Typically, the average battery buyer walks into his favorite dealership, asks the counter guy for a battery to fit his 1923 GRINDLAY-PEERLESS 500, the one with the sleeve valve engine, not the four-valve Rudge variant, thank you, and takes whatever is handed to him. Unfortunately, that's not always the best way to select a battery.
Every battery manufacturer has some sort of catalog that, along with a listing of OEM supplied batteries, has page upon page of pertinent battery information that describes and pictures everything from terminal shape and location to physical dimensions. Very often, the catalog even lists upgrades in a particular category, which makes it simple to upgrade your battery if it's listed. But what if it isn't?
Let's suppose, for example, that you'd like to upgrade your motorcycle's battery from the factory-recommended wet cell to something a little larger, or perhaps to a maintenance free version, and there's no specific interchange listed.
First, you'll need to know how much room you've got, and yeah, leave yourself a little extra-shoehorning the battery into a space that's too tight is never a good idea, and neither is sticking a too-tall battery into a too-short battery box. In either case you run the risk of physically damaging the battery or perhaps shorting the terminals against the frame or seat base.
Once you have the basic dimensions, you'll need to check Cold Crank Rating and the Amp Hour Rating, as you don't want to replace the recommended battery with one that has less capacity.
Using the catalog as a guide, first select the type of battery you'd like to use (i.e. conventional or maintenance-free, standard or advanced technology). Next, find one with the proper physical dimensions, or as close to them as possible. Slightly smaller batteries can be used; you can always shim the battery box with foam, rubber or even soft wooden strips, but as we said, pass on ones that may be too large. Once you've found a battery that fits, check the terminal location and style. You'll want terminals that match the OEM battery's as closely as possible. Lastly, make certain that chosen battery's Cold Cranking Amps and Amp Hour Ratings meet or exceed the motorcycle manufacturer's specs.
In some cases the process may take a bit of detective work; for instance I recently wanted to upgrade my freshly restored 1968 BSA 441 Victor Special's OEM wet cell battery to a maintenance-free type, and was initially stymied. After about ten minutes, I found a YUASA AGM battery that fit perfectly. It had to be ordered, which was to be expected, but if I can find a maintenance free battery for that my old crock, you can find one for yours.