Month: April 2017

Proper Use of Li-ion Battery Pack

Marvelous Power Comes from Proper Handling of Li-ion Battery and its Packs

Li-ion battery is becoming the most promising energy storage solution to the future. It has proved its superiority over the NiCad and NiMH batteries since its was commercialized in 1991. Nowadays, many worldwide advanced researches are still booming to improve the performance, safety, reliability and affordability of the existing Li-ion batteries. To fully utilize the tremendous  power delivered by the evolving Li-ion batteries, users should understand how to handle it properly. We are describing some useful information from battery cell to battery pack for your reference below.

Cell Charging:

  • A CC/CV (Constant Current / Constant Voltage) charger is required for charging a Li-ion cell. Unless a cell is specially specified, the charging voltage of a normal Li-ion cell is limited to 4.20 V/cell. Over-charging a Li-ion cell will reduce its nominal capacity and obviously shorten its cycle life. A Li-ion cell may be excessively charged if it is left on a charger unattended for long time, and it may lead to cell failure or a serious safety issue!
  • Never use a DC power supply to charge a Li-ion cell because a continuous charging device will over-charge the cell with trickle currents. When a Li-ion cell is fully charged at 4.20 V/cell, the charging current should be completely cut off when the charging current drops below 0.02C level.
  • The standard charging current is o.5C after a Li-ion cell is discharged and its OCV (Open Circuit Voltage) is above 3.0 V. The OCV of an aged cell may stay below 3.0 V after excessive discharge. For such a case, a pre-charge at 0.1C is required before its OCV rises over 3.0 V. Dispose a Li-ion cell if its OCV no longer exceeds 3.0 V.

Cell Discharging:

  • The discharge current must obey its specification. Never abuse a Li-ion cell by discharging it over the specified continuous maximum discharge current.
  • The discharge voltage must obey its specification. Never abuse a Li-ion cell by discharging it below 2.0 V.
  • Use caution to monitor the body temperature of a Li-ion cell when it is drained at high current. A Li-ion cell will shorten its life if you allow its body temperature to rise over 60 degree C.

Storage:

  • A Li-ion cell should be stored in a dry area without any corrosive gas.
  • It requires paper board crate to store each cell separately in a sturdy box so that no excessive mechanical force can shock or deform the cells in storage.
  • The ambient temperature from -10 ~ +25 degree C is proper for long-term storage (e.g. 12 months). Wider temperature range from -20 ~ +45 degree C is also acceptable for a short-term storage (e.g. 3 months).
  • The SOC (State of Charge) of Li-ion cells should be controlled between 40% to 50% before they are put into storage.

Cycle Life:

  • If a Li-ion cell is charged and discharged per its specification, it will deliver the promised cycle counts and capacity level according to its cycle life test data.
  • Deep cycles are not suggested for the normal use of a Li-ion cell or pack unless it is necessary for a special application. Reduce the voltage swing will optimize the cell performance and lengthen its life.
  • Cycle life of a Li-ion cell is determined by the conditions of charging, discharging, operating temperature and storage.

Battery Pack Design:

  • The design of a Li-ion battery pack must consider a structure that can effectively prevent water and static charges from spreading through around the assembled cells.
  • If a PCB/PCM (Protection Circuit Board/Module) is required to be assembled with cells, the leakage current must be less than 1 micro Amps and the short circuit protection must be detected less than 0.5 ms (shorter detection time is better). Other features like over-voltage protection, over-discharge protection, and over-current protection should be considered, too.

Battery Pack Assembly:

  • The usage of damaged cells must be prohibited! Never use abnormal cells, which have been damaged during the transportation. Damaged cells may be resulted from over stress or deformed housing, short circuit, twice spot welded, or venting with electrolyte odor.
  • Inspect OCV, IR (Internal Resistance) and Capacity before doing the cell assembly. Only precisely matched cells are allowed to be assembled into battery packs.
  • Use spot welder and nickel strip or plate to connect cells in series or in parallel. Never solder directly onto a cell, which may be damaged by the overheat!
  • Accidental short circuit may occur during the pack assembly. It is dangerous and may hurt the operators. Proper wiring layout and assembly procedures must be integrated into well-trained workmanship to effectively avoid any possibility of a short circuit.