Adding secondary circuit protection to ensure safety when using Lithium-Ion and Lithium polymer batteries discussed in this app note from Littelfuse. Link here (PDF)
The introduction of portable devices with advanced features and the demand for longer run times and has resulted in innovations in the battery technologies used for portable applications. The latest generation of battery technologies has met these needs by increasing the energy density of the battery cell. The increase in energy density allows low voltage applications such as mobile phones to operate off of a single cell, and multiple cell applications to achieve longer operating times. The energy density change has also required that protection circuits be included with all rechargeable Lithium-Ion and Lithium Polymer battery packs. Protection circuits in packs include a control IC, MOSFET switch, external capacitor for IC timing to prevent inadvertent MOSFET operation, capacitors or varistors to protect against ESD and system transients, and secondary protection devices. The control IC and MOSFET provide primary protection for the pack during both charge and discharge cycles. This protection circuitry is offered in a single package solution for battery pack protection. While the primary protection circuit is reliable, to ensure complete user safety secondary protection devices are included to protect in situations outside of the normal range of operation. The secondary protection is provided by the use of overcurrent protection devices.