Dual Battery Jump Wiring

[Fishhawk]

After my recent GPS issues and a dead house battery, I am wondering if any of you have installed permanent jumper cables to the second battery.

I'm interested in knowing:
Can the negative poles on the batteries be be permanently connected without a switch?
What size wire is needed for "jump" starting a Honda 40?
What is the best type of switch for connecting the positive poles and where is it best located?
Are there any good reasons not to do this at all?

The wisdom shared here is remarkable.

Thank you

Capt Dan

[Wandering Sagebrush]

Hi Dan,

Yes, the negative posts on the batteries can be tied together. That in effect is putting the batteries in parallel. The positive posts should lead to the battery switch. One wire to position one, the other to position two.

I have a group 24 starting battery in my stbd lazerette (spelling?), and a group 27 in the port side. The negatives are tied together with appropriate gauge cable, then to system ground on the engine (as I recall). I also have a Guest charger that I have going to each battery via the hot side of the switch, with the negative leads going to the stbd battery.

If you need, I can draw you a schematic that I can scan. It may take a few days, as I am just about out the door to chase salmon. You can call my cell if you would like... 503 705 3063.

Hope this helps.

Steve

[Captains Cat]

Dan, here's a LINK to a diagram. The first one is the one you want. It's simpler if both batteries are co located (in the same place). Wires are shorter. The switch can be nearly anywhere convenient.

I know you'll have more questions, ask away.

Charlie

[Fishhawk]

Thanks very much guys,

I think I am going to deploy the KISS principle (keep it simple stupid) and just tie together the negative poles, and then tie the positive poles together with a good switch in that line.

The only time the switch will be thrown is if the house battery is dead. And then only to start the second engine or use the VHF.

Capt Dan

[Captains Cat]

Dan, when your engines are running, close the switch. That way, the alternator on the engine will charge both. I'd move all the house functions (the things that run on the battery when the engine isn't running) to that new second battery. When you anchor, open the switch. That way, the house things will run the second battery down, not your original battery.

Don't close the switch when you start the engine. If the second battery is depleted, it may not start. Close the switch only after the engine is running.

Would be best to connect as the diagram says, not as you described.

Charlie

[Fishhawk]

Charlie,

Perhaps I should have better explained my setup or perhaps I am missing something.
Osprey has twin 40 Hondas.
The starboard engine's battery is also the house battery.
The port engine has it's own battery that, at this time, powers nothing else.
There is no new battery.

My desire is to have power available to the house battery and starboard engine from the port engine's battery only when it's needed, and that would only be when issues in the boat have drained the starboard battery. A permanent Jumper cable system if you will. Nothing more.

Does this alter your view on it?

Dan

[thataway]

Even with the twin 40 set up, you are best off with a 1/2/all/off switch. This way you can alternate, the house loads (which is a good idea), plus you can always leave one battery isolated when you stop for some time--such as fishing with bait pump, sterio, GPS, Fishfinder on....

Yes KISS is an excellent principle, but the difference in cost between a simple on and off marine type switch and a 1/2/all/off switch is only a few dollars, and it gives you much more versitility. (and safety).

The 40's don't take a very heavy wire--probably a #1 or #0 would be more than enough. Your question about the negatives has already been answered. You don't want to use jumper cables for a number of reasons.

[Byrdman]

You need to address the constand drain from the GPS on your battery..and any thing else that may have been wired to it that is "always on" type set up. Now, like most folks boats, some times they sit for long periords of time with no charge or use... and draining any battery all the way down a few times is never real good.

If you can not cut that off, as most of us can not without putting some type of simple in-line switch or breaker, then you just may end up with two dead batteries.

A simpe inline switch for your GPS and a pair of jumper cables. That is pretty simple and also handy if you are traveling and come up on someone who needs a quick jump.

Just a thought.

Byrdman

[BRAZO]

I realize that this solution is an expesive KISS, but we have BEP Twin VSR. http://bepmarine.web03.co.nz/Twin-Outboards%2C-Three-Battery-Banks-180-1465.html

This cluster will automatically keep all the batteries seperate during discharge and tie them together during charge. Works great for us.

Enjoy your day.

[edwardf]

I'd be very careful about charging parallel batteries.

For starters the two batteries should be as electrically identical as possible. Same group, manufacturer, and build date. The goal is identical internal resistance.

Both batteries should be at a similar state of charge. Remember that not only the battery charger but also the battery with greater charge will source current to try and charge the other battery. Large amounts of current could potentially flow from the fully charged battery into the rundown battery. That means heat.

If there is any chance the rundown battery actually has a dead cell, ie a shorted cell, the good battery will try like mad to ruin your day. Chargers are designed to deal with this but batteries are mindless.

Whatever you do use a safe switch. No arcing allowed.

I vote strongly for something like a BEP VSR switch.

Hope this helps.

[Chester]