how the capacity will change depending on the storage temperature and the battery’s level of charge:

Temperature , оС With 40% charge With a 100% charge
0 2% over a year
6% over a year
25 4% over a year
20% over a year
40 15% over a year
35% over a year
60 25% over a year
40% over three months

Quick chargers, which charge batteries in less than an hour, never charge them completely. Upon completion of the procedure batteries, as a rule, have a charge of 80-90% of their capacity. 

The lithium-cobalt battery can store more energy than other batteries, but in terms of specific power (load) and thermal stability, lithium-manganese and lithium-phosphate batteries are superior. As we advance in the direction of the electric power-supply unit, safety and life cycle will become more important than power.

In spite of various chemical differences, lithium-ion batteries can generally be divided into two groups: lithium iron phosphate (LFP, LiFePO4) and metal oxides (NCM, NCA, cobalt, manganese). The table shows the differences between the two groups of batteries.

 

LFP

LiNCM

Voltage

3,3 V

3,7 V

Energy density

300 Wh/l

735 Wh/l

Specific energy

128 Wh/kg

256 Wh/kg

Power

1 000 W/kg

512 W/kg

Number of charge-discharge cycles

2 000 – 100% discharge

3 000 – 80% discharge

750 – 100% discharge

1 900 – 80% discharge

Average lifetime

6 years

8 years

The maximum recommended temperature

40 ° C

55 ° C

Safety level

High

Moderate

Commercial vendors A123, Valence, BAK, BYD, K2, Lishen, many Chinese vendors Sanyo, Panasonic, Samsung, DowKokam, Sony, LG Chem, Moli

Battery life is significantly increased if the depth of each discharge is limited to 80% of the nominal power.

Pin It on Pinterest