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Outboard Alternator

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sfein
1st Mate

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36 Posts

Initially Posted - 06/29/2008 : 21:46:48

Hi All,

So we finally got her launched and everything works great. A few kinks to work out with the new engine.

So my new engine has an alternator on it. I have a two battery system. My questions is simple, is there a way to have the alternator charge both batteries while on or must it be restricted to one battery.

I have a jumper that runs from the negative to negative, I believe this is called in parralell.

If I put the positive from the outboard on one battery and the negative from the outboard to the opposite negative on the battery, will this charge both batteries?

Thanks

Stuart Fein
Blonde Over Blue
1980 Catalina 25
East Hampton, New York

Edited by - on

Voyager
Master Marine Consultant

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USA
5352 Posts

Response Posted - 06/29/2008 : 23:02:05

Stuart -
On his website http://www.boatus.com/boattech/casey/30.htm, Don Casey shows a method for charging two batteries while keeping them isolated by installing a pair of high power low loss Schottky diodes in line with the positive or (+) lead from a solar panel. A typical diode would be an SR 360.

There is no reason why the power source could not be an alternator instead.

This will allow the power delivered by the alternator to be divided to supply both batteries with current. The diodes would isolate the backward current coming from each battery.

Note that a separate positive wires should remain connected from the plus terminal of each battery to the battery switch. Note that if the battery switch is set to "BOTH", then this circuit is bypassed.

Good news is that if one of the two batteries has a much lower voltage than the other, the one needing more charge will take most of the current.

Bad news is that the more charged up battery could fool the alternator's regulator into thinking that both batteries are fully charged, so you should monitor both battery voltages to make sure that the alternator is charging them correctly.

If, for example, the more discharged battery sits at 12.0V for a long time and the more charged up battery is sitting at 12.6 or more volts, the regulator is not charging either battery.

In this case, you might be best off to charge each battery separately.

Standard lead acid battery voltage fully charged should be about 12.6V. Contrast that to a 3/4 discharged battery at 12.0, or a fully discharged battery at 11.8V.

End of the day, as Casey says, " .... most of us find electricity as incomprehensible as the tax code."

Edited by - on

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 06/30/2008 : 07:33:57

While I'm no Marine EE, I believe your present parallel setup should work fine. With no diodes, the two batteries will present a voltage that is something like the average of the two, and when the circuit is closed (on) the battery with the greater charge will tend to charge the one with the lesser charge (and be drawn down by it), which is partly what the diodes prevent.

Do you have a 1-All-2-Off switch? That allows you to use and charge either battery or both in parallel. Otherwise, leave them in simple parallel and make sure they're in equal condition--that is, replace both at the same time with identical batteries.

Edited by - on

JohnP
Master Marine Consultant

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1520 Posts

Response Posted - 06/30/2008 : 08:40:44

One important detail - don't run the outboard with the battery switch "off". That can apparently blow the diodes in the outboard.

Happy wiring and smooth sailing!

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Sloop Smitten
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1181 Posts

Response Posted - 06/30/2008 : 10:07:47

<blockquote id="quote">quote:<hr height="1" noshade id="quote">don't run the outboard with the battery switch "off"<hr height="1" noshade id="quote"></blockquote id="quote">

With the switch at "OFF" you have an incomplete circuit so I cannot see how that would damage a charging circuit. If you switch it off while running you could spike the diodes and potentially damage them. I run my Honda 8 once a year with the charging circuit attached when I make the passage from Oceanside to Catalina. the rest of the time it stays disconnected. Maybe it is a manufacturer specific requirement. Whatever your owners manual states should be followed.

Edited by - on

JohnP
Master Marine Consultant

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1520 Posts

Response Posted - 06/30/2008 : 12:54:21

Well, I have little knowledge about diodes and such, but...
<blockquote id="quote">quote:<hr height="1" noshade id="quote">...Whatever Voyager, our Forum's resident marine electrical engineer, states should be followed.
<hr height="1" noshade id="quote"></blockquote id="quote">from Voyager, in a related threadPosted - 04/26/2008 : 23:44:38
-------------------------------------------------------

Two comments on this thread (just in case you wanted to know): (a) what happens if your alternator is not hooked up and (b) how to tell whether your battery is charged up by reading the voltage.

(A) If your outboard has an alternator, but you never use it - that is, if the cables aren't even connected to the outboard, the alternator will not generate. Simply because the alternator's rotor is connected to an open circuit, so current can never flow and no current can be generated.

Now, the problems occur when you connect the alternator to a battery or circuit THEN you disconnect the circuit. This is bad, because the inductors and magnetic fields in the alternator are still pushing current - due to induction - they cannot stop delivering current instantaneously.

This current causes the terminal voltage go way up, so it arcs over causing a short - blowing out the rectifier diodes.

Moral of the story - its not the open circuit that causes the damage - its the OPENING of the circuit that does it.

(B) ...deleted
-------------------------------
Bruce Ross
Passage
Marine EE

Edited by - on

Dave5041
Former Mainsheet Editor

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3758 Posts

Response Posted - 06/30/2008 : 20:18:17

Considering the low output of your alternator, I think you are better off just using the battery switch and charge your starting battery until it is full and then switch if you are still motoring. No matter how you divide it, running your motor for an hour only gives you 6 -7 amp hours of charge

Edited by - on

Voyager
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5352 Posts

Response Posted - 06/30/2008 : 22:30:08

You could use Dave Bristle's approach. You would probably be ok most of the time. The reason for the diodes is to prevent the lower battery discharging the stronger battery, so you'd wind up with two batteries at 50-60% charge.

If you plan to use Dave5041's approach and charge each battery sequentially, take care not to open the alternator circuit while it's charging. If it is convenient to stop the engine while you switch batteries, you can be sure not to harm your alternator's rectifier diodes.

Edited by - on

Dave5041
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3758 Posts

Response Posted - 06/30/2008 : 23:19:32

As Bruce notes, stopping the engine is definitely safest if you you can, but if it is awkward to do so for some reason, then the sequence is "1-both-2" or "2-Both-1", not "1-Off-2".

Edited by - on

Stardog
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319 Posts

Response Posted - 07/01/2008 : 08:42:45

You guys have got me thinking; I've always hooked up to my charger at the marina, but it would be nice to be able to recharge when out for many days. Back in the day I tried solar panels and they were inadequate and didn't put on much of a charge. I even considered a small generator that could be fired up every few days to recharge. It seeems that most long trips entail running the engine at some point, so this would be an opportunity to charge batteries. I have a new Tohatsu with alternator output, so I would just have to provide the proper circuitry to the batteries. Does anyone know how long it generally takes to charge batteries from an outboard engine?

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 07/01/2008 : 09:52:37

The 1-All-2-Off switches are designed to maintain continuous contact when switching between any combination except to Off--no need to stop the engine. One good strategy with the switch (more important for me than you) is to charge both when running--then when not running, switch to one battery for house use. That means the other battery is "full" if the first gets too low. I do that because if I have no battery power, there's absolutely no way to start my 3.5L engine!

Edited by - on

Dave5041
Former Mainsheet Editor

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3758 Posts

Response Posted - 07/01/2008 : 13:49:56

It didn't come with a crank?

Of course you have enough amps to charge a few batteries at once.

Edited by - on

John Russell
Master Marine Consultant

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3444 Posts

Response Posted - 07/01/2008 : 14:56:41

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 ... there's absolutely no way to start my 3.5L engine!
<hr height="1" noshade id="quote"></blockquote id="quote">Can't ya git it a goin downstream and pop the clutch?

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Voyager
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5352 Posts

Response Posted - 07/01/2008 : 21:49:47

Clifford -
<blockquote id="quote">quote:<hr height="1" noshade id="quote">...Does anyone know how long it generally takes to charge batteries from an outboard engine?<hr height="1" noshade id="quote"></blockquote id="quote">

You can figure it out along these lines. If you have a 100 Amp-Hour (AH) battery, this means roughly you can draw one amp for 100 hours, or two amps for 50 hours, or four amps for 25 hours.

Typical lighting in the cabin or an anchor light takes about 1A per bulb or 12 W, so if your battery were fully charged, you could run four 12W light bulbs for 25 hours before your battery was fully discharged. If your lights were 24W bulbs, you'd be discharged in 1/2 the time.

Now, if your alternator puts out 10A and if your battery were mostly discharged, it would take you about 10 hours at 10A to fully recharge the 100 AH battery.

Your numbers may be different: your alternator, your battery, your light bulb wattage. If you were to recharge 2 batteries, you'd be looking at 20 hours of charging.

This takes quite a bit of time.

Occasionally, I carry the battery home and put it on the charger in the garage.

Edited by - on

Stardog
Captain

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319 Posts

Response Posted - 07/02/2008 : 11:00:06

Thanks for the information, I'm always looking for ways to extend battery life. I recently had to replace my stern navigation light, so I installed an LED type. The old one was 10W, the new one 1W, so that's a nice saving in power requirement. I'm thinking of all the lights I use onboard and how much longer my batteries would last if I replaced them with LEDs. The only ones I'm not so sure about are the bow forward navigation lights, they are Perko and have festoon type bulbs, mounted horizontally. I haven't seen any replacements that are rated at two mile visibility, nor have I seen any that would probably not rotate in the fixture and face downward instead of outward.

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OLarryR
Master Marine Consultant

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USA
3440 Posts

Response Posted - 07/02/2008 : 11:44:23

There are LED fixtures rated at 1NM and 2NM USCG approved navigation lights, however, most that replace the bow light are super expensive. Obviously, just a bulb replacement seems so reasonable but not sure if you can find a festoon bow light replacment and if it is rated to throw light 1NM/2NM.

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Stardog
Captain

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USA
319 Posts

Response Posted - 07/02/2008 : 21:43:54

That was my concern too Larry, I'm not going to replace a 2-mile lamp with something less, it's just not a good idea. Also the festoon replacement LEDs I've seen have the LEDs all mounted facing in one direction, which is bound to make them rotate in their sockets, then they would end up shining down into the water, which would be very bad. Maybe some kind of tab could be affixed with epoxy; something the LEDs would rest on, to keep the assembly from turning. This might be an easy fix.

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John Russell
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3444 Posts

Response Posted - 07/03/2008 : 10:48:23

Make me smarter about these rectifier diode thingies. Are these something a guy (that calls them "thingies") with a screwdriver set could find and replace? I have #1, #2 AND #3 phillips head screwdrivers!!!!!!!!!

Just don't ask why.

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Voyager
Master Marine Consultant

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5352 Posts

Response Posted - 07/04/2008 : 22:33:52

John -
Do you have a Honda 8 HP engine, or another brand? The Honda BF8 has a "regulator rectifier" block that houses the diodes. It looks like the AC from the charging coil goes into this block and comes out as a pulsing DC current. This current goes to the battery to recharge it.

If you blew your diodes, you'd have to replace this component.

If you have another engine type, check the circuit diagram for either a discrete alternator or a charging coil as part of the flywheel assembly.

The AC voltage comes out of the alternator or charging coil, then goes to the rectifier diodes or to the regulator. At the output of these, you'll have your battery charging current.

You can tell if you've burned out your diode thingies if you have a voltmeter. If you remove your battery and use the pull start and if you set your voltmeter to the DC 20 volts range and measure the output voltage of the engine, you'll get zero volts. If you set the voltmeter to the AC 20 volts range, you may get 15-20 volts AC, or you may also get zero.

This means that the diodes are either shorted (which is bad) or open (which is also bad).

You will have to purchase a new regulator and or rectifier diodes and replace the module exactly as instructed on the package. You will need to have a nice pair of wire strippers and a crimper, plus several crimp-on connectors, and of course, black tape and a screwdriver.

Edited by - on

John Russell
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3444 Posts

Response Posted - 07/05/2008 : 11:40:56

Thanks, Bruce. I guess this looks to be a project for post-haul out ---- at the Tohatsu dealer.

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