Power Inverters

[MalibuC]

So here goes, having fulfilled a (nearly) life long dream and purchasing my first sailboat, I am undergoing several lifestyle modifications switching from power boat life to sailing life. None of these changes are unwelcome mind you, however, painful none the less. After a night of refueling my generator twice, I have been searching for a way to break away from AC. In this process, I have (or think I have) found a way to break free from my generator as well (at least away from running it all night).

To get any sleep at all, I need to be cool. So I purchased a high velocity fan to create gale force winds in my sleeping berth. The plan was to power this off the generator as well as whatever else is on board. Upon further research, the idea of installing a power inverter to run the fan, without the generator during the night while using the engine or generator to recharge my batteries as needed during the day.

I need some assistance in confirming the power usage estimates and deciding on the type/size of inverter to use.

The fan operates at 1.3 amps which is roughly 150 watts. I divide 150 watts by 10 to determine the amp draw on the battery through the inverter. This comes to 15 amps per hour times 8 hours = 120 amp hours. The equates to almost half of the amp hour ratings on my 2 battery house bank. So what I am thinking is that I should be fine as long as I recharge my batteries daily.

Someone that is in the know about calculating power load/drain using power Inverters check my math and let me know if I'm in the ballpark. Also any feedback regarding inverter use/selection/installation would be appreciated.

 

[Benny17441]

I ran the numbers slightly different than you but we came up with the same approximate result. 1.3 amps at 120V X 10 = 13 amps at 12V. At 85 to 90% inverter efficiency you may be looking at 14.3 to 15 amp/hour. In 8 hours you will consume close to 120 amps as you had also calculated. The kicker is that an electric motor can require up to 5 times its rated power at startup. That may be up to 75 amps for a couple of seconds but translates to the output of a 900 watt inverter. Perhaps with fully charged batteries you may get it to kick over with a 750W inverter but I would consider that the minimum. For maximum efficiency it is recommended that you match the load to the size of the inverter but when talking about electric motors their startup requirements make that very difficult to achieve. You now have two other issues to address; 1) an inverter larger than 400W needs to be hardwired and fused and it is also recommended that it be installed as close to the batteries as possible. You can run an extension cord on the 120V side but the 12V circuit should be as short as possible. 2) According to the figures you will be likely exceeding the recommended 50% threshold for discharging your batteries. Deep discharges reduce battery life. It is very hard to recharge batteries to their maximum capacity especially as they age so I would not count on having the rated maximum amps from the batteries. A large discharge rate, ambient temperature and other factors also affect battery performance. I would definetely add a third battery to the bank. Mind you that these batteries should be connected together in parallel when working the fan for extended hours.

Looking at costs you are probably looking at approximately around $350 to cover the cost of an inverter, wiring and fuse assemby for the installation and an additional battery. Mind you an inverter capable of running delicate electronics will run you more than the one I have considered for the fan in my estimate. I have seen some 1000W inverters from $130 to $150 in auto part stores and discount stores. Just some food for thought.

 

[rbietry]

I found the following website:
http://www.lifelinebatteries.com/marinesizingac.php
It is by Lifeline Batteries and it is a battery size calculator for inverters. It takes all the work out of the problem.
They also have another link:
http://www.lifelinebatteries.com/marinesizing.php
It will do the calculations for your DC load.

 

[rochesterdriver]

I bought two 12v fans from an RV supply and they certainly do the trick for me.

I'm just sayin'.

Rich

 

[MalibuC]

Thanks for your comments Benny. I feel a lot more comfortable with my math on this and with my choice to utilize an inverter. I picked up a 1000 watt inverter with a 2500 watt surge from west marine today and am getting ready to install. After further consideration, I will add, not one, but two batteries to my bank as I also have an Adler fridge conversion on the boat as well. I haven't pulled the documentation yet to see what it will draw but I am confident a 4 battery bank with a total of 500 amp hours will do the trick.

Thanks again for the comments/advice. Greatly appreciated.

 

[MalibuC]

I bought two 12v fans from an RV supply and they certainly do the trick for me.

I'm just sayin'.

Rich

Rich, let's just say I'm a big man and those small 12 V fans don't cut it for me!

 

[MalibuC]

I found the following website:
http://www.lifelinebatteries.com/marinesizingac.php
It is by Lifeline Batteries and it is a battery size calculator for inverters. It takes all the work out of the problem.
They also have another link:
http://www.lifelinebatteries.com/marinesizing.php
It will do the calculations for your DC load.

Thanks for the link, checked it out and it did the math for me...happy to see I came up with the same answer as the computer. Now I can start adding up the usage for each of the electronics I will be using onboard.

 

[MalibuC]

Here's my next question:

IF I fuse the connection, is there any reason I can not use a double ended plug to connect the inverter into one of my 110 plugs and power the entire boat rather than running extension cords?

I know you can hard wire the inverter directly through the outlets. And I know that anything plugged into the 110 outlets can create a drain if it draws power such as a clock. I also know that the shore power can not be connected while the inverter is in use.

Outside of these considerations, can/should this be done?

 

[Maine Sail]

Here's my next question:

IF I fuse the connection, is there any reason I can not use a double ended plug to connect the inverter into one of my 110 plugs and power the entire boat rather than running extension cords?

I know you can hard wire the inverter directly through the outlets. And I know that anything plugged into the 110 outlets can create a drain if it draws power such as a clock. I also know that the shore power can not be connected while the inverter is in use.

Outside of these considerations, can/should this be done?

You don't want to do this if you also have a shore power inlet. You want an inverter transfer switch so you can't inadvertently have two "sources". The easiest way is to wire your inverter is directly to it's own outlets.

Remember even with 500 Ah's you'll only have about 250 Ah's usable and if off cruisng even less becuase you're rarely recharge back beyond 80-85% state of charge unless you really run the motor for long periods.

Using that much energy will take a lot of resources to put it back in.

 

[MalibuC]

You don't want to do this if you also have a shore power inlet. You want an inverter transfer switch so you can't inadvertently have two "sources". The easiest way is to wire your inverter is directly to it's own outlets.

Remember even with 500 Ah's you'll only have about 250 Ah's usable and if off cruisng even less becuase you're rarely recharge back beyond 80-85% state of charge unless you really run the motor for long periods.

Using that much energy will take a lot of resources to put it back in.

I guess the shore power outlet would be "hot" if I did it this way. I will still carry a generator to recharge as needed when I don't run the engine. I'm not sure how long under generator it will take to recharge the batteries to full. I'm sure there is a formula for this.

I have a 30 amp charger. Does that mean it puts 30 amps in per hour? If so it will take 5 hours to put 150 amps back in after running my fan for 8 hours. That's certainly not feasible. Perhaps my chargers owners manual will provide info on how quickly it will recharge 4 batteries.

I am going to look into the inverter transfer switch option.

Thanks Maine

 

[Maine Sail]

I guess the shore power outlet would be "hot" if I did it this way. I will still carry a generator to recharge as needed when I don't run the engine. I'm not sure how long under generator it will take to recharge the batteries to full. I'm sure there is a formula for this.

I have a 30 amp charger. Does that mean it puts 30 amps in per hour? If so it will take 5 hours to put 150 amps back in after running my fan for 8 hours. That's certainly not feasible. Perhaps my chargers owners manual will provide info on how quickly it will recharge 4 batteries.

I am going to look into the inverter transfer switch option.

Thanks Maine

It will actually take longer than 5 hours to fully recharge your batteries. A 30 amp charger can put out a max of 30A. As batteries become more full they begin to accept less and less current. At 80% +/- of charge is when the batteries will start accepting less and less (dependent upon charge rate) until they are taking about 1% - 0.5% of their Ah capacity. The last 10-15% of charge can take many hours to get back in. Most cruisers cycle their banks between 50% SOC and 80-85% SOC...


You might look into running two or three Bora Caframo 12V fans.... I use a Vornado at home but the Bora's do fine on the boat..

 

[rochesterdriver]

What a great idea, why didn''t *I* think of that.

Not all 12volt fans are 'small'.

 

[Fly_H23]

I think I would look for a more efficient fan. Sounds like a power hog to me. My 12 volt fan only pulls 15 watts.

 

[Claude L.-Auger]

FWIW, I connected a Marinco cable to the outlet of my 1500W / 3000W surge inverter. The other end of the cable is the outlet to connect to the Marinco 110V inlet.

This way I can have 110V throughout the boat through the normal circuits. And I cannot accidentally connect shore power & the inverter together. It has to be one or the other.

But you will still be putting your batteries through hell if your run them through deep discharge night after night. Inverters are great for a short period of time but as Benny and MS posts mention, you will not be able to recharge them 100%, and eventually they will not be able to give you their full power.

Looking into different fans might be one way. I have a small fan, but if extremely hot, I will either shower or go for a short swim (I am in freshwater) before going to bed, which helps cooling me off. Good luck