Is 3.5kw Generator OK for a 16,000btu AC?

[Sailmonkey]

Locked rotor amps are what the rotor will draw if the compressor seizes and the motor tries to turn it. A normal 16k btu unit has a locked rotor rating of around 40-50 amps almost always installed with 10awg wire and a 30 amp breaker. At no time during normal operation will you see this kind of current draw. I have not yet run across a "healthy" 16K a/c unit that a 3.5 kw gen wouldn't start and run with no other loads applied.

[Highlander40]

While putting in a new Fisher Panda 4.2 kw AC generator, I noticed they make DC units also.
I used the smallest unit possible and load it up most of the time its running.
I can see the advantage of running a DC gennerator at 80% output during the whole charge cycle.
The problem is that you then rely on an inverter for ALL AC loads.
Thats a huge inverter. Mine will run one 10A load but not the AC.
Most of my loads are about 10-12A at 120V. Thats 2 stove burners 1 microwave, water heater and the AC.

My max output is 32A cont so the loads are stagered.
Charging goes on first and stays on but it only draws 22A for a little while.
Then cooking, then water heating, then AC if needed.
The goal is to get to the float stage. This is usually longer than the meal prep, eat, cleanup time.

The question is, can you charge lithium batteries while you are doing all the heavy lifting and be done by the end of breakfast or dinner?

A separate issue is the weight, my water jet drives love to push a lightly loaded boat. The difference can be 4 kts. 28 vs 32 kts

[Mule]

SailMonkey has seen 3.5kw push 16kbtu air conditioner. Ohms law says it will with quite a few amps to spare, the NextGen company rep says it will, the PO of my boat says it will...

I am gonna go out on a limb here and say, "Yes, a 3.5kw made by NextGen will start and run a 16kbtu modern, trouble free air conditioner."

[OceanPlanet]

Originally Posted by Highlander40

While putting in a new Fisher Panda 4.2 kw AC generator, I noticed they make DC units also.
I used the smallest unit possible and load it up most of the time its running.
I can see the advantage of running a DC gennerator at 80% output during the whole charge cycle.
The problem is that you then rely on an inverter for ALL AC loads.
Thats a huge inverter. Mine will run one 10A load but not the AC.
Most of my loads are about 10-12A at 120V. Thats 2 stove burners 1 microwave, water heater and the AC.

My max output is 32A cont so the loads are stagered.
Charging goes on first and stays on but it only draws 22A for a little while.
Then cooking, then water heating, then AC if needed.
The goal is to get to the float stage. This is usually longer than the meal prep, eat, cleanup time.

The question is, can you charge lithium batteries while you are doing all the heavy lifting and be done by the end of breakfast or dinner?

A separate issue is the weight, my water jet drives love to push a lightly loaded boat. The difference can be 4 kts. 28 vs 32 kts

Relying on an inverter (or inverters) isn't a big deal...you can use the Victrons (MultiPlus / 800VA - 5kVA - Victron Energy) and others in parallel to provide whatever you need, or even bigger individual units like the Dimensions (24 VDC Single Phase Pure Sinewave DC to AC Power Inverter Specifications).

Regarding how fast you can charge while running everything at the same time, it is simply how big a generator you want to have. But if it's a DC genset capable of putting all it's output into DC amps straight into the batteries it's just plain going to be a lot faster.

It would take some research to chose the right DC genset. There are of course many more choices for AC units, but there are at least Polar Power, Ample Power, and as you mention Fischer-Panda.

How heavy is your current battery bank and what is the capacity?

[bobfnbw]

Its not a big deal if your rich....
but most of us cannot afford to spend that kind of money.
I have a victron inverter charger 3K, but to have 2 or 3, with a dc gen, plus the lithium batteries would cost more than I could ever do...
but the price will come down.

[OceanPlanet]

Agreed, up front the costs are currently high. They will come down eventually, but even now on certain high-load systems, over the lifespan of use and factoring in fuel savings and performance gains it is worth considering for some.

One example of a system that I am currently involved with is a new diesel-electric hybrid powerboat built by French & Webb (Belfast, Maine) that is using a Steyr hybrid engine/generator-motor. It can motor at 7kts or so under electric power only (much faster with the engine of course).

They were going to use 1000 lbs of AGM batteries for a 300AH x 48V pack. We provided them with 320AH x 48V at 400 lbs, saving them 600lbs in a not very big boat. Yes, the batteries were expensive, but getting that 600 lbs out of the boat any other way would have cost even more.

Another take is this: that boating at all is obviously not really a sensible activity from the economic perspective (yes, I know living aboard can stretch the $, I grew up doing that, but still...). It is an asthetic choice of how we want to live. For most of us to make the choice we have to cut a lot of corners just to get there, but many make choices that allow them to do it the way they really want to.

For instance, not buying the biggest boat they can get for the $ and then struggling to keep it maintained. The folks getting the hybrid boat mentioned above could have a bigger boat, but for what they want to do, and how they want to do it, the smaller one makes more sense.

Different solutions for different folks!

[imagine2frolic]

I believe you will find that going without a/c on the hook will be fine. We spent four season going in, and out of the Bahamas. We used South Beach as our hub. Even being in Miami in August. If there was a zephyr of a breeze I still took my daily nap after lunch. There were only 2-4 nights in 4 years that I lusted for a/c. Those 2-4 nights don't justify the cost in my mind.

A good windscoop, and screens at times will keep you comfy. The screens won't always be needed also. Without them you get much more airflow.Now at the dock we just use cheap window a/c units with insulation board to direct airflow. Cost of about $100.00 each including unit, board, & tape........i2f

[Highlander40]

AGM's are 100AH and 65 lbs each.
House bank is four and starting bank two for a total weight of 390 lbs.
Are the lithiums good for starting also?

If I was building a new boat, the lighter weight would make the decision easy.

[OceanPlanet]

Originally Posted by Highlander40

AGM's are 100AH and 65 lbs each.
House bank is four and starting bank two for a total weight of 390 lbs.
Are the lithiums good for starting also?

If I was building a new boat, the lighter weight would make the decision easy.

I hear that.

You might have seen my post on page 3 where I calculated a Genasun system with the same "useable" AH as 4 x 100AH AGM's. The system weight was 31 kg, or 68 lbs. Hmmm....why do you need two for the start bank? For an LFP start battery you could get away with a RaceCell LFP90 (29 lbs) if you were really on a serious weight loss plan. 68 + 29 = 97 lbs.

The RaceCell would not have as advanced a BMS as the Genasun house bank, but is less $. With an emergency parallel switch you could always start off the house bank, but remember to never run LFP's dead or you can shorten or end the life of the cells! In this sense they are less forgiving than lead/gel/agm.

The LFP's can handle fast discharge much better than lead/gel/agm so you don't need nearly the total capacity there for starting.

[Highlander40]

Just to beat this to complete death. The lithium starter bank will go through a DC - DC converter with enough capacity to start a 440 HP diesel?

[OceanPlanet]

Hmm...DC-DC converter? What voltage are the starter/systems on the engine? I'm guessing 24V as you had 2 agm's (in series?) for the starter bank? What cranking amps are typically required for that engine?

For a lightweight lithium starting bank, you'd be able to use a much smaller bank (AH-wise) as they handle high discharge rates very well.

We're still talking LFP's here for marine use...but for the high-end auto scene I've seen lithium nano-phosphate batteries the size of a deck of cards that will start a full-size freight truck engine (I think it was made by A123, the folks who make the batteries for the Tesla EV sports car).

Crazy stuff. I've also seen a "fake" racing stock car battery that was a few tiny cells inside an otherwise completely hollow battery "shell". It looks just like a regular car battery on the outside (apparently to fool the officials or whoever else looked at it), but it weighed only a few lbs.

[Highlander40]

Little Harbor used group 27 batteries so they would fit in the space and were light enough to get into that space. I've done it and even at 65 lbs I nearly died trying to lift them at arms length to get them under the deck and over the battery box, not much extra room.
So, everything is 12 V.

[bobfnbw]

Its pretty short sided how builders plan for things like that. I have yet to see a well designed battery installation in a production boat... but of course I haven't seen them all..

On our E40, the PO placed 800 AH of golf cart batteries on top of each other in a plywood box, in the engine space. access was pretty much impossible to the lower set, and with the heat generated by the engine they were guaranteed to fail early, and they did. I have moved the battery space to under our aft berth, easy access, low in the boat, switched to agms, so they won't gas or leak, and went from the complexity of a 6v parallel series setup to just series. The less connections the better. I also went from the 1/0 cable that was poorly done, to 4/0, and used proper buses to distribute the load.

The Lithium batterys sound great, and will be on my list when I change out the batteries, in say.... 2015. Hopefully they will be much cheaper by then.

Bob