ARM with integrated battery charger?
Page 157 gives a clear hint: Charging resistor 5k or 1.5k. 

VBat in a microcontroller isn't meant to let you operate the entire thing from a 50Ah Li-Ion. It's a back-up battery pin, that uses extremely little power when the main power goes away, to keep a clock or calendar running and possibly keep some or all of the RAM to its value.

It can be connected to a non-rechargeable Lithium coin cell, or to a small rechargeable battery, or to a large value capacitor.

If you connect a high-value capacitor to Vbat, such as a 1F one, that can keep the memory and/or functions in tact a while on the few-dozen-&mu;A it needs to stay "prepared". The internal resistor can then safely recharge that capacitor once power comes back. Many of these large capacitors don't fare very well for very long when they charge with high currents, so ST just decided to include the power-path and protection resistor in the chip for your convenience.

If your entire device works on a battery, obviously it'll be big-ish, and charging it with 1.5k would take weeks if not more. Not to mention that batteries of Lithium types and several other chemistries don't do very well with a fixed resistor from a fixed voltage as a "charge system". Of course with 1.5k or 5k, if you had the patience, it might work well enough, since that's hardly any current.

Note:
Do be aware that (I didn't read more than page 157) many controllers need the Vbat pin to be powered, so if you don't use a back-up battery, check the datasheet if you then need to connect the pin to Vdd, or what else they advise. (Maybe just a 1&mu;F cap and turn on the 1.5k?)