Originally Posted by wrcsixeight
https://www.amazon.com/Cole-Hersee-24213-Continuous-Solenoid/dp/B005K2429I
For best possible recharging of accessory battery, Hook it right to alternator via 4AWG or thicker. Use a large ANL fuse close to secondary battery + terminal. If you groud accessory battery to frame, make sure the connection is solid, and then add a frame to alternator ground. I prefer to avoid extra frame grounds as they becomme problematic all too soon and too often.
If you hook Accessory battery to engine battery with solenoid switching, then both sides of the cable should be fused close to battery. You might want to thicken up or add a parallel circuit from alternator to engine battery.
Don't cheap out on the cables or the ring terminals. genuinedealz.com will make custom cables professionally, meaning no Hammer crimps or copper clad aluminum wire.
Triggering the soleoid can be done automatically, or manually or automatic with a manual override.
The Ideal trigger circuit would activate solenoid only after engine is running.
Avoid diode based 'isolators' they drop too much voltage slowing recharging greatly.
YOu can also use a simple On off Switch like Blue seas. 300 amp continuous rating.
https://www.bluesea.com/products/6004/Single_Circuit_ON-OFF_with_Locking_Key_-_Red
There are dozens of different products which can do what you want.
I use a Blueseas 6007m manual switch myself
There are "smart" solenoids that isolate the batteries until the car is running. And only when the starting battery is up to say 12.5 volts (charged ENOUGH to re-start the engine), will it close the loop and then allow the auxillary battery to be charged. This is always giving the starting battery priority.
They also monitor each battery's voltage and if one is draining, it will isolate the two batteries protecting one of them from being depleted.
Here is what I'm looking at:
https://www.amazon.com/Cole-Hersee-48530-Battery-Isolator/dp/B00LMGPHW2
From Product Description:
Prevents loads on auxiliary battery from draining the starting battery.
It works with all types of alternator.
Smaller and lighter than traditional isolators.
There are no efficiency losses due to diodes. It allows bi-directional charging from the alternator or from shore power.
It reduces the load on the charging system by not connecting the auxiliary battery until the primary battery is charged to 13.2V.