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Old 12-16-2020, 07:00 PM   #1
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Electrical question for full time liveaboard

The boat came with 2 8d batteries,I replaced one of them,bank 2 which is dedicated to start engine,bank 1 needs replacing before heading offshore but can get the rest of winter outta it being tied up at dock,anyway when I replace bank 1,I was thinking of going down to 4d possibly and one more factor is by summer I will have either wind turbine or solar panels hooked up to bank 1
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Old 12-16-2020, 07:21 PM   #2
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The boat came with 2 8d batteries,I replaced one of them,bank 2 which is dedicated to start engine,bank 1 needs replacing before heading offshore but can get the rest of winter outta it being tied up at dock,anyway when I replace bank 1,I was thinking of going down to 4d possibly and one more factor is by summer I will have either wind turbine or solar panels hooked up to bank 1
I recommend you calculate your current loads, anticipated usage, active and passive charging- then purchase batteries based on current needs +.

Where are you heading "offshore"? You'll want to prep for worst case scenario, not on doing things piecemeal.
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Old 12-16-2020, 07:22 PM   #3
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Use two 6V golf cart batteries wired in series in place of your 8D. They will fit in the same space. These are true deep cycle batteries. Most 8D and 4Ds aren't.

Read more in the Library section of this forum.

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Old 12-16-2020, 07:23 PM   #4
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Starting to explain an answer would be an entire book on cruising electrical systems..

You need to read A LOT more on electrical systems currently used on cruising boats.

Even thinking what you have or will have in your post is a mistake in my mind. Especially if you are talking using the boat beyond marina to marina cruising.
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Old 12-18-2020, 06:49 PM   #5
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Two standard golf cart batteries will run a compact DC refrigerator and the needed accessories on a small boat for approximately 4 days in moderate summer weather, using the batteries' full capacity. Two GC batteries provide about 10% less capacity than one 8D.
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Old 12-18-2020, 07:41 PM   #6
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We do a pretty fair amount of that work. You couldnít give me an 8 D these days.
The weight of them is not worth the risk of getting hurt while handling them around boat type spaces.
Golf cart batteries belong in golf carts.
Please take a look at AGM types. Two group 31ís or thereabouts, in parallel will start most any six cylinder diesel.
Another pair for house loads will do a good job for most cruising. Charging relays are available now. They bring a new dimension to battery arrangements and life.
Hope this helps a bit.
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Old 12-19-2020, 02:37 PM   #7
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If you search the forum, you'll find numerous threads about converting from 8D to golf cart GC2 and other battery forms. 6 GC2s fit into 2 8D boxes for a ~50% increase in AH capacity. Lots of good into out there already.
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Old 12-19-2020, 03:57 PM   #8
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8D is old technoligy - look into Group 31 batteries

The guys telling you to step away from 8D batteries are 100% correct.

As they die a natural death, that is the time to make the change to dual 31's.

I changed by boat over from 8D & 4D batteries & went to Group 31 units & have never looked back. Better technology available as all the big rigs now have Anti-Idle laws, so have to turn the engines off & run off their batteries in the sleeper cabs, so they have switched over to 31's & that is a market every battery manufacturer is looking to be in.

Batteries of the group 31's sizing & style are available in Starting type, AGM type & Deep Cycle types as well as industrial types & others & are available up to 1000CCA each for starting batteries & 700CCA typically for the deep cycle batteries as those have High Reserve style internal plates.

Two of the physically smaller but powerful group 31 batteries fit perfectly in your 8D battery box & can be wired in parallel for max AH capacity & still be 12-Volt system. usually comes out the same or less for better power.

Here are some examples group 31 Lead Acid Flooded batteries HOUSE Bats.
Included # cycles for Deep Cycling specific part numbers.

Part # -------- AH --- CCA --- RC ---Cycles
31-900CT ---- 105 -- 950 -- 195 -- 280
1110 ---------- 99 --- 750 -- 180 -- 320
1150 ---------- 99 --- 700 -- 180 -- 320
31TSFleet- ---105 -- 700 -- 190 -- 450
DC31 -------- 105 --- 700 ---190 -- 523
31AGMDCT--- 100 --- 700 -- 200 --- 950

Examples - group 31 Starting batteries examples.
31-901CT ----- 105 --- 950 -- 195 -- not deep cycle
31-1000 HRAP - 105 --- 1,000 --- 200 --- not deep cycle - so no spec

You pull out that 8D which is probably 1200 CCA & put in two group 31- 1000 CCA batteries in same battery box & then would have 2,000 CCA starting power in same footprint.

Good luck,

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Old 12-20-2020, 03:43 AM   #9
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Actually three six volt golf cart batteries will fit in the space where 180 lived. That means you could have six golf cart batteries around 600-625 amphrs of true deep cycle batteries
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Old 12-20-2020, 07:26 AM   #10
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OP has a sistership to my Willard 36 which I've owned for 22 years, so have gone through a few battery iterations. 20 years ago I replaced the 8Ds with a pair of 8Ds. Was an absolute bitch to get them past engine in both directions. I then went to golf cart batteries. If you go that route, the brand name Trojan T105s are worth the extra money - the Costco interstate knock off did not last long for me. I'm on wave three with LiFePo4 going in as we speak.

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Old 12-20-2020, 07:57 PM   #11
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Actually three six volt golf cart batteries will fit in the space where 180 lived. That means you could have six golf cart batteries around 600-625 amphrs of true deep cycle batteries
Not sure what '180' means but I replaced 2 separate 8D batts of 225 AH ea with 6 Costco GC2s for a total bank of 660AH. That's a 46% increase in AH capacity in the same footprint. If I had more headroom, I could have put in L16s for even more juice. I typically get 5 years or more from a GC bank for about $600 Costco bucks.

Then I added a single $100 Costco G31 to start both Perkins 4.236s. It's never skipped a beat in 4 years of heavy use.
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Old 12-26-2020, 02:47 PM   #12
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I replaced two 8D batteries with 4 group 31 Firefly Oasis carbon foam 110 AHr AGM batteries 4 years ago and have found them outstanding. We have a high capacity alternator and 400 watts of solar which nicely maintain the battery charge and support our refrigerator and small freezer and other small boat loads quite nicely. Each battery is 75 lbs, which is quite manageable for us old guys.

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Old 12-26-2020, 03:24 PM   #13
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Why choose a AGM battery vs a traditional flooded battery?


An AGM battery has many advantages over a traditional flooded battery. It has more power, longer life, a slower discharge rate, a faster charge rate, and is more durable. Research shows that AGM batteries are more efficient because they have low internal resistance, deliver high currents and can handle deep cycles. All of these features lead to a longer service life as AGM batteries typically cycle better than a comparably rated flooded battery.
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Old 12-26-2020, 03:25 PM   #14
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The benefit of firefly batteries is not only their depth of discharge, their recharge rate, but also the abuse they are able to maintain. These batteries were designed to be used with large Caterpillar heavy duty equipment. The Caterpillar units were left out in the cold, vibrated to death, in the blazing heat, ignored for months on end, only partially recharged multiple times. The batteries were designed by Caterpillar for their stuff, not for boats, we are just the happy recipients of a great technology. They don't sulfate. These batteries are the closest to lithium that aren't lithium.
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Old 03-01-2021, 12:56 AM   #15
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A previous owner replaced the two 8D's in my Tradewinds 43' with four Group 31 AGMs. I added a fifth for the generator, which is typically isolated, but can be tied in if the others were allowed to run down. As a liveaboard who spends most time at the dock, I don't need a huge house battery. Also agree about the significant weight difference between 8D and group 31 or GC2 batteries. Such heavy batteries are not worth the risk of injury. Conversely, if I were cruising or anchoring out most of the time, I would want AGM GC2s for house battery as opposed to the Group 31s. The golf car batteries are a true deep cycle with the heavier plates.


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Old 03-01-2021, 08:34 AM   #16
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More plife?- Nope. Batteries, whether FLA or AGM have a certain 20 hour rate. An AGM does not have more "power".

Longer life? - Nope. All batteries are only capable of delivering a certain amount of energy over time. AGMs last no longer than FLAs.

Faster charge rate? - Yes, sort of but one must have a charging system capable of delivering more amps relative to the bank size. I have a 920 amp-hour FLA bank charged with either a 125 amp inverter-charger or 200 amps (nominal) of alternator (100 amp each engine). The charge acceptance rate on FLA at .2C, that's 20% of the size of the bank, suggests an input requirement of 180 amps, about what can be expected from my two 100-amp alternators.

Now, AGMs generally can be charged at .4C, 40% of bank size. In my case that would be 360 amps but I only have 180 amps of charging capability. So, that faster charge rate, useless in my case.
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Why choose a AGM battery vs a traditional flooded battery?


An AGM battery has many advantages over a traditional flooded battery. It has more power, longer life, a slower discharge rate, a faster charge rate, and is more durable. Research shows that AGM batteries are more efficient because they have low internal resistance, deliver high currents and can handle deep cycles. All of these features lead to a longer service life as AGM batteries typically cycle better than a comparably rated flooded battery.
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Old 03-01-2021, 11:44 AM   #17
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AGM batteries have two real advantages and one disadvantage.

They are not vented. This means they can be located in living spaces were FLA batteries should only be located in vented non living spaces.

They require no water maintenance. This is bonus if your battery location is hard to reach or if battery maintenance is something you would rather not do.

They cost more per amp hour than FLA batteries.
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Old 03-01-2021, 01:39 PM   #18
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AGM batteries have two real advantages and one disadvantage.

They are not vented. This means they can be located in living spaces were FLA batteries should only be located in vented non living spaces.

They require no water maintenance. This is bonus if your battery location is hard to reach or if battery maintenance is something you would rather not do.

They cost more per amp hour than FLA batteries.
A couple more advantages.

They discharge less than FLA batteries when sitting for extended period with no load or charge.
There is no danger of acid spillage, and less corrosion buildup on terminals etc.
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Old 03-05-2021, 07:30 PM   #19
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I briefly read through the updates on this subject. Someone may have mentioned the biggest reason to use AGM and I did not see it. If so, okay, it’s worth mentioning again. AGM’s do not explode. Think that as not substantial, then have a battery blow up in your face. Some notes, amps x volts equals watts. 750 watts is one horse power. 13.5 volts ( which is low) x 80 amps equals 1080 watts. That means pulling 1.44 horsepower from an alternator drive belt. It also means side loading the rotor bearings in the alternator by that amount. It’s agreed the auto and truck industry has come a long way with big alternators. But their mountings and drives are highly engineered. And still fail. Additionally, it’s more than once we have seen a diesel wet stacked and glazed due to it ( a full sized engine) being used as a battery charger. And, remember, each energy source must have proper over current protection. That means a fuse or breaker at alternator output and at the battery. There are plenty more considerations I have not mentioned and I’m sure plenty I don’t know about. It’s a good subject and so critical for safety of persons and vessels.
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Old 03-05-2021, 07:59 PM   #20
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I briefly read through the updates on this subject. Someone may have mentioned the biggest reason to use AGM and I did not see it. If so, okay, itís worth mentioning again. AGMís do not explode. Think that as not substantial, then have a battery blow up in your face. Some notes, amps x volts equals watts. 750 watts is one horse power. 13.5 volts ( which is low) x 80 amps equals 1080 watts. That means pulling 1.44 horsepower from an alternator drive belt. It also means side loading the rotor bearings in the alternator by that amount. Itís agreed the auto and truck industry has come a long way with big alternators. But their mountings and drives are highly engineered. And still fail. Additionally, itís more than once we have seen a diesel wet stacked and glazed due to it ( a full sized engine) being used as a battery charger. And, remember, each energy source must have proper over current protection. That means a fuse or breaker at alternator output and at the battery. There are plenty more considerations I have not mentioned and Iím sure plenty I donít know about. Itís a good subject and so critical for safety of persons and vessels.
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