Battery Question

[Analgesic]

My apologies to the electrical engineers for a stupid question.  I have a 5 year old 2 AGM house bank of 100 amp hours each that has been adequate for my needs but is getting very tired. I looked through the site and see many references to substituting 4 x 6V batteries for better capacity.   On the Defender web site I saw a Sportsman wet cell 6V with 220 amp hour capacity for only $125. The replacement cost for what I now have would be $420 each!   It sounds like I could get 2 of the 6 Volts for much less and get a much greater capacity than I am accustomed to (might even turn on the fridge when the engine is off!)  I can't imagine needing the capacity of 4 (880 amp hours).  Am I missing something here?  Is there a trade off in life expectancy or some other drawback?  I never see anybody mentioning getting only 2x6V batteries. 
Brian McPhillips

[Stu Jackson]

Brian, not a stupid question at all.  And, as you can imagine, we've covered that well here on the website and the MB.

The basic concept with 6V batteries is that you connect two of them in series to make a 12V battery, so the 4 six volt batteries at 220 AH each would give you 440 AH capacity in one bank.  See Reply #13 here: http://c34.org/bbs/index.php/topic,4843.0.html.

The additional issue is your energy budget.  If you feel that you have to turn off your fridge when your engine isn't running, you either need to reconsider how you use your boat, or get extra battery capacity to enable running the fridge 24/7 or, as we do, turning it off at night to extend your "range".  But, your 200 ah house bank (with your (2) 12V batteries at 100 ah each) should give you a full day of running the boat even with the fridge ON.  100 ah is usually the daily use of the boat's electrical system based on experience, others uses and plain math from an energy budget for an overnight on the hook.  You don't want to discharge your house bank more than 50%, so you're good with your 200 ah for one day of 100 ah use.

The energy budget worksheet is here:  http://c34.org/bbs/index.php/topic,3976.0.html  Just click on the PDF file.  I have it in QuattroPro which almost no one else still uses and I haven't moved it over to Excel, yet, so you'll have to make your own - there are many out there, including the West marine Advisers DIY section, here: http://www.westmarine.com/Images/wa/Elecbugt.pdf.

Many have installed (4) 6 volt batteries in their battery compartment just north of the galley.  Here's an example picture.

Do you have a separate reserve bank?  Sure, you can buy just (2) and get a 220 ah house bank.

[Analgesic]

Thanks.  After I posted the question, I went searching for bargains and came across a US Battery website that has an excellent visual on configurations.  My only other battery is a 100 amp hour starter that is still strong.  Normally the limited engine time it takes for me to get on and off my mooring fully recharges any loss from my weekend day sails.  The bigger capacity sounds nice for our annual Summer 2 week cruise when we really use the fridge.  The 4 x 6V configuration certainly seems like the best bang for the buck.  I just wanted to make sure there was no major disadvantage that I hadn't yet read about.
Brian

[Ted Pounds]

For even better deals on 6V batteries check out Sam's Club or Costco.

[Roc]

I just bought (4) 6 volt 'Energizer' (US Battery) batterys at Sam's Club for $72 each.  They charge $9 for a core fee if you don't have a battery to return. that's a bargain!  I'm not a member, but a friend saw a one day pass in the newspaper that I used, so it didn't cost me the membership fee.  If I didn't have that pass, I had co-worker lined up to go with me to purchase them using his card for entrance.

I checked BJ's and Costco and they did not have 6 volt batterys.  Sam's club had a whole pallet load of them.

[Stu Jackson]

There have SOOOO many posts recently on battery costs, it may be worthwhile for you all to add the phrase "battery bargains" to your posts.  That'd make it a lot easier to search for those bargains.  Thanx. 

[Ted Pounds]

Stu, I think you just did that for us.  Thanks.

One additional note (which I think I mentioned before) is to look for the manufacture date on the battery.  I think it's in the form of a letter for the month and the last digit of the year.  You want the freshest battery you can find.

[Roc]

Sam's Club just had a new shipment, so the ones I purchased had 4/09 stickers on them.   

[Bobg]

For those of you on a budget.  Go to the local golf course, I got mine from a guy who changes his golf cart batteries every summer , and gave me his one year old batteries.  Have been using them for the last 2 years effectively, I understand I got another batch coming this year.  just a thought.

[Ron Hill]

Bob : That's a GREAT idea !!