BATTERIES: how your recorder is powered depends on various factors but basically there are three groups:

1) recorders powered by conventional AA batteries - sure Duracell batteries will last longer than a cheaper brand, but by far the best option is to invest in some high capacity, power holding re-chargeable batteries & possibly a re-cycling charger that depletes & then restores the cells from time to time. Your standard Duracell battery is 1500 mAh capacity, whereas the better recyclable AA's will be 2100mAh, 2500mAh and higher these days - meaning they'll last, on average, 20-40% longer. Two brands that I personally know to be good are:

Sanyo Eneloop
Recyko

on handheld recorders there are various things that will eat battery power quickly, such as phantom power being turned on - so if you're not using it turn it off in the menu.

2) recorders powered with standard li-ion batteries - some hand held recorders use these batteries & whilst generic batteries can be significantly cheaper, they often don't hold their charge very long or last a large number of re-charges. It's better (& cheaper) to spend a bit more & get a decent one (+ a couple of spares).

3) recorders powered with Sony NPF / L style batteries (such as the Sound Devices recorders). I buy mine from Hawk Woods, which do cost a bit more but seem to last longer than the sony or generic ones. The biggest capacity from H-W is 7200mAh - which gets you about 8-10 hours on a full charge with phantom power on etc.

lots of folks decide to fit a shoe to their recorder so they can use NP1 batteries - these are fairly high cost (from £100-£200 typically + the charger) but provide much longer powering.

With the Li-ion & NP1 batteries there's an increasing problem of taking them on planes in hand luggage - one is usually ok, but turn up at an airport with 4 or 5 in your hand luggage or indeed in your suitcase & it could be a problem - & given the cost of them you don't want to be having to leave them behind at the airport.