Parallel Why NOT?

[AmzngGrace]

Older simple boat with two “dual-purpose” batteries.
Is there ANY reason that a simple boat owner with only two combo batteries WOULDNT want to go to parellel hook up?
A broader question:
Is there ever a reason why someone would not want to hook there batteries in parallel?

[JPA Cate]

When/if you flatten those parallel batteries, how do you start the engine? We have, for many years, kept a separate start battery, and also have it rigged so the engine may be started from below on a separate switch.

Ann

[sailormed]

"Is there ever a reason why someone would not want to hook there batteries in parallel?"
Reasons: batteries of dissimilar chemistry, dissimilar capacity, dissimilar age, .....
There must be more of course.

[AmzngGrace]

Is there any reason other than using up both batteries and not able to start to preclude hooking two batteries together in parallel THAT ARE EXACTLY THE SAME AGE, CAPACITY AND CHEMISTRY?

Why I am asking...
I am thinking about my weak links, (many). I want to hook up a 1000-1500 w (cayman vs tigris) windlass to my boat that has 2 100aH batteries. I am moving mentally from the tigris 1500w to cayman 1000w to make it easier for my system to run (55a alternator) while engine running. Can I, and does it make sense to wire (optionally when using windlass) both batteries in parallel? I would separate them afterwards to minimize impact on ability to start boat the next morning after using my house battery for lights and various other circuits...

[thomm225]

Mine have always been in parallel since 2011

After I removed the diesel from my boat, I put the starter battery in parallel with a so called deep cycle battery and they lasted about 4 years.

Now I have two of the so called deep cycle batteries in parallel and they run everything on my boat and are charged via solar (115 watts to 155 watts) They run the inverter, computers, autopilot, lights, GPS, depth, phone charging, HDTV, etc

I have a pull start outboard so no starting battery needed.

These batteries are 90 ah each.

When they wear out, I may go with a couple 200 ah or better 6 volt batteries in series but have a charged 12 volt for backup and this will be about the time I retire and start cruising long distance

[Pete7]

A popular mod for older boats with a basic electrics fit is to parallel them and then add a third engine start battery. That doubles the house bank capacity instantly for all the electronics you need I did ours when we first got her in 2007.

So two batteries and the engine running, that should power a 1kw windlass. You might put a el cheapo battery monitor in the house bank circuit to monitor what's going on and ensure the batteries are fully charged afterwards/

https://www.amazon.co.uk/Battery-Mon...8781207&sr=8-5

[rslifkin]

Personally, I'm of the opinion that engine starting power is safety critical. Any boat with one or more engines or generators should always have at least one battery dedicated to starting. It only starts engines and gets recharged and never powers anything else.

[Lake-Effect]

Originally Posted by AmzngGrace

Is there ANY reason that a simple boat owner with only two combo batteries WOULDNT want to go to parellel hook up?
A broader question:
Is there ever a reason why someone would not want to hook there batteries in parallel?

Paralleling batteries is only actually necessary when you are occasionally pulling more current than one battery can comfortably deliver. Otherwise, you will still have the same overall Ah capacity whether you run the batteries in parallel, or run from one and then the other.

Downsides of a fixed parallel battery arrangement:

• batteries need to be of the same type, size, age, condition
• a failing battery -eg shorted cell - pulls the entire bank down, and neither battery can be fully charged
• if the bank is discharged, you are now out of DC power

For the price of a 1-2-both-off switch, you can have the batteries isolated, and you can parallel them if desired.

But my preferred arrangement is Pete7's suggestion - a house bank and a dedicated start battery.

[AmzngGrace]

So 1-2-both-off switch automatically gives me twice the current?

FYI left as shown is to starter, right is from battery 1, center is battery 2.

[Lake-Effect]

Originally Posted by AmzngGrace

So [batteries in parallel] gives me twice the current?

In theory, if you have two identical batteries, each capable of delivering X amps, then the two in parallel could deliver 2 times X amps.

The question is, of course, are you ever demanding 2 times X amps from your batteries? Have you ever measured your highest current draw? If you never need more current than one battery can deliver, there's really no point to leaving your batteries connected in parallel.