So my battery went recently. The electronics were doing weird things. The windows wouldn’t roll up or down for the first 10 seconds after starting the car, my locks would double lock, and the engine, of course, had a harder time turning over.
The MKV stock battery has absolutely no size markings on it. A part number lookup at a local autoparts store will likely yield nothing. A 2.0t Jetta, with the same size engine, is listed as a size 48 but that is too big to fit in the GTI’s box. A size 47 will fit just fine and is probably closest to the stock battery size.
So cliff notes:
The closest battery to the stock size is a Group 47. I’d like a bigger battery but that would require not using the battery box. A 48 will not fit.
The stock battery is supposedly rated at 480 CCA (cold cranking amps). More is better in this department. The one I just got has 590 CCA. This is the important number. Most batteries have another number which is cranking amps or some variation of; that number is usually way larger than the CCA’s but is measured at higher temperatures. That won’t help you when you need it the most.
When you purchase a battery, make sure that the terminals fit when everything is in place. The wires on the GTI require some room to fit back in the box.