There's no guarantee a "new" EZ Flash IV will have a brand new battery. I got one on eBay that has something wrong with it that makes it a battery-eating machine. Battery only lasts weeks in this cart, so I gave up replacing it.
You can play (and save game) any game up to 16MB (very few games are bigger than16MB) with no battery provided the game is compatible with soft reset (L + Up + B). Save in game and then soft reset for your save to be written to SD. I've been doing that way for a while with an EZ Flash IV (microSD version). All the games I want to play lately have worked, even Phantasy Star (have to start with L+A).
Even if you don't get a defective one, you have no idea how long the cart (and battery) has been sitting. I don't know about other people's experiences buying on eBay and from big Chinese stores, but it is better to buy used from individuals: 3 of my 5 EZ Flash IV cards are white (bought used in 2012) and still work. The black microSD model (bought from an eBay store) was a dud and I'm a little disappointed with EZ Reform(bought from a big Chinese store): the contacts were dirty and it is hard to take apart/insert in DSL because the small case does not fit properly. Nothing a dremel couldn't fix, but for the price people are asking new it ought to be perfect. My sample is too small to draw any definite conclusions so I'll not point the bone, but it seems to me like EZ Flash quality is going down. Hopefully Omega will be better, but I plan to wait and buy it used.