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Old 10-14-2019, 10:20 PM   #1
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Any reason not to share engine and gen start batteries?

Other than the obvious reduction in redundancy. I'd like to recover the space. Most of the objections to two alternators on one battery seem to center around overworking one while the other idles. Since a rarely run both at once, and the engine alternator has higher set points, seems like a non issue in this case.

Something I haven't thought of?
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Old 10-15-2019, 12:07 AM   #2
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Is this also your house bank? Our setup is one 12v bank for electronics, one 24v bank for house/start and one 12v bank for the genset. We keep the genny battery completely isolated. It is the emergency backup, if the genset starts we can get everything else to function via chargers.
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Old 10-15-2019, 01:08 AM   #3
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We have 2 start batteries and a house bank. The genset starts off the house bank. I can get the genset off one of the start batteries. I also carry a jumper pack along just in case.
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Old 10-15-2019, 01:56 AM   #4
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There's also a house bank, and a paralleling switch so if the start battery went dead I could still start the engine or genset from the house bank. I'm planning on re-battery-ing, leaning towards two G24 for engine and genset start, three L16 Firefly for house. That would take a much smaller footprint than the existing setup, more capacity, less complication.

Currently the bow thruster is powered from the 4D engine start battery, but two good G24 have more CCA and lower internal resistance than a 4D.
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Old 10-15-2019, 06:59 AM   #5
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My engine and generator use the same start battery. I don't know enough to be able to say if you should do that or not, but it seems to work well enough for me so far.
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Old 10-15-2019, 07:38 AM   #6
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IMO, it depends on your cruising style. I have read on TF about people who have one house bank and start everything off of it. If you're a local cruiser within easy reach of Seatow or Tow Boatus, I guess that's fine. As you venture to more remote locations, I think separate and redundant is more prudent. Don't know how big your generator is, but a properly sized battery is pretty small in the scheme of things. Maybe you could find a better mounting location. My engine, house, generator, and bow thruster batteries are all separate, although I can jump the engine battery off of the house bank with the flip of a switch. That same switch also allows me to charge the house bank off the engine alternator when the house bank alternator regulator failed (have 2 alternators on the engine for engine battery and house battery).

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Old 10-15-2019, 08:19 AM   #7
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I have 2 start batteries. Each engine and the generator has a 1-2-both-off switch, so any of the above can be started from either battery. Normally the port engine starts from battery 1, stbd and generator from battery 2. I've had no issues with this sharing arrangement. In my case, the house bank is separate and nothing can start from it without jumper cables (it only ties to the engine batteries via ignition-interlocked ACRs for charging with engines running).
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Old 10-15-2019, 09:16 AM   #8
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You need a backup of some kind. There is at least a hundred ways to wire them up. Learn what works for you in the simplest and safe arrangement.
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Old 10-15-2019, 09:47 AM   #9
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With proper battery management and maintenance all sorts of setups are possible. The trouble is most folks aren't going to put in enough of the gear AND monitor it effectively. Batteries are more tedious than you might think. Temp, types of cells, dis/charging rates, float, etc.

So, can you do it, sure. Should you? Only if you're really going to set it up right and monitor it.

Just how much 'space' is it you think you'll be recovering? And what is it you anticipating putting there?
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Old 10-15-2019, 10:05 AM   #10
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No problem sharing batteries. However you will have to run cables to the gen that will probably cost as much as a small battery. I like having the genny on a separate battery because I can use it to charge dead start batteries if necessary. If you have separate house bank then you can use that if youcan connect them.
I always kept jumper cables so I could jump anything in an emergency.
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Old 10-15-2019, 10:17 AM   #11
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Our genny has its own separate battery (with a pair switch to the other batteries). It's a nice bit of redundancy, but our Xantrex controller has an adjustable setting that will shut off our inverter if we go below a certain voltage to keep from running our batteries too flat, so I don't see us ever having to use it (unless our twelve volt systems ran them down).
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Old 10-15-2019, 11:38 AM   #12
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I have separate gen, start and house batteries, and house can be paralleled for start. I see no reason why the start batteries couldn't also be used for the genset, 2 group 31's in parallel in my case. I also carry jumper cables for any unplanned cabling permutation that might pop up.
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Old 10-15-2019, 09:58 PM   #13
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My boat comes with a separate genset battery. When my genset battery died a couple days after I bought the boat, I didnít know any better so I just replaced it. In retrospect, I probably should have simply gotten rid of the genset start battery and used the engine start batter to start the genset as well. I have the ability to start the genset from the house bank with a cross connect switch.
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Old 10-15-2019, 10:39 PM   #14
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Our start and house is one combined bank of 2 8Ds. Sub-optimal but IG tradition. Genset has separate N150(?6D) battery,so it and genset are backup if the dual use bank gets messed up(which it hasn`t).
Genset batt has its own regulated solar panel so it is good backup, otherwise it only gets charged when genset runs. I like the self contained stand alone backup.
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Old 10-15-2019, 11:09 PM   #15
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House bank is as the name implies... 4 deep cycle bats hooked in parallel. Runs all our 12 V general needs. Charges from charger activated by gen set or shore power. Also charges from starboard engine alternator

Both engines start off the house bank

Gen set has independent starter batt. Charges from solar panel in front of fly bridge. Also charges from 2 amp float charger that runs whenever 120 V AC is on during gen set or shore power activating.

Have isolated starter batt in its own batt box - for emergency if all else fails. Float charger same as the one on gen set batt is also activated whenever 120 V AC is activated.
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Old 10-15-2019, 11:16 PM   #16
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I have 3 groups
house 6 6volts also wired to Solar
two grp 31 one for each engine
Gen 1 grp 27

The charger onshore power has 3 legs
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Old 10-15-2019, 11:21 PM   #17
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I run everything off the 4 x 220Ah house bank and have one separate 220 Ah backup battery (charged by an ACR) that can be switched for house or start.

So far I haven't had to use it.
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Old 11-01-2019, 01:47 PM   #18
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As others have said, it really depends on your cruising style, location, redundancy requirements, technical knowledge, etc. etc.

My boat had two separate main engine starting batteries that couldn't be combined and a single generator starting battery when I bought her. I combined the two main engine starting batteries in a way that allows me to use them individually or together, and replaced all of the batteries with more modern and smaller AGM batteries.

I usually leave the two main engine batteries combined all the time and start both engines from that. I have not seen any issues with that setup. I like having the generator battery completely separate as it is my "last resort" if for some reason I drain my house bank, and somehow mess up my main engine start bank, I can at least start the generator and have power to start figuring out what mess I got myself into, and charge other banks back up.

Current diagram below

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Old 11-01-2019, 01:58 PM   #19
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Wow, I wish that I'd thought of that. Last summer i had to replace my gen battery, in part because it was used so little.
With my setup it would have worked fine with one start. I can always stsrt engine with house bank of needed.
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Old 11-01-2019, 02:03 PM   #20
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I can think of one reason for two separate batteries. It has just happened to me.
The engine battery failed and I could not styart the engine. I am at a bulkhead with no power and use solar to charge both batteries. The GenSet started and I was able to recharge the main engine battery enough to start the engine.
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