There is no such code for "saving battery life". Not for other Nokia phones either.
On some phones there are codes that can be used to change, e.g., the default voice codec.
Some have the illusion that changing the default from the default EFR (Enhanced Full Rate) to HFR (Half Rate) would somehow use less power, but changing the setting won't really have an effect, because in practice it is the network that's really deciding what codec is used (and the network always uses the best quality codec it can).
Main point is: The phone and its software already has been designed to use as little power as it can get away with, and there is nothing you can do (except to use it less; make less calls, run less applications, etc.) to save battery.
If one battery isn't enough, get a second one that you carry with you & which can switch to when you run out of juice with the first one.