Switch for parallel batteries

[Kfedka]

My current setup has two deep cycle 12 volt batteries, one is specifically for cranking(starting outboard) and the other standard deep cycle( for trolling motor), the are both same brand and same size and purchased about one year ago. One battery runs only the electric trolling 55# trolling motor, the second battery runs the rest (outboard starter, fish finder, pump, navigation light etc). There is a two bank battery charger that is connected to each battery, which I charger after each fishing trip.

My outboard is a two cycle 40 HP that has a built in rope pull start on the exterior, so in the event the battery is low or dead the motor can be started.

I'm looking to be able to spend more time on the water with the trolling motor, especially on all day outings. I could purchase another battery, but prefer not to as I don't have that much extra storage space for an additional battery on the boat.

I'm looking into connecting both batteries in parallel, but wanted to install something such as a disconnect switch between the two batteries(using 4 gauge wire). In this manner the batteries only run parallel when I'm out of juice on the main trolling battery and the switch can just be turned "on" to make the batteries run in parallel.
Was looking at this switch: Battery Doctor Battery Disconnecthttps://www.amazon.com/WirthCo-20138-Ba ... disconnect,

Now if my trolling battery is almost dead and the second battery is at full change when the parallel feature is turned on by the switch would the fully charged battery automatically "recharge" the dead trolling battery until both batteries are holding the same level of charge(about half charge each)?

Looking for input if this is a good idea or not and what are the advantages and disadvantages of this setup. If there is another setup that would work better then this please let me know. Thanks for your assistance.

[Amx]

That style could produce a spark when it's first engaged or disengaged, a bad idea in a boat where gas fumes could build up in the compartment. They have the proper type/style switch available on Amazon. Do a search for switches and get one there, or your local boat store.

[Amx]

This one should work;

https://hydrillagear.com/shop/master-ba ... witch-kit/


This similar to what I have;

http://www.bbcboards.net/attachment.php ... 1453227984

[hewesfisher]

You just need a simple battery selector switch for marine application. Either the BEP or Blue Seas will work, I prefer Blue Sea Systems for marine electrical.

What you want to do will require a 4 way switch, OFF, Bat 1, Bat 2, Bat 1&2. This would allow you to completely disconnect all electrical power (OFF), select Bat 1 (Bat 2 isolated), select Bat 2 (Bat 1 isolated), or select BOTH to combine Bat 1 & Bat 2.

To answer your question:

"Now if my trolling battery is almost dead and the second battery is at full change when the parallel feature is turned on by the switch would the fully charged battery automatically "recharge" the dead trolling battery until both batteries are holding the same level of charge(about half charge each)?"

No. Connecting a fully charged battery to a nearly drained battery will not "recharge" the nearly dead battery. Depending what is connected to the other battery, you will see the capacity of the fully charged battery plus whatever capacity is left in the nearly discharged battery.

Need to keep in mind the charging system of your 40hp engine is not designed to function as a battery charger. Outboard charging systems are designed to maintain the fully charged state of a properly rated, fully charged battery.

Also, routinely running a battery below 50% depth of discharge is highly detrimental to longevity.

An exposed blade switch has no place on a boat. In addition to corrosion concerns in a marine environment, you also, as AMX pointed out, run the real risk of igniting fuel vapors.

AMX - Your BBC link shows an ON/OFF switch and I don't think that would work the way Kfedka want to use his battery setup. The BEP 701s is a 4-position switch (OFF, Bat 1, Bat 2, Bat 1 & 2) similar to the Blue Seas 6007 linked below.

[Amx]

All he has to do is wire neg. to neg. with a cable. Then use a cable from the switch from pos. on one battery, and another cable from the other battery's pos. to the switch. That's it.

[Kfedka]

Thanks for the input Amx and hewesfisher. The first option I was originally looking at was Perko 8501DP Marine Battery Selector Switch; https://www.amazon.com/Perko-8501DP-Mar ... ine+switch.

I was at a local boat store and the guy behind the counter said that the Perko 8501DP Marine Battery Selector Switch is not designed for what I'm trying to accomplish. He said there is going to to much power draw from the electric trolling motor going through the Battery Selector Switch when running in parallel. That was one of the reasons I was looking into the knife switch, but I'm glad I asked since it's not safe with gas fumes.

Now if what the guy at the fishing store said is incorrect and if the power draw of the trolling motor is safe to continually transfer the power through switch, then I can go back and look at the option of installing the Perko 8501DP Marine Battery Selector Switch.

Is the Perko 8501DP Marine Battery Selector Switch as good as the BEP 701s as they are both a four way switch.

Amx the Master Battery Switch Kit is similar to what I was thinking purchasing but again the guy at the boat store said to much power drawn. Also I was looking at the second link you posted, bbc requires login to access. Is there a was to view without registering with that forum?

[Amx]

Blue Sea Systems PN 6006.

[4n6fisher]

I like the Blue Sea combined with automatic charging relay, gives the benefit of the switch, but no changing back and forth unless you want to combine. It will also charge both batteries with only one hooked up to the charger. This also allows for charging of both batteries from the motor, if main battery is full it will start charging secondary battery as you motor around. Although as Hewesfisher pointed out the motor charging (especially lower HP outboards) is not really designed to charge, just maintain.

[Onmygame]

Hewes pretty much nailed it.

onmygame