Battery questions

[Jeffmorrell7]

The 36 GB I’m buying has a 8D AGM battery at each motor and an isolate smaller Gen battery. The two 8Ds are also “house batteries .” The survey shows one of the 8D batteries needs to be replaced.

So, should I replace the single 8d and everything will be OK? Remaining 8d shows 80% life. Or, since a new 8D is $1000, is there a better plan for batteries that I should consider now? Currently we do little cruising and most time away from shore power is on an island mooring. We’re in S Cal so heat and AC isn’t much of a consideration. Thx in advance.

 

[KnotYet]

While it is possible to pay $1k for an 8D battery (even $2k+) you don't need to.
A more typical price is $400 or so. For your kind of use, just match the existing
8D battery if you're sure it's in 80% good condition. That way you can parallel them.

 

[jgwinks]

Two group 31 batts are about the same cranking capacity as one 8D, and will fit in the 8D box. Cheaper and most important half the weight each. Much easier changing them out next time.
I get all my batteries from Sam's Club. We had the membership anyway, but the savings on 4 GC2's just about covered it.

 

[jgwinks]

While it is possible to pay $1k on an 8D battery (even $2k+) you don't need to.
A more typical price is $400 or so.

Haven't priced agm's in that size but I wouldn't be surprised.

 

[DavidM]

Well the simplest thing to do is replace the bad 8D with another 8D, but $1000 is a lot of money. Unless you have really big engines, like an 8.3 Cummins 6CT or bigger then a good Group 31 will start most engines just fine, but for safety and more amp hours if using it as a house battery, a pair of G31 AGMs is what I would do to replace the 8D.

Renogy, Windynation, Weize are all low cost but decent brands for G31, AGM batteries. With AGMs they have good starting and deep cycle performance, so you can save one bank of two while on the hook to start the engine in the morning and then switch to the other bank next time.

For the cost of a single 8D, you can replace the two 8Ds with two pairs of G31s and they are lots, lots easier to handle.

David

 

[jgwinks]

I pulled the two 8D's out of my boat but I had to build a little crane arrangement to do it. Replaced them with 8 GC2's.

 

[KnotYet]

I agree with the above posts. My old boat had 4 8D batteries and 1 4D when we
bought it. I replaced them with six large 6 volt batteries and doubled the capacity.

 

[Bacchus]

I'm with most others... replace them with a pair of GP31 AGMs for each 8D.
AGMs do both start & deep cycle well and you will likely find both CCA & AHs speced for AGMs reflecting the dual capability w/o the compromises found in FLA battys.
I'm also a fan of East Penn that mfg Sams Club Duracells and hard to beat the price.

 

[PM]

I've said it before, I'll say it again.

"8Ds do not belong on a boat" They are expensive, heavy, bulky and don't seem to last.

pete

 

[Comodave]

My 6CTAs have 2 group 31 batteries for each engine and they start them fine, no hesitation. I would get rid of the 8Ds and use 2 group 31s for starting and then setup an entirely separate house bank. That way the only thing the starting batteries do is start the engines. Maybe 6 volt GC batteries for the house bank. Now is the time to do it, don’t buy another 8D.

 

[hmason]

I purchased 2 x 8d AGMs at Batteries plus for $1600.

 

[sunchaser]

I've said it before, I'll say it again.

"8Ds do not belong on a boat" They are expensive, heavy, bulky and don't seem to last.

pete

Once into big motors and decent space 8Ds are quite common if not necessary. Our last set of FLA 8Ds were still doing fine after 6 years, which is our pre-set change out interval.

 

[ranger58sb]

The 36 GB I’m buying has a 8D AGM battery at each motor and an isolate smaller Gen battery. The two 8Ds are also “house batteries .” The survey shows one of the 8D batteries needs to be replaced.

So, should I replace the single 8d and everything will be OK? Remaining 8d shows 80% life. Or, since a new 8D is $1000, is there a better plan for batteries that I should consider now? Currently we do little cruising and most time away from shore power is on an island mooring. We’re in S Cal so heat and AC isn’t much of a consideration. Thx in advance.

Short version is probably yes, you can replace the single 8D and call it good. Wait to replace others until necessary.

Guys here who've said 2x G31s are sufficient are semi-correct... for your starting function.

OTOH, 2x G31s usually gives you about 200 Ah for your house functions, where a single 8D might vary somewhere between 225-245 Ah... and if you anchor or moor out a lot, every Ah you can get is probably useful.

Your $1100 projection seems very high. Or maybe that includes installation?

Reputable flooded lead acid (FLA) 8Ds from Deka/East Penn (for example) don't cost that much.

We even replaced a pair of older 8Ds with Lifeline AGMs, $785 each. (If you don't have them already, you'd have to decide whether you benefit from AGMs or not.)

And we had a yard guy do it. Easy. Simply wrote a check, very little effort required.

-Chris

 

[mvweebles]

Question to the group: OPs two 8Ds are start batteries, and house batteries. Is there an argument to dedicate one 8D (or one pair G31s) to start, and the other dedicated to house? Meaning both engines would start off one battery (or one pair of G31s)?

I'm a single engine guy so don't think about this much. But seems a pair of G31s should have enough reserve capacity to start an engine several times over. I realize the CCA is high, but duration is short. Of course, cold weather or engine issues could debunk so there is a risk. Thoughts?

Peter

 

[Portage_Bay]

Question to the group: OPs two 8Ds are start batteries, and house batteries. Is there an argument to dedicate one 8D (or one pair G31s) to start, and the other dedicated to house? Meaning both engines would start off one battery (or one pair of G31s)?

I'm a single engine guy so don't think about this much. But seems a pair of G31s should have enough reserve capacity to start an engine several times over. I realize the CCA is high, but duration is short. Of course, cold weather or engine issues could debunk so there is a risk. Thoughts?

Peter

My take on that subject is that as long as you have backup a single start battery for twins is sufficient. I know many will disagree. I start a pair of Cat 3208s with a single 8D. The backup is the house bank can easily be switched in. The backup to the backup is fire up the gen set or use the gen battery. And because I want even backup options more I pack a jump starter to help with the gen if need be.

The single start battery got a good test recently after fuel system work. The mechanic had to crank both engines quite a while to get things going again.

 

[DavidM]

Group 31 batteries usually have an amp hour capacity of 100 Ah and a CCA of about 800. So two such batteries wired in parallel gives 200 Ah and 1600 CCAs. That is close to what an 8D offers, so I don't see any performance downside in switching to G31s.

And if they are AGMs they are equally as good at providing starting amps as deep cycle amp hours. So if you want to keep it simple with minimal rewiring, switch the two 8Ds with two pairs of AGM G31s and either bank can be used for starting or deep cycle house use. I would trade back and forth to keep them fully charged as much as possible.

The G31, AGM batteries I mentioned in post #5 can be bought from Amazon for less than $250 each. I have a pair of Windynation G31/AGMs on my RV and they have perforemed very well for the last two years.

David

 

[MYTraveler]

I am a fan of 6 volt, golf cart if they fit, FLAs.

 

[Bacchus]

Question to the group: OPs two 8Ds are start batteries, and house batteries. Is there an argument to dedicate one 8D (or one pair G31s) to start, and the other dedicated to house? Meaning both engines would start off one battery (or one pair of G31s)?

I'm a single engine guy so don't think about this much. But seems a pair of G31s should have enough reserve capacity to start an engine several times over. I realize the CCA is high, but duration is short. Of course, cold weather or engine issues could debunk so there is a risk. Thoughts?

Peter

I'll offer my opinion only but won't claim it's the only answer.
First point... important to be clear whether battys are FLA or AGM when discussing applications & alternatives. I agree with DavidM that AGMs perform start & deep cycle equally well and are better than a " dual purpose" FLA ( BTW both East Penn and Trojan agree and report no start or deep cycle performance difference between AGM GP31s and AGM GCs... not true when talking FLAs.
I was a proponent of using one batty bank for starting, even when twins involved and saw little need justification for each engine start to be separated... until recently when Steve DAntonio provided a very good argument for separating start battys for each of the twins. Steve's rationale, which makes a lot of sense, is that while a single 8D or pair of GP31s have the cranking power to crank a second engine over, today's electronic controls may not do so well with the V sag when running one engine and starting a second. That's the best argument I have seen for separate starting battys for each twin.
My MS 34HT came with the OPs arrangement - 2 8D for combo start (single eng) and house with SW to choose 1-2-all-off. My electronics did not like the V drop when starting on one 8D and I normally ran with both connected to avoid problems with electronics and to only draw the DOD half as much overnight. My back up was always gennie w separate gen batty.
I have since moved my single eng start to my thruster bank to leave a pure house bank. So far that solved the V drop issue w electronics run from house. I have not had any issues w my Yanmar when running and operating 1 or both thrusters.
I added a monitor to track house SOC and start/ thruster bank V. It gives me a rough idea of SOC for thruster/ start bank by watching the V recovery provided by eng alt and my ACR.
Long winded answer with my set up / practice and the whys.

 

[tiltrider1]

The $1000 for an 8D made me think we were talking about an AGM battery. As usual we appear short some critical data. This probably explains the large variance in recommendations.

I am in the same camp as Bacchus. I would like better details before stating an opinion on what should be done.

 

[ranger58sb]

Question to the group: OPs two 8Ds are start batteries, and house batteries. Is there an argument to dedicate one 8D (or one pair G31s) to start, and the other dedicated to house? Meaning both engines would start off one battery (or one pair of G31s)?

Given the designed-in dual-purpose-ness-hood (?) of the combined start/house battery architecture... I've just not found it worthwhile to change the design.

It was easier in our last boat to add battery capacity to each main/house bank, and call it good. In that case, we went to 3x Odyssey G31 AGMs on both banks (~300 Ah), then about 11-12 years later replaced the oldest G31 bank with 4x 6V AGMs (~440 Ah).

Messing with the panel and/or pulling new wires just didn't seem worth the trouble.

Group 31 batteries usually have an amp hour capacity of 100 Ah and a CCA of about 800. So two such batteries wired in parallel gives 200 Ah and 1600 CCAs. That is close to what an 8D offers, so I don't see any performance downside in switching to G31s.

Our Lifeline 8D AGMs are 255 Ah... and for us, that extra 55 Ah (over a pair of G31s) is significant enough for house loads to go that route. In our case, that's also guided by these being pairs of 8Ds at 24VDC and physical architecture isn't all that easy to modify.

This particular 8D pair was originally for our thruster, and we repurposed to also support an inverter... which in turn services all the AC outlet loads as well as the freezer and two fridges...

Haven't replaced the FLAs for port main/house and stbd main/house yet, but the plan is to continue with the Lifeline 8D pairs for those too... whenever the time comes. Those aren't so heavily loaded with 24VDC and 12VDC stuff, but I reckon more capacity is usually better at anchor... especially to better protect the start function... even when there's a parallel switch handy.

Not a recommendation, just an observation.

-Chris

 

[catalinajack]

And another option is no starting batteries at all. Just use the house bank. My boat has eight golf cart batteries. They start the Lehmans down to 35% (test, not practice) state of charge. Backup is the generator starting batteries. Fewer switches, less wiring, fewer points of failure. Not right or wrong, just different.