Re: [electricboats] Silly question - Hybrid what-if'ing

Yep John it would be really hard to find a DC power supply at a reasonable price that would supply enough amps continuous compared to a battery.  It's also hard to get a setup from generator to battery charger(s in parallel) that will keep up with battery consumption, but the approaches Dan outlines are both routes that should work. 

If your stumbling block is initial cost of the battery bank, I'm not sure that these approaches will save you a whole lot of money, but what either approach will do is give you a path out of the range limitation of a smaller battery. 

On May 25, 2021, at 21:41, Dan Pfeiffer <dan@pfeiffer.net> wrote:



Yes, I think what you describe can work but from what I have found (and been advised by Thunderstruck) you need/want the battery in the system rather than powering the motor controller (of whatever type - AC, DC, BLDC,...) directly from the generator.  But the battery can then be smaller.   It is not a solution to head offshore with but for the way most people use their boats I think it can work.

I would look at propane or dual fuel generators rather than gasoline.  Propane on the boat has the same issues as gas with regard to accumulation in the bilge but it is a lot easier to swap a propane bottle at sea in rough weather than to re-fill the gas tank on a portable generator from a gas can.   I know of one 6-1/2 ton monohull that runs with a 2000 watt propane generator and reports 4 kts and 8 hrs on a 20lb bottle (about 4.5 gallons of propane).  You would use about 4 gallons of diesel in that boat over 8hrs at 4kts.  So that cost per mile is about the same?   A 20lb propane refill is about $20 and so is 4 gallons of marine diesel?

As far as the power supply, the charger is exactly what would do the job.  You run the propulsion motor from the battery and you keep the battery topped up with the charger that is running from the generator.   If you want to go 4 kts you need a 2000 watt generator and a 2000 watt charger. 

There is a boat in Seattle that a list member here has that is running three 2500 watt Thunderstruck chargers from a 220V AC diesel genset to get 7500 watts for about 6 kts I think.   The Thunderstruck chargers can run up to four from one controller so the system is scalable.   But note that it is running at 220V.  Those chargers only deliver 1500 watts at 120V.   So to be a practical solution you really need the 220VAC genset. 

 

I don't think a 3000W power supply would be more cost effective and with a charger you should have the controller to properly manage the system and not blow anything up.   

I have in mind a similar system to the one is Seattle but I am looking at a 48VDC genset to take out the middleman of the chargers.  I am planning on using a BLDC motor for the generator and a 12kW diesel (Kubota 722).  But it will require a beefy rectifier and regulator and I have not gotten to the research on that yet.  That's phase 2 or 3 and I am a few weeks away from getting phase 1 up and running which is the electric propulsion part.  I am almost done with machining the running gear.  After that's up and running I'll start working on the genset. 

For what it's worth, I have a 13kWhr battery and estimate I will have a 25 mile range at 4 kts.  If I add a propane generator to power the charger (a possible interim solution while I get the 48VDC genset built) I hope to be able to run at 4kts with zero draw on the battery.  I should have real data in a couple months.

Dan Pfeiffer

On 2021-05-25 11:05 pm, john via groups.io wrote:

 

Yes, power is power.

The thought was that it's relatively cheap to buy a portable generator and carry a can of gas as backup.

 

Looking at this a different way...

 

If I go with traditional 48V motors (maybe even 10Kw so that I have plenty of room to grow) - can I power those (at low power) from a DC power supply running off a portable generator?   Is it possible to buy a 2000W or even 3000W 48V DC power supply that runs off 110VAC?   This would (in theory?) allow me to run off a very small battery bank, or no battery bank at all initially.

 

Then there's the electrical question:  If I connect a 3000W power supply to a small lithium battery (which would be very unlikely to have exactly the same voltage - higher when charged, lower when discharged - will I blow up anything?

 

I'm assuming a 3000W power supply would be significantly more cost effective than trying to find a 3000W charger (which would be massive overkill, and likely damage a small battery bank).

 

John

 

 

 

 

On Tuesday, May 25, 2021, 09:53:32 PM CDT, Dan Pfeiffer <dan@pfeiffer.net> wrote:

 

 

Setting aside efficiency differences it still takes the same amount of power to move the boat at a given speed.  AC, DC, gasoline, diesel.  Doesn't matter.  The boat wants what the boat wants.   Higher voltage will be lower amps.   Lower voltage will be higher amps.  The watts remain the same.   The sizing of the wires may change but the power needs remain the same. 

An AC motor can indeed be used with a controller that allows for forward and reverse and all the rest.  Your battery bank challenge will remain.   It may even get larger from more losses through added inverter stages.  

The generator can also work.  For a 6 ton boat you'll need something in the neighborhood of 2000 watts to go 4 kts?   Doesn't matter where you get it or what conversions in may go through making it's way to the propeller.   If you're going to loose half of it on the way you need to start with 3200.   An AC motor will work but it won't pick up the check at lunch. 

Dan Pfeiffer

On 2021-05-25 9:19 pm, john via groups.io wrote:

 

 

This is probably a really silly question, but....

 

Is it possible to find a 120V AC motor that can be speed controlled (and maybe even reversed??) for at least a proof of concept on a boat?

 

For those of us that prefer electric motors to gas or diesel engines, but are challenged by the investment of a huge battery bank, wouldn't it be cool if we could run an AC motor off an inverter from whatever battery bank we have handy, and use a generator to supplement - or just run off the generator entirely at first?

 

We've discussed the efficiency gain of running off a higher voltage / lower current.

I'm guessing that there just isn't a suitable way to control and/or reverse an AC motor??

 

John