Honda BF8 (1990) electrical system

[walt]

My boat has a 1990 or so Honda BF8 (8 hp) outboard which had a bad charging system and I finally got around to fixing it.

What I found was that the bridge rectifier (shown as "SR" on the attached picture) was blown. The main way that this gets blown is by disconnecting the battery from the charging system WHILE the engine is running (you can run the engine all day with the charging system not connected to a battery - just dont disconnect is while the engine is running). What happens is that the altnernator coil has is "generating" current and when you disconnect the battery, the current has no where to go and creates a large voltage spike. If the voltage spike is large enough, it reverse breakdowns the rectifier diodes and the burn out. Turns out my PO had installed a switch between the outboard and the battery located in the cabin and likely using this switch while the motor was running burned the rectifier out.

One very interesting thing I found with the Honda charging circuit is that it has NO regulator. A regulator measures the output voltage on the battery and will shut down the charging current when the battery voltage gets high enough. The little Honda does not have this (not on the schematic and I also verified by a running test) so the Honda WILL over charge and damage batteries if you are not careful (but at normally less than 5 amps and limited motor useage on a sailboat, maybe not so important - but good to know).

One final note - if you have a two battery switch, these are usually "make before break" so should not have the same problem the little switch the PO installed on my boat.

 

[walt]

the cludge fix

In the attached picture is the SR which came out the outboard. I dont know what a new one costs - but suspect it is MUCH higher than the $5 I spent on my fix.

What I did was to simply by-pass the rectifier in the outboard (very simple - just connect the plugs together) so now I have AC comming out of the outboard.

The new bridge rectifier (pretty easy to spot in the picture).

Since the outboard does not have a regulator, I still want to use the switch the PO put in. An example of when I need this switch is if I take an extented trip which requires a lot of motoring the first day. The first day I typically will have the battieries topped off and if I left the outboard charging on all day, I could over charge the batteries.

What I added to was a 100 uf 100 volt capacitor as shown on the schematic. What this does is suppress the voltage spike when the batterie is disconnected while the engine is running. In the picture, there are four caps but this is only because I had some 25 volt caps handy so had to "series" to get the voltage up. Note that I measured over 50 volts open circuit from the outboard charging circuit hence the 100 volt rating of the caps.

I tested this by connecting and disconnecting the outboard charging circtui to a battery over 50 times at various engine RPM's. No problem - works fins.

 

[walt]

Test for the regulator

I am of course not at all suggesting anyone test their outboard as you could damage something.. but here is what I did to verify my Honda does not have a regulator.

I simply put a 24 ohm resitor in parrellel with a "large cap" on the charging circuit output. If the outboard had a regulator, more than likely I would not have even got any voltage out since the regulator uses the battery voltage to charge a stator coil which makes the alternator work in the first place. Second, if I did see any voltage, the regulator should have limited the votage to something in the mid 13 to 14 volt range.

What I saw was voltage accross the resitor change dependent of engine RPM - higher RPM, higher voltage. I saw over 20 volts at fairly low RPM and it will go higher than this..

Picture from my backyard yesterday... why post all this - I had assumed all these small outboard had alterators with regulators so you could just hook up to the battery and forget about it. I dont think this is true - at least with the old Honda...

 

[David22141]

Honda Charging Circuit

Walt: I have a BF 8 (1988 I think) and have had some probs with charging circuit. Thank you so much for clearing up how it works. I probably blew the rectifier trying to determine if any current was flowing I ordered a new rectifier from Honda but (thankfully) never installed it. I decided I really did not need battery charge much anyway. I remember getting some strange readings on the rectifier circuit (trying to verify shop manual specs) that I couldn't interpret and put the project on hold. Now that you've "splaned" it, I can probably make some sense of things. David

 

[David F]

What about the Nissan 8BEF

Does anyone with a Nissan 8 HP with electrical smarts know if this is the same situation as the Honda? I am new to sailing and have an 88 26D. Curious to know for sure the Nissan was charging the battery, I placed a clamp-on ammeter onto a battery lead. There is little perceptable currant flow at idle or just above, but at maybe 1/3 open throttle, there is about 3 amps. However, after a couple of days.... off and on running, the battery is down to about 11 volts and would not turn the motor over after sitting 3 weeks.

 

[walt]

testing?

Once my Honda alternator was working, I can easilly see a voltage rise on the battery when the outboard is charging so this is a pretty simple test to see if its working or not. To do this test, just take a voltmeter and measure the battery voltage. Then run the outboard with some "mid level" RPM's and you should see the battery DC voltage rise maybe 1/2 or 1 volt - dont have exact numbers but it should be easy to measure. And of course, dont disconnect the outboard charging wires while the outboard is running to see if the voltage drops as this is what likely damaged my rectifier.

Diodes (or rectifiers) fail in different ways but damaged diodes can certainly have increased reverse leakage current. Normally this current is very small so leaving the charging wires hooked up to the battery all the time has no effect. However, if the rectifier is damaged, it could draw current out of the battery when the motor is not on. I think you should be able to test for this by first having the motor OFF - disconnect the outboard charging wires and measure battery voltage. Then connect the charging wires and measure battery voltage. You should see no voltage change if the rectifier is not leaking current. The voltage change might be small and difficult to measure however - and the option below might be better.

If you really want to test for "motor off" leakage current, put about a 1K or even 10K ohm resistor in series with the outboard charging wire and the battery terminal (with the outboard NOT running). You should measure almost no voltage accross this resistor for normal leakage current.

If you dont read anything else, never disconnect the charging wires to the battery while the outboard is running. FYI, this is not that hard of an enginneering problem to protect for (the cap I added on mine makes it very robust) so maybe its not such an issue with newer outboards? It adds a little cost however so who knows..

 

[walt]

cludge

John, Ive learned a lot here (including many of your posts), happy I can participate. My biggest problem with the old Honda is getting it to start but that is also at 8600 feet el. Overall, its a good ol work horse.

Cludge... if I ever sell my boat, the next owner is probably going to be doing a lot of WTF's.