If you limit your smartphone usage to the capabilities of a 2003 Nokia (turn off data and wifi, only use calls and SMS) the battery will last 2 weeks and never crash or reset. Before I got a phone with dual SIM capability I used to bring an old spare phone to keep my home SIM in with data off only to be able to not miss calls/SMS. They’d typically last the whole trip without charging when they’re not keeping connections alive for email, push etc.

Before I got a smartphone I used a j2me IRC client to keep connected with my friends, and I had to carry 3 batteries to swap throughout the day for it to last, the battery life was horrible if you actually did anything on it.