Lpc2000

I don't understand the idea of VBAT in LPC2138. 

I want to have RTC running in case of main power supply failure. 

I didn't find any information about a battery backup switch in LPC. So
if I connect a battery to VBAT, RTC is powered from the battery all the
time. I'm afraid that the life of my battery will not be long. 

I can add an external battery switch (one of ADM.. or MAX..) but
Li-Ion batteries are 3.0V or 3.6V. In case of a 3.0V battery the
voltage drop on the switch causes that VBAT is outside of
specification. In case of a 3.6V battery the switch doesn't work
(battery voltage is higher then main power voltage!). 

Have anybody solved this problem?

Darek