House Batteries

Sep 25, 2008
7,449
Alden 50 Sarasota, Florida
If you buy 12v batteries, they are wired in parallel.

As to how many you need, that is a function of how much power you require which we don't know.
 
Aug 2, 2009
651
Catalina 315 Muskegon
It really depends on what your needs are. How much peanut butter should you have in your cupboard?...same thing.

We'll assume you use your batteries for starting your motor, beyond that, who knows? How much 12v do you use? Do you spend a lot of time away from the dock overnight? How many days in a row? What 12v devices to you run, and for how long? How do you charge them...alternator, charger, solar, wind?

Ideally, you'll have enough batteries to meet or exceed your calculated needs. If you don't want to calculate your specific needs, if you give the forum a general idea of how you're using your batteries, it's possible that some ballpark generalizations might be made.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
You need more. if you have 10 you need 11. You can NEVER have too much storage capacity. So the real question is how many batteries can you cram in the space and can yo afford that?
The only exception to this is the boat that only has an anchor light, no cabin lights, reefer/freezer,......appliances and never sails at night. They can get by pretty well with just a starter battery.
One big bank has advantages in charging, battery life, and simplicity. Does leave you responsible for getting the motor started and managing the banks charge though.
 
Feb 26, 2004
23,051
Catalina 34 224 Maple Bay, BC, Canada
Do an Energy Budget (and search for the phrase here) and find out. Good luck.
 
Sep 20, 2014
1,329
Rob Legg RL24 Chain O'Lakes
Although I will not tell you to not wire them in parallel, there are some dangers you should be aware of. If one battery shorts out, the other batteries will dump gobs of current into the shorted battery. After a short time there will be a significant explosion. If the batteries are not contained in an explosion proof container, it could do serious injury or cause a fire. I had this happen in my motorhome. I was about 10 feet way when the battery exploded. Fortunately the battery compartment contained the explosion, but if the batteries had been exposed, I could have been injured from flying plastic. It may be wise to isolate each battery with a 50 or 100 amp fuse. This way if a battery shorts out, it just pops the fuse, rather than causing other damage.
 
Apr 8, 2016
114
Beneteau 361 Clipper Garrucha, Spain
How many house batteries should I have for my 373, and can I just wire them in parallel?
I have a 361 with 3 AGM totalling 200ampH house and a starter. I think the boat came with 2 house batteries originally, I think 3 is a good starting point as a minimum.
 
Sep 20, 2014
1,329
Rob Legg RL24 Chain O'Lakes
Don't worry about the number of batteries but rather the total reserve capacity for what ever combination you use. Often times you can get more capacity running two 6 volts in series, rather than two twelves in parallel.
 
Feb 6, 1998
11,709
Canadian Sailcraft 36T Casco Bay, ME
Although I will not tell you to not wire them in parallel, there are some dangers you should be aware of. If one battery shorts out, the other batteries will dump gobs of current into the shorted battery. After a short time there will be a significant explosion. If the batteries are not contained in an explosion proof container, it could do serious injury or cause a fire. I had this happen in my motorhome. I was about 10 feet way when the battery exploded. Fortunately the battery compartment contained the explosion, but if the batteries had been exposed, I could have been injured from flying plastic. It may be wise to isolate each battery with a 50 or 100 amp fuse. This way if a battery shorts out, it just pops the fuse, rather than causing other damage.
When this happens the most dangerous time is when the system is charging and the charger is attempting to push voltage above static resting voltage. This is one reason I really dislike leaving unattended vessels with large 80A plus inverter chargers running with no one on-board. IMHO this is a very unsafe practice regardless of series, series-parallel or parallel banks. The smallest charger to maintain charge and supply DC dockside loads is always the safest choice. An 80A + charger is always the least safe choice for unattended charging.

The risk of internal shorts and large chargers also highlights a failing of the charger industry in not allowing for, or designing for, a temp sensor for each battery. This type of sensing system is not hard to do, and it was a topic of conversation just two weeks ago, over lunch with a few of the industry Guru's..

A member here had a short happen three weeks ago with a 6 battery 900Ah parallel string. IIRC he had two 40A chargers running (80A) and the shorted battery got to 159F. Once charging stopped the battery began to cool and it was then disconnected from the string...

If it's just the bank, not charging, then an internal short usually just depletes the entire bank through the short in one cell. It does cause that shorted cell to gas and explosions are usually a result of non-ignition protected devices in the battery compartment. In a situation where it's unattended heat can cause connections inside the battery to melt and arc too but this type of explosion is much, much rarer. I had it happen on a commercial guys work boat when he left a rolling 80A shop car charger attached to a single battery in a 22' aluminum work boat at a boat yard dock. The battery had shorted internally and then the charger dumped what ever it could into for a period of time until it exploded.

The current derived between parallel batteries is limited by internal resistance and voltage differentials so a bank not charging is at considerably less risk of a dangerous explosion than one that is charging. One charging, especially with a huge charger, is at a significant risk of thermal runaway. Of course this can happen, when charging, in a series packs too. When not charging, a short in a series only pack will have no current fed to it, but during charging it will have charger potential feeding the short.

It would be extremely hard to design a fusible system where by the fuses would trip if one battery became a 10V battery, as opposed to a 12V battery, without also tripping during normal use. I actually tried this in my shop, with a shorted AGM battery, and no matter what I did I could not get a properly sized fuse to trip, between batteries, under internal battery shorting conditions.

All that said shorted batteries are very rare when the batteries are taken care of and tested regularly. Even when ignored these events are still quite rare. The highest incidence I see, of internal shorts, occurs in flooded 4D & 8D batteries and I have not been able to get a good explanation as to why other than;

"Our 4D & 8D product are not designed as cycling batteries. Deep cycling this battery probably led to an early plate failure."
...
 
Sep 20, 2014
1,329
Rob Legg RL24 Chain O'Lakes
My batteries were not charging, as the cut off switch was flipped the day before. Could be rare. I always assumed the risk was pretty low until it went bang. I went for 12 years and was on my third set of deep cycle batteries before it shorted. I suppose if you are anal about never running them too dead,and never over charging them, then the risk is low. But is everyone that careful?
 
May 24, 2004
7,174
CC 30 South Florida
Rare events happen from time to time. That don't mean you have to refrain from certain activities; just be diligent in maintenance and frequent inspections and keep up your insurance coverage.
 
May 10, 2007
42
Beneteau 45 Annapolis
I highly recommend installing a battery monitoring system if you do not already have one. A volt meter does not cut it. It will enable you to treat your batteries well, know how much power you consume, know when to recharge, and help you figure out if you need to add more batteries. You are pretty much in the dark if you don't have one.
 
Sep 6, 2011
10
Beneteau Oceanis 373 Starr, SC
I own a '07 B373 also. It has four 6V golf cart batteries. They are hooked in series to create two separate 12V banks. The boat also has a separate starting battery.