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Old 27 December 2009, 13:23   #1
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Constant voltage out of 18 HP Tohatsu?

Constant voltage out of manual start 18 HP Tohatsu?

Planning to add running lights and a small chart plotter on my SIB. As is, the engine
(TOHATSU MFS18B2S 2008) can not deliver 12V/DC but by adding a rectifier-kit(as an example http://www.fairweathermarine.com/cha...r-kit-93-p.asp) 12 DC will be available.

Does anyone know is this type of kit also regulating the voltage(constant voltage), can I connect the chart plotter to the kit without having a battery between? The lights will be ok but how about sensitive electronics as a chart plotter?

If this is not considered OK, next question is about the battery. The engine delivers max 130 W, almost 11 Amps, system is designed for 60-70 amp batteries. Due to the nature of this boat, would not like to install a bigger battery than a motorcycle battery(http://www.thebatteryshop.co.uk/yuas...a-bs-683-p.asp). Is there a risk of overheating/exploding of the battery as the charging power of the outboard engine is way over battery specification. Any workaround this one or is it only a theoretical problem.

Thanks for any advice.
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Old 27 December 2009, 13:57   #2
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Originally Posted by C-NUMB View Post
Constant voltage out of manual start 18 HP Tohatsu?

Planning to add running lights and a small chart plotter on my SIB. As is, the engine
(TOHATSU MFS18B2S 2008) can not deliver 12V/DC but by adding a rectifier-kit(as an example http://www.fairweathermarine.com/cha...r-kit-93-p.asp) 12 DC will be available.

Does anyone know is this type of kit also regulating the voltage(constant voltage), can I connect the chart plotter to the kit without having a battery between? The lights will be ok but how about sensitive electronics as a chart plotter?
I don't know for sure - but it is unlikely that it actually regulates the voltage. In any case the rectifier may not like being run without a significant load on it (like a battery), and it can be quite useful to have a plotter when engine is off (either to plan next stage of journey at anchorage or to know where you are when the engine is broken and you need help!) - so I would be adding a battery.
Quote:
If this is not considered OK, next question is about the battery. The engine delivers max 130 W, almost 11 Amps, system is designed for 60-70 amp batteries. Due to the nature of this boat, would not like to install a bigger battery than a motorcycle battery(http://www.thebatteryshop.co.uk/yuas...a-bs-683-p.asp). Is there a risk of overheating/exploding of the battery as the charging power of the outboard engine is way over battery specification. Any workaround this one or is it only a theoretical problem.
Firstly "130 W" is probably more like 9 Amps than 11 Amps (12V systems usually seem to run at closer to 14!); secondly the engine will only output maximum power at maximum rpm. At tickover it could be putting out less than 1/6th of this. Thirdly - are you sure about the spec (e.g. is the "alternator" capable of that - but the engine never spins it fast enough?) ? My engine is than yours (and its virtually identical big brother has same output), with electric start etc but IIRC only has a 6 Amp (at 6,500 rpm) output.

For what its worth my own system is wired to a 21 Ah battery (used in 'jump start packs') and I've never had any problems with overcharging it even after several hours running. I would have no worries about connecting it your engine (and wouldn't even have checked the output current) unless you have some sort of special very high output alternator.

P.S. you can usually find a jump start pack cheaper than the battery that is found inside it (I chose to leave it in the pack giving me a master switch, accessory socket, light in the locker its stored in, etc).
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Old 27 December 2009, 15:04   #3
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A properly regulated charging system will supply charging current depending on the state of charge of the battery. So, unless the battery is so small that even the minimum charge current is too great there shouldn't be a problem. To simplify and clarify a wee bit, let's say the charge current has a potential of 14volts feeding into the battery, the battery (fully charged) will have a potential of approx 12.8volts feeding into the regulator. Therefore, the charge voltage will only be 1.2volts, on a discharged battery at a lower potential (say 12volts) the charge voltage will be higher at 2volts. So, for a given system, the power going into the battery for charging will depend on the state of charge of the battery.
This isn't the whole story btw.
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Old 27 December 2009, 15:47   #4
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RS components do suitable bridge rectifiers that are loads cheaper than the kits listed above.
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Old 27 December 2009, 16:22   #5
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I used to run a tractor battery on a 40hp Honda motor. No problem.

The current that the motor will put out is dependent on the load; with a charging battery, the load decreases as the battery nears a charged state.

jky
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Old 27 December 2009, 17:21   #6
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Thanks, a lot of good comments. Will go for the battery, as pointed out, it might be quite useful to have electricity also when engine is off.

Based on the product page and manual, this 4 stroke model seams to deliver the 11 amps "Alternator (OPTIONAL): Optional 12V 130W 11A (installation required)" , in my case less would be sufficient.

I guess that the regulation is not that sophisticated, the standard coils and then the few components seen on the web link. But if I understand correctly, by using a precharged battery, the amps going into it will not be that high as the resistance will be there from the battery.

Good point also with the jump start kit, sometimes such kits can be priced very economically indeed.

"RS components do suitable bridge rectifiers that are loads cheaper than the kits listed above" That's another good advice. However, litle bit lost faith when got 400 hits on the web page, might be too long time since keeping a soledring tool in my hand.....
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Old 27 December 2009, 18:15   #7
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Electrex will sell a voltage rectifier complete with regulator for a very reasonable price. It's usually the best option as you avoid the excess voltage getting dumped through any electronics you have connected if the battery gets acidentally disconnected while running.
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Old 27 December 2009, 18:25   #8
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It's worthing reminding that the original kit above, and the ones I've used from RS are rectifiers only, which is all my old merc 150 had. Completely different from a regulator!
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Old 27 December 2009, 22:26   #9
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Well, not that much different; just short a few components.

The output from the alternator will be AC sinewaves. The rectifier flips half of the waveform, outputting a DC voltage with pulses at twice the alternator output frequency. That's most likely what the OP is looking at.

Generally you want some kind of smoothing to get a reasonably constant pure DC voltage; in this case the battery will do that. Alternatively (no pun intended) you can add in capacitors, filters, and regulation, such that the output voltage is *always* a known constant DC voltage (i.e. a voltage regulator.) Most automobile regulators are not that sophisticated, and still vary somewhat depending on peak input pulse voltages and frequency (or rpm, which is what drives the input voltage and frequency.)

For boating applications, that smoothing is generally not necessary - lights don't really care, and most electronics will work fine from about 11 to well over 18 volts (though you should check your manual, rather than take my word on it.)

You could, I suppose, graft in an automobile voltage regulator assembly if you're really worried about overvoltage, but in most cases, I'd say it isn't worth the hassle. But, a small car regulator should run you about $10US, and has only three wires: Input, output, and ground. Not really sure it would get you any real benefit, though.

My $.02;

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Old 28 December 2009, 08:25   #10
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Not really sure it would get you any real benefit, though.
At high revs, I fried a VHF and got an awful lot of over-voltage warnings from the GPS along with display fade when I had a master switch play up while I was out. Without the battery in circuit (when the master switch died) I was seeing nearly 24 volts at times.
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