# Converting 28VDC -> 14VDC at around 200A How do you pull that off with that kind of current?

Current boat has 110AC air conditioners and some small appliances but the rest of the systems are on 12VDC. Genset ate a jellyfish and has to be replaced, so I'm working out options to upgrade/change the electrical system. Converting the whole boat to 24VDC is probably not doable as that seriously limits the places you can get replacement parts.

I've found a DC Generator with a ~200Amp output @ 28VDC that I need to convert down to 14VDC to charge the house batteries.

How do you do that efficiently at such high amperage? Lots of parts out there, but they are typically 5A or less and running them in parallel is just not a very good idea.

Rest of the boat is on 12V as well, trying to use 24VDC to halve the current from the Genset to the battery bank and power panel.

Wondering aloud if it makes more sense to go with alternator instead of a Generator and convert from AC -> DC closer to the battery bank as that would cut the current flow for the longest part of the wire by 6-10x. The problem with AC is the speed of the engine affects the 60Hz portion of the equation. I've considered adding one alternator per Diesel but can't figure out how to make that work since the RPMs of the engines that run the props are subject to change based upon propulsion needs and getting reliable 60Hz out of that setup seems nearly impossible. And combining the 2 AC alternators when the motor speeds can't be synced seems like inviting disaster.

Any ideas how to solve this kind of problem without just replacing the existing generator with another?

• Old genset was 6500Watts @ 110VAC
• 200 Amps @ 28VDC to provide about 400 Amps @ 12VDC which is only 4800 Watts but sufficient for the aircon load
• With a fully DC system, I'd be feeding a larger Inverter to run the Aircon from the generated DC or the batteries.
• Hang on. Your stuff doesn't require 28V * 200A = 5600W, right? – Dejvid_no1 Sep 6 '14 at 20:10
• Make your own switcher from a controller and discrete switches. It will handle how ever much current you want to pay for. – Ignacio Vazquez-Abrams Sep 6 '14 at 20:26
• What current do you need at 12V - forget about what the generator is capable of - what do you need? What is a genset? I know what a jellyfish is BTW. – Andy aka Sep 6 '14 at 20:47
• @Andyaka genset = an electrical generator paired with an engine to turn the generator. Fuel (usually diesel) in, electricity out. – hobbs Sep 6 '14 at 23:36
• You mention 110 VAC aircon. I assume this means that you have 12 VDC -> 110 VAC now? But you mention 60 Hz so was genset AC? A more complete description of the system and components would assist good answers. – Russell McMahon Sep 7 '14 at 2:45

Ample Power has a diesel genset with a 12 volt DC 280 amp alternator.

You can get high-output 12 volt DC alternators to mount on your main engines. For best charging effficiency, you also need an external "smart" regulator to control the alternator output.

In automotive/marine terms, an engine-mounted alternator will normally produce DC, so engine RPM is not important (although an alternator won't produce full output at idle.) (For the past 50 years or so, all cars have used alternators with DC output to charge the battery.)

Conversely (or perversly) in marine use a generator or genset is an engine-driven device that produces household-style AC power (120V 60 Hz or 240V 50Hz depending on area of use).

• So even though they are called alternators, most of them are producing DC? On a car, the distance from motor to battery is pretty short. On a sailboat that distance is NOT insignificant. Turns out 200Amps DC requires some pretty big wire. – boatcoder Oct 12 '14 at 20:16

"28VDC" will charge two 12V batteries in series, but I doubt that will be of much help.

If the generator is capable of 200A at 28V and if you only need 200A/14V, you can probably fiddle the regulator on the generator to get 200A at 14V (half power) out of it. That would be the cheapest solution if it's feasible.