8D Battery Connections

[Sidclark]

Ok, so my latest boat, a 1993 Carver 350 Aft Cabin, has two 8D batteries that are start and house. This setup is configured with a three way battery switch. Bat 1, Bat2 and All. I just installed two new 8d's and I want to tie them together to increase the amp hours. Here's the question, can I connect them together and leave the 3 way switch in play? I would never move it from the "all" position. Or, do I have to remove it from play and rewire the system?

 

[Comodave]

While I would not do this since combining the start and house banks could lead to the start battery going dead along with the house battery. But if you are set on doing it I would remove the switch. If you are going to leave them permanently connected the switch just adds connection points and more places for corrosion to form.

 

[Sax]

My boat came configured with three 8D batteries wired together in parallel to perform as the “house bank” and “start” for starboard engine and a single 8D battery for “start” for port engine. I suggest you keep the three way battery switch to isolate or combine all four batteries as required. I normally operate with the selector on the house bank and monitor the battery status.

 

[SteveK]

While I can agree with what Dave said about more/less connection points. I would just leave it as is and use the 1-2-all in the ALL position to accomplish your goal of combined bank.

 

[Bacchus]

For 2 - 8Ds you likely fall in the ABYC " large batty bank" category and require special fusing considerations. See

Battery Banks & Over Current Protection - Marine How To

Battery Banks & Over-Current Protection A 300A (MRBF)Battery Terminal Mounted Fuse by Blue Sea Systems In this article we will discuss fuse selection, fuse types, wire ampacity, Amperage Interrupt Capacity (AIC) and even potentially unsafe ABYC “exceptions to the rules”. In short,

marinehowto.com

Especially the AIC fusing reqmts for banks >2200 CCA or 500 AH. These require properly sized class T or MRBF fuses.

My MS 34HT was factory equipped with 3 8Ds... 1 thruster and 2 combo start/ house. I have since moved start to the thruster bank and permanently combined 2-8Ds for dedicated house. I recently replaced the house with 4 - GP 31s in parallel and added MRBF fusing to protect individual batty from short circuits.

 

[Delta Riverat]

I'm a dedicated fan of isolated house and start batts. With an emergency shunt switch that really should never be used. I've even used that one. You don't need to fuse the start battery.

 

[HeadedToTexas]

I’ll second (or fourth?) keeping the start battery separate from the house bank. With relatively little capacity, you’d have to be really vigilant any time your 1-2-All switch has them connected.

Pretty good bet those batteries are not far from their replacement date. Maybe it’s time to study up on lithium ion technology and go that way to address house bank capacity? It’s worth the effort.

 

[ranger58sb]

Ok, so my latest boat, a 1993 Carver 350 Aft Cabin, has two 8D batteries that are start and house. This setup is configured with a three way battery switch. Bat 1, Bat2 and All. I just installed two new 8d's and I want to tie them together to increase the amp hours. Here's the question, can I connect them together and leave the 3 way switch in play? I would never move it from the "all" position. Or, do I have to remove it from play and rewire the system?

It's likely that wouldn't really "increase" your available capacity. Your DC systems are probably split, half on one battery, half on the other.

Might be possible to increase capacity with different battery sizes/shapes. 3xG31s might fit if you have a bit of space around each 8D. Or maybe more, 4xGC2s wired series/parallel. Or even much more, 4xL16s if you have available height. Et cetera.

Or maybe even better... change battery chemistry to LFP. Lots of threads here about that, and some offerings are beginning to sound viable as duel-purpose start/house batteries.

I'm a dedicated fan of isolated house and start batts. With an emergency shunt switch that really should never be used. I've even used that one. You don't need to fuse the start battery.

I’ll second (or fourth?) keeping the start battery separate from the house bank. With relatively little capacity, you’d have to be really vigilant any time your 1-2-All switch has them connected.

Agree, but... bazillions of boats are delivered like Sid's boat, and the dual-purpose configuration generally works well enough. I've not ever found it worthwhile to do all the work necessary to convert from the original architecture. Adding an inverter/charger on another bank has been more useful, especially to reduce DC loads for fridge/freezer... in turn leaving more capacity in the original banks for the rest of the DC loads.

-Chris

 

[tiltrider1]

OP’s set up is pretty standard for a pre 90’s set up. While many people have gotten by with that set up, it just doesn’t make sense to me to risk continuing with it. I would upgrade to a more modern LFP house bank and AGM start bank.

 

[DDW]

I'd leave it, but if you are going to rewire, the best solution in my mind is to parallel the two 8D, and add one or two G24 or G27 to start the engine. These won't take much room and are plenty big to start an engine. Then use the existing switch to isolate or parallel the larger house bank with the small start bank.

 

[Sidclark]

I really need to get in there and figure out if indeed one 8d is for starting and the other for house. I'm trying to get as many amp hours as I can with the current setup. The 8d's I have now are brand new along with a Sterling 50 amp charger. Victron is the monitor. The genny has its own g31, which I could use in a pinch for starting if I lost both 8d's. The reason I need the amp hours is once we leave our home dock, we rarely plug back in to shore power and rely on the genny for charging. Lithium is out for now as an option.

 

[SteveK]

I see the GEN has its own G31. IMO, you can combine the two 8D for house in parallel, but take off +/- from opposite ends and keep it starting too since the GEN can be started if needed to charge up the house/start combo.

 

[Sidclark]

I see the GEN has its own G31. IMO, you can combine the two 8D for house in parallel, but take off +/- from opposite ends and keep it starting too since the GEN can be started if needed to charge up the house/start combo.

That's what I was thinking, but the three way switch was confusing to me. I figured if it was left in the "All" position it would work fine the way you described. I just wasn't sure. I do most of my own work, but fall short sometimes on the electrical end...

 

[Bacchus]

A consideration might be location of existing gen batty and proximity to starter. When I moved mybstart from my pair of 8Ds that were combo start / house like yours I was able to cut the run to starter in half and use a 4/0 cable that previously ran one of the 8Ds to my selector sw. When modifying my wiring I added a battery monitor that needed to go directly to the combined bank of 8Ds and wouldntnwork through the switch. The repurchased cable replaced a 2/0 cable that was 2X the run length... a real win - win in my case.
If there is a place for a combined gen / eng start batty close to eng starter decreasing run length is an advantage with very hi amp draws like starting.

 

[SteveK]

That's what I was thinking, but the three way switch was confusing to me. I figured if it was left in the "All" position it would work fine the way you described. I just wasn't sure. I do most of my own work, but fall short sometimes on the electrical end...

If you leave the 1-2-all in play then leave the + on the 1&2 input as before but cable jump the - between batteries if not already done.

If you remove the switch as a transfer point to choose a bank then take + from BAT1 to switch and also jump + to BAT2.
Then - from BAT2 to BAT1. Now take the - load from BAT2. This makes two batteries one bank.
Continue to use the switch as a main shut off with only BAT1 + in/out at switch.

With first method you combine with ALL but keep available 1-2.
With second method you create a large bank with charging balanced.

 

[Sidclark]

I want to make sure I understand the wiring correctly. Leave the connections as is at the switch itself. Leave the switch in the "All" position. Connect the 8d's in parallel. Connect the two positives going to the switch to one end of the bank. Negative cable from opposite end of the bank to ground. Does that sound right?

 

[SteveK]

I want to make sure I understand the wiring correctly. Leave the connections as is at the switch itself. Leave the switch in the "All" position. Connect the 8d's in parallel. Connect the two positives going to the switch to one end of the bank. Negative cable from opposite end of the bank to ground. Does that sound right?

Not exactly.
As is now the positive from each battery to the switch 1&2 and they are only in parallel in the ALL position. If you leave it like that then just ensure the two negatives are joined together before going to a common ground location to ensure solid connection in parallel. You end up with two batteries in parallel as one bank when switch is in ALL position OR just BAT1 OR just BAT2. Two banks combined with ALL.

 

[tiltrider1]

Lithium is out for now as an option.

I don’t see why. LFP batteries are cheaper than FLA or AGM. An 80amp LFP charger is only $250. You could easily get 400 Ah in the space of one 8D and leave the other 8D in place as a start battery

 

[Sidclark]

Ok, this is what I have minus the battery monitor and charger. From what I understand this should be ok and I'll have 480ah as each 8d is 240ah.

 

[tiltrider1]

That will give you two separate banks that can be used either or both.

 

[ranger58sb]

Ok, this is what I have minus the battery monitor and charger. From what I understand this should be ok and I'll have 480ah as each 8d is 240ah.

Good. Two points:
- 480 total means more like 240 usable, with lead-acid batteries
- 240 usable for all house loads (your combined config) is pretty much the same as 120 usable for each half of your house loads (your original config)

Latter might make more of a difference if both fridge and freezer are on the same half, otherwise it can all come close to a wash...

-Chris

 

[Tjohn]

This won't answer the question but solved my problem. In fifteen years, I have had one emergency battery bank replacement and two old age replacements. Guess what? I don't have the strength anymore to lift the 4D's, let alone 8D's. My answer was to switch to 4 grp 31's. It required a few more connections, but I did all the lifting myself and ended up with a couple extra amp hours as well.

 

[RickyD]

If you have room above like I did I converted 2 8Ds for 6 6v L16s. They give me a little over 1300 amp hours with a little over half as usable