What kind of battery should I use?
Removing & installing your battery
Learning more about batteries
There is a good Battery Tutorial at batterystuff.com. They also have a link to this Battery FAQ. Another good source on lead-acid batteries and their care and feeding is the comprehensive Yuasa Technical Manual (PDF).
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. Wet batteries are often reported as lasting longer, but you never know with batteries.
The picture on the left shows the difference in size between the 9 amp and 6 amp models (the bigger one is actually a PowerStar, discussed below). On the right is shown a maintenance-free model with the sleeve around it.
Amp hour explanation: The amp hours listed for the battery (quite often right on the case) is the battery's capacity. For a 6 Ah battery, this means that it delivers 6 amps for one hour, or 3 amps for two hours, or 1 amp for 6 hours, assuming a perfect battery.
The rule of thumb is to charge at the 10 hour rate: 6Ah / 10h = 0.6A on the charger. This is often not possible, but you don't want to exceed it by much.
One thing to watch out for on wet batteries is getting the vent hose correct. If it's too short, acid vapors could blow back on the exhaust, swingarm, and wheel. Run the vent down close to the ground with the other tubes. Even then some vapors could blow back; make sure you wash and inspect things once in a while.
Checking the water level
You can remove the caps with the battery clamped in, but it's real hard to see the level. It's much easier to just disconnect the battery and pull it out of the bike, then fill it on a level bench. Start out checking it 3-4 times a year and see how it does. If the metal plates inside aren't covered with water, add enough distilled water so they are.
It is usually possible to remove the caps from a maintenance-free battery, too, although don't do it if it looks like it will harm the battery. Sealed batteries have designs to minimize water evaporation while still being able to vent hydrogen gas (hydrogen comes from water, hence water loss over time). Inside, they are still standard flooded cell batteries, so it's still a good idea to check them periodically.
Auto parts stores and battery shops should be able to get what you need. People have had less than stellar luck at Wal-Mart. Parts Unlimited batteries, available from your local shop, have been reported as being good value, especially in the "wet" models.
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.
Some clueless store staff may be able to find a generic battery more easily if you just give the numbers as B9L-A2 (wet) or X7L-BS (maintenance-free) instead of the Yuasa YB9L-A2 or YTX7L-BS. Knowing your dimensions and how the posts are located will also help.
One of the problems with buying a battery online is that many places don't ship ready-to-install. Some have an acid pack that you have to open up and pour into your battery. This may have limited appeal for some people.
If you want something that's plug-n-play, get one of the standard batteries listed above. The ones below don't fit without some futzing. People have been using the normal-sized ones for decades without any trouble.
About bigger batteries: A big battery really won't help starting much; it will just crank for longer if it won't start! It will also give you slightly more reserve (before it dies) if you're running a bunch of accessories (heated vest, heated gloves etc... - see here) or if you take lots of very short trips that don't give the battery time to recharge after starting.
Under normal conditions there is little or no observable difference between properly charged batteries in good condition that are rated at 6, 9 or 12 amp hours.
The 12A-A is a bit taller than the OEM battery, but the hold-down strap will stretch and there reportedly isn't any problem with clearance with the seat. It fits well once you remove the rubber sleeve spacer that's designed to hold the 6 amp batteries in place, but it's a very tight fit and you'll probably want to put some kind of lube on it to make it slide in.
There are a couple other things you need to be aware of. The positive and negative posts on this battery are opposite from those on the Ninja, so it will have to be installed with the posts at the back. See the PowerStar battery information below. The positive (+) cable needs to be bent a little to clear the side vent tube, which is normally on the other side, due to the fact that you have to put this battery in 180* from stock. You will not need to bend it back should you decide to go back to the smaller batteries. Also, because the vent tube is now on the opposite side of stock, you'll have to re-route it. See above The stock vent tube should work, and there should also be a clear tube that comes with the battery. You can also get regular plastic tubing from a hardware store.
If you have any doubts, make sure it fits before you hook it up. Dimensions are 134mm long x 80 wide x 160 high. The extra capacity should provide an extra couple seconds of cranking, should you need it.
This is the Advance Auto Parts version:
PowerStar PM12-PA: This is the same size and has the same capacity as the older 'wet' model, but it's maintenance-free. It has a better-than-average 2 year warranty, and when it stopped working for one of our riders after 16,000 miles in 15 months the company said it should not have failed and replaced it. It cranks over every time, including in below-freezing weather. This battery cross-references to the Yuasa YB9L-A2.
Note - June 25, 2010: Powerstar seems to have changed the part number for this battery. If you are interested in it, contact the distributor directly and check the dimensions and part numbers.
There are a few kinks with the PowerStar, only related to installation. These are more than compensated for by its increased capacity/power and good warranty. To correctly install this battery, you have to put the posts at the back of the battery box, instead of the front. The cables are long enough and will still reach. The traditional installation, with the posts at the front against the airbox, is shown below.
These photos show the orientation of the PowerStar, posts at the rear. (The large extra black wires are for a battery charger.)
So, in the pictures below, the side with the lettering faces towards the back of the bike.
Something you may want to do to make installation easier is to get regular 6-sided nuts to use on the terminals instead of the rectangular one. This makes it easier to keep the nut from turning when you tighten it, as the bolt inserts horizontally instead of vertically, and there is nothing on the battery to hold the nut in place.
One other thing while you're installing: Now that the posts are in the back, the cutout in the little red boot that covers the positive terminal is now oriented the wrong way. You can get a new cover, go without one, or just stick it on and know that it's not really covering like it should, though there's really nothing for it to short to there under the seat.
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.