Charging the House Battery Bank with a Small Outboard

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O’Day 222

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Nov 2, 2022
9
O’Day 222 Oriental
My 300 amp hour house battery bank consists of 3 x 100ah lithium LiFe4 batteries in parallel.

My outboard is a Tohatsu 6 hp SailPro equipped with a 12V 60W 5A alternator.

I charge the house bank with a Sterling-Power 120VAC 60A dry-mount marine battery charger using either AC shore power or a small 2000 watt suitcase-style gas generator.

How would I connect the two leads of the outboard alternator to the house bank in order to also charge while underway? Is it as simple as directly connecting the two leads from the alternator to the two leads of the house bank?

Thank You,

Rod
Oriental, NC
 
D

dlochner

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Jan 11, 2014
12,691
Sabre 362 113 Fair Haven, NY
I think it would depend on your BMS and the particular brand of battery. The 5 amp out put is pretty insignificant with a 300ah battery bank.

What are you running on a 22 foot boat that requires a 300ah battery?
 
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O

O’Day 222

.
Nov 2, 2022
9
O’Day 222 Oriental
dlochner said:
I think it would depend on your BMS and the particular brand of battery. The 5 amp out put is pretty insignificant with a 300ah battery bank.

What are you running on a 22 foot boat that requires a 300ah battery?
Click to expand
These are Ampere Time 100AH LiFe4 batteries.

I totally agree that a 5A alternator is very small but when motoring 8-12 hours it could provide approximately 50AH’s per day…..providing I don’t fry that little alternator first with the AC charging system.…hence my question.

Boat is being outfitted for extended overnighting away from shore power on the Great Loop.

Thank You,

Rod
 
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D

dlochner

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Jan 11, 2014
12,691
Sabre 362 113 Fair Haven, NY
Never heard of that company. The prices on their batteries seem exceptionally low.

The company touts good service and support. They would be the people to contact.
 
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J

JBP-PA

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Apr 29, 2022
576
Jeanneau Tonic 23 Erie, PA
I believe the charger on these engines is just a coil and rectifier with little or no voltage regulation which is fine for flooded lead acid, but not so great for lithium. I think one safe way is to get a small FLA battery and a dc-dc charger to charge your lithium at a more controlled voltage. The voltage output on mine varies greatly with the RPM, being well below 12V until the RPM's are high. I do have it connected to my FLA battery however I've never noticed how high the voltage gets at wide open (I rarely motor at full throttle) and I've never noticed any significant charging ability in addition to my solar.
 
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stickinthemud57

stickinthemud57

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Apr 11, 2020
780
MacGregor 26s Scott's Landing, Grapevine TX
I was considering getting a magneto for my Tohatsu 8 hp 2-stroke, but a boat motor mechanic told me that it would not really help much in terms of keeping batteries charged and recommended a solar panel instead.
 
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P

Project_Mayhem

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Sep 24, 2018
3,260
Catalina 30 MKIII Chicago
The alternator in your outboard should be able to keep the battery full enough to start the motor the next time around but in most cases I wouldn't expect it to keep up with much more than that. Since you'll be motoring most of the time down the river you'd probably charge your batteries much more than the average sailor. I believe in most cases, you will get unregulated voltage coming from the motor. Most of the so called "regulators" on outboards are a bunch of diodes to convert AC into DC

You definitely need some sort of charge control or battery management system. I agree that solar would be a great option. I can post a pic of how I supported my 100w panel on my O'Day if you'd like.
 
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Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,149
Hunter 310 West Vancouver, B.C.
O’Day 222 said:
Averages about 350 watt hours per day (29AH’s).
Click to expand
Somewhat smaller boat but it sounds like you've got all the energy sucking devices I've got on board. 250 AH FLA in one bank which lasts 3 days on the hook. That takes it down to 12.2V and 50% SOC. I understand that your LiFe4 batteries will go much lower that 50% SOC without any complaints. Don't know what your voltage gets down to but that could cause a problem with the fridge. Mine cuts out at 11A.
 
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O

O’Day 222

.
Nov 2, 2022
9
O’Day 222 Oriental
Ralph Johnstone said:
Somewhat smaller boat but it sounds like you've got all the energy sucking devices I've got on board. 250 AH FLA in one bank which lasts 3 days on the hook. That takes it down to 12.2V and 50% SOC. I understand that your LiFe4 batteries will go much lower that 50% SOC without any complaints. Don't know what your voltage gets down to but that could cause a problem with the fridge. Mine cuts out at 11A.
Click to expand
Each of the batteries is 12.8V at 10% capacity and the “Under Voltage Warning” is at 11.6V.

3 days is great for 250AH FLA on a big boat.

3 days is about all I’m currently “comfortable” with before charging.

My goal is to only get dockage/shore power one night per week.

Rod
 
D

dlochner

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Jan 11, 2014
12,691
Sabre 362 113 Fair Haven, NY
O’Day 222 said:
Yes, Iceco VL45 which is very efficient. Averages about 350 watt hours per day (29AH’s).

Rod
Click to expand
125 watts of good quality solar with a good MMPT controller will yield about 375 wh a day on average.

We have 300 watts of solar and can stay on anchor almost indefinitely with just 232 ah of battery capacity using Firefly batteries, and our refrigeration draws about 70+ah per day.
 
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Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,149
Hunter 310 West Vancouver, B.C.
O’Day 222 said:
Each of the batteries is 12.8V at 10% capacity
Click to expand
So you have 270 AH of usable current as opposed to my 125 AH.

And I'll thank you NOT to start getting me interested in converting from FLA to LiFe4 :cuss: . At first glance the cost is, and always has been prohibitive. However, a single 125 AH LiFe4 (is such a thing exists) would produce 113 AH of usable amperage which is pretty close to my present setup back in its prime. I'll have to check on the LiFe4 life expectancy some day. Not that I'm interested in buying, just for the knowledge you understand :facepalm: ?
 
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Stu Jackson

Stu Jackson

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Feb 26, 2004
22,977
Catalina 34 224 Maple Bay, BC, Canada
Just about every single LifePO4 discussion says: DO NOT CHARGE DIRECTLY FROM AN ALTERNATOR.

What part of that memo didn't you get?
 
B

Benny17441

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May 24, 2004
7,164
CC 30 South Florida
I would not bother, the current provided by those small outboards is mainly intended to run navigation lights in small boats. It would be useless to charge that big of a battery bank.
 
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