Charging House Bank from Mercury 9.9hp Outboard

[Seafarer 23]

Hello all,

I'm hoping someone can help me with the following. I bought a new Mercury 9.9 hp outboard last year with electric start, capable of delivering 6 amps. I used to charge my deep cycle batteries from shore power, which was sufficient given my limited power requirements. Just some lights, the depth sounder, VHF, and knot meter. I'm hoping to wean myself off shore power entirely by using the outboard to charge my batteries. I motor for about 1.5 hours each round trip to reach open water. My question is this: Can I hook the alternator directly to my house bank or do I need some form of surge protection for my electronics. I would like to be able to start the motor with the pull string (to save power) while the electronics are on. I would appreciate your thoughts.

[GordMay]

Greetings and welcome aboard the CF, Seafarer 23.

The 6A Alternator may actually be an AC lighting supply, not suitable for battery charging, which would require a rectifier circuit. Check /w your dealer.
Most systems are rated at wide-open throttle, and charge substantially lower amounts at slower engine speeds.

[S/VSkylark]

For the power requirements you noted above, your best bet would be a solar panel or two.

[Pete7]

Tread carefully, an alternator (actually called a stator on an outboard) produces AC, you need a rectifier in the circuit to then produce DC. Some of the outboards will have the rectifier already fitted, but the dealer should be able to advise if it has.

Also that 6 amps will be at full RPM and your electrical draw with those items could easily be 6 AH.

You ought to consider a solar panel which can be left to charge the battery up mid week and then give you sufficient power for weekend use.

Pete

[capngeo]

To add to the above, 1.5 hours each way, once a week (assuming the outboard put out 12vdc@ 6 amps the whole time) would not likely be enough AH considering what you plan to run, AND the batteries' need to be fully charged. A small Honda genny may be more suitable for your needs

[s/v 'Faith']

Originally Posted by Pete7

Tread carefully, an alternator (actually called a stator on an outboard) produces AC, you need a rectifier in the circuit to then produce DC. Some of the outboards will have the rectifier already fitted, but the dealer should be able to advise if it has.

Also that 6 amps will be at full RPM and your electrical draw with those items could easily be 6 AH.

You ought to consider a solar panel which can be left to charge the battery up mid week and then give you sufficient power for weekend use.

Pete

The 'charging circuit' on your outboard is designed to charge batteries. The electric start will draw current, and the minimal charging capacity of the outboard is really just barely enough to replace the current used to start the motor.

Your Mercury does have a rectifier, but keep in mind that it does not make 6 amps unless the conditions are perfect... I have cruised with an outboard in the well with a similar set up, (and a link 10 monitor).

Pete is right when he suggests that you view the capacity as a 'lighting circuit' in that it will replace the current used to start the motor and maybe run a couple lights.... It is nothing like having a 6a battery charger.

It is not ac, but it is dirty DC... the rectifier is pretty crude. You would not want to run electronics off of it without a battery.

Pete is also right, that you would be well advised to look into a solar panel for your real charging needs.

[Seafarer 23]

Thank you all for your quick response. I take your point about the solar panels. I am considering adding some perhaps next year but was hoping to make do for now. I will have it hooked to a deep cycle battery and was hoping that by using the pull cord to start it would give me enough juice to get by. It runs at about 80% full throttle up and down the river each way. In response to your mention of dirty dc, with the battery hooked to the starter and a steady 14 volts across the terminals with the motor running, do I still need to worry about hooking my electronics to it? Also, is there a surge I need to be concerned about if I start the motor with electronics running off the battery? Thanks again

[s/v 'Faith']

Originally Posted by Seafarer 23

Thank you all for your quick response. I take your point about the solar panels. I am considering adding some perhaps next year but was hoping to make do for now. I will have it hooked to a deep cycle battery and was hoping that by using the pull cord to start it would give me enough juice to get by. It runs at about 80% full throttle up and down the river each way. In response to your mention of dirty dc, with the battery hooked to the starter and a steady 14 volts across the terminals with the motor running, do I still need to worry about hooking my electronics to it? Also, is there a surge I need to be concerned about if I start the motor with electronics running off the battery? Thanks again

You should have no problems with your electronics with the battery hooked up... the problem would come if you tried to hook a radio to the output of the outboard without the battery...

[motion30]

You already have everything you need except for some wire , hook up the merc to your battery and use whatever little amps it has to offer If you find you need more then decide between solar, wind or a little gen set To then me 6amps is better then 0 amps

[noelex 77]

There is no need for any surge protection , but you do need to keep a watch on battery voltage which could become too high if you have a near fully charged battery at the start of the trip.
My first boat had a similar arrangement, until I could afford a solar panel. The output from the Yamaha 2 stroke was rated at 70w, but gave about 4 to 4.5A at cruising revs. At idle it was less than 0.5A. I suspect your outboard will be similar.
1.5 hours motoring (each day ?)will give you about 6 Ahrs.

Its always difficult to do an energy budget for someone else, but it might look like this

VHF 0.5A for 2 hours =1Ahr
LED lights 3X for 6 hours @ 0.2 hours each =3.6Ahr
Instruments ).0.3A for 2 hours= 0.6Ahr

Allow a bit for residual battery loss and you may just be OK. But there is no allowance for anything else, no navigation lights, no phone charging, no music.. Nothing other than what you have mentioned.
This is assuming you use energy efficient gear for the minimum time. It’s a very tough budget, its just possible, but as others have said a solar panel would make things a lot easier.

[Seafarer 23]

Thank you all for your advice and for taking the time to help me out. I really appreciate it.