Will my car battery run down?

Chopin

We are planning to take the Tab on the SS Badger, the steam ferry across Lake Michigan. From what I understand, the ferrymen actually drive your vehicle onto the boat and park it. This means I won’t have an opportunity to disconnect the 7-pin for the duration of the 4-hour crossing. Does this matter? Thanks. 

Chopin

I should add that it’s a 2019 Subaru Outback and a 2021 320S. Also We can assume no solar as the vehicle deck is covered. 

Sharon_is_SAM

I know our charge wire remains hot when we turn off the ignition - potentially draining the TaB battery and TV battery.  If you have an isolation relay installed in the TV, the charge wire turns off with the ignition.  You can get a 7 pin tester or do this to see what you have:  Turn off the TaB battery, plug into your tow vehicle. Crank the engine. Turn on a light on your T@B. If the light comes on you have a charge line. Turn off the engine, if the light goes out, you have an isolation relay. If the light stays on, you do not have an isolation relay.

Can you just put frozen bottles in your frig and turn it off?  Then when you get out of the TV, just disconnect the 7 pin and turn off the battery?

Chopin

@Sharon_is_SAM Thanks for the suggestions. I will try the light test you suggested. The issue is that they drive the car onto the deck, so I guess I would have to ask them to disconnect and then reconnect the 7-pin. I assume they deal with RVs all the time so this can't be the first time this comes up!

Sharon_is_SAM

I guess I am suggesting that they can drive to park the rig without a 7 pin connection.  Then you know your TV battery is protected.  

dragonsdofly

@Chopin, haven't made the USS Badger crossing since the 60's, and never before with a trailer, but I remember that vehicles line up to be loaded onto the ferry. While lined up,  you can get out, shut down the fridge and anything else you need to. Wait until just before they load you to do this. When you need to leave your vehicle, simply step to the rear and disconbect the 7 pin. Make sure you secure the trailer end so it doesn't drag. Also ahead of time turn off your propane tank. I remember them asking folks with trailers if the propane was shut off. They can drive the car and trailer the 100 or 200 ft without the 7 pin hookup connected. As you get back to your vehicle, plug in the 7 pin and drive a short distance to a place where you can comfortably stop. Power up your trailer, slowly open your propane valve and be on your way. Enjoy your trip.

pthomas745

What size battery?  Which fridge?  Which trailer?  If you are trying to run the 3 way fridge with a stock battery, you will be using about the entire battery for a 4 hour tour:  16 amps per hour of the 3 way fridge with only 40 amps of a stock battery available.  48 amp hours would be no bueno.

A two way fridge might use about a third or less than that. (Assuming 3 amps per hour on a 2 way fridge, which is probably too much.) I'm assuming no isolation relay.  

So, which fridge, which battery?

Chopin

@pthomas745 It is a 100 AH lead-acid battery that is in good shape, and the 12V (only) Dometic fridge that comes standard with the 2021 Tab 320S.

Based on the advice received so far, I think I will disconnect the 7-pin before giving them the car, and turn off the fridge. It should stay cold enough for a few hours, especially since no one will be opening it!

pthomas745

Your battery and fridge will handle that power use for a few hours just fine, if you discover the need to transport an antique piece of wedding cake or something.  The test that Sharon is Sam posted about the "isolation relay" will also be useful.  Just a nice thing to know about your vehicle.

Chopin

Thanks everyone for the great suggestions. I ended up disconnecting the 7-pin before giving them the car/trailer. I left the trailer battery connected and the fridge running. Everything was fine. Here's a photo of them coming off the boat