What kind of battery should I use?
Learning more about batteries
Your bike does need a battery
While the charging system on the EX250 is similar to that on a car, it's not exactly the same. No doubt you have experience with driving your car while the battery is dead. However, due to the mixture of diodes, flux capacitors, and dilithium crystals in a motorcycle's charging system, you can't do that on your Ninja. While this can get technical very fast, suffice it to say that the DC voltage coming out of the regulator/rectifier is not a nice flat line, but rather a stream of pulses. This must then be "smoothed out" -- that is where the battery comes into play. It provides voltage stabilization in the bike's electrical system. See here for more information.
You can replace your original Yuasa battery with one that's just like the one that came on your bike. They're reliable and usually last a long time. (YMMV) Actually, since about 2004 the EX250 has been supplied with batteries made in Thailand. Their average service life seems to be about two years. Sometimes they will have the classic dead-battery symptoms, but usually they just quit completely. D-E-A-D
The easiest way to find a battery is to take this Yuasa part # to whatever store you want to buy from and cross-reference it to whatever off-brand it is that they sell.
Wet or dry?
The main difference (besides maintenance) between the early and late series batteries is size. The '88-94 "wet" models are bigger, which gives them more storage capacity (9 amp hours "wet" vs. 6 "maintenance-free" for the Yuasas). The battery box was not changed when the smaller batteries were introduced, so a rubber sleeve fits around the later models as a spacer. You can put either type of battery in your bike by either adding or removing the sleeve, depending on the year of your bike. The sleeve is available from your dealer.
These companies sell ready-to-install products.
The following companies ship their batteries dry, with an acid pack you have to install. They sell Yuasa and cheaper brands.
Searching the internet will bring many more options, as will shopping close to home.
Wal-Mart, auto parts stores, and battery shops should be able to get what you need. Go in armed with your Yuasa part number, and they can cross-reference it to what they have. Bargain brands tend to come and go. Sometimes they're OK, sometimes not.
If all else fails, or you like to do things your own way, any 12v bike battery that fits will work.
Measure the length, width and height of the battery box.
Note which side of the battery has the + and - terminals. It's possible to mount the battery if they're wrong, but it's easier if they are correct.
If the battery that you're considering is not maintenance-free, then it's much easier to route the drain hose down the right side of the bike. It can be done on the left, but the hose pulls out of the battery when you remove the battery from the bike.
If you've removed the airbox, you can build your own battery box and get even more choices. Leon's rally bike has an Optima dry cell battery in it.