Charging batteries with a tohatsu 6hp saildrive outboard

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johnbmandala

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Folks, I've bought a tohatsu 6hp 4 stroke 'saildrive' for my Hunter Duette. It comes with the ability to charge - but when I tested the output voltage at modest revs, it was only 8v. I couldn't test at high revs, as it's still running in, but I'm sceptical whether the voltage would get high enough to charge! Has anyone else found themselves needing a voltage regulator to get a worthwhile output?

Regards,
John B
'Mandala' Hunter Duette
 
Sounds typical. It probably is not regulated and you will find it will need quite high revs to get up to a voltage where it will give any worth while charging. At full revs you will probably find the open circuit volts will rise above level you expect for battery charging (unless it is regulated) but since the current output is relativley small (compared with a proper alternator) that will not matter unless you have a very small battery or one that is very sensitive to charging conditions or you will be running for prolonged periods at high revs which is unlikely.

Basically the open circuit volts from a simple system like this is proportional to the engine speed. (Some schooldays physics in there somewhere)

Welcome to the forum !
 
I bought a Tohatsu 5hp Saildrive last year, and get a far higher voltage than that.

I get 14+v at quite low revs. At higher revs (but still not thrashing it) my DSC radio gives an over vlotage alarm! I have seen the voltage over 16v.

At anything much over idle you should see more than 8v. If not then I'd take it back & get it checked out.
 
Hi John

I've got the same (5hp) engine which chucks my Corribee 21 along at design speed and gives 15 n/mpg. I have an ammeter wired in, and that reckons I get about 1.5amps at leisurely cruising speed. (Mind you, it could be lying.) Not brilliant, but enough to put something back I suppose! The chargers on these engines are only really meant to run a small pair of nav lights. I do know of river boaters who add a second alternator working off the flywheel of some smaller four stroke engines. Honda 9.9's used to lend themselves to this, but it adds a lot of weight to an already quite heavy engine.
 
I think you need more information before you send the motor back to the agent. Are you sure that it is DC out of the motor. My old Johnson has AC out which can be used to light nav light globes. But requires a rectifier to produce DC for battery charging.
Your voltmeter may be trying to measure AC on a DC range. The regulator that is made for the motor may in fact contain the recifier diodes. A simple trick would be to try a 12 volt globe about 8 watts and see if it lights up. Of course the system could be made to feed a 6volt nav light.
As already suggested the open circuit voltage may not be so significant compared to the current which goes into your battery. So an ampmeter is far more use.

sorry for rambling but I think you need more information from Tohatsu. good luck 0lewill
 
Hi John,

I have the Mercury branded version of the same engine although I have a 7" pitch weedless prop unlike your 6" saildrive prop so I probably run slightly fewer revs than you. (BTW 8" is standard on non saildrive versions)

You should definitely be getting battery charging type volts. Like Jake I have an ampmeter. I see > 2amps at 4 knots and 3 amps at 5 knots. It is still 3 amps at 6 knots and doesn't seem to vary with the battery state of charge.

I have acquired a rev counter and voltmeter this winter but they won't be installed for some weeks yet so I can't be more specific about volts per rpms.
 
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