There is no way a fish finder needs a group 27 battery. I run my, admittedly, smaller less powerful finder off of a small sealed lead-acid battery like this:

https://www.amazon.com/UB1250-Sealed...ub1250+battery

I'd look for a larger battery for your Garmin. It depends on how you are going to use it. How many hours a day do you use it? Can you charge the battery at the end of the day? The specs for the Striker 7sv says it draws .67A .