# All electric tin boat multi batteries



## gnappi (Jul 31, 2018)

I have two main batteries in my tinny, and I liked the idea of them being totally isolated so I installed two switches, one red, one black. 

I don't "think"having them both isolated is necessary and I could have used a common ground like on larger boats but a friend also has an all electric boat and wants to be able to switch them, so what's the consensus??? A common ground and a single switch? or separate switches for ground and positive?


----------



## Scott F (Jul 31, 2018)

There really is no purpose in switching the negative side. You don't have to have a "common" ground either. You can run a wire from the negative side of the battery directly to your device without using the hull as a conductor, and install a switch and circuit breakers to the positive side.


----------



## gnappi (Jul 31, 2018)

Scott F said:


> There really is no purpose in switching the negative side. You don't have to have a "common" ground either. You can run a wire from the negative side of the battery directly to your device without using the hull as a conductor, and install a switch and circuit breakers to the positive side.



Maybe my terminology is off so call it what you will but let's say you have 2 or more batteries on an all electric TM only boat there's no way a boat will function without a "common" ground.


----------



## gnappi (Jul 31, 2018)

Regarding my switching scheme. If I'm carrying one battery, replacing one, or one fails catastrophically one thing for certain is that when one cell is out of the boat, no ground wire is ever free to touch the hull or other live wires. 

Anyone who has replaced one battery of a bank and claims to have NEVER crossed / shorted wires is... well (IMO) not being honest. Can't happen with my rig. Serve a purpose? maybe that's it?


----------



## eshaw (Aug 1, 2018)

All the batteries on my boat (4) each have their own ground, except the trolling motor batteries, they share a common ground since it's a 24 volt system.


----------



## MrGiggles (Aug 1, 2018)

So I gather that by "common ground" you mean wiring the batteries in parallel, as compared to wiring them separately, each tending to their specific devices. Trolling motor on battery 1, sonar, nav lights, bilge pump on battery 2 for example.

On an all electric boat I think it would be smart to isolate the TM battery, as you don't want your electronics running it down, since it's essentially your gas tank.

If you do feel the need to combine them, all you should need is a relay on the big ground were between batteries.


----------



## gnappi (Aug 1, 2018)

MrGiggles said:


> So I gather that by "common ground" you mean wiring the batteries in parallel, as compared to wiring them separately, each tending to their specific devices. Trolling motor on battery 1, sonar, nav lights, bilge pump on battery 2 for example.
> 
> On an all electric boat I think it would be smart to isolate the TM battery, as you don't want your electronics running it down, since it's essentially your gas tank.
> 
> If you do feel the need to combine them, all you should need is a relay on the big ground were between batteries.



Yup, right on... One battery runs my instruments exclusively and can be pressed into service as a "spare tank" if either of the two other separate TM batteries die. Those I consider analogous to tank 1, and tank 2, completely independent of each other. 

I don't run them in parallel for multiple reasons. With three separate cells which have more total Ah capacity with less overall weight than one large battery (plus a smaller spare) that I can more easily distribute their weight, I also know how far I can go when one is depleted and switch to the second. If I ran them in parallel and one failed it might take the other down leaving me my spare cell which has been running my electronics all day to get me home, without rowing that is. 

I tried running with one large battery and a spare, and it just did not work for me. Beside the weight penalty, I tended to run the one cell to a point that when I switched to the backup battery it was a distance stretch on the spare cell. I wanted to keep the DOD of the cells within the recommended limits for best battery life. 

After over a year of running all electric on most days 6-8 hours on the water I haven't been stranded yet  I think this setup is pretty much ideal.


----------



## gnappi (Aug 1, 2018)

I should clarify my battery weight / Ah issues above. 

My larger group 27 92Ah battery is 63 pounds, that along with one spare 35Ah battery at 21 lbs is 84 pounds. My three 35Ah cells are a total of 63 lbs saving me 21 pounds for a lighter boat, easier weight distribution and a bit more run time (3x35Ah=105Ah vs. 92Ah) capacity. 

My initial Q was more for my friend who will have only one main cell for the TM and the 35Ah for the instruments. When the main cell runs low he wants to be able to switch without moving batteries or re-attaching them separately. Hence the Q about a shared / common ground setup. So I guess input from others who are running a similar setup my friend has will have suggestions on how they switch batteries.


----------



## water bouy (Aug 2, 2018)

Maybe this will help.

https://newwiremarine.com/how-to/wiring-a-boat/


----------



## gnappi (Aug 2, 2018)

water bouy said:


> Maybe this will help.
> 
> https://newwiremarine.com/how-to/wiring-a-boat/



Umm.. No, but thanks.


----------



## johnbt (Aug 5, 2018)

"Anyone who has replaced one battery of a bank and claims to have NEVER crossed / shorted wires is... well (IMO) not being honest."

Believe what you like about people being liars. Doesn't make it true. IMO.

Fwiw, I got lost initially when you started using battery and cell interchangeably. Two or more cells make a battery. I suppose you could have single-cell 35Ah batteries, but I haven't seen them. 

I have 4 group 29 batteries on my boat and no common ground or switches. Two for a 24v trolling motor, one for a 12v trolling motor and one for the other stuff and as a spare, although I've never needed it.


----------



## gnappi (Aug 6, 2018)

johnbt said:


> "Anyone who has replaced one battery of a bank and claims to have NEVER crossed / shorted wires is... well (IMO) not being honest."
> 
> Believe what you like about people being liars. Doesn't make it true. IMO.
> 
> ...



When you need to change over batteries are you saying you have a connectivity method with some type of QD?


----------



## gnappi (Aug 7, 2018)

johnbt said:


> "Anyone who has replaced one battery of a bank and claims to have NEVER crossed / shorted wires is... well (IMO) not being honest."
> 
> Believe what you like about people being liars. Doesn't make it true. IMO.
> 
> ...



Since I asked about MULTIPLE BATTERIES on a single motor, you decided to tell me about your irrelevant setup added more heat than light to the subject. 

Now if you say you have changed batteries around without QD cables and NEVER had an issue shorting cables I'd call YOU less than truthful.


----------

