calculating inverter use

[buck454]

What is the proper way to calculate the safe use of inverter for overnight use. Lets assume one has a 2000 watt on their motorhome and camping in the mountains with a generator curfew. Wife wants to watch her HD TV for a couple of hours and run a small fan all night. How do you know you are safe. Fans may not always give wattage but might just read 120 v 0.65 A 60 HZ, TV I guess may vary from model to model.

b

[gcampbell]

A voltmeter is the way its mostly done. Some folks say that when your batteries get down to 12.4 volts that's when to charge them up. I am going to start going down to 12.2 volts. I had a good talk with a Xantrex tech the other day and he was reassuring that this wouldn't harm my battery banks at all and they would last just as long. This is a difficult topic because batteries do things differently than most other electrical things.

[SUMDALUS]

buck454,

There are two different ways to test for battery state of charge voltage.

One is called the Open Circuit State of Charge. The battery bank must be disconnected from any load or charger before testing. The battery bank should "sit" for 6 to 12 hours. A 50% discharged/charged battery bank will register about 12.24 VDC on a volt/ohm meter.

The other is called an Under Load State of Charge. A little more useful when reading your voltage level at the inverter panel. A 50% discharged/charged battery bank will register about 12.06 VDC at your panel. Remember, this is under a load. This is why gcampbell's Xantrex tech said what he did.

Also, if the wattage of an appliance (e.g. fan) is not specified, just multiply volts times amps to get your wattage number. In your noted case -- 120V X 0.65A = 78W. More about Volts-Amps-Watts at this blog.

Besides your inverter manual, perhaps some other iRV2 members can recommend some simplified "inverter" books.

Hope this helps.

[buck454]

S, great information. Here's another question, probably once again showing my ignorance on matter (college physics was over four decades ago), but in example of fan, is the 78W per hour meaning one could safely run the fan via 2000W inverter for up to 20 hours. I realize that during a 20 hour, even 12 hour, period other little RV devices might be used briefly (fan on refrig coils, light on ging to bathroom, night lite, etc). If I am figuring correctly one is safe with fan over night but more than a couple of hours of television might be pushing it.

[FDNYdave]

Quote:

Originally Posted by buck454 S, great information. Here's another question, probably once again showing my ignorance on matter (college physics was over four decades ago), but in example of fan, is the 78W per hour meaning one could safely run the fan via 2000W inverter for up to 20 hours. I realize that during a 20 hour, even 12 hour, period other little RV devices might be used briefly (fan on refrig coils, light on ging to bathroom, night lite, etc). If I am figuring correctly one is safe with fan over night but more than a couple of hours of television might be pushing it.

The size of your inverter is not what you need to be concerned with, you want to know the size of your Batery bank in Amp hours and how many Amp's/hr your inverter uses to run your TV and fan.

[SUMDALUS]

buck454,

Check the wattage rating of your TV. Most are around 200 -250 Watts. Low compared to the microwave, toaster, or coffee pot.

Some inverters have a "safety" setting which will shut off the inverter if the level of the battery charge gets too low. Does yours?

A great deal about how long you can run the inverter has to do with the total amp hour capability of your battery bank, the age and condition of your batteries, as well as the state of charge.

Trying to explain how an inverter "works" (as well as the battery system) is a little problematic in a simple forum such as this. Hence, my previous suggestion about finding a good resource book. Unfortunately, mine is in the works, but not published, yet.

Hope this helps.

[RJay]

buck,
Keep in mind that your primary source of power are your batteries, your inverter is simply a medium to to get the power from your batteries to your AC appliances. Using your example of your 78 watt AC fan will draw 0.65 amps from the output of the inverter so the batteries will have to supply 7.2 amps to the input, assuming a 90% inverter efficiency. If your battery bank has a capacity of 400 amp/hrs your fan could run for 28 hours before your battery bank would reach 50% discharge.

[CraigAllegro]

The deep cycle batteries can be discharged deeply. The more frequently you do it the shorter the life span of the batteries. Here is some gleaned information. By the way when you check your voltage make sure you are not discharging or charging the battery while taking the measurement. The other thing that is important is this
Voltage X Amperage = Wattage

Wattage is king and remains constant - where it is really important is that if you are on crappy shore power and are using a 1500W heater and the voltage drops to say 97V then 1500/97=15amps = breakers breaking!

Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell.

Shallower the average depth-of-discharge (DoD), increases the battery life. For example, a battery with an average of 50% DoD will last twice as long or more as an 80% DoD; a 20% DoD battery will last five times longer than a 50% DoD. For example, golf cart batteries will average 225 cycles at 80% DoD and increase to 750 cycles at 50% DoD. Try to avoid DoD that is less than 10% or greater than 80%. Industrial traction and stationary deep cycle batteries are designed for 80% DoD and most marine an RV deep cycle batteries are designed for 50% DoD.

[Dave78Chief]

The approach is:

AC Wattage = 120 x 0.65 = 78 Watts
That must be converted to DC usage plus account for converter efficiency
DC amps = (78 watts / 12VDC) * 1.1 (efficiency) = 7.15 DC amps
The fan will be drawing 7.15 DC amps per hour (Rjay rounded up to 7.2)

Now for Fan plus HD TV (assuming 250 Watt TV)
78 Watts (Fan) + 250 Watts (TV)= 328 Watts
DC amps = (328 watts / 12VDC) * 1.1 (efficiency) = 30 DC amps
The fan plus TV will be drawing 30 DC amps per hour

What your forgetting is lights, Refrigerator controls, Water Pump, Microwave control, etc. All of this has to be added up to determine total usage. You have to account for both AC and DC loads. Once you know your usage needs, you can then determine if the battery you have is big enough or if you need to either cut back or add battery storage capacity.

Dave

[buck454]

Thanks all, I am learning a lot here. I am currently just not up on inverter use. When camping near ALBQ for one of the big matches I attend the nights are very nice at 6500 ft but my wife likes a little fan going so in the FW I have been running a small DC fan with no problem. With the purchase of the new MH I have comfort (I think) of Inverter so I am wondering if I can upgrade to common AC fan. Sometime it is a little trying to make her happy in dry-camp situation, especially when there is a generator curfew keeping her from watching TV til 11 or 12. ha.

When I pickup MH I have noted to inquire about battery bank, capacity, etc.

b

[Dave78Chief]

If you do a lot of dry camping then it is worth it to purchase one the RV Electrical books. Contains several AC/DC reference tables that will allow you to calulate your usage so you can determine what your battery size needs are.

Summer Breeze - Oh - Managing 12 Volts - Repair Books - Camping World

Dave

[jspande]

There are a couple things that could reduce your power under this usage.

First, if your TV is an LCD (or LED) the power consumption can probably be significantly reduced by adjusting the picture settings, and you'll actually improve the picture quality to boot. Typically TV come set up to "grab" a customer when on display in the store - brightness and contrast are often way too high. You can either have the store "calibrate" the picture (I believe Best Buy charges $300 or so for this), or you can do a Google search for your brand and perhaps find guidance for settings. I did the second way on my Samsung 40" LED a couple years ago and really like the results (memory fails, but I believe power consumption was decreased around 40% - don't hold me to that, my memory is pretty fuzzy - and the picture is much easier on the eyes for extended viewing).

Second, using a large inverter for small loads is inefficient. They have a "baseline" power consumption that is constant regardless how small the load, which is probably around 15 to 20 watts on your 2000 watt unit. So when you're running just your 75 watt fan, you're actually pulling 90 - 95 watts, and your actual efficiency may be only around 70 - 80%. It might be worth it to get a small inverter just for the fan (make it SW, a MSW inverter is inefficient running electric motors). Or get a 12vdc fan.