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Old 01-02-2020, 12:38 PM   #1
4mo
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Combine house and start?

I'm going to split this out from the 8D thread:

I have a new to me Navigator 53 with 2 8D's as the starting and another 2 8d's as the house bank that all need replacing, we've been running them combined on the small cruises, but planning on being out in the salish sea for a couple weeks on anchor moving every other day or so and would like to keep genset runtime down.

Option 1: 6 Dyno GC batteries for the house in one of the double 8D boxes, and then new 8D's starting or 2 group 31's to start the TAMD 63's in the other double 8d box. If possible I'd love to have 2 more 6v's in that box tied into the house bank.

Option 2: Combine the house and start banks with 12 6v GC2's. There will be a dedicated start battery for the generator so if somehow I deplete the bank I should be able to eventually start the engines. Maybe hard wire a switch to start the engines off the same group 31 gen start battery?

They are charged by a ProNautic 40amp charger as well as an alternator.

Thoughts?
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Old 01-02-2020, 01:08 PM   #2
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As long as you have a way to start the engines if the house bank goes down, I donít see a problem with combining the house and start. Maybe wire a battery switch to the genset battery for times that you need to start off the genset battery so that you donít have to wait hours for the 40 amp battery charger to charge the huge house bank. The 40 amp charger is probably too small to efficiently charge that large of a house bank so you should try to upsize it.
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Old 01-02-2020, 01:17 PM   #3
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Personally, I like to keep primary engine start power separate from house to avoid the house electronics seeing a voltage dip at engine start. Depending on battery state of charge and how much cranking power your engines need, that dip can be enough to cause some items to power-cycle.

In your case, I'd see no issue with having 1 combined start bank for both engines and possibly even the generator too. Just include a way to start from the house bank just in case.
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Old 01-02-2020, 01:49 PM   #4
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I drug anchor two summers ago while coming back from a walk in town and needed to start the engines quickly to avoid another boat so being able to start the engines immediately without going down and flipping switches is a priority.

Ideally I would like to keep everything separate, but increasing the size of the house bank to 700 usable amp hours with 12GC2 batteries is appealing.

I think it might make sense to have a separate starter bank (2x Group 31's)on an ACR for the genset and engines then wire a way to start off the house if they somehow die.

I'm sure I could fit a couple 31's somewhere close.
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Old 01-02-2020, 02:56 PM   #5
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I use a combined start/house bank. I think you'll get a lot of people that say this isn't a redundant enough way of doing things....but it has served me well as a weekender. Cruising with significant time on the hook may require a more robust set up. And if your batteries cause "power cycling" of your electronics during engine start, that is a sign that your batteries are at the end of their life.
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Old 01-02-2020, 03:15 PM   #6
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And if your batteries cause "power cycling" of your electronics during engine start, that is a sign that your batteries are at the end of their life.
It can be. But some electronics also have a fairly high cut-out threshold and will drop out if you get a dip down to 10.5 volts or so. Unless you've got a massive battery bank, starting a decent size engine with the house bank drained to 60% can easily cause a second or 2 of voltage down to that range (even more so if there are other loads being powered). It's not necessarily a big concern, but for me, it's reason enough that if there's going to be more than 1 battery bank on board, the house should only be a backup for starting, not the primary source of engine starting power.
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Old 01-02-2020, 03:33 PM   #7
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Originally Posted by rslifkin View Post
It can be. But some electronics also have a fairly high cut-out threshold and will drop out if you get a dip down to 10.5 volts or so. Unless you've got a massive battery bank, starting a decent size engine with the house bank drained to 60% can easily cause a second or 2 of voltage down to that range (even more so if there are other loads being powered). It's not necessarily a big concern, but for me, it's reason enough that if there's going to be more than 1 battery bank on board, the house should only be a backup for starting, not the primary source of engine starting power.
I guess that is what we are talking about to some degree....a MASSIVE battery banks. I am not saying to keep your modest sized house bank and eleminate your start banks. I am saying keep your modest sized house bank and add your start batteries to it.....equals a fairly large combined bank. I once screwed up and had my inverter on when I thought I was on shorepower. I have my inverter programmed to cut out at 40% battery power. Of course after the surprise of all AC loads shutting down, I thought to myself.....hmmmm....I wonder if my engines will start on this depleted battery state. I didn't notice any sort of depleted performance from the starters. The engines fired right up....granted I did not have any electronics on as I didn't not think to add them to my impromptu test. I Also have AGM batteries...not sure what their performance curve is on a fairly discharged battery....if they stay level and drop off rapidly or if it is a gradual slope.

I will also echo what was said above. A 40amp charger is WAY too small for a boat that big. Just get you another one(bigger is better) and they can both charge the same bank as long as more than 40amps is needed. Once it gets below that the lesser charger will kick off. It is very possible a boat that size can be burning more amps than the battery charger can keep up with. I have a friend that is a marine electrician. He called on a boat that had an electrical "problem". It was a Carver 53 with a 40amp charger. He went around the boat and counted all of the amps that were being used....50 amps with a 40amp charger. THAT was the problem! AGM batteries also desulfate by rate of charge. They like a good hard fast charge. To give you an idea, Odyssey(maker of the batteries on my boat) recommends a charger that is half the size of the bank!!!!! So a 600amp bank would require 300amps of charging. That is what they call optimum...obviously that is tough to fulfill. But the bigger the better.
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Old 01-02-2020, 03:54 PM   #8
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AGMs tend to be better at delivering large bursts of current than good deep cycle wet cells, so an AGM bank may mitigate the voltage drop issue to some extent.
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Old 01-02-2020, 04:05 PM   #9
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I've started my two Lehman 120s off my eight golf car battery combined bank depleted to 20% with nary a hiccup. Separate bank, two Group 24s for the generators. I do not have these wired through a switch to the starters. I may do so in the future but I do have a set of heavy jumper cables aboard in the unlikely event I need them. As for OP, well, with twelve golf cart batteries, it would be a no brainer to me to have a combined bank.
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Old 01-02-2020, 04:19 PM   #10
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We have a 6x6v golf cart bank it has no trouble starting my Perkins 6.354 even down at 50% SOC. 12 GCs would be an outstanding bank. My recommendation would be to install an inverter-charger in the 2800 watt/120 amp range. The 40 amp charger can be a backup.
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Old 01-02-2020, 04:45 PM   #11
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I'm in the minority here, but I like having separate house and start batteries as a hedge against the unlikely scenario of the house bank going flat and not being able to start. We are frequently in remote areas of BC and Alaska and I feel like I need that level of redundancy.
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Old 01-02-2020, 06:10 PM   #12
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I'm with Ken. I try to keep battery functions as purpose specific as possible. I had the boat space to do it. True, the start banks on each engine each did some house bank duty (but not inverter), but they were each composed of four massive Rolls 8 volt batteries the size of a 12 volt 8D, and had a parallel switch at each helm. To me a standalone well maintained generator start battery and charger are a must, as that is your fail safe if you screw up one of the other banks.

If you are seeking more house capacity, consider using L16 batteries instead of GC2 if you have the vertical space. I used 4 very happily for my inverter bank; we spent weeks at a time living on moorings and/or at anchor.
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Old 01-02-2020, 07:12 PM   #13
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My '08 mainship always combines house & start with a selector sw to run off either / both 8Ds.
Initially I switched them daily to keep one 8D in reserve just in case I needed it.
I have decided to run it combined with house / start off 2 8Ds and have never been stranded.
I do have a separate gen start batty and replaced my gen on/off sw with a 1-2-all sw so I can easily switch to gen to supplement main start or vica-versa.
My ultimate fall back is a separate thruster 8D that I could easily jumper to start when all else fails.
My theory w/ combining start & house 8Ds is that under normal house use I will only be drawing them down half of what I would if separated.
I know others will disagree but it works well for me.
I am planning to replace 8Ds with 6V GC2s when the time comes.
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Old 01-02-2020, 08:04 PM   #14
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If you have the room and budget for 12 GC2s (Assuming a battery watering system) for use as both start and house and also have concerns about starting - why not just buy a spare G31 and keep it charged and in reserve? With that and a generator batter plus jumpers you’d have great redundancy plus a really large reserve for the house. I used to look down on flooded batteries but using the battery watering system with properly set Magnasine configs has changed my mind whilst saving significant bucks in the process (Costco Interstate GC2s at $95 ea plus core charge). Excellent performance so far, almost no fluid consumption, easy peazy top off.
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Old 01-02-2020, 08:31 PM   #15
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What works on a 30’ boat won’t always work on a 50’ boat so be careful with some of the advice given. I don’t like having just one battery bank for both start and house, it just limits your options in time of emergency. Since I know my boat will make a trip or two from PNW to Alaska I set things up as if every trip is going to ‘Alaska. With that in mind and considering the size of your engines, I would want 4 G31 or 2 8D’s for starting. Sure, you can start both engines with 1 G31 but what happens when you get a bad cell in a remote anchorage. If you have no desire to go beyond the Salish sea and own a fast dingy, then 2 G31’s is fine.

Trying to create a bank out of 8 6v batteries with 2 of them being remotely located is going to cause longevity issues for your house batteries. Would be much better to upgrade from GC 6v’s to L-16 6v’s. Better yet, maybe now is the time to redesign the whole system. Starting with determining how much house bank you need rather than going with what fits in the current battery box.
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Old 01-02-2020, 09:04 PM   #16
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I am another that has a dedicated starter set. I also have a means of jumpering, actual good cables, but also the usual switch to parallel the starter and house sets.

My boat is of course a lot smaller and I don't have a generator so I cannot afford to play around. THat engine MUST start.
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Old 01-03-2020, 12:20 PM   #17
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re: 6V GC batteries vs. L16 or group 31

While I do have vertical space, I believe I can get 6 GC batteries into one of the double 8D boxes, but only 4 L16 in that area and that would be fairly tight, but certainly worth considering.

Units Cost Usable AH Other Costs Total Cost $/AH
Dyno CG2 6 $1,140 345 100 $1,240 $3.59
Dyno L16 HC 4 $1,600 390 520 $2,120 $5.44
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Old 01-03-2020, 12:39 PM   #18
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Another factor is weight. L16's I think are about the same weight as 8d's. Gut busters even with some vertical room. GC's weigh much less and are more portable.
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Old 01-03-2020, 12:40 PM   #19
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I've considered a full rewire, but as the boat is new to me there are other fun things to spend money on (new anchor!) and it bashed its way up the coast in November so we've discovered many other less fun things to spend money on this year (vaculfush, bow thruster relays), so I'm hoping to make due for now and kick this can down the road 5 years when these batteries are ending their life.

This is 12v house bank only, there is a separate inverter bank in front of the engines. I've switched the lights out to LEDs, so this is mostly fridge, lights, icemaker and electronics.
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Old 01-03-2020, 12:48 PM   #20
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Another factor is weight. L16's I think are about the same weight as 8d's. Gut busters even with some vertical room. GC's weigh much less and are more portable.
L16s are heavy, but still better than an 8D. About 120 lbs vs 160. Plus, I find the L16 form factor a bit easier to lift (handles aren't as far apart and they're up higher, so usually a better lift angle).
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