8Ds versus Diesel Truck Batteries

My boat is an 1988. Back then 8D batteries were common in boats. I have two 8Ds for start batteries for the CAT 3208T/As. The start batteries are NOT connected to a battery charger but are charged by its respective engine. I am diligent and start both engines every Sunday.

The 8ds are 5 years old and I have noticed the stbd side is getting weak. The 8ds are in the back of my ER and are a bear to lift them out to the dock and then reinstall.

I am thinking of replacing them with two, per side 850 CCA, Group 65BCI batteries. These are the same as the batteries in my diesel truck with a 6.4 engine. So two batteries per side. They would take up about the same space as an 8d, yet easier to handle.

Thoughts?

Well, when I bought my boat 5 years ago, it needed batteries. It had 4 8d wet batteries. I replaced them with 4 Barracuda AGM's at a cost of a little over 2k. This fall they gave up the ghost and 4 new barracudas were now over 3500 bucks. Since my boat is all 120AC electric, as in stove, refrigerator Etc. no inverter and I always run Iron Mike when away from the dock anyway. I replaced them with 4 AGM Group 31 batteries from Tractor Supply company for 104 dollars a piece with an 18 month free replacement warranty. Both sets of batteries are wired in series. Each G31 is 995CCA, The Barracudas were 1350CCA. The Detroit's crank right up, not spending over 800 bucks a battery and the fact that I cannot get them in and out by myself. 8d's weigh 170lbs a piece where as the G31's are 65lbs a piece and I can get them in and out easily.

Are your cats hard to start? Glow plugs?
If not, a single 8D should be more than you really need for both.
Do you also have a separate start batt for your genset?
I have all 3 on a single 4D. My boat came that way, with slow starting, glow plug, TMD40s and it was still A-OK. Now that I run easy starting TAMD 41s, I know I have way more batt than I need for starting. My last 4D replacement was after 12 yrs.

In case of failure, I also have a combining solenoid so my 4 6vGC House bank can be cut in for starting engines.

I also have flood house batteries. Anything special to keep in mine when switching to AGMs?

No I had no issues and my 24 year old year old at the time, Sentry battery charger worked fine with them.

koliver said:

Are your cats hard to start? Glow plugs?
If not, a single 8D should be more than you really need for both.
Do you also have a separate start batt for your genset?
I have all 3 on a single 4D. My boat came that way, with slow starting, glow plug, TMD40s and it was still A-OK. Now that I run easy starting TAMD 41s, I know I have way more batt than I need for starting. My last 4D replacement was after 12 yrs.

In case of failure, I also have a combining solenoid so my 4 6vGC House bank can be cut in for starting engines.

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Well the stbd battery was a little low and no glow plugs. Separate battery for genny. Genny used to have a 4D, replaced it with a truck battery and no worries.

I have 2 Costco type 31s. Tossed the 8Ds.

My boat had 8Ds for the house. PO changed to 6volts. So glad he did. I am too old to be moving 8 Ds. I do have 2 4Ds for start batteries, but they are very easy to access.

The brand of flooded batteries makes a difference. Fishermen on the West Coast have been using Dyno batteries (Seattle) for many years. When I bought my current boat, it had 20 year old Dyno batteries for both starting and inverter. No they weren't in anything near new shape, but I could anchor for several days and still start 2 Detroits and the inverter would run lights, reefer and a box freezer overnight.
My experience fishing, the Dyno batteries lasted 8-10 years of good service. When I look at the difference in cost for other type batteries, I don't see the new style worthwhile. Current 8d price is $350 for starting and $415 for deep cycle.
I understand and agree with the difficulty of changing them, but I just hire a couple dock kids to move the ones I had. Then I'm good for 8-10 years.

Xsbank said:

I have 2 Costco type 31s. Tossed the 8Ds.

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You're just boasting! Tall buildings in a single bound as well?

Use some Odyssey's. One for each engine, have them able to be paralleled but I doubt you will ever need to.
PC2150 MS Odyssey Battery

8D's make no sense at all these days.

I have New Interstate 8D starting and another for house loads, actually a year old now. I bought them from the marina and they installed them on the boat, more expensive but cheaper than a dr. visit. Next time I’ll switch to 4 grp 31 deep cycle for house and a 31 start battery.

Alaskan Sea-Duction said:

I also have flood house batteries. Anything special to keep in mine when switching to AGMs?

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Go deep cycle when you do switch.
Also check charging profiles available on your charger to match AGM charge profile specs

Alaskan Sea-Duction said:

I have two 8Ds for start batteries for the CAT 3208T/As. The start batteries are NOT connected to a battery charger but are charged by its respective engine. I am diligent and start both engines every Sunday.

I am thinking of replacing them with two, per side 850 CCA, Group 65BCI batteries. These are the same as the batteries in my diesel truck with a 6.4 engine. So two batteries per side. They would take up about the same space as an 8d, yet easier to handle.

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I get the "easier to handle" part!

Your Cat manual will say how many cranking amps you need, minimum.

I dunno G65s, but the combined cranking amps from two G31s will likely start your 3208s. I'd guess 1700 CCA from those G65s will do so, as well.

We've had good luck with Odyssey PC-2150s (G31s) and it happens they offer huge cranking amps; maybe a single of those would work for you, too. Almost certainly two would... An AGM advantage: they don't self-discharge very quickly. We've left ours over a 4-month winter storage without significant self-discharge... so starting your engines every Sunday could become a thing of the past if you choose that...

-Chris

ASD
I'm the outlier here I guess. I use 8 Ds for start batteries and local strong backs to install. Then a charger and BMK for same. The start batteries power the dinghy davit too. The ultimate sin, I even have an 8D for the genset.

I use a pair of group 24’s to start my Lehman 135. The engine jumps to life on starting. Why would you need more than a pair of 31’s to start both engines? Once one is started, you have your alternator brining up the volts to start the other engine.

JDCAVE said:

I use a pair of group 24’s to start my Lehman 135. The engine jumps to life on starting. Why would you need more than a pair of 31’s to start both engines? Once one is started, you have your alternator brining up the volts to start the other engine.

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My Lehman's are started with the 900+ amphour house bank. I have two Group 27's to start either of the two generators. If the house bank happens to get too far depleted to start the Lehman's, I can use the generator start batteries to get going or use the generator to sufficiently recharge the house bank. In a pinch I could move the Group 31 thruster battery to the engine room and use jumper cables.

sunchaser said:

ASD
I'm the outlier here I guess. I use 8 Ds for start batteries and local strong backs to install. Then a charger and BMK for same. The start batteries power the dinghy davit too. The ultimate sin, I even have an 8D for the genset.

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You are not alone Tom, we have 8Ds too. But our genset batt is a "6D"( smaller than 8D, bigger than car batt), and the Onan manual stipulates it. You genset is probably bigger than our 6.5kw.

knotheadcharters said:

I replaced them with 4 AGM Group 31 batteries from Tractor Supply company for 104 dollars a piece with an 18 month free replacement warranty. Both sets of batteries are wired in series. Each G31 is 995CCA, The Barracudas were 1350CCA. The Detroit's crank right up, not spending over 800 bucks a battery and the fact that I cannot get them in and out by myself. 8d's weigh 170lbs a piece where as the G31's are 65lbs a piece and I can get them in and out easily.

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Huh, I never thought to look there for batteries. Their price for a group 31 start battery is almost half of what WM wants! The reviews aren't glowing but for the $$ saved, it's worth a shot.
Thanks for the info.

I had two 8ds for my 3208s. After dragging those sob's out of my bilge, I replaced them with a couple group 27. If your cats are like mine, they start before the key is fully turned. The space I freed up in the battery box added two more batteries to the house bank which is now about 1200ah. No need for 8d and I can always parallel the house if I need more cranking.

"Next time I’ll switch to 4 grp 31 deep cycle for house"

GP 31 are mostly created as Starts, good luck finding them in Deep cycle.

Expensive but the double tall golf carts (L16)are great if you have the height.

Sadly a batts power comes from the lead weight , so moving the L-16 is as much a PIB (pain in back) as the old 8D.

My battery source (local) has recommended US Battery #US31DCXC2 for a group 31 deep cycle. I have not bought yet, waiting for the snow to melt, but will give them a try in April.

BruceK said:

You are not alone Tom, we have 8Ds too. But our genset batt is a "6D"( smaller than 8D, bigger than car batt), and the Onan manual stipulates it. You genset is probably bigger than our 6.5kw.

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I have an 8KW Onan. It had a 4D, until it blew up a few years ago. I replaced it with a group 27(?) and it starts the genny every time.

Follow Up

I thought I would share my fix. I had a STBD 8D battery go dead. Measured 10.5Vdc. Joy Joy!!

So I huffed and puffed and dragged that beast out of the engine room up 2 sets of stairs to the sundeck, then down to the cockpit, then down the dock up the ramp and then some more stairs then one big huff to the back of the truck!!! Why do they put these piece of crap ropes for handles WTF!!! That hurt!!

The old 8D was 1200CCA and was 5 years old. I bought two Group 31 batteries 1000CCA each with two cables to tie them together. They actually have handles and I got the studs instead of the old big connections. Easy getting down to the boat and down to the engine room.

Turned the key to start and the STBD CAT 3208T/A fired off with a half turn of the engine!!

Yeah! I can now cross number 500 off the list. Only 499 projects to go.....

Alaskan Sea-Duction said:

I thought I would share my fix. I had a STBD 8D battery go dead. Measured 10.5Vdc. Joy Joy!!

So I huffed and puffed and dragged that beast out of the engine room up 2 sets of stairs to the sundeck, then down to the cockpit, then down the dock up the ramp and then some more stairs then one big huff to the back of the truck!!! Why do they put these piece of crap ropes for handles WTF!!! That hurt!!m:

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I paid two high school kids to haul my 8D’s off the boat. It was easy and worth every penny. LOL

Post #23
Good fix and one that I have been recommending and doing for several years. It is my goal to remove every 8D beast that I encounter on boats and replace them with Group 31's in parallel.
One 80' boat that I work on has 1200 hp MTUs. They are started with 4 Group 31 batteries in series/parallel. Engines start instantly and nobody's back is strained when it comes time to replace a battery.

When I did my complete electrical and charging make-over i set up a "cranking" bank. It starts 2 x 6bta5.9's and a 10kva genset; and powers bow thruster, windlass and davit.....all short-duration high-draw loads. It comprises 3x gp31 spiral-wound AGMs (Optima 34M Start). One of the 3 can be isolated and dedicated to the genset and I do this on extended cruises, but don't bother for a weekend away. I've been delighted with the longevity of these batteries and they are more tolerant of charge voltage than other agm's. $$ are right too. They are charged from the House bank only via 2 x Balmar digital duo-chargers programmed for these specific batteries (and though I've never had to do it, could be parallel'd to the 8x6v FLA house bank). I'd be pretty upset if I had 8Ds that lasted only 5 years as reported by several people above. I'm on my 10th year and expect at least 2 more.

No question the multiple thin pates of an auto or truck start batt is nice and fast , and will start a cold soaked engine , where crank speed really matters.

The question becomes how well the batt will do if a fuel problem occurs.

Some folks have an installed pump to re-prime all the hoses , and filters others hope the tiny lever on some fuel pumps will work.

IF the engine needs to be cranked to restart , the 170 lbs of lead in an 8D will last far longer than flyweights.

"Stuff happens" cranking capacity is your insurance.

the 170 lbs of lead in an 8D will last far longer than flyweights.

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If by "flyweights" you are referring to two Group 31 batteries in parallel, please note that eh easier to rig Group 31s in parallel have essentially the same MCA and Ahr rating as the demon Group 8D.

I replaced my 8d with two 31s. Except, I need to swap my battery boxes around, so one is not installed yet, so both engines ran off the single 31 last season. No complaints have been lodged yet. I'll isolate them from each other when I eventually get around to it. 3208s don't take much to start.

Feeling like the odd man out as I just purchased a third 8D to:

1. Act as my start battery for both FLs. (Bat1)

2 Allows me to wire the two existing 8Ds in parallel to act as the house bank increasing usable amp hours. (Bat2)

3. Along with the other two 8Ds provides needed ballast (relocating all batteries from port to starboard to correct a port list).

Very inexpensive battery overall ($240 USD) but they sure are damn heavy.