Calculating Diesel Generator Size

[beebopbogo]

I'm looking at buying a DC diesel generator and need some help with the sizing. I will only use the generator to charge the batteries. I have no need to connect directly to the unit. Can I get away with a small, quiet, portable 1 kW diesel generator? (Planning a long cruise on my Cal 39 and want the generator as a second source of power.)

Here is my expected usage:

Amps / Hrs / Ah
0.90 / 12.0 / 10.8 Running Lights
2.00 / 12.0 / 24.0 Tricolor
8.00 / 00.5 / 04.0 Spreader Lights
0.10 / 12.0 / 01.2 Binnacle
0.40 / 24.0 / 09.6 Instruments
0.40 / 24.0 / 09.6 VHF
1.10 / 04.0 / 04.4 Cabin Lights
0.80 / 01.0 / 00.8 Galley Lights
0.70 / 02.0 / 01.4 DC Outlet (Phone)
4.70 / 02.0 / 09.4 DC Outlet (Laptop)
3.50 / 24.0 / 84.0 Fridge 24/7

Volts: 12.0 V
Ah used per day: 159.2 Ah
kWh used per day: 1.91 kWh (V*Ah/1000)
Battery Capacity: 420 Ah (4 x 12V AGM 105Ah Cap each)
Battery Cap (Useful): 126 Ah (80% - 50% capacity)

It seems I'm short on battery capacity with the usage listed above. Charging above 80% capacity is less efficient and a waste of fuel (it becomes harder to force a charge into the batteries as the batteries become more charged). The lower limit of 50% is set to ensure a long life on the batteries. Which leaves me with only 126 Ah of useful capacity while I am using 159.2 Ah per day. Looks like I'll have to either run the generator more than once per day, install solar, or cut down on my usage.

Here's more detailed info from my website:
Electrical Diagrams | Sailor's Life For Me

[colemj]

1KW is only going to be able to handle a 30A charger. Maybe 40A, which still isn't much for a charger.

I don't know of any diesel gensets that small. Typically, they start at 3-4KW.

Your above usage assumes you are on passage all the time?

Mark

[Captain Bill]

Charging agm's to only 80% will kill them in short order. If AGM's aren't charged to 100% fairly often they will loose capacity. I have seen this happen on my own boat after not getting them past 90% for a month. Mine are Concorde Lifelines and fortunately can be given an equalize charge that restores most of their capacity. Not all AGMs can take an equalize charge. If you are going to insist on using the 50-80 charge regimen you would be better off with FLA golf cart batteries.

[beebopbogo]

Your above usage assumes you are on passage all the time?

Yeah, my numbers assume I'll be at sea for a 30 day crossing, then cruising the South Pacific. My goal is to run the smallest, quietest, most fuel efficient generator possible for the least amount of time. Ideally, 1 hr per day. I'm pretty weak on the theoretical calculations to find the lowest kW generator required. Any electrical engineers out there?

1KW is only going to be able to handle a 30A charger.

That reminds me. Will I need to run the generator output through a charge controller?

I've been talking to a buddy who has a 1 kW diesel generator for his motorboat ice-maker. He was thinking it should be enough, but I want to see the actual numbers before I shove off for the big trip.

[Stumble]

Beebop,

You cant have a 1hr/day run time or a small generator, not both. They are diametrically opposed demands. If you want to put 150ah back into a 12v back in an hour (let's assume only during bulk charging phase) then you need a generator capable of putting out 15amps at 14v. Which works out to a 2.1kw 12v DC generator.

If you are using an AC generator and battery charger then you need to size it to the power draw of the charger. Bu you would need a massive battery charger to do this, like 150amp charger.

[colemj]

OK, I was thinking AC generator and I think you are talking about a DC generator. Even so, 1kW will be pretty anemic charging for what you want - maybe 70A.

Here is an example of a DC genset. This one is ~3kW and drives a 200A alternator: Aquamarine, Inc -

An AC generator produces AC power - like in your house. You will need a separate charger powered by it if you use one.

What generator does your friend have? Strange that he needs a generator for an ice-maker. Is he freezing fish holds, or does he have a margarita problem?

How large is your engine? Your simplest route may be installing a large alternator (200A) on it. You will need a good regulator and robust mounting and a ribbed belt system. This will still be costly.

Solar and wind are other options, but they too are costly and have mounting issues.

Mark

[beebopbogo]

If you want to put 150ah back into a 12v back in an hour (let's assume only during bulk charging phase) then you need a generator capable of putting out 15amps at 14v. Which works out to a 2.1kw 12v DC generator.

Colemj, how do you arrive at these numbers? I get the 14 charging voltage (my AGMs like 14.4V Bulk Charge), but why 15A? And how does that work out to 2.1kW?

I need to go back to high school physics, haha. Thanks for the refresher.

Charging agm's to only 80% will kill them in short order.

Captain Bill, whoops! I did not know that! A bit of extra fuel in the genny is a lot cheaper than new AGMs! Thanks!


P.S. I don't want to veer off into a solar conversation, but here's an image of my current setup. I already have 200W of solar installed. The wind generator threw a blade and is off the boat at the moment. I hate the noise, anyway!


Electrical Diagrams | Sailor's Life For Me

[colemj]

Originally Posted by beebopbogo

Colemj, how do you arrive at these numbers? I get the 14 charging voltage (my AGMs like 14.4V Bulk Charge), but why 15A? And how does that work out to 2.1kW?

It was Stumble who wrote that, not me, but let me try to explain. AC produces power at 120V, while DC is at 12V. You want to produce 150 DC amps in one hour.

Power (in Watts)=current x voltage

150A x 14.4V = 2,160 watts.

To produce this with an AC generator: 2,160 watts÷120V = 18A AC current needed.

Stumble's 15A is probably correct, because I have used nominal values and ignored some details.

It is all moot, because I don't know of any 1kW diesel gensets. Mostly because they do not really make diesel engines that small, and it doesn't make sense to put a tiny generator head on a diesel engine that can run a larger one.

Mark

[CarstenWL]

Stumble wrote

then you need a generator capable of putting out 15amps at 14v

Would say a zero got lost ... 150A at 14V makes more sense (thus 'the confusion').

Carsten

[Dockhead]

Originally Posted by colemj

OK, I was thinking AC generator and I think you are talking about a DC generator. Even so, 1kW will be pretty anemic charging for what you want - maybe 70A.

Here is an example of a DC genset. This one is ~3kW and drives a 200A alternator: Aquamarine, Inc -

An AC generator produces AC power - like in your house. You will need a separate charger powered by it if you use one.

What generator does your friend have? Strange that he needs a generator for an ice-maker. Is he freezing fish holds, or does he have a margarita problem?

How large is your engine? Your simplest route may be installing a large alternator (200A) on it. You will need a good regulator and robust mounting and a ribbed belt system. This will still be costly.

Solar and wind are other options, but they too are costly and have mounting issues.

Mark

This is good advice.

There is no such thing as a "small, quiet 1kW DC generator". To get quiet, and I might add, smooth, you need 3 cylinders and low speed, and those start at about 4kW, at least in AC they do.

The smaller ones use one-cylinder Kubota rototiller engines and typically run them at 3000 or even 3600 RPM -- awful. Some of the DC ones have variable speed depending on load, but they will still be screaming their little lungs out when you load them up.

For 1kW, or even two, the quietest and smoothest generator you will find will be a Honda gasoline powered one.

If I were you, I would start with a high output, "hot rated" school bus alternator on your main engine, and then take it from there. Supplement that if necessary with a Honda 1kW or better 2kW gasoline generator. And only if that does not prove satisfactory, start looking at diesel gensets.

[darylat8750]

I don't see a water maker in your list of power required. I would think you would want at least 2kw. so you could charge while making water?

[beebopbogo]

Ahhh, ok. It's coming together:

Buy a small 2 kW gasoline generator in case of engine problems. (Here's a Honda EU2000i, $1200 new: Honda EU2000i Model Info | Super Quiet 2000 Watt Inverter Generator | Honda Generators with DC output 12V, 96W, 8A. The EB2000i is AC output 120V, 2000W, 16.7A. I'm assuming the DC out would be simplest to plug into my battery charging circuit.)



Some confusion is injected when talking about an AC generator. I have zero AC on my boat. (Well, AC outlets, but I never turn on the inverter. So inefficient!)

I believe my engine alternator is the old 100A original from 1978 along with the 50HP Perkins 4-108. I'll look into adding a larger alternator, but I don't think I need more than what I've already got.

And yes, I do have a Pur 40E Watermaker sucking down almost 5 A when running. Forgot to add that to my list.

[colemj]

Originally Posted by beebopbogo

Ahhh, ok. It's coming together:

Buy a small 2 kW gasoline generator in case of engine problems. (Here's a Honda EU2000i, $1200 new: Honda EU2000i Model Info | Super Quiet 2000 Watt Inverter Generator | Honda Generators with DC output 12V, 96W, 8A. The EB2000i is AC output 120V, 2000W, 16.7A. I'm assuming the DC out would be simplest to plug into my battery charging circuit.)

No, you will not want to use the DC output of that unit for charging your battery. You will want to buy a larger output battery charger and use the Honda's AC output to run it. A good compromise in price/output for that use may be a 75A Iota charger. But even this would require 2hrs of running to get your nominal 150Ahrs into the bank.

Mark

[Stu Jackson]

Mark's right.

you also need to educate yourself some more on boat electrical systems and simple Honda Generators.

The DC output of those is useless for battery charging, you use the AC output to a boat shorepower charger via the shorepower inlet already on the boat, and use a pigtail to plug the generator into the boat.

Have you downloaded the Honda generator catalog and manual yet?

Might wanna do that, and research good battery chargers, like this one:

Wiring a new charger From Maine Sail: Marine Battery Chargers - Installation Tips & Considerations - SailboatOwners.com

The "stuff" here might help, too

Electrical Systems 101 Electrical Systems 101

[savoir]

If you decide to go with a diesel generator I can recommend one of these. At around 5 hours/gallon they are very easy on the fuel. They are made in Florida from Kubota parts.
NEXT-GEN - Marine Power Units