Step up voltage convertor

[Hawkcat]

Hi my 8hp saildrive outboard generates a current of 6.5v which I want to us to charge/top up my 12v battery. Does anyone know where I could get a suitable convertor to ensure that this is done effectively? Any other possible solutions?

CHeers

 

[savageseadog]

6.5V is a voltage not a current. I assume this is DC which will make conversion difficult unless there is a rectifier. If there is you would be able to connect before the rectifier and use the AC at that point with a 1:2 transformer to double it. The other way would be to rewind the coils in the dynamo/generator with twice the number of turns. Looking at your expertise you may struggle with this without some local help.

 

[Swagman]

Are you sure with those figures? If it's 12v at 6.5Ah things get easier.
JOHN

 

[VicS]

6.5 volts sounds a bit odd! One would expect it to have an output suitable for 12 volt lighting, in which case it would be unrectified AC, or for charging a 12 volt battery in which case it would be rectified, ie DC. In either case one would expect the "no load" voltage to be considerably above 12v.

Are you quoting the hand book figure or is this a figure you have measured?

If you tell us the make, model and year of the engine maybe someone will have some specific knowledge of it. Does it have a rectifier to provide a DC output or is it an AC output.

 

[VicS]

[ QUOTE ]
If it's 12v at 6.5Ah

[/ QUOTE ] Now you are adding to the confusion. Did you mean 12volts at 6.5 amps?

That would be a reasonable quoted output for an outboard engine. In which case it would be intended to charge a battery directly if it is rectified or via an add on rectifier.

Some outboards have a centre tapped coil which can provide AC power to lights in a series/ parallel circuit or battery charging via special rectifier that incorporates 6 diodes.

 

[Guest]

Sounds wrong ... as most outboards supply AC ... and that means if you are measuring with meter on DC - you will likely get this wrong result. Try metering with meter set to AC ... you'll likely find its 12v or slightly higher.

Any outboard shop can sell you a rectifier box to connect to it - to supply charge to battery's .... but a) it's only a holding charge at best ... b) the coil is usually designed for "lighting" ... low power.

 

[Hawkcat]

Thanks all for your responses, as you may have guessed I am not the sparky on board. Have sent your posts to a more qualified person than I for his response.

CHeers
Simon

 

[Swagman]

Sorry Vic. Yes meant 12 v at 6.5 amps.
JOHN

 

[William_H]

The problem being that the rating and intent will possibly be for 12v lights at 6.5 amps. Probably AC to the lamps. (and probably the 12 volts would drop to 11 or less at that current and of course only at full revs. To add a rectifier to charge a battery needs about 14 volts DC.
So the only usefull way to test the system assuming it has a rectifier is to connect it to the battery via an ampmeter and observe how much current you actually get into the battery. good luck... olewill

 

[Anonymous]

[ QUOTE ]
Any outboard shop can sell you a rectifier box to connect to it - to supply charge to battery's .... but a) it's only a holding charge at best ... b) the coil is usually designed for "lighting" ... low power.

[/ QUOTE ]Looking at the Mercury 8hp details online, as you say, it has a 6A 'alternator' output which is, I presume, unrectified. An idea (which I don't have the time to work right through) that might help folks in this situation is to buy the rectifier and feed that into a cheap laptop converter. These things produce variable voltages from pretty poor dc inputs. Mine, for example, produces among other choices, 14V and 15V. If one chooses 15V and puts a decent power diode in series then the output would be a pretty good charger for lead acid. Ideal for a outboard where maximum output is required but won't be run for many hours at a time. Anyway, 14.5V isn't going to harm the battery. Cheap, effective and cheerful. BUT....I don't know how these converters will like raw dc. If the chopper frequency is of the order of 10kHz ++ then maybe it will be fine. It's a suck it and see job.

Edit:- Hmmm - Rethinking this already. It might overload the alternator on raw dc. Probably essential to smooth the rectified dc which needs an engineer to choose the right value and type of capacitor. I'll do the calculation for anyone if they post the engine type, make and model of rectifier and rated alternator output here.