2 battery 12v hook up

I’ve 2 new 27 size batteries and would like to know which side one hooks up first, I tried to place the neg-neg after I’d put the pos-pos on and the terminal sparked?? Do I need to do the neg-neg first?

Make sure all other equipment is disconnected before you attach the parallel cables.

I’ve never payed attention to which one I put on first and never had an issue. If I remember correctly there was always a small spark. Maybe someone smarter than I can confirm.

I would also recommend connecting all of your equipment to one battery. Not including your charger. One bank for each battery is ok.

Just a little nerve racking but never resulted in any harm in my experiences.

It won’t matter what way you hook them up. If there is a load on the system, you will have a spark. More importantly, both batteries must be at the same voltage. If one is discharged any the fully charged one will try to charge the other. This can be very dangerous depending on how much current is being transferred.

It usually doesn’t matter in our use because the boat isn’t grounded however it is good practice to connect the positive first, then the negative. Electrons flow out of the negative terminal and are attracted to the positive polarity, input.

Inversely, disconnect the negative first.

Force of habit but I always attach the negative after the positive is attached and disconnect the negative first.

Tom Sawvell wrote:

Force of habit but I always attach the negative after the positive is attached and disconnect the negative first.

Yes!

I’ve had a battery explode in my face one time. Hopefully never again.

Kyhl wrote:

because the boat isn’t grounded

There is a whole wealth of information available about the difference between “grounded” and “bonding” in regards to boat hulls.

Suffice it to say, If you short a metal wrench from the aluminum hull of an aluminum boat to the positive post of the starter battery, you WILL get, at minimum, a major spark.

Huntindave wrote:

There is a whole wealth of information available about the difference between “grounded” and “bonding” in regards to boat hulls.

Suffice it to say, If you short a metal wrench from the aluminum hull of an aluminum boat to the positive post of the starter battery, you WILL get, at minimum, a major spark.

Do you have a marine radio?

I’ve not heard of there being a circuit through the hull. You should measure 0 volts between the hull and both the positive and the negative battery terminals. I’ll have to measure mine tonight.

A lead to the positive terminal and an aluminum hull will read the battery voltage. It is what is referred to a hot boat and will repel fish.

Kyhl wrote:

I’ve not heard of there being a circuit through the hull. You should measure 0 volts between the hull and both the positive and the negative battery terminals. I’ll have to measure mine tonight.

The thing to keep in mind is there is a difference between “using the hull for ground” and bonding of the hull to the negative side of the electrical system. It is usually never recommended to use the hull as the ground return for any electrical circuit. This does not mean that there is no electrical connection between the hull and the negative battery post.
Some additional information if you desire.

I have a 1985 Merc 60 hp as part of the motor harness the negative from the battery is connected to ground (ie the engine block). I would expect most outboards are wired that way. Then my outboard is bolted to my aluminum boat.
So either directly or indirectly the negative of my starting battery is connected to the hull.

So, the safest thing to do is connect the positive first in case of a short in the system. Then the negative.

If you are connecting two new batteries negative to negative and positive to positive, and nothing else is connected. And you get a spark that would tell me that one of the batteries has a higher level of charge. This is OK, the higher voltage battery is “charging” the other battery.

Thanks for the answers! Elmar Fudd

Thanks for the answers! Elmer Fudd

Kyhl wrote:

You should measure 0 volts between the hull and both the positive and the negative battery terminals. I’ll have to measure mine tonight.

Kyhl,

Did you check your hull with your meter yet? If so, what did you get for a reading?

I got to it last night. The motor uses a negative ground that leaks back through the hull. Learn something new all the time.

Kyhl wrote:

I got to it last night. The motor uses a negative ground that leaks back through the hull. Learn something new all the time.

It doesn’t “leak”, the system is designed that way. The hull is “bonded” (electrically) to the negative side of the battery. Again this does NOT mean that the hull should be used as a return path for any electrical component used on said boat.