This morning I replaced the 12V battery in my 2007 HCH II.

I've read several posts here about using the larger 51R battery (2 inches wider than OEM battery, so OEM tray and shroud doesn't fit). Also, some posts about using a tractor or lawn mower battery.

I called around and found that our local SF Bay area independent parts chain (Monument Auto Parts) carries a direct replacement, a perfect fit for the OEM battery. It's the AC Delco 151RPS. Price was $79.95 after core exchange.

Although it's less CCA than the 51R, the advantage of being a perfect fit was a plus for me . . . and the price seemed OK compared to current 51R prices, and especially compared to the OEM price.

 

As you mention the direct replacement battery is Group size 151R. This smaller battery is standard in several smaller Japanese cars including the Honda Fit. In North America it would seem that they are now starting to produce replacement batteries so you are no longer forced to buy an OEM battery from a dealer. As you say, the larger 51R will fit, but the plastic covers and tray are too small.

I noticed recently that Costco lists the size 151R in their battery guide but so far I haven't seen one in stock. As I've found before, replacement items for newer import models seem to take several years before they become available at the usual auto supply stores.

As you may already be aware, a reliable 12V battery can help avoid some of the "computer" issues that can arise when the battery is weak. To me, it's good insurance at a modest cost.