? 2 12v batts in parallel ?

[timmyk78]

My current set up is like this-
1 battery on starboard side-It's #1 on the switch and starts starboard engine and runs house loads. I only use the swith in the #1 setting.
2 batteries on port side-One of them just starts the port engine. It is isolated from the rest of the system, not on battery switch and charges from port engine alternator
The other battery on the port side is #2 on the switch. I only use this battery if I run # 1 so dead it won't crank my engine.
Promairiner 20sport plus charges all 3 on shore power.

Since I run my house loads alot and I'm putting in a new stereo with an amp I would like to add more amp hours.

What I want to do is just install another battery in parallel to my #1 now. Connnect DC+ wires to one of the batts dc+ posts and run the DC- wires tot he other batts DC- post.
What gauge wire sould I use for connecting the batts in parallel? Should I fuse the DC+ parallel connection wire? Anyone think this is not ok?

 

[boatmailster]

Easiest way is to install new battery parallel to your house battery.
You should use heavy duty battery cables - same size like battery cables already installed on your boat. No fuse is needed (it will probably blow one day during engine starting).

 

[tobnpr]

The "usual" configuration would be to have two of your batteries as dedicated starting batteries, and the third would be a large deep-cycle to handle your house loads.
There are starting batteries and there are deep-cycles; they are different and each is designed for its intended purpose. Starting batteries should not be repeatedly drawn doen and recharged as it will kill them, and deep-cycles aren't designed for the large CCA's required for engine starting. Can you use them for either purpose- sure, but that's not optimal.

With this setup you would install a battery combiner that will charge the house battery from one of your engines' alternators.

My $.02

 

[Four Suns]

One of the issues you need to look out for when hooking a new battery in parallel and using a converter is both batteries on the same converter/charger circuit should be in the same shape (i.e. same make/model/age). The reason is the smarts in the converter/charger will probably ending up charging both batteries wrong as it gets a reading that is essentially the average... It can kill/overcharge a new battery if hooked in parallel with an old battery.

 

[Go Aweigh2452]

If you plan to add a new house battery, you should add two new. Never add a new battery with an older one. You'll eventually kill both when the older one dies. Use heavy cables and go + to + and - to -.

From another website:

Marine Batteries and Charging.

by Harvey Gilbert (Captharv)

I will attempt to explain which batteries that you need, why you need them, and how to properly charge them. I will speak of regular flooded electrolyte batteries. The gell and special batteries are similar, but require a slightly different charging voltage.

Marine batteries come in 2 general types: Starting and Deep Cycle. A starting battery has many thinner plates for much current, for a shorter time period. Their total amp-hours are much lower than the deep cycle for the same size. You will see instructions in the engine and/or battery literature about not cranking the engine for more than 15-20 seconds. During this very high current state, the battery is venting hydrogen gas, among other things, and if it gets hot, can ignite. So, it is designed to produce the kind of current to allow the engine to start relatively fast. Problem is, some engines don’t know that…….

The deep cycle battery has fewer, much thicker plates and is designed to give a small current for a long time period, like 5-10 amps to run a refrigerator, lights, TV, etc Their starting amps are low, but the amp-hours (AH) for smaller currents is much higher. Fact: most “deep cycle” batteries are a sort of compromise: they fall somewhere between a starting and a true deep cycle battery, and can start an engine. The total amps is slightly less, but usually they are just plain big enough to do the job.

Another fact: batteries are designed to be discharged less than 25% of AH capacity. Running even a deep cycle completely dead harms it. They are called deep cycle, not run-dead. Why? When a battery is run down, one cell will be dead before the others. The remaining cells are passing current thru the dead one, and trying to charge it backwards. Think about it! This causes a condition called sulfation. Sulfation is not reversible, regardless of the snake oil ads. The battery may not immediately accepting a charge, but it may after a few hours. This battery cannot be trusted. It will fail, but how long is merely a guess. Replacement should be done.

Charging. Here’s where it can get a little complicated. Simply put (I hope) is the battery has an internal voltage that is proportional to it’s state of charge, while being charged. A dead battery is about 12.6 (remember, while being charged) and a fully charged about 14.4. The voltages are dependant on temperature, but the alternator knows that and automatically compensates for it. The alternator is regulated at about 14.4 volts and is obviously connected direct to the battery. The difference between the batterys internal voltage and the charging source determines how much current flows. The battery voltage starts coming up fast, and the charging current drops off in proportion. The battery receives a tapering charge until it and the source are somewhat equal. Then the circuitry considers the battery charged. he current tapering off does not put a lot of heat into a battery. Heat is also an enemy of the battery.

The tapering causes the battery to take a lot longer than one expects for charging. An example here: Say you anchor overnight, and the combination of accessories draws 55 AH. You run an hour back to the marina and have a 65 amp alternator. The battery should be 100%, Right? Wrong. First of all, the engine electricals take about 10 amps, and lets say your electronics take 5. So, now you are down to a 50 amp. The alternator will bring the battery’s initial charge up fast, and the current tapers off to 25 amps, then 10, etc. It would take about 4 hrs to top it off. Some boaters have a bypass switch on the regulator to hot charge the battery. Bad. You will overcharge and overheat the battery, and ruin it. Simply put, the alternator can not fully charge the battery, itself.
This brings us to chargers. Don’t even think about a cheapie auto charger or a “trickle” charger. They can ruin a battery, because they are not regulated. Get a “smart” charger. It has 3 stages: 1. A bulk mode. As much current as possible, for a very short time, 2. An absorbsion mode, where it delivers the taper charge, like an alternator, and 3. A maintenance stage where the circuit reduces the charge voltage to about 13.6 to avoid boiling the electrolyte out. These chargers are a plug in and forget until you go out again, and the batteries are near 100%. It will also make the batteries last longer. As I said above, improper charging is the #1 killer.

For the size of the battery: starting batteries are rated in CCA (cold cranking amps) or MCA (marine Cranking amps) Bigger is better, and at least 1 CCA for each engine cubic inch. Again, bigger is better. For the house battery(s) calculate the amp hours by taking the current of the device times how long it’s run at anchor. Like a 4 amp refrigerator, times 20 hrs = 80 AH. When you come up with the total, double it. That’s the size to get, or even 2 in parallel if over 115AH (size 27). Again, do not discharge the battery below 25% and this also allows for the battery aging. What I have in my cruiser is a 230 AH battery bank for house battery and another separate starting battery. An overnight with the fridge, and intermittent TV, radio, lights, etc draws about 110 AH. I let the alternator do its thing, and have an automatic shore power charger, which runs while the boat is on the trailer in my yard. Takes it about a day and a half to be back to 100%.

The simplest installation would be a sport boat, without a cabin and accessories like a refrigerator that would draw a lot of current when the engine is not running. A stereo would draw about 3-5 amps, unless you have a godzillion watt amplifier (please don’t anchor near me). The simple installation would involve 2 starting batteries, with a 1-2-all-off switch. When running and starting, “all” when the engine is off, either 1 or 2. This way, if the stereo runs one dead the other can start the boat. For the gozillion watt amps, get 2 batteries, one starting and 1 deep cycle, and a battery isolator. The stereo unit and amplifiers to the deep cycle, the engine to the starting. The isolator will charge both batteries as needed.

Next we would have a small cruiser with 1 engine. A starting battery. A house battery (system) as above. Now here’s a way to make it boater proof: an isolator. What it does is to divide the alternators output to the batteries. Remember what I said above about the battery’s internal voltage vs state of charge? If obe battery is down and the other is charged, the one that is down has a lower internal voltage, and current will flow thru it. The one that is up will not take current. This holds true for good batteries.

The isolator allows you to have a dedicated starting battery hooked to the engine only, and the house battery for all else. This way, if the house battery is run dead, the starting battery does it’s thing. This is as automatic as it gets.
Being that the alternator can’t replace the charge during the trip home, a “smart” charger is a necessity. When you get back to the marina, or your home on the trailer, simply connect the boat shore power up, and forget about it. The batteries will be 100% the next time out. Another advantage of the charger is the batteries will have a longer life. The worst thing for a battery is to store one discharged. I have been getting 5 years out of my batteries using this method. Almost boater proof.

With twin engines/alternators, recommend a 2 alternator, 3-battery isolator; a separate house battery (bank), and a smart charger. Not only do you have the above advantages, but also it will charge all batteries if one of the engine/alternators quits working. If there is a genset, let it have it’s own battery, and get a 4 bank charger.

Battery maintenance. Note: battery electrolyte (sulfuric acid) is some bad stuff. Please wear safety glasses when working on or near the battery. If you do get splashed, rinse with a lot of water. 1. Clean the top of the battery. Use a baking soda/water mix to neutralize the acid. Keep the terminals clean and tight. 3. Maintain the electrolyte level. NOTE: Use only distilled water to fill to mark, and only top it off when it is charged. Discharged batteries have a lower level because of the lower specific gravity.

If the covers are removable, check the specific gravity of the cells every few months. The exact reading will depend on state of charge and temperature, but should be around 1.280 at 70 deg F. However, the reading difference between cells is important. A large variation, more than .020, shows that cell is going bad. Replace it.

The batteries are an important part of boating. Have you ever tried to rope-start a 5.7L engine?

Harvey Gilbert Captharv

 

[charlieft]

Tim,

It is usually not a good idea to leave batteries connected in parallel. Two batteries are never exactly the same voltage and, as they age, the difference in resting voltage between the batteries will most likely increase. This means that there will always be current flowing between the batteries because the lower voltage battery will always be pulling the higher voltage battery down to it's level.
You can connect them in parallel while they are being charged because the charger will have a higher voltage than either battery but, as soon as you stop charging, current will start flowing between the batteries again (current loop).
When the batteries are brand new you will probably not notice any problems but, as they get older and go their separate ways, the current between them will increase which will heat them up prematurely wear them out.

 

[rondds]

mine, and i believe all older vintage mid-large gas SRs, have three batteries. port handles starting the port engine and genny. stbd is a bank of two in parallel and handles house and starting stbd engine. nice thing abt SR setup is that all batteries charge with the converter and when engines run. i have friends with other boats that have a dedicated battery for the genny which ONLY charges when the genny is on (dumb). another great SR feature not found on all boats is the "emergency start" switch. so if you run down your two house batteries on the hook, just flip a switch and use the port battery to start the stbd engine.

i replaced the two house batts with identical deep cycles. you really have to watch the water in them for the reasons described above. each may call for different slightly different voltage from the converter and, over time, the stronger battery could get significant water boil-off.

i was also told, "you cant start big blox with deep cycle batteries!!" bull-oney. three full seasons and never an incident. hell, i stuck a 7 year old tiny little automotive battery in the boat during the winter just to run the lights when i'd go down for a look-see. it sat in the boat all winter and in the spring, i charged it overnight on the converter. who'd a thunk it but it started both engines without a hitch.

 

[Sorrento 25]

Definition of a battery: A battery is two or more electrochemical cells connected in series which store chemical energy and make it available as electrical energy. Common usage has evolved to include a single electrical cell in the definition. ...

A 12 Volt battery is comprised of 6 2V cells. Two identical 12 volt batterys in parallel is simply a 12V battery comprised of 12 cells in a series/parallel arrangement with double the capacity instead of 6 in series alone.

My house, inverter and start banks all consist of two batteries in parallel for a total of 6. They were easier to install, yielded more capacity per dollar, and were easier to fit.

The house bank is the original (identical) start and house batteries that were in my boat when I purchased it. Although they didn't serve the same purpose they've been happily married for two years now.