Re: What is this in my engine compartment
Yes it looks like a homemade battery isolator / combiner. West Marine sells or used to sell something similar. I think the purpose of the solenoid was to disconnect the two batteries when the ignition is off. Both battery are charged when the engine is running, and disconnected from each other when it's off. The isolator has reverse current leakage, so if you had one bad battery, it can still draw down the good one over time. The solenoid keeps that from happening. I'm not sure I have that 100% correct. It's been a long time since I've seen or used one.