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Old 02-28-2021, 04:27 PM   #1
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Switching to connect drained with full battery bank

I have a general question about battery banks. I have 4 6V Dyno 260Ah batteries, two in each bank to get 12V. The PO said that's plenty to start my 80hp Lehman without the need for a separate starting battery. I have left both banks on overnight and used them as house batteries, but I am concerned that if anything went wrong I could drain both banks and not get the engine started. But then I thought about what actually happens when I use only one bank and connect them in the morning before starting the engine.

Say I've drained the overnight bank of batteries to 50% SOC. Should I connect the banks with the battery switch and then start the engine? What happens when you connect a drained bank to a full bank? Any issues on the "rush" of juice from one to another? Does that count as a "cycle" with the full bank discharging into the 50% SOC bank. Would the full bank be discharged to 25% over time if the engine isn't started? Any issues with the alternator bulk charging to batteries at different SOCs? Am I overthinking this?
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Old 02-28-2021, 05:27 PM   #2
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Do you have a gen with a separate gen start batty?
Many are proponents of one large combined bank and run that way.
Until recently my Mainship had 2 8Ds as a combined house / start bank. I did have a sel switch to isolate house overnight but I kept them combined and never needed my gen as an emergency start. I recently switched eng start to my thruster bank which left a pure house bank.
My primary reason for leaving the 2 8Ds combined was to for ability to monitor them accurately with a Balmar monitoring system. Switching batty in/out just confuses the monitor and provides bad info.
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Old 02-28-2021, 05:54 PM   #3
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I have a generator with its own starting battery, but . . . it's not running right now. If I can get it up and running that will solve most of my issues by having a self rescue backup.

I also was leaving both batteries on in part because of my battery monitoring system. I have it set for the number of amp hours available in both banks so, in theory, I could guesstimate that I have 1/2 of that when using just one bank. I think that I have already experienced the "bad info" problem with switching banks on a battery monitor system. Since my alternator doesn't always fully charge like the dockside charger, I wasn't sure if it was really accurate when going back to both banks.
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Old 02-28-2021, 06:00 PM   #4
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Should I connect the banks with the battery switch and then start the engine?
Yes, a lot of boats have this feature via a switch and/or a solenoid for exactly this purpose.
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Old 02-28-2021, 06:16 PM   #5
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If I did not have a dedicated start bank, I would make sure that I had a battery jumper pack so I could use it to start the engine if the house bank was too low.
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Old 02-28-2021, 06:39 PM   #6
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I have 4 8ds in a single bank for starting 2 Detroits and 2 generators. One of the generators has a separate battery as an emergency source for starting. The starting bank is used for nothing but starting.

I use to be a fisherman and often shutdown nights on the ocean. I suppose I worry too much.
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Old 02-28-2021, 07:35 PM   #7
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F.L. 80's generally "pop" right off but it is conceivable you could drain your batteries down low enough that they won't do the job.

I vote for a dedicated starting battery. It doesn't have to be strictly for the engine. Just an extra battery that will not be drained by a night on the hook that turns into two or three.
My genny has a devoted battery, so does my windlass. I also carry a jump pack with a small air compressor to blow up air toys. It can start my 120 easily, even if it hasn't been topped off all summer.

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Old 02-28-2021, 07:48 PM   #8
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I bought a new jumper pack, haven’t tried it yet. They claim it will start a 6 litre diesel. It is Lithium and incredibly small and light. You can also use it as a power supply and to charge things like a phone. It also has a light and flashing light for breakdowns on the highway.
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Old 02-28-2021, 08:06 PM   #9
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Originally Posted by Marco Flamingo View Post
I am concerned that if anything went wrong I could drain both banks and not get the engine started. But then I thought about what actually happens when I use only one bank and connect them in the morning before starting the engine.

Say I've drained the overnight bank of batteries to 50% SOC. Should I connect the banks with the battery switch and then start the engine?
I'm probably missing something obvious, but...

In the latter scenario, where you have used one of the banks overnight as a house bank (and drained it), and the other banks is still fresh. Why would you combine them to start the engine? Why not just start it with the fresh bank?

For other reasons, I'm more a fan of one big house bank and then a separate smaller start bank (with means to back that up), but I'm specifically asking about the setup you describe above. I can't see anything positive about -- or any reason to -- combine the drained "half" of your two banks with the good "half" before engine starting?
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Old 02-28-2021, 08:28 PM   #10
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Why not just start it with the fresh bank? . . . . I can't see anything positive about -- or any reason to -- combine the drained "half" of your two banks with the good "half" before engine starting?
That's one of my questions. If my "used" bank is drawn down overnight to 50% SOC (about 12.2V) and my other bank is still fully charged (12.7V), do I reconnect to start? Does the used bank still provide extra "poop" even though not fully charged? Does that kick out more cold cranking amps even though the "blending" of the banks may lower the overall voltage to 12.45V?
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Old 02-28-2021, 08:33 PM   #11
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Originally Posted by Marco Flamingo View Post
That's one of my questions. If my "used" bank is drawn down overnight to 50% SOC (about 12.2V) and my other bank is still fully charged (12.7V), do I reconnect to start? Does the used bank still provide extra "poop" even though not fully charged? Does that kick out more cold cranking amps even though the "blending" of the banks may lower the overall voltage to 12.45V?
Okay, I see. I'd be curious about that just because I like to know things (maybe I'd set up an experiment), but beyond that, if one of the banks is adequate for starting by itself, it just seems counter intuitive to harness it to a depleted bank until after starting the engine. (But my intuition has been wrong before.)
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Old 02-28-2021, 09:14 PM   #12
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All this begs the question, "What does the engine's starter cable connect to?"

I have a start/thruster bank and a house bank aboard. The boat came to me with only a single bank. Now my generator and single main engine starter cables connect to the starter bank.

One time while moored at a no-power pier, I pressed the main engine starter and nada - dead start bank. I was forced to use the manual parallel swith I installed when I added the extra bank to start the main. I immediately switched it back off after start to prevent the house bank being adversely affected by the dead start bank. New start batteries were installed ASAP. That's the reverse of the OP's theoretical dilemma, but the point is, .. er ... somewhere on the top of my haid.
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Old 03-01-2021, 08:08 AM   #13
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"Why would you combine them to start the engine? Why not just start it with the fresh bank?"

A run down house bank will usually do fine starting an engine , unless it was run to almost 0 Volts. Fridges the big killer usually shut off about 11V leaving lots to crank the engine.

Many folks have a wimpy near stock alt , so starting and charging just the house batt is fine for a few hours.

After lunch joining the batts to top off the unused house batt works fine .Just remember to cut one batt out of the house , after engine shutdown.

A 3 or 4 stage V regulator will help charge the low batt , if the alt is up to the task.
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Old 03-01-2021, 12:04 PM   #14
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Quote:
Originally Posted by Marco Flamingo View Post
I have a general question about battery banks. I have 4 6V Dyno 260Ah batteries, two in each bank to get 12V. The PO said that's plenty to start my 80hp Lehman without the need for a separate starting battery. I have left both banks on overnight and used them as house batteries, but I am concerned that if anything went wrong I could drain both banks and not get the engine started. But then I thought about what actually happens when I use only one bank and connect them in the morning before starting the engine.

Say I've drained the overnight bank of batteries to 50% SOC. Should I connect the banks with the battery switch and then start the engine? What happens when you connect a drained bank to a full bank? Any issues on the "rush" of juice from one to another? Does that count as a "cycle" with the full bank discharging into the 50% SOC bank. Would the full bank be discharged to 25% over time if the engine isn't started? Any issues with the alternator bulk charging to batteries at different SOCs? Am I overthinking this?
I do knot know how your electrical system can be switched. Ideally you would not want to combine the banks for starting or charging. It is even possible for the first bank to pull the second bank down so low that you couldn’t start the engine. Even with charging one bank is going to get over charged due to different state of charge each bank is at.

In the past it was very common to have two banks that could be switched and we would just combine the banks for charging. Battery life was 5 years and we didn’t know any better. Now we have learned bigger banks draw down less and over charging is bad. By having one big bank and an isolated emergency/start battery we now get up to double the life out of batteries.

In the end it all depends on what is important to you. In my case I have a very big bank for house. I have a big start bank and my generator has an isolated start battery. All charging sources go to the house. The start bank is charged by a SmartACR and the generator battery is charged by both the generator alternator and by it’s own battery charger. I can also switch engines to start off of the house bank, the start bank or both. I also have a jump box for all kinds of emergency options.
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