Charging a second battery

[LAB3]

Here's the deal. Picked up my XS a bit less than two months ago and it came with an older yet still functional Motobatt battery. They have a great rep but I'd just bought a brand spanking new AGM battery for a Goldwing I had finally given up on and it fit the XS perfectly so in it went.

Since then I've added a trailer hitch and still have the Motobatt battery which I'm using to charge my phone and run lights when I'm camping which is about 95% of the time.

I'd like to be able to charge the Motobatt off the XS while running down the road and am somewhat savvy when it comes to electrics. My understanding is that if I where plug in the Motobatt in a state of discharge that it would draw a whole heap of current to try to level out to the same charge as my main battery, not a good scenario when your working with 12ga or similar wire!

So here's the question then. Should I be looking to use a battery charge isolator like an RV would use or would some kind of high amperage switch turned "on" after the bike is started do the job? Price is somewhat of a concern but size is what matters most, I don't want to mount something the size of a brick on the bike that needs to be kept dry. TIA.

[DiverRay]

Look for a battery iisolater at your local auto parts store or this one on ebay: https://www.ebay.com/itm/36366371854...RoCd6wQAvD_BwE They will allow you to keep the starting battery charged, but still charge the spare when you have available current.

[LAB3]

Now that's in the size range and price point I'm looking for! The ones suggested by the RV crowd at the campgrounds all have these huge boxes that cost $150 plus. Thanks for the link.