Lithium batteries are commonly used in many device applications that require backup power, such as real-time clocks (RTCs) and storage devices. When a lithium battery is not the sole source of power in a circuit, there is a risk of fire or explosion if the battery is accidentally connected to a power source that can charge the battery. This application note provides the information needed to connect a lithium battery in a backup power switching circuit so that the circuit complies with Underwriters Laboratories (UL) standards. Specifically, UL Standard 60950-1 describes guidelines for lithium batteries.

Intersil RTC Protection Circuit with Battery Switchover
All Intersil RTCs with battery switchover, such as the ISL12026 series, have internal protection circuitry to prevent reverse charging. Figure 1 shows the internal switching circuit, illustrating complementary control that disables one power input while enabling the other. Two series MOSFET switches provide safe switching and redundancy. It is also the path from VDD to VBAT, through the MOSFET (M1) and two 200Ω resistors (R1 and R2) used to pull up the gate of the VBAT switch. If the gate of M1 is shorted, the current flowing through resistors R1 and R2 will limit the charging current of the lithium battery (up to 9.25mA when VDD = 5.5V, VBAT = 1.8V).

To meet full UL safety requirements, a series Schottky diode needs to be added to the VBAT input of these products. The external diode is used to provide protection in the event of a failure of the internal MOSFET and series resistor. Figure 2 shows the actual circuit with an external protection diode. (Note that a high value series resistor will provide similar protection, but will limit the normal operating range.)

Use only silicon diodes or Schottky diodes with low reverse current. The typical reverse current recommended by UL is 1μA. Some diodes with low reverse current that can be used include, but are not limited to, BAS40, BAS70, and BAT54 diodes.
Where Q is the nominal capacity of the battery in mAh and tp is the total allowed charging time in percent. For coin cells, tp is 3%. For cylindrical cells, tp is 1%.
Example: A 1000mAh coin cell lasts five years.
Based on Equation 2, Ic is 30mAh (1000mAh x 3% (coin cell) = 30mAh).
Based on Equation 1, a diode with a reverse current of 0.7μA or less is needed (30mAh ÷ time of use (5 years x 365 days x 24 hours) = 0.7μA).