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Old 01-31-2015, 01:49 PM   #1
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Battery questions

I have a 37 ft 2006 Nordic Tug. It has a Onan 9kw generator and a Xantrex tc 40 multistage battery charger. When we bought the boat it had 4, 6 volt Interstate deep cycle workaholic u 2400 golf cart batteries in a series and parallel connection ( 12 volt system )(251 amp hours each) .The batteries were at the end of their life and it was suggested that I replace them with 4 Optima AGM Blue top SC31DM 12 volt batteries. They are rated CCA 900, Crackling amps 1125, AH 75 and a reserve capacity of BCI 155 minutes with a 25 amp discharge. I have 2 Optima AGM SC34M as my start batteries. When we are under way the alternator will handle the charging. When we are on the hook/mooring and we are on the boat ( generally 7: 30 am to 9: 00 am and 5:00 pm till 11:00 pm) we will run the generator that will run the battery charger. When we are off the boat sightseeing and asleep at night, the house batteries will handle our power. This will include a small nova-kool refrigerator 4.3 cu ft and a small freezer 3.5 cu ft and additionally at night, an anchor light, vacuum flush head, Garmin 7212 and a couple lights as needed.

So my questions are:
1 Are there any advantages to 12 volt over 6 volt or 6 volt over 12 volt?
2 Why is the 6 volt battery 251 amp hours but the blue top battery only 75 amp hours?
3 Are the 4 SC31DMs enough power for my intended usage?
4 For house batteries which is most important : AMP hours or reserve capacity ?

Any and all suggestions welcome!
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Old 01-31-2015, 01:53 PM   #2
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If I was in your situation I would consider the Trojan T105's or equivalent. Worth looking into.

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Old 01-31-2015, 03:13 PM   #3
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1 Are there any advantages to 12 volt over 6 volt or 6 volt over 12 volt?

Real flooded cell, 6V golf cart batteries are the cheapest battery that you can use for house purposes. All golf cart batteries are "real" deep cycle unlike many 12V batteries. AGM batteries like your Optima's are good for both starting and house use and there is little difference between AGMs designated for each.

2 Why is the 6 volt battery 251 amp hours but the blue top battery only 75 amp hours?

The blue top battery is 12V whereas it takes two of the 6V batteries to make 12V. So the comparison is really 251 amp hours vs 150 AHs comparing two to two.

The blue top has a cylinder arrangement that doesn't pack as much power into the battery. They are probably much smaller physically than the GC batteries. Also 251 amphours is higher than any GC battery I have seen- typically 220 AHs for Trojans.

3 Are the 4 SC31DMs enough power for my intended usage?

Probably yes if you run your generator daily to recharge them.

4 For house batteries which is most important : AMP hours or reserve capacity ?

Amp hours!!!!! Reserve capacity is a spec that means the number of minutes that you can draw 25 amps without the voltage dropping below 10.5 volts. It is more useful for sustained high loads than you find in house use which is typically more like 5-10 amps.

And finally a 40 amp charger will take too long to recharge your batteries. You can easily charge you current batteries at a 100 amp rate which will cut down genset running time. So consider a bigger charger or a combination inverter/charger. It will load your genset better and cut down running time.

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Old 01-31-2015, 04:56 PM   #4
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Hi 96,

I've had a number of Optimas as start batts, in boat, diesel truck, and cars. They're great for starting. But if you look at the specs it's pretty clear that they are not so great as house batts - not enough AH per pound or per $ as compared to 12V g31 AGM's. My house bank has been 12V g31 AGM's, either Deka or Sears Platinum - both work very well.

So if you choose AGM house batts I'd look at 12V g31's. If the 6V's fit your battery box better, there are 6V AGM's as well.
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Old 01-31-2015, 05:24 PM   #5
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Ditto everything djmarchand said! Definitely go REAL deep cycle of you can. Lead-acid requires maintenance but that's part of boat stewardship/ownership. My Costco golf carts cost me $75 a piece for 6 of them with 660AH of 12V house power. That's a lot of battery for about $500. Today they're closer to $100 per battery.

AGM's are available for the mx-free approach, but you pay for that convenience. Also, they accept a larger charge for a longer period, so you'll recharge faster with suitable charger.

My L-As typically run from about 60-85% on the hook between chargings. They can only take the max output from my 55A charger until they reach about 75%, then the acceptance rate tapers down, so in my case, a larger charger wouldn't save me much gen run time. If you have AGMs, a much larger capacity charger could be more beneficial.
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Old 01-31-2015, 07:54 PM   #6
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Yup. . . . I agree with FlyWright.

For the amount of time you're running your generator, you should have no problem keeping your lead acid batteries charged up and they will give you the best bang for your buck too.

For me the refrigeration is my biggest draw, when the main engines aren't running and I shut them down during sleep time and they seem to hold over night just fine. I also use the gen. set in the morning and evening, maybe a little less than you in the evening.
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Old 02-01-2015, 07:12 AM   #7
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Overnite can sometimes be helped with packages of Blue Goo, that freezes and works as a eutetic buffer in the system.
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Old 02-01-2015, 03:10 PM   #8
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Battery

Thank you all for your suggestions.
One other question, I have a Xantrex TC 40 battery charger. Can I charge a AGM battery( my start battery) and a flooded battery (house and bow thruster batteries ) with this charger? The owners manual recommends not to mix Flooded, AGM or Gel but i have heard many believe its not a problem and I spoke with a Xantrex representative and that person didn't believe I should have any problems.
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Old 02-01-2015, 03:55 PM   #9
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Is this a Xantrex Truecharge 40 unit. Yellow case with silver trim? I just switched one of those out for a newer Xantrex Model. Mine would do all three battery forms, AGM, Flooded, Gel, but not at the same time. It had a slide selector switch on the face of the charger for the different battery types. I believe the manual says not to mix battery types even if they're in different banks. There are differences in charging rates and max charge voltage and float voltages between the different types. (see below)
-----------
Page1-2 Xantrex owners manual

"Simultaneous Three-Battery Bank Charging Truecharge+ has three separate DC positive terminals to allow charging of three separate batteries (or battery banks). The total current into the batteries is a maximum of either 20 amperes or 40 amperes, depending on the model, which is divided amongst the batteries according to their state of discharge."

"Note: The three outputs are not independently voltage regulated so it is important to avoid systems with mixed types of batteries."
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Old 02-01-2015, 04:15 PM   #10
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Yes it is the Yellow case with the silver trim. I have read the manual and seen to avoid mixing batteries. But I have heard its not a issue. This is all new to me.This is why I was asking about AGM house batteries in my first post. I was looking to have all AGM batteries on my boat
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Old 02-01-2015, 04:31 PM   #11
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Set your charger for AGM type batteries and don't worry about a flooded cell battery also connected to the charger. The charging profiles for each are close enough that it won't matter, particularly if the charger is set for the more sensitive AGM battery.

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Old 02-01-2015, 04:54 PM   #12
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Quote:
Originally Posted by 96kpmjmm View Post
Yes it is the Yellow case with the silver trim. I have read the manual and seen to avoid mixing batteries. But I have heard its not a issue. This is all new to me.This is why I was asking about AGM house batteries in my first post. I was looking to have all AGM batteries on my boat
Also that charger is very sensitive to input line voltage/condition. Very narrow window of 60 cycle operation. Sometimes when running on my Onan 7.5, the charger would switch to standby and not charge at all until it was reset.

What I found, was If you check your gen. voltage and it will be a few volts high or low which means it's running too slow or too fast (needs to run at 1800 RPM's = 120 volts, 60 cycles). That changes the AC cycle rate enough to shut that particular charger down. My new Xantrex charger will accept a wide range of AC cycle rates.

The old Xantrex is now mounted in my shop maintaining my collector car batteries.

As a beginner, whatever you decide to do, keep it simple. Don't over complicate your systems until you gain some experience and are more familiar with trouble shooting problems.
Good Luck
Larry B
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Old 02-01-2015, 05:46 PM   #13
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I might suggest you check out this thread. I'm still learning myself. Calder's book is excellent.

A path to redemption for an electrical idiot

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Old 02-01-2015, 08:31 PM   #14
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Sears makes a Group 31 Marine Die Hard that is made by Odyssey. Odyssey makes some of the best batteries on earth. The Sears version is exactly the same as the Odyssey version...just cheaper. I called Odyssey and they straight up admitted to it. I just googled the Dies Hard it it is listed at $309... Not cheap. I found mine on sale at $249...likely similar to your Optimas and a SIGNIFICANTLY better battery. Call your local Sears...they are likely priced better than what you see on line. I went thru this gyration about a year ago. This is the thread that was generated. Pretty much all you need to know about marine batteries is in this thread. But in the end, it is your decision.

Batteries

PS...My system includes four house G31s that I usually leave combined for house and starting. .I did have an event where my shore power was turned off....for a week. The refrigerator ran until it dropped off line due to its low voltage protection(and subsequently defrosted itself). I was curious if the boat would start after that sort of treatment. Started without even struggling. I know that does not prove much other than if my charger falls off I can still start my boat. But so far the batteries have been completely trouble free. And they are a very pretty blue...
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Old 02-01-2015, 08:34 PM   #15
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Ps...the Die Hards are three year full replacement NON prorated warranty...something that could be processed at you local Sears store.
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Old 02-02-2015, 09:03 AM   #16
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I agree with Baker, post 14; sounds basically same as to how my house bank and starter situation is set up with 31's. Sears batts sounds a good choice.

I recommend Batteries Plus. East Penn, largest and one of the best in U.S., manufacturers their "branded" bats. Good prices and service. I've not had one fail in six years. Including boats, cars, SUV, and truck... I have 12 batts in service.
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Old 02-02-2015, 09:51 AM   #17
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96...
I agree w/ djmarchand reply #3 and would add that how you wire banks makes a difference.
Here is a link to the best presentation on wiring banks that I have seen

Smart GaugeHow to correctly interconnect multiple batteries to form one larger bank.

I haven't tried to verify the calculations but I do agree in principal.
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Old 02-02-2015, 10:10 AM   #18
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Quote:
Originally Posted by Art View Post
I recommend Batteries Plus. East Penn, largest and one of the best in U.S., manufacturers their "branded" bats. Good prices and service. I've not had one fail in six years. Including boats, cars, SUV, and truck... I have 12 batts in service.
Deka, the g31 AGM's with which I have the most house bank experience, is East Penn's brand name. They are also available under a variety of other brand names, including Duracell at some Sam's/Walmart stores. I get mine at a local industrial battey supplier, about 2/3 of the price I find at marine stores, with the Deka Intimidator brand. My first Deka AGM house bank lasted through 11 summers of cruising, anchoring out most nights, no generator. If you see a g31 AGM with model number 8A31DTM, it's probably an East Penn Deka.

This time around I decided to try the Sears Die Hard Platinum AGM's (Odyssey). Not too much more expensive if you catch the sale price, twice a year or so.

Another top quality AGM is Life Line, but quite a bit more expensive, IIRC.

All three of the above have g34 size AGM's that are a fine alternative to Optima for a starting bank.
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Old 02-02-2015, 11:40 AM   #19
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I swapped out 3 Group 31 flooded batteries for 3 Die Hard platinum AGM. The group 31 make up a start/thruster bank and the extra cranking amps provided by the Die Hards come in useful when I am driving both thrusters. I have 2 Victron 4D Gel batteries for my house bank. I wouldn't mix battery types within a bank. If you have multiple banks with different types, make sure the charger switch is set not to exceed the voltage requirements of the most restrictive battery type. My Victron charger is set to gel (Victron recommends the gel setting for both AGM and Gel batteries)

I had the Xantrex TC40 charger when I got the boat. After a while, I noticed that I was having to add water quite often (my boat originally came with 3 group 31 flooded batteries) I tested the charge voltage from the Xantrex and found that the voltage was .2 - .3 volts higher than the profile called for. The charger was out of warranty and Xantrex support was useless. I got rid of the charger and installed the Victron Phoenix charger.

I have used Deka batteries in the past and have had good service from them. I went with the Diehards this time because of the higher cranking amps and the better warranty.
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Old 02-02-2015, 01:21 PM   #20
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I just bought 2 Deka 8d agm batteries from a local supplier for $485.00 ea. Same battery with the West Marine label was $799.00 ea. in the store.

Don't ask me why I had to buy them.
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