isolating / charging batteries?

[Doghouse]

I currently have 2 group 27 batteries on the boat. One for starting 1 for house. My battery charger can handle up to 3 (batteries/banks), and currently I do not beleive it is hooked up correctly but that is another topic.

I have been thinking about adding a third battery, and to start my planning I read the charger's owners manual.

It states that I should have only the same type / size batteries for each of the connections to the charger. So I would have to either replace 2 and buy 1 or just buy another group 27. Hmm... which is cheeper?

If I do that, how do I add this third battery to my second for more house amps? I understand the +2+ -2- rigging, just wondering if one of those should have a diode of some sort to prevent the charger from over cooking one of them or do I have to install a second battery switch to isolate them while at dock?

I am sure I am overthinking things, but help me out a little here!

Thanks
John

[hellosailor]

John, it might help if you told us the brand & model of charger.

When they say to use only the same "type" of battery, that usually means battery "type" as in wet lead, AGM, or gel cel, because each of those needs a slightly different voltage and charging profiles. "Group 27" is a "group size" not a "type".

If it actually says SAME SIZE for each battery...that's not really a 3-bank charger in the typical sense.

Adding diodes is a kludge, but replacing your whole system with a better one may be the best long-term investment--after you understand how whole systems work and the pros/cons of each. There are a number of good explanations on the web at Balmar.net and BlueSeas and Ample Power among other vendor sites. And a couple of good slim books, THe 12V Bible and The 12V Doctor if I recall the titles correctly.

In a perfect world (ha) you match the capacity of your alternator to your batteries, so they recharge efficiently, and use an external regulator to control that properly. (Internal ones are intended for cars, not deep cycle batteries.)

The modern trend seems to be using one large battery for "house" needs, and one smaller battery as JUST a starting battery. That also makes the starting battery readily replaceable (cheap) while ensuring you are never tempted to use it as a house bank. I kinda appreciate that logic. Or, one small starting battery, two large house--if you need that much power.<G>

A lot of mixes "work well enough" but if you read up on how to match the different parts of the system, you'll get an "AHA!" moment when you realize that designing it as a SYSTEM can get you an easy 2x-4x longer life from the batteries, shorter engine runs times, all sorts of pleasant side effects.<G>

[Doghouse]

"John, it might help if you told us the brand & model of charger."

The charger is Protech4 model 1220.

OK so adding a larger size (group) battery is not a problem as long as I keep the same wet/agm or gel.

My goal is to change boats in the next 5-7 years, and don't want to invest a huge amount of money in this one if I can avoid it. Additionally there is a minuscule amount of storage space on this boat so there is not enough room in the bilge to put golf cart batteries. Although, I have very small power requirements, I would like to not have to worry as much when I do overnight at anchor.

I do understand that it is better to have a person learn how themselves than to just give an answer, but I have to keep SWMBO happy or kiss the idea of cruising good-bye. I guess what I am trying to ask is how do you "properly" hook up the 3 batteries?

Currently I have the above listed charger, the stock alternator on a y2gm20, 2 group 27 batteries, and 1 battery switch.

Currently, the charger and alternator and batteries all connect at the switch. I don't think that is correct. Should not each of the batteries + connect to the charger and to the switch. while the 1 goes to the bus?
If I add a third battery, and it will be in parallel to the second, do I need to hook up the third bank of the charger to this battery even though it is hooked up to the second? Won't that potentially cause the charger to incorrectly see those batteries?

I hope this post makes my goal more understandable.

Thanks for the help
John

[hellosailor]

And what is that "stock" alternator's output? 45A? 60A? 80A?

How much engine running time do you consider allowable for charging each day/night?

What is your energy budget? That is, how much power do you consume overnight? Just cabin lights? Or refridge and disco sound system?<G>

If the answer is just "I need more cheaply" then another Group 27 or Group 31 (more amperage) on the third position could be the simple but perhaps unsatisfactory answer. Then again...what kind of shape are your current batteries in?

"If I add a third battery, and it will be in parallel to the second," Uhuh. Paralleling batteries is never a great idea, and mixing an old one with a new one that way is a total waste. You'd typically separate the starting battery out with a "battery combiner" (West Marine, Yandina, Hellroarer) and then use the A/B switch for house batteries, or switch them manually in/out as three banks. Some of those web sites should have schematics for that.
Or, replace the Group 27 battery with two of the biggest 6V batteries you can find to get one higher amperage battery out of the pair. Moving from Group 27 to Group 31 in a 12V battery can move you from 75AH to 100AH with a battery that looks "just a little bit" larger, not a bad way to get 30% more capacity.

It is really hard to make any suggestions without running all the numbers (and needs) beyond saying "well, if you get more battery you can go longer before you need the shore charger" but...that's not a very elegant solution.<G>

[Doghouse]

Some more facts:
The boat is a 2005 Hunter 27, my first sailboat.
We rarely spend more than 1 night on the hook. Usage is usualy 1-2 cabin lights and the anchor light. We often use either the sterio installed on the boat or a laptop computer. Oh, I almost forgot the fan, that runs all night also. I have never run out of juice, but don't want that to happen. Hunter installs 1 group 27 on the boat as standard. The dealer installed a second one. So the batteries are about 9 -12 mths old. At some point, I plan to add a voltage meter to the electrical panel so I can see where we are for battery condition. I removed the "koolatron" style fridge to save amps and use ice.

I will look into the battery combiner thing.

[hellosailor]

Well, we all know what ASSuming does, but I guess that's all I can do here.<G>

"1-2 cabin lights and the anchor light."
Maybe hat's 3x20W bulbs...60 watts. = 5A
"the sterio installed on the boat"
Similar to a car stereo, could be 5-10 amps draw, let's say 7A.
"a laptop computer."
Most use a 75W power brick, the "desktop replacements" 140W, let's be an optimist and say your's is 75W. = 6A
"Oh, I almost forgot the fan,
I'll guess one amp and hope that's a small fan.<G>

So...that's 18-20A from "moored" till "asleep" five hours? That could be 100AH right there, which is more than a Group27 battery has to give. The assumption is then that you have a hidden nuclear pile on boat to make that battery last throught the night.<G>

Dunno, really, but a typical Group 27 deep cycle has maybe a 70AH capacity and doesn't live long (i.e. maybe only 50 cycles instead of 1000) unless you only cycle it 50% (300-500 cycles) for 35AH of drain between charges. Your best "cheap bang" for the buck, unless you look at real power consumption, etc., is just to buy two hiking huge 6V batteries to make up a bigger house bank on the third position. Set them up as "B", set up your existing Group27 house battery as "A", and split off the starting battery with the isolator, as the primary battery that gets charged first all the time.

Typically, the wiring would be:
1-Alternator to starting battery directly
2-Starting battery via combiner to battery switch common terminal
3-Shore power set up separately to all three batteries, main battery switch OFF while using it.

I'd do it that way despite a number of flaws (suboptimal charging, etc.) because that's about the only q&d solution. And use the dual-6V- new house bank as your primary bank, because you're probably cycling the Group27 too deeply. When the Group27 dies, replace it with another 2x6V bank.