Page 140 - Lindens Handbook of Batteries

BATTERY DESIGN 5.17

thus, discharging the battery beyond its safe cut-off could cause permanent damage to the battery. 6
Similarly, on charge, accurate control, as discussed above, will enable a maximum charge under safe
conditions without damage to the battery.
Modern battery packs and devices will utilize electronics in the device or battery to provide
monitoring that ensures the operational limits of the battery pack are not exceeded. These electronic
circuits can also provide enhanced safety and reliability, fuel-gauging, warranty data recording, and
long-term battery health information. Some devices such as cellular phones, PDAs, MP3 players and
digital cameras may use system-side monitoring and protection circuits, while larger devices such as
notebook computers and power tools use monitoring circuits located inside the battery pack.
The ability to interrupt charge and discharge current to protect the battery may also reside in a sepa-
rate location from where the monitoring occurs. Power tools, for example, may only interrupt charge
current inside the charger, while a laptop computer may contain a battery pack that can interrupt both
charge and discharge current inside the battery pack itself. A laptop computer, however, will often still
contain circuitry to allow it to interrupt both charge and discharge currents independently.
Discharge and charge control are especially important for a lithium-ion battery for which the
preferred charge protocol for a high rate charge is to start the charge at a relatively high, usually
constant current to a given voltage and then taper charging at a constant voltage to a given current
cutoff. Exceeding the maximum voltage is a potential safety hazard and could cause irreversible
damage to the battery. Charging to a lower voltage will reduce the capacity of the battery, although in
some applications such as uninterruptible power supply (UPS) systems, charging to a lower voltage
is preferred. As the charge is continuous, the lower capacity is an accepted side effect in this usage

since the life of the battery is more critical. 7
Another interesting example is the control of charge for hybrid electric vehicles (HEVs). In this
application, it is advantageous to obtain close to 100% charge efficiency or charge acceptance rather
than maximum battery capacity since the battery is essentially used as a capacitor or power reser-
voir instead of an energy source. The charge acceptance for a nickel-metal hydride battery at the
low state-of-charge (SOC) is close to 100%. As the nickel-metal hydride battery is charged, charge

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acceptance becomes progressively poorer, particularly above 80% SOC. (At full charge, the charge
acceptance is zero). In the HEV application using nickel-metal hydride batteries, the charge control
keeps the state-of-charge, under normal driving and regenerative braking conditions, as close to 50%
SOC as possible, and preferably within 30 to 70%. At these states-of-charge, the coulombic charge

efficiency is very high. 9
Other useful information can also be monitored and recorded by any charge or discharge con-
trol electronic circuitry either in the battery pack or on the device (system) side. This information
can be used for warranty return analysis, life-time predictions, fuel-gauging, and similar advanced
features.

5.5.3 Lithium-Ion Batteries
Special controls should be used with lithium-ion batteries for management of charge and discharge.
Typically, the control circuit will address the following items that affect battery life and safety:
Cell Voltage. The voltage of each individual cell in the battery pack is monitored on a continuous
basis. Due to safety concerns, secondary cell voltage monitors are often employed in the event that
the primary monitor fails. These secondary monitors typically only look for an overvoltage condition
and once detected, activate a permanent fuse in the charge current path. Depending on the specific
lithium-ion battery chemistry that is used, the upper voltage limit on charge, as specified by the
manufacturer, is usually limited between 4.1 to 4.3 V. On discharge, the cell voltage should not fall
below 2.5 to 2.7 V. Newer lithium-ion and polymer formulations have significantly different over-
voltage and undervoltage limits. Phosphate-based lithium-ion voltages have maximum limits near
3.8 V and minimums below 2.0 V. Titanate-based chemistries are even lower, with maximum limits
near 2.5 V and minimums near 1.8 V. In all cases, the tolerance for primary detection is typically
+/- 25 to 50mV for overvoltage conditions and +/– 50 to 100mV for undervoltage conditions.