8D starting battery replacement

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I think not exactly. Two G31s will likely fit inside an 8D box with room to spare (and 3 G31s won't fit). Two G31s in parallel will usually give you about 200 Ah capacity, where a typical 8D might give you somewhere between 230-255 Ah.

For starting loads, a pair of G31s often (usually?) provides more cranking amps than a single 8D. (I think.)

For house loads, compare 6x 6V GC2s, which are said to fit inside an 8D box... and might give you something like 660 Ah.


You are correct Chris.

I could not fit two G31 batteries into the old 8D box so I created a new box that was slightly wider than the 8D box. In almost the same footprint, I could then accommodate 3 x G31 batteries. This gave me a new start bank with ~ 2200 CCA & 330 Ah of storage: Significantly more power than the 8D in almost the same footprint.

But the point remains that the advantages of using an 8D over 3 x G31’s are minimal. The 8D may be a little cheaper with less complex wiring vs the G31 plan with the need for a couple of Busbars and some cables.

The G31’s are easier to work with and provide more power.

(In my case, I had an 8D that was dead and had six good G31 Fireflies looking for a new application so the choice was easy. 3 went to form the new start bank & other 3 joined the stern thruster bank as an auxiliary bank.)
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I start my SP135 with two group 24’s. It starts instantly. They lasted 8 years with some abuse. Never any trouble. Always on float charge at the dock.

You don’t need an 8D to start most engines.

So a buddy and his high school football playing son and I switched out the old 8D for new 4D start batt yesterday morning. Slid it in the box to a place we could sort of tip it out then lifted up through the hatch with dock lines thru the handles onto a rug, out the back, through the transom door onto the dock and in the cart. Reverse for the new seemingly light in comparison 4D. Pathmaker green light came back on, same voltages on both banks of the link 20 monitor, engine fired right up.

Then on the celebratory family cruise the flybridge Murphy gauge acted like it lost contrast — it was on, but no writing visible. Seems wholly unrelated, except for the timing? Have not checked connections on the back but I will. Any other thoughts on that? It’s original….Thanks for advice, I’ll be back.
Used the packaging for the new one to fill space in 8D box


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It appears one of the positive cables is resting on top of the negative terminal of the battery. That terminal appears to be square with corners.
I would want some chafe guard between the positive cable and the negative battery terminal. If possible reroute in some manner so the positive cable is not just resting on a bare negative terminal. May be over kill and 4 or 5 years from now I would not want to deal with an event that was due to vibration, chafing through the insulation of the positive cable.
Choosing a replacement battery I check the terminals are in the same places. Otherwise you end up pulling/twisting the cables to relocate them to the correct orientation. From the box design I`d say the + and - cables were intended to exit either side of the box.

I prefer "cause and effect" over "coincidence". If something electrical changes after you do something electrical it`s likely related.

This is drift but relates to decisions(some not so good) around battery change out.

I didn't think much of it at the time, but now consider it proof that I married well.
DOMINO was on the hard in the Sea of Cortez for hurricane season when we were notified by the yard that a contractor had left a breaker on in our ship that was not connected to shore.
The house bank was gone.
It was a bank of 6 Odysee PC1800 AGM 214 AH, 770CCA suitcase batts that weigh around the same as a lead acid 8D.
Since we could not buy these in Mexico, we flew to San Diego, bought the batts, loaded them in a rental car, drove to Tucson and then 5 hours down the cocaine highway into mainland Mexico.
The house bank is under a bunk forward in the basement of this boat. With only my wife and I and a couple of cargo straps, we wrestled 6 of those beasts up into the saloon, out to the back deck, and then using the straps lowered them to the ground(a good long way). In between every batt it was down the ladder to get the straps off and then back up to fetch another one.
Then, we did the same in reverse another 6 times up onto the back deck, into the saloon, and then down into the basement.
This was a 90 degree weekend day in a dirty dusty boat yard on a weekend with no yard help or equipment in a not too nice part of Mexico.
Then we loaded the used batts back into that SUV and dove back about 11 hours to San Diego to turn them in.

Can you believe I am still married and happily boating?? What a dumb-a$$.
The 8d that was in there before had the same flag terminals at the same end. Those cables were like that- they are bundled together with that spiral chafe protection, outside the box.
I wondered about the red ones resting on the black one thing. They were already sort of bent like that, wanted to lay like that. I connected them without twisting or manipulating them at all. I can re do the pos with a lil curve -Don’t want to say arc! - above the neg terminal. What is good chafe protection? That spiral stuff? Elec tape with something stiff but not sharp under it?
Box has notches all around for various access options.

I agree about the timing on the Murphy. Too coincidental for me. But how/why could new smaller start batt cause that to go dim ? Engine ran great, batt monitor info all good, other gauges worked, lower helm Murphy gauge was fine ….
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Too late likely but something I should have mentioned. Your existing battery for the engine, is it getting a proper charge?
To often people blame the battery or alternator when it is actually the wiring that is at fault. Wire too small, connections POORLY done and/or loose, connections corroded creating resistance all of which adds up to a lousy recharge.
Before tossing the batteries check the charging, the wiring. It only takes a volt or two loss to turn an otherwise decent installation from a good one to a bad one.
My buddy just went through starting problems. Stbd starts, port no good. He thought he had AGM batts because that is what his survey said and though there were plugs, they were flush with the surface. Batteries ran out of fluid, and were failing. So he bought new. Still no start. I helped him by tapping the starter and solenoid with a hammer to see if it was jammed and it worked, twice. Then not. So he cleaned up all of the connections at the starter, still no good. Starter was looking rusty so he pulled it and put in a rebuild. Still no good. Then a friend bypassed the starter ground with more wiring and it started. Now he has to find where the ground has gone bad. We both have Cat 3208s. Is the main engine grounding at the starter or some place else? I’m not at the boat. I need to find where they ground.
I was rereading this post and realized I did not comment about cabling and poor start up cranking operation.
Check all of the connections , both positive AND negative connections at the battery and where they lead to. Do not just look at them. Both + & - are equal parts of a circuit and if one is poorly done then it will cause problems for the entire circuit.

Your engine requires a L:ARGE amount of current to crank the engine. Often slow or no starts are the result of poor connections.

Those connections must be :
----no corrosion on any part of the connections. The wire itself including inside the crimps and the bolt clamping area.
----Properly crimped crimps . Doesn't need to be fancy but well done and tight.
----application of a coating such as NoALox or a dielectric grease, not just any grease, which will prevent or a least hugely slow any corrosion. Applied as you assemble and before the crimp is done. You don't need gobs though.
----sealed crimps using good heat shrink tubing to close the gap between the crimp barrel and the wire insulation.
----keep the wiring out of the bilge so no water can get to it.
A good job will last years , 20 to 30 or more , a poor job won't.

I am sure I have missed something. I know I keep harping on this but aboard my own boat I have corrected many connections that were giving some trouble early on in my ownership. Less and less trouble now to almost none. I have had my boat for 39 years and still counting, maybe.
I have seen to many questions blaming batteries alone for poorly operating equipment. If those electrical connections are not RIGHT then they are WRONG especially if they are expected to last for years.
Resistance from corrosion or poorly done joints will reduce the current then it will also interfere with recharges. You lose on both directions
Update, my buddy with the ground problem polished up the buss bar and connections and is happily starting his engines.

Since we are talking about moving batteries, when I bought my new start batts 8Ds the guy I bought the new ones from did the removal and installation with a helper. Worked for me.
We just swapped out our bow thruster 8D for a new one. Used an 8’ 2 x 6 as a ramp and with the help of a dock line and our 2 teenage sons we slid the old one out of the forward bilge and the new one in. It actually wasn’t too difficult.
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