Aux Battery question

You have the right idea, but there's not enough information to tell you if the battery is sufficient. There's no stats on how many reserve minutes or Ah it has, or even CCA. We don't know what your aux usage is, either. If you're using a dc-dc converter to connect the aux to the traction pack, then you won't really need a big battery, just one large enough that you're not totally killing it with the current draw.

The CCA is 300. I will be using a dc-dc converter to connect the aux to the traction pack.

you don't really NEED an aux battery at all... you can wire your dc-dc 'always on' and go direct to your 12v main fuse block. but the tradeoff is a small power drain all the time, and dead in the water if the dc-dc fails.

Use a UPS backup battery, something like a 20AH, 12V. You don't need cranking amps but a backup in case of the DC/DC failure is an important safety item. You don't want to lose lights, wipers, etc driving home in the dark and dead of winter.

my opinion of course.

I'm with Madderscience on this one. I strongly recommend a small 12v backup battery and I generally use the Zappy size AGM battery. These have a footprint of about 3 by 7.25 inches and are about 6.75 inches tall. It is a common size, generally available for less than $40. Something like this and a lot more can be found by searching the web retailers. Being a sealed battery it can be shipped easily.

The battery you where looking at appears to be a U-1 size unit. That should work, but it may be larger than you need. That would be a good size to consider if your DC to DC converter cannot keep up with your vehicles 12 volt system loads.

Before someone calls me on this, it is true that my EV Buggy doesn't have a backup battery. It has few accessories and is generally not built for safety. It also doesn't have seat belts!