# USING 2ND BATTERY FOR ROADTRIP IN SUV. NEED ASSISTANCE

#### BRIN

Joined Dec 15, 2020
5
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

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#### LowQCab

Joined Nov 6, 2012
2,054
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.
.
.
.

#### bassbindevil

Joined Jan 23, 2014
422
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.