1984 Honda B75 Alternator Output

[sbbloom69]

Hi,<br />I got a 1984 B75 with the Mac 22 I just bought. Runs well. However, the alternator output voltage is not enough to charge my battery, even wide open. Voltage wide open only gets to about 13 volts (at my boat panel). I'm going to re-wire the harness in the boat, and I"m going to check the wiring harness on the motor this weekend to look for bad connections. I'll also check voltages at the motor jack and at the coils (AC). Motor has spent its entire life in fresh water.<br /><br />I'm an electrical engineer and have repaired, designed, rebuilt and tested alternator / field control systems, so I know how they work.<br /><br />I notice that the later Hondas (BF8, BF10, etc) have the alternator coil system AND a voltage regulator. The B75 has no regulator, which means the voltage level output at the jack is directly controlled by engine speed. <br /><br />Either way, can a later coil kit be fitted to the earlier B75 (since the B75 and BF10 have the same engine displacement) so I can get a voltage regulator? I'm assuming the newer version coils put out a slightly higher voltage, so that the regulator can chop it down a bit to a proper level (usually 14 to 15 volts) over a wider engine RPM range.<br /><br />I may also have an RPM issue since I'm pushing a sailboat. Should I consider a lower pitched prop, so I can get a slightly higher engine RPM (and thus higher output voltage from the generator)?<br /><br />Ideas??<br /><br />Stu in New Mexico

 

[cp]

Re: 1984 Honda B75 Alternator Output

Had hoped that someone who’d actually done what you’re asking about would’ve replied by now. Since no one has, here’s some food for thought as you consider whether/how to proceed.<br /><br />The on-line owner's manual for a B75 engine makes no mention of any charging capability; the manual for a BF75 specifies a 5 amp capability. (Was that a 'typo' where you say you have a "...1984 B75..."?) Anyway, a sampling of wiring diagrams in owner’s manuals for BF8 and BF100 models finds that all rectifier/regulator units are shown simply as two-wire AC in and two-wire DC out, usually to a plug. There is no field excitation control as on an SSTG or SSMG or even an automobile alternator. This seems to be clear indication the outboard’s charging system does not have a wound rotating field, but rather a permanent magnet for excitation. This would also lead to the possibility that the ‘regulator’ might be something simple like a zener diode. As you’ve said, the regulating action would be to ‘clip’ a higher voltage down to the nominal voltage. The desired effect would be to have a higher voltage available at a lower RPM, yet not an excessive voltage at higher RPM. Given permanent magnet excitation, one way to achieve a higher voltage at a given RPM would be to increase the turns in the stator windings as you’re considering (or install stronger magnets). <br /><br />But the purpose of a higher voltage would be to drive more charging current into the battery would it not? Consider then that the ratings for the charging systems on the BF8 and BF100 are 5 amps, the same as a BF75 (and presumably on a B75?). And that rated output would still only be attained at rated RPM. When the current draw from radios, navigation aids, lights and other loads is factored in, there’d be little capacity left for charging a battery. Would you really gain anything by switching to the stator winding(s) and rectifier/regulator from the BF8 or BF100, if that were even possible? Perhaps, but IMHO it wouldn’t be worth it ‘cause you’d still have what amounts to just a float charging unit (no freshening or equalizing charging here). And you’d have to run the outboard at high RPM for long periods to make much difference. You could achieve the same effect with solar panels and save the fuel to boot.<br /><br />With regard to a prop pitch change, the specified WOT RPM for a BF75 is 4800-5200 and for a B75 it's 5500-6000. If you're not within the band at WOT, then a pitch change is advisable. A reduction of an inch will give about 200 RPM increase and conversely. It's not advisable to run the RPM higher than specified.<br /><br />Hopefully your work on the outboard over this past weekend has led to some improvement in your charging system, but the bottom line IMHO is that these small alternator coils were never meant to truly charge a battery, but rather to run your navigation lights. Good luck.

 

[sbbloom69]

Re: 1984 Honda B75 Alternator Output

Hi,<br />Thanks for the reply, I was only looking to make up for some battery losses from nav lights, small loads. I know 5 amps isn't a lot. I'm also looking at solar, wind, and water (dragging a prop to turn an alternator) power. I'm experimenting now, since I want to take two weeks in the Sea of Cortez next summer. <br /><br />I haven't had the chance to check the motor out again yet. I suspect I'm not getting to the rated engine rpm, but I haven't checked that out yet either.<br /><br />I'll check again, but I believe I have a B75, not BF75. The earlier engine electrical diagrams just show wires coming from the coil windings to the fuse to the connector with no regulator. There has to be a rectifier somewhere I would think. On the later models I'm sure the "black box" has the rectifier/regulator like so many motorcycles, etc.<br /><br />Anyway, I'm not going to break something that's not "broke", but this motor came with the DC power system, and it was semi-permanently wired to the boat. I can't believe it only works at WOT. Of course, it may be like you said, only to power nav lights at a much wider voltage spread.<br /><br />Thanks,<br /><br />Stuart<br /><br />BTW, Do I sense a Navy Nuclear Power background??<br /><br />I served on the USS Narwhal from 83 to 86 and the USS New York City from 89 to 90.

 

[sbbloom69]

Re: 1984 Honda B75 Alternator Output

Hi PCHONDA,<br /><br />You were right, it IS a BF75, not a B75. I still haven't had a chance to check things out yet.<br /><br /><br />STu

 

[cp]

Re: 1984 Honda B75 Alternator Output

Hi Stu,<br /><br />When you measured the 13VDC at the boat panel at WOT, was that under load, i.e. with the battery connected and other loads on? If yes, then IMHO that's about all you can expect; just make sure all connections are clean and tight to minimize any voltage drops. If that was at no load, then suggest you check that the charging coil is not shorted or grounded since it should put out about 13.8VDC at no load, even at lower RPM. The BF75 coil should read 0.12-0.15 ohm.<br /><br />My sailboat experience is very limited, but seems that you'd be under sail power unless maneuvering was restricted. When under sail, would you run the BF75 in neutral to charge the battery? Running outboards at high RPM in neutral is not good for their longevity. You've already recognized that solar, wind and water are viable alternatives to running the BF75. You may also want to consider a portable generator as another option. One of those can provide a higher battery charge rate while at the same time providing some AC for your use. Just a thought. <br /><br />Honda makes some quiet, light, portable generators that you might want to check out: Honda Portable Generators<br /><br />I'm not a Honda salesman or anything like that, but did work on ships' electrical systems at Pearl for 30+ years, including the "Big Apple" in the mid-to-late '80s. Good luck.