battery and inverter advice

Hello everyone! Im in the process of designing my boats electrical system and had a question for you! I need to have a 3000 watt inverter on the boat (for this example assumer I use the full 3000 watts). Lets also assumer there is not a DC system for this example and we are just concerned about the inverter. I need to find out how many AH I need in the batteries so could you tell me if my logic is correct and if not please fix it.

So 3000 watts / 12volts = 250 AH
so not letting the batteries drop below 50%
250AH * 2 = 500AH as a minimum?

I know that does not account for efficiency of the inverter (90%), How do I account for that and am I missing something else?

Thank you for your help!

 

No, start by evaluating your mains energy loads and time used then work back... How long will you run on batteries before charging? - then you will be able to work of the size of the batteries and the generator needed.... mine delivers 97% for 4000 watts and cost in excess of $4000, the batteries will FLOOR you.... on such heavy loads AGM spiral wound for small capacity 400A 48V system or NiFEPO4 I think is the latest technology as used in hybrid cars... for higher capacity loads - BUT they both require special management systems for charging...

3000W at 12V is 250Amps... and needs heavier cabling than 250A rated cable... (NOT AmpHours) - - for that think VERY heavy cabling and for marine applications ALL WIRES MUST BE 'tinned' as copper only will last less than a month if any humidity/salt moisture is present...

3 KWH is 3 k watts for 1 hour ..... 3000 watts may also be called 3KVA (3000VoltAmps)

If you are thinking of "house" battery system (for galley and comforts only), then the usual is to hold 4 days normal use as a minimum, and then you need a diesel backup, as we just had 7 days of below capacity PV charging weather at home (home system has 4200w of panels...) Get professional advice - it is VERY complex... for example charging parallel banks of batteries poses expensive management problems, especially if you are drawing energy out at the same time...

 

You forgot to mention the duration of the load. 3000 watts @ 12V = 250 A, not AH, as the H stands for Hours.
250 A is the average consumption of a starter motor, that is quite acceptable for a few minutes, but if the load is present much longer, use 24V instead, that is more efficient and costs about the same.

24V with 250 AH capacity will allow you to drain 3 KW for 1 hour. Multiply that by the number of hours of autonomy you need and use that figure to calculate cost, weight and space. If the disappointment hasn't choked you, add 10% to cover the cable and inverter losses and find a way to finance your project.....

 

Im really not sure what the duration would be, the inverter is mainly for a 120 volt fridge that uses 350 KwH/year. I divided 350000 by 365 days and it came to a little less than 1000watts per day. Should that be divided by 24 hours to get the amount of watts per hour? As for the charging system, I was planning on using lots of solar, I can fit 600 watts on deck easily with out having to get creative. I will also be using dual wind generators as my area very rarely has wind less than 10mph.

Thank you all for your help again!

 

Hi Jakegator,
Maybe this will help, A test that I performed.......

1-Interstate 8D-MHD 12 volt lead acid battery (Industrial Deep Cycle)
......1,700 cranking amps
......1,400 cold cranking amps
I think the amp hour rating is a little over 200 for each battery

1-5,000 watt (10,000 peak) Aims 12 volt Inverter 90% efficient (?)
......120 volt AC output

1- Home Depot 24" shop fan, 120 volt AC, amp draw 3.0 high speed

Run time on one battery (I have six) until voltage dropped to 11 volts, 13 hours and a few minutes. The low voltage safety in the Inverter switched it off.

In reality, the fan was likely just short of 120 volts, the Inverter was just a little less than 90 %, which would mean the battery was giving up just a little more than 30 amps. (on a 12 volt system the ratio is 10) if a tool draws 10 amps the batteries must give up 100+ amps.

If I recall correctly the 20 hour rating for those batteries is 75 amps, down to 50%. Someone might be better informed on this last comment.

An actual test that will help you on the cheapest kind of battery (my cost per battery + all the ad-on's $217.00 US)

Hope this is a help to your thoughts.

RonL

 

That's a totally different situation.
1 KW for 24 hrs means 100 AH @ 12V in your case. One 200 AH battery will supply enough power for a very dark day without wind. If that frequently happens in your location and you do need the fridge, double the capacity.

When choosing an inverter, first check the power consumption of the fridge with the compressor running. That is usually between 80 and 200 W. The inverter must be able to deliver at least 10 times that value to cope with the inrush current of the motor.

 

3kw would be a huge inverter for a small craft. HUGE. Dont know the load for your refer but its not 3000w. recalculate. Manytimes its best to have a dedicated inverter for continuous duty equipment , then an auxiliary inverted for general ships chores like making a cup of tee.

 

Things to consider about those cheap inverters

Here's a link that I find very helpful and a lot to think about.

http://ludens.cl/Electron/chinverter/chinverter.html

I have opened a few smaller ones and find his information to be quite accurate.
For example 2,000/4,000 watt units that safety trip at less than 1,500 watts, one other thing I find is many cheap inverters have cooling fans that run at all times, buy a unit that has heat activated cooling fans. Just my preference for what it might be worth.


RonL

 

CDK - Could you explain your math im a little confused how you got 100AH, did you round?. Im going to need to reproduce it later on for different systems. thanks!

 

Watts are voltage times current. In your case the outcome would be 83.3333 AH, but there is also 10% for inverter losses to consider, which makes a total of 91 .64 AH.
That is what a 100 AH battery can store after a few months of service.

 

I see no real reason to use 120 v fridge on a boat unless you have a houseboat, floating house, or shantyboat. DC fridges with holding plates make excellent use of solar. Two speed compressors that switch speeds based on input voltage (run on high and freeze the holding plate when lots of sun or wind charging power is available, then let the plate melt overnight, then run on low off batts if plate melts and box temp rises and no solar or wind charging happening) are best. Marine fridges are usually designed to be a lot better insulated than household models. If you still want to use household fridge, get a bigger one and add 2" foam panels on the inside. Make sure it does not have digital controls. Units which run on both 12 and 120 are available as well. Not cheap, but neither is an inverter. The reason you spend the money on these appliances is because it cuts down on the battery bank. Battery power is expensive and annoying.

You need to get a book on boat wiring and load calculations so you can make some estimates of the "rest of system" implications when you want to add a gadget. Batteries have to be charged. The charging system is a huge part of the cost of battery power. Anytime you can work it out so the power is consumed as it is generated without flowing through the batteries, you should try to do this. The battery charger that is needed to front-end a 3000 watt inverter is more expensive than the inverter.

 

Yah... for your main refrigertion a dc cold plate is the way to go... Keel cool it and you can greatly economize on the coolant circuit energy needed to run the unit. Well insulate the box and make it a top loader and you will cut run time.

Air cooled units are cheap and very compact but heat the boat up. Add in the inverter needed to generate AC then DC looks even better. .

 

fridge heating up the boat is good for at least 2/3 of the year.
I have a midsized air cooled fridge 120 VAC in the boat and dont notice that extra heat until mid summer. Spring, fall, winter is no problem and then the heat is good. Mostly boat sits at dock on shore power. Otherwise 3,000 watt 6,000 watt surge invertor or gen set can run fridge. I think it all depends what type of use the boat will get as to how miserly you should be with power needs.

I have thought if you could add some water cooling to the heat coils on this air cooled fridge that would be worth doing, for the summertime use.

Any ideas on how that could be done?
If you took some 1/4 copper tubing and wire tied to the heat condenser and used a small water pump would that work?
What type of pump, I though perhaps a 12 volt bait well pump could be used. Perhaps run from a small 120 volt adaptor, or rig up a rectifier bridge and resistor? Run the pump when compressor comes on.
I would think it would improve the efficiency of the cooling.
The compressor surface also can get hot.