SailNet Community banner
1 - 20 of 22 Posts

·
Registered
Joined
·
994 Posts
Discussion Starter · #1 ·
I want to install a 2500 Watt inverter on my boat with a single 110V outlet in the Galley.
a)My battery bank is only Two 100 amp hours deep cycle batteries.
b)My intention is to use the outlet (inverter) while the engine is running to run either the microwave(1000W) or fancy coffee pot(1500W).
c)Not sure the exact output of my alternator on my engine, but i'd guess it's something like 30 amps.
My question is: For short uses of under 5 minutes, would my battery bank & alternator together do the job???

I also assume that while plugged in (110V)at the dock, the inverter could be used without worry full time, as my battery bank is being charged? (again, not sure the specs of my battery charger, oops).

Are my "close enough for government work" assumption going to work in these senarios???? thanks gang!
 

·
Registered
Joined
·
116 Posts
The short answer is no. If you are on dockside 120VAC, then skip the inverter, just plug directly into the shore power.

The current draw from the inverter will overwhelm your system. You will have to size conductors for the maximum current, (2500 w @ 12.5 VDC) which is enormous at 200 Amps. Engineering factor and voltage drop will push that value to nearer 250 Amps.

That current will melt your batteries, and they will run down into your bilge, where you will have to clean them out with a shovel. Obviously your engine alternator is hopelessly under sized - so no help there.

(Although your batteries are 100 ah, they will not put out 100 Amps for an hour. Nowhere near. So the "just for a minute" scenario won't fly, either.)

Either stay with shore power, or buy a genset that can deliver that kind of power. Sorry.

Best Regards,


e
.::.
 

·
Registered
Joined
·
285 Posts
You would be surprised on how many amps a coffee pot pulls. I have a Grind & Brew, and the pull on it is almost 100 amps to heat the water. Running the engine helps, but not if you don't have the housebank sized to meet the needs of the job.
 

·
moderate?
Joined
·
1,000 Posts
Lets do the math. A 1500 W coffee pot will draw 125 amps at 12Volts continuosly from your battery. For 5 minutes or 1/12 of an hour this would be roughly 14 amp hours which is no big deal. Of course there are coversion losses and you cannot rely on the 20amphour rating of the batteries at such a high draw level... But even at twice the draw level you are still ok regardless of your alternator size. A 30 amp alternator would take an hour of run time to recharge your bank.
I'd feel more comfortable if you had a bit larger bank but I am thinking you are OK with this approach as long as you are careful in your use
 

·
moderate?
Joined
·
1,000 Posts
The current draw from the inverter will overwhelm your system. You will have to size conductors for the maximum current, (2500 w @ 12.5 VDC) which is enormous at 200 Amps. Engineering factor and voltage drop will push that value to nearer 250 Amps.

(Although your batteries are 100 ah, they will not put out 100 Amps for an hour. Nowhere near. So the "just for a minute" scenario won't fly, either.)
Well I had a 2500Watt Inverter and everything was sized correctly and we could run even greater than 1500W (125amps/12V) loads. The big difference in my set up was battery bank size but there is no reason you can't draw 125 amps of current for 5 minutes.
It is correct that you will get far less than the rated 200 amphours (1.5 hours to flat...or .75 hours to 50%) which is calculated on a 10 amp draw.
Wet cell batteries prefer to be discharged at not more than 20% of rated amphours or they heat up and can self-destruct. Thus a 600amp hour bank would be preferrable....but for 5 minutes, I don't think any harm will ensue.
If that is a concern...asailor could switch to AGM's or Odysseys for significanlty lower internal resistance and no worries.
 

·
Registered
Joined
·
285 Posts
Cam, the issue I saw was that even running the engine, the draw on the housebank is large. The output from the alternator won't keep up and with only 100 amps in reserve, there will be a large voltage drop as well. Plus, a ten-cup coffeemaker runs about 10 to 15 minutes. Makes it a tough setup for coffee as that is around the outer edge of the opperation curve.
 

·
Registered
Joined
·
311 Posts
The real question would be, why? Why do you need to run a microwave and coffee maker away from the dock? Of course there is the theoretical question of whether it can be done. The practical answer is yes, but it will probably fry your batteries and be a PITA to recharge and eventually replace them. And asailor thinks he needs to run an inverter while on shore power. Maybe he should figure out how to run the microwave and coffee maker at the dock, before he tries at anchor.

But of course, I like instant coffee and warm beer. So who am I to give advice?

Scott
Gemini Catamaran Split Decision
 

·
Registered
Joined
·
285 Posts
Scott, you are talking to a guy who's first purchase for the new boat was a Cuisanart Grind & Brew. I have to have my good cup of coffee (more like a pot of coffee to be honest) in the morning, and it tastes even better when on the boat! It's why I learned about inverters and how to make sure I was always able to grind and brew that great cup of coffee.
 

·
Registered
Joined
·
311 Posts
Windward,

To each his own, to each his own. I'm glad you enjoy your brew! Everything tastes better on a boat.

Scott
 

·
Registered
Joined
·
141 Posts
The few times I'm overnighting on the boat, I have a stainless steel French press coffee maker. I boil a couple of cups of water & add it to the pre ground coffee. In 5 mins. I have great coffee with no use of the battery at all (I use my 2 burner Coleman Stove to heat the water).
 

·
Registered
Joined
·
994 Posts
Discussion Starter · #13 ·
The Keurig makes one hell of a great cup of coffee in about 2 minutes and chicken nuggets need to be ready pronto sometimes when the little ones are hungry... The whole issue was, "what can i get away with" without having to reach very deep into my pocket this year... luckily a weeks trip on the boat is coming fast, and the few extra gadgets i can use AWAY from the dock, help me win the battle from the Admiral about if we have to be at the dock or whether or not i can persuade the family to stay on the hook out in some beautiful cove for the night. Making the most, with very little... Thanks All!
 

·
Registered
Joined
·
45 Posts
Another opinion - I agree with Cam, you should be able to do this. The important thing will be the wires between the battery and the inverter and recently someone posted a nice web site calculator for determining wire size given current and run lenght. As the batteries age, the output voltage will drop more at the high current level so I would calculate the wire gauge for minimimun voltage drop - like less than 3%. If the wires are too little, the voltage drop could cause the DC voltage into the inverter to drop low enough to where the inverter shuts down (most have some sort of minimun voltage shut off so they dont drain batteries past some threshold). Worst case is that the wires between the battery and the inverter are too small, heat up, melt the insulation and weld together and then burn up but I would guess the inverter would shut down due to low voltage before this happened.

This is likely all covered in the manual for the inverter and it may have even come with its own wires to connect directly to the battery?. With this setup, you will end up with some big A$$ wires between the batteries and the inverter and putting a big A$$ fuse close to the batteries just in case you short the wires near the inverter would be a good idea. The inverter probably has some sort of internal fuse to protect itself but the fuse near the battery is still needed to keep the wires from burning up during a short - I dont know how many amps a 200 amp hour battery bank will put out in a short - but its a lot.

That inverter will likely draw at least a few watts even when it has no load so putting it some place with easy access to the power on/off switch might be useful when you need to be frugal with amphours.
 

·
Registered
Joined
·
45 Posts
One other point (which Cam also already made) is regarding "can the 200 amp hour batteries put out enough current".

The intended usage time frame is short which I think is important here for safety because I would guess its an unkown for everyone what a 200 amp hour battery would do putting out 125 amps for a long time? Heat up and melt, I dont know.

But in the short time frame (ie, couple minutes), you need to deliver the 125 amps to the inverter and it likely still has to be above 11.5 volts or so at the DC input of the inverter. So you need both low impedance wires and also low internal battery impedance.

How to get low battery internal impedance (besides adding more amp hour capacity)? AGM will have lower internal impedance than wet cells. Also according to the "battery bug" web site, as the battery charge goes down, the internal impedance goes up. As the battery ages, the internal impedance goes up.

So if we assume you have 200 amp hour of wet cell batteires, I think there is a very good chance everything would work just wonderful with newer fully charged batteries. However, I think there is also some chance that as the batteries age, you will have issues sooner with the batteries not being able to deliver 11.5 volts at high current than if you only used them at light loads. Also, if for example your batteries were down 40% and you tried to get the big current from them - you might have an issue getting the 11.5 volts minimum to the inverter.
 

·
Registered
Joined
·
311 Posts
Asailor, obviously I and others don't think running a microwave and coffee maker off an inverter is a good idea. But if doing so will make the difference between having to stay at marinas with your family, or being able to anchor out, that may be a good reason to try it.

Others are giving good advice as to the set up you should have, and some theoretical and practical limitations. A lot will depend on the age of your batteries. I would try it at the dock first. Disconnect from shore power and pretend you are at anchor for the night and do everything as if you were at anchor. That way, if you have an "Oh sh*t moment", at least you will be at the dock. And then when you are at anchor, you should have a plan B in case you fry your batteries or something. (Have a spare battery on hand, etc.)

Scott
Gemini Catamaran Split Decision
 

·
Registered
Joined
·
80 Posts
I have a 500 amp battery bank. I run a coffee pot an microwave off my inverter not at the same time ofcourse. The mircowave seems to use more power then the coffee pot. Either way the time of useage is so short the impact on the batteries is no big deal. I have a 100 amp alt so when running this is not an issue. I have lived aboard and cruised for 5 months and this is my experience as yours may vary
 

·
Registered
Joined
·
116 Posts
Hey, here's a little-known story from the WWII U-boat war:

The German technology was the best in the world - one of the things that the US Navy learned from the Germans was that you could "Overdrive" the diesels and the e-motors in a U-boat (referred to as "emergency speed") for a limited period of time, like if a lousy American airplane was trying to bomb you.

In the case of the electric motors used when submerged, these were big series-wound motors, and when "emergency speed" was ordered, the technicians in the E-room basically dead-shorted the batteries to the motor. The Germans knew this was risky - the batteries could melt, the motor could run away (and sometimes did) at high rpm and disintegrate, and misc flames, arcing, and general excitement. All of the above was considered acceptable if the boat and crew were in imminent danger

There was a lever that did this, and it was attached to a counter which indexed every time they pulled the lever. When the counter reached a set number, usually around 20, the batteries were considered at the end of their useful life, even if they were only a month old. The batteries would be replaced next time the boat was in port. This was an expensive and long procedure, so the captain had to have a damn good reason every time he ordered emergency speed.

Anyhow, the lesson is that yes, you can stress your batteries by sucking giant amps out of them in a short period of time, but you only get to do it a certain number of times, and there is no question that each time you do it, it shortens the battery life. Also, lead-acid batteries used for starting purposes have an unfortunate tendency to fail catastrophically; not gradually - so prepare for "surprises" when starting the iron jib.

just FYI

Best Regards,

e

.::.
 
1 - 20 of 22 Posts
Top