I'm taking a road trip soon and need help connecting the 2nd battery. Right now, I'm using a solenoid to separate the two batteries. I also have a DC to DC charger which I plan on using if the 2nd battery was heavily used. Once the battery is charged 80-90%, I will activate the solenoid to allow the alternator to complete the charge. I think I may need to install a diode between the batteries. Some of the battery isolator reviews complain about failures, and there is some discussion about a voltage drop across the diode, but I can live with a less than a 1.5V drop.
I would appreciate it, if anyone can review my drawing and provide comments.
Thank You,
Mike

 

Running a Lead-Acid-Battery very low will shorten its life, unless its specifically a "Deep-Cycle-Battery".

You have a lot more complexity than is really necessary.

Would You like to build a simple automatic Switching-Circuit for your Solenoid ?
( I hope that You are using a proper "Continuous-Duty-Solenoid", and not a "Starter-Solenoid" )
This could eliminate the Charger, a Switch, and the 40-Amp Fuses.
It would also insure that both Batteries are properly Charged, as fast as possible,
whenever the Engine is running.

A ~12-Volt to ~14-Volt Charger will waste ~15 to ~20%, (or more),
of the Power that it's transferring from Battery to Battery.
.
.
.

 

Consider investing in a LiFePO4 battery; they appear to be far better than lead-acid for deep cycle. If you also have solar, Renogy makes a unit that combines a solar charge controller with the ability to charge the RV battery from the engine battery.
The thermoelectric cooler is a waste of money and energy (they are very inefficient and have very limited cooling ability). An insulated cooler with ice blocks uses no power and stays cold regardless of ambient temperature. Or, spend more for a real 12V fridge/freezer; small ones from China are under $300 shipped. Or salvage an electric/propane fridge from an old Westfalia or RV.

 

Consider investing in a LiFePO4 battery; they appear to be far better than lead-acid for deep cycle. If you also have solar, Renogy makes a unit that combines a solar charge controller with the ability to charge the RV battery from the engine battery.
The thermoelectric cooler is a waste of money and energy (they are very inefficient and have very limited cooling ability). An insulated cooler with ice blocks uses no power and stays cold regardless of ambient temperature. Or, spend more for a real 12V fridge/freezer; small ones from China are under $300 shipped. Or salvage an electric/propane fridge from an old Westfalia or RV.

 

Thanks guys,
I looked into the refrigerators and I agree. I purchased another model with much better specs.
My "HOUSE" battery is a deep cycle. I am also using a 40A power relay for the connection between both batteries. The reason I am using a DC to DC charger, is that I read that if the "HOUSE" battery is discharged significantly more than the starter battery, they will 1st try to equalize, possibly reducing the capacity of the starter battery. I do not want to take that chance, so I will charge the HOUSE battery using the DC to DC converter during the day while driving until voltage is above 11.7 or so, then switch to the main battery using the relay to complete the charge and not worry about discharging the starter battery. This setup is for a roadtrip for a month or two, then I will remove.
Thanks again all,