New 8D batteries

I'm thinking my 2 8D batteries on my '01 MS 390 are ready for replacement. Cummins 6bta 330 HP. Any suggestions on good quality replacements? Brand/model/price?

You will get lot's of answers to your question but I'm going to suggest you use a couple of Group 31 batteries instead of one 8D. At some point those damn things have to be moved and they will destroy your back.

I think Johnson Controls AGM batteries are the best you can buy. Notice I said "you can buy". I can't afford them.

Three 6volt golf cart batteries fit in the space of one 8d and are true deep cycle

Let me combine the two foregoing answers. Your boat probably uses one 8D for starting and the other for house loads, but probably on a one,two,all switch so that either can be used for either service.

So select one for starting and replace it with a Group 31. Then replace the other with two 6V golf cart batteries wired in series and designate that for house loads.

You will then have the best battery for the job.

David

Vulcanbike said:

I'm thinking my 2 8D batteries on my '01 MS 390 are ready for replacement.

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Maybe comment on how your battery bank are set up? Twin engine boat? Maybe a common Luhrs Group configuration where each engine starting battery also runs approx half the house and one of the bridge loads (electrics on one, electronics on the other)?

If so...

And if you don't want to reconfigure to have separate start and house battery banks...

And if you have some extra space in the battery areas...

3x 12V Group 31 dual purpose batteries will give you more capacity than one 8D (~300 Ah vs. ~245 Ah)... and each individual G31 will weigh less so installation may be easier. These would fit in a space slightly larger than a single 8D.

4x 6V deep cycle golf cart batteries in series/parallel will likely start those engines (check Cummins specs), will give you even more capacity (~440 Ah), each weighs less than an 8D, and these will fit in a space just slightly larger than 3x G31s.

There are taller version of the G31s and the GCs, even more capacity if you have extra height to play with.

And if you have even more horizontal space, 4x G31s or 6x GCs could be even better.

Note increasing battery capacity may in turn want you to increase your charging capacity, depending on how you use the boat, access to shorepower after cycling, etc.

Or... If Dave's right and you have one 8D for start and one for house... what he said! Except 4x GCs instead of two.

-Chris

You have good advise here. Look at the requirements in your engine operators manual. Your engine probably requires a starting battery of 1000MCA rating, and some, but not all group 31 batteries will supply that. Then fill the rest of the available space with house batteries, either group 31 deep cycle, or 6V golf cart in pairs.

Note that there is a big difference between a start battery and a deep cycle house battery.

You will probably want to go with AGM batteries.

A good place to buy batteries is Sam's Club.

8Ds are too heavy for me at my age. When mine failed 6 years ago, I replaced them with a combination of East Penn 6v golf carts. These provide 2 banks of 12v @ 230 amp hours each. Oh-- purchased them from Sam's for about $85 each back then.

I'm the outlier here. My 8 Ds work quite fine and provide good ballast. This is a trawler forum where ballast is not a bad word.

I use three 8Ds, 2 for engine starts and 1 for genset. and am quite happy with them. There is an already setup space for each and every five years the local yard spends about an hour lifting the main engine starts in and out. The first genset went 8 years with no problem.

Get some prices, question the crowd advice and make your own decision. 8Ds can be very cheap. Sticking with what works has no downside unless you insist on being the strong back rather than using a young buck.

If you prefer, replace for 4D batteries.. way less expensive and easy to move around the boat. Group 31 is another good option

sunchaser said:

I'm the outlier here. My 8 Ds work quite fine and provide good ballast. This is a trawler forum where ballast is not a bad word.

I use three 8Ds, 2 for engine starts and 1 for genset. and am quite happy with them. There is an already setup space for each and every five years the local yard spends about an hour lifting the main engine starts in and out. The first genset went 8 years with no problem.

Get some prices, question the crowd advice and make your own decision. 8Ds can be very cheap. Sticking with what works has no downside unless you insist on being the strong back rather than using a young buck.

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My golf carts have the equivalent weight of the replaced 8Ds because it requires 2 of them to provide the service of 1 8D.

Here are the weights of the batteries discussed so far:

Group 31 57 lbs
4D 100 lbs
8D 127 lbs

GC 62 lbs

Any of the first three will start a Cummins 6BTA. Pick your poison.

And as foggysail notes above, two GC batteries weigh the same as one 8D and for house service are true deep cycle batteries, unlike most 8Ds.
David

My boat was set up with 2 x 4D from the builder. One was the start and one was the house. I added one group 31 to be the start and combined the 4Ds into a house bank. My engine is a Cummins 6BT5.6.

My 3988 came with 3 8D batteries, 1ea. for starting the Cummins and 1 for house. I have since changed to group 31 for starting and GC for house but had problems with the Group 31 overcharging and using too much water vs. the GC. I now have 2 GC for each starting bank and 6 GC for house resulting in no more problems with charging using combiners. And I can lift them.

My boat started out its life with me with 3 8Ds - 2 house, and 1 for start. I replaced them all in Phase 1 of my electrical mod project and stayed with 1 8D to start both engines and 6 GCs in the other 2 8D boxes for the house. My house bank grew by almost 50% in weight and AH capacity. 7 years later in Phase 2, I replaced the 8D start with a Group 31 start...one battery to start both engines. I figured I'd give it a try and could always add another G31 if needed. It works well with my Perkins 4.236s. Since that start battery is charged by the port alternator, I always start the port engine first.

djmarchand said:

Here are the weights of the batteries discussed so far:

Group 31 57 lbs
4D 100 lbs
8D 127 lbs

GC 62 lbs

Any of the first three will start a Cummins 6BTA. Pick your poison.

And as foggysail notes above, two GC batteries weigh the same as one 8D and for house service are true deep cycle batteries, unlike most 8Ds.
David

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Good info here, David. Sunchaser, if you replace each 8D with 3 GCs, you'd increase your ballast and your AH capacity by almost 50%!

The Other Gary said:

My 3988 came with 3 8D batteries, 1ea. for starting the Cummins and 1 for house. I have since changed to group 31 for starting and GC for house but had problems with the Group 31 overcharging and using too much water vs. the GC. I now have 2 GC for each starting bank and 6 GC for house resulting in no more problems with charging using combiners. And I can lift them.

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Good point! I watch for this overcharging of the G31 through my combiner, too. The start G31 is always full when I return to the slip. If I return to the slip and the house is not at or near float (<5A), I leave the combiner switched off until I return. I like this idea of using 2 GCs for the start bank and might consider this as an option for the next start battery change.

FlyWright said:

Good info here, David. Sunchaser, if you replace each 8D with 3 GCs, you'd increase your ballast and your AH capacity by almost 50%!

:

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NO! It only requires two GCs for one 8D. Remember, you have to connect the GCs in series 6v + 6v = 12v. Of course you could connect two GCs in parallel and series those two with another pair in parallel to have 12v at 460 amp hours.

foggysail said:

NO! It only requires two GCs for one 8D. Remember, you have to connect the GCs in series 6v + 6v = 12v. Of course you could connect two GCs in parallel and series those two with another pair in parallel to have 12v at 460 amp hours.

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I probably didn't explain that well. Take 2 8Ds out of the boxes and replace each 8D with 3 GCs. Now connect them in series-parallel and you've got 50% more weight and AH capacity. I did this with my 3 pairs of GCs and went from ~440 to 660AH.

FlyWright said:

I probably didn't explain that well. Take 2 8Ds out of the boxes and replace each 8D with 3 GCs. Now connect them in series-parallel and you've got 50% more weight and AH capacity. I did this with my 3 pairs of GCs and went from ~440 to 660AH.

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YOu say you replaced one each 8D 12volt battery with three golf carts. Please explain how you wired your three golf carts to make this replacement. I am still confused

I wired 6 GC into a 660 AH bank via series-parallel. These 6 batteries fit in the boxes formerly used by 2 8Ds. This makes up my house bank. The third 8D box that formerly held an 8D start battery now only holds a single G31 start battery.

FlyWright said:

I probably didn't explain that well. Take 2 8Ds out of the boxes and replace each 8D with 3 GCs. Now connect them in series-parallel and you've got 50% more weight and AH capacity. I did this with my 3 pairs of GCs and went from ~440 to 660AH.

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I do not use 8Ds for house , so no advantage. Also, the cost, associated risk and hassle of redoing something that works is well down my list as compared to other boat projects that take precedence. The beauty of battery discussions is there are so many different ways to do it.

FlyWright said:

I wired 6 GC into a 660 AH bank via series-parallel. These 6 batteries fit in the boxes formerly used by 2 8Ds. This makes up my house bank. The third 8D box that formerly held an 8D start battery now only holds a single G31 start battery.

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I thought I understood and started to reply. I understand 6 GCs and two 8Ds.

If you truly did as stated, I am still lost. 2 GCs are needed for 1 8D, 4 GCs are needed for 2 8D replacement

Yes, You can wire two GCs in parallel and series those two with another GC for 12v BUT YOU ONLY HAVE 230 AMP HOURS CAPACITY/BANK. If you have this wired as I just described, you are wasting 2 GC batteries. They are doing no good what so ever.

Now of course if you have 3 banks of GCs, that is just fine!!!

Also you could have just one big bank where there are three pairs of GCs connected in parallel for 690 amp hours

Whatever you have, if you're happy with it ..........

The 6 GCs form a single house bank of 660 AH. Each series pair of GCs provide me 12V-220AH. I have 3 of these pairs combined in parallel to form a single bank of 6 GCs providing 12V and 660 AH. I'm not wasting any power and my battery bank is working well.

Imagine this bank shown below with TWO MORE GCs in series-parallel. 3 GCs occupy one 8D box and the other 3 GCs occupy another 8D box. That's my HOUSE bank.



My start battery is a single G31.

FlyWright said:

The 6 GCs form a single house bank of 660 AH. Each series pair of GCs provide me 12V-220AH. I have 3 of these pairs combined in parallel to form a single bank of 6 GCs providing 12V and 660 AH. I'm not wasting any power and my battery bank is working well.



Imagine this bank shown below with TWO MORE GCs in series-parallel. 3 GCs occupy one 8D box and the other 3 GCs occupy another 8D box. That's my HOUSE bank.

My start battery is a single G31.

Click to expand...

That is my House bank as well. 900 ah with Lifeline AGMs.

FlyWright said:

The 6 GCs form a single house bank of 660 AH. Each series pair of GCs provide me 12V-220AH. I have 3 of these pairs combined in parallel to form a single bank of 6 GCs providing 12V and 660 AH. I'm not wasting any power and my battery bank is working well.

Imagine this bank shown below with TWO MORE GCs in series-parallel. 3 GCs occupy one 8D box and the other 3 GCs occupy another 8D box. That's my HOUSE bank.

My start battery is a single G31.

Click to expand...

Of course!!!!

I was stuck thinking you were maintaining 2 banks rather than 1 larger bank.

I replaced my single 8D starting battery with a single group 31, and used the extra space for additional GC-2's. Two starting batteries plus generator battery plus house bank does not make sense to me. I'd jump them or charge them from genny if it was ever necessary. I did need to get a beefy starting 31 tho like so some models are only 900CCA ish. YMMV.

What type of posts are on GCs? Same as 8Ds or..?

Thinking about the wiring changes I'd need to make.

kev_rm said:

Two starting batteries plus generator battery plus house bank does not make sense to me. .

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I once owned a planing vessel similar in size to the 3870. Space and weight limitations negated the use of lots big batteries. Now, I am on a much bigger non planing boat and a roomy ER. The T105 house, thruster AGMs and 8D start battery systems have worked well (except for the crazy starting cable runs) as delivered new from the factory.

There appears no reason to move away from 8Ds. But if I had a different cruising style with fewer charging hours and spent lots of time on the hook, I'd use the 8D space for more house power like Flywright has done. For the way we cruise now starting redundancy is very high on the battery set up list.

Yet another option. I went with US Battery 305 AGMs for the house wired in series parallel ala Al.



They are true deep cycle floor polisher batteries, an inch longer than GCs and the same width. Taller than CGs (about 11 inches) but not as tall as L16s. 312 AH per 6-volt pair and--at about 100 lbs each--I can still move them around when I need to (carefully). For the real estate they occupy, they provide a lot of capacity.

My start battery for both engines and genset is a Goup 31 AGM (under the socket set). I can switch to the house bank for backup starting power and a Balmar Duo Charge makes sure the start battery stays topped off.

djmarchand said:

Here are the weights of the batteries discussed so far:

Group 31 57 lbs

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Hmmm.... Our G31s weigh 77.8 lbs.

And I felt every pound of each when I replaced the first bank myself.

-Chris

ranger42c said:

Hmmm.... Our G31s weigh 77.8 lbs.

And I felt every pound of each when I replaced the first bank myself.

-Chris

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And feeling every pound offered by 8Ds is why I went with East Penn GCs. These are working out well for us and bang for buck, they were less than $85/each from Sam's when purchased.

Just for S&G... I'll add in - Group 31 deep-cycle wet batts. Affordable, powerful, long lasting, easy to handle.

https://www.batteriesplus.com/batte...31m/sli31mdc?gclid=CMeZ_qbk9tICFQIdaQodY9EAJg

Ours batts [same as on the link] are in their 8th yr... still going strong. I use four of them in parallel for house bank as well as for starting either engine. Have independent group 27 starter batt for gen set and an isolated (in its own separate battery box, always kept full charge) group 27 starter batt; for if per chance emergency arises. I believe in battery redundancy!

I do not recommend multi purpose starter/deep-cycle wet batts. Have experienced too early failure from that type.