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Lithium-Ion Polymer Batteries

Charging

from http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

  • Turn off while charging

    • A portable device should be turned off while charging. This is to prevent the circuit that the battery is powering from drawing current while the battery is charging.
    • The load of the device (known as a parasitic load) will confuse the charger by drawing current and preventing the battery from reaching the saturation stage.
      • A battery may be close to or fully charged, but the parasitic load will prompt a continued charge, causing stress.

  • Fully charged

    • (1) Full charge occurs when the battery reaches the voltage threshold and the current drops to ~3% of the rated current.
    • (2) A battery may also be considered fully charged if the current drops to a lower level and is stable and cannot go down further. In this second scenario, an elevated self-discharge may be preventing the battery from completing the end of the saturation stage.

  • Charge rate

    • Increasing the charge current does not linearly cause the battery to charger faster.
      • The battery may reach peak voltage faster, but the saturation charge time will be increased as well.
    • A high current charge can be used to fill the battery to ~70% at a faster rate.

  • Is a full charge required?

    • Li-ion batteries do not need to be fully charged (unlike lead-acid batteries).
      • There are no benefits to a full charge, besides a longer runtime for that single cycle.
      • In fact, it is better not to fully charge the battery because a higher voltage stresses the battery.
    • Choosing a lower voltage threshold, or eliminating the saturation charge prolongs battery life.
    • To achieve maximum runtimes, most consumer devices choose a full charge over providing ideal conditions for battery life.