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Old 06-09-2015, 11:18 PM   #1
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I have my first boat without generator. Have 4 8-D. Batteries, two for house and two for starting and bow thruster. How often do you start engine to charge battery, and what voltage should I start engine and charge batteries?? Help
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Old 06-10-2015, 04:31 AM   #2
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One rule of thumb is that the voltage should not drop below 12.2.
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Old 06-10-2015, 06:19 AM   #3
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Use the search function here and read up on SOC (state of charge) meters.

Most old engines use a 12V system .

You will need some form of seperiation between the engine and house batts (usually a rotary switch ) to not discharge the start batts , only the house batts , while anchored.
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Old 06-10-2015, 06:27 AM   #4
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Quote:
Originally Posted by restless View Post
I have my first boat without generator. Have 4 8-D. Batteries, two for house and two for starting and bow thruster. How often do you start engine to charge battery, and what voltage should I start engine and charge batteries?? Help
That was the setup on our Krogen when we bought her two years ago.
The first thing we did was change it by adding one start battery, group 31, and making the house bank the 4 8-D's.

I also added the Victron SOC.

That setup has worked great.
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Old 06-10-2015, 06:56 AM   #5
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I think I answered a similar question over on the Mainship forum this morning.


With two 8Ds which is about 440 amp hours you can probably hang out for 3-4 days before the state of charge drops below 50% which is the limit for best battery life.


Relying on the voltage is a very poor indicator of state of charge. Look at it in the morning when there are no lights on. You will still have cycling fridge loads which will skew the result, but 12.2 volts in this situation is probably a good indicator that it is time to recharge.


Also using your propulsion engine to recharge batteries with the OEM alternator is very poor practice. First you are not loading the engine sufficiently to keep it up to operating temperature. Secondly an OEM alternator charges very slowly.


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Old 06-10-2015, 08:14 AM   #6
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It is impossible for anyone to answer the question correctly without knowing what the loads are going to be, and exactly what the specs and condition of the batteries are. Then what the charging capability is of the alternators at various engine speeds. I strongly suggest that the OP obtain a copy of Calder's "Boat owners mechanical and electrical manual", which covers the issue involved here thoroughly in the first few chapters. You'll come back to this book time and again for all kinds of things beyond this one.
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Old 06-10-2015, 11:50 AM   #7
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If you do an internet search, you can find some pretty good advice about how to manage a 12v house bank. From what I recall, in a nutshell:
1) The less deeply you discharge, the longer the life.
2) Discharging 100% (whatever that means -- I think down to 10.8 volts) is near instant death to lead acid batteries.
3) Damaged and even dead batteries can be substantially restored, at least a few times, by "equalizing", a process that requires a charger with that function.
4) The more fully-charged a battery is, the slower it will charge; ie, recharging a battery from 50% charge to 75% charge is WAY faster than recharging from 75% to 100%.
5) In balancing competing demands (primarily, house bank size, recharge time and battery life), many people adopt a 50% approach. The run the batteries down to a 25% charge, then charge back up to 75% and stop there in order to not continue to run the gen set. I used that approach for years on my boat and got 4-5 years life out of the batteries, "equalizing" the batteries about once a year.
6) A good state of charge meter (which will include a bunch of useful ancillary functions) is necessary to maximizing the efficiency of your house system. I used a Link 30 (maybe 20) and liked it alot. Come to think of it, they had (still have?) very useful materials on line.
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Old 06-11-2015, 06:18 AM   #8
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Trojan and other bat mfg will have a chart that relates the DOD (depth of discharge) to batt service life.

Only useful after the SOC meter is installed .
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