The battery which comes with Galaxy S5 would last if used normally. You would like the battery to last more than a day to make the communication smooth. This is necessary in emergency situations or you are in a situation where the phone cannot be charged. Use the power saving features which comes with stock software. This way Galaxy S5 can last few days if used normally. Starting from new Ultra Power Saving mode to taking managing apps and screen brightness there are several ways to get better battery life without buying one or ceasing to use the features you are in love with.