RV Electric 101 questions

[rich85704]

I've been puzzling over adding an inverter to my rig on a different thread. But in the process, I've had to evaluate what I think I know about RV matters electrical. I'd appreciate comments on any of these. I'm assuming the battery disconnect switch is not in the "store" position and that I have NOT installed an inverter.

1. Dry camping: When just sitting there and not running the generator and not hooked up to "shore power" the 12V system in the coach is running off the battery. AC outlets and appliances don't work. Yes? Assuming a fully charged healthy battery, normal use of the water pump, the (LP Gas) stove and hot water heater and refrigerator, and two or three 12V lights for three hours in the evening, how many days can I camp like this before the battery is depleted?

2. When running the generator or plugged in, AC outlets and devices are live; the battery is being charged and the 12V circuits are live, via the converter. Yes?

3. When on the road, AC outlets and devices are dead and 12V circuits are live and the battery is being charged by the alternator under the hood. Yes?

If I do install an inverter, any AC devices plugged into it are effectively running off the battery. The size of the "draw" on the battery from these devices depends on the devices plus a small amount the inverter draws just sitting there. If an AC device is rated at, say 50 watts, what is the approximate draw on the battery from connecting this device through the inverter.

I know these are real basic questions, but I'm a pretty basic guy.

Thanks!

Rich

[Water-Bug]

To get technical, P = I X E or Power (watts)= Current (amps) X Energy (volts) . Since the ratio of AC voltage (120v) to DC voltage (12 volts) is 10 to 1, the approximate difference in current (amps) is 1 to 10, minus the loss to heat produced by the inverter , to produce the same power. Just where E = MC squared fits into all this is another topic.

[Fishinghat]

wa8yxm wrote:
Fish you typoed and said 50 watts at 120 = 500 watts at 12, you meant to say 500 watts at 120 = 50 AMPS at 12

And I think I was the first to advocate use of 10x or x/10 in these forums since this takes into account several variables like line loss (power/voltage loss in the wires) inverter effiency and such,, Not 100% accurate but as it turns out very very close up to around 1000 watts..

But it was clearly a typo type mistake to me.. (But then I'm the King of typos).

You're right. I apologize.

[wa8yxm]

Fish you typoed and said 50 watts at 120 = 500 watts at 12, you meant to say 500 watts at 120 = 50 AMPS at 12

And I think I was the first to advocate use of 10x or x/10 in these forums since this takes into account several variables like line loss (power/voltage loss in the wires) inverter effiency and such,, Not 100% accurate but as it turns out very very close up to around 1000 watts..

But it was clearly a typo type mistake to me.. (But then I'm the King of typos).

[Fishinghat]

kaydeejay wrote:

Fishinghat wrote:
..........As a rule of thumb, you can figure a draw rate of 10 times, or 50 watts at 120 volts equals 500 watts at 12 volts. Most of us don't use our inverters to run any high wattage 120 volt appliances, like coffee pots, crock pots, hot plates, etc. TV sets, radios, lights, etc. should work just fine........

NO NO NO NO!
50 watts is 50 watts!
At 120 volts that is approx 0.4 amps, at 12 volts that is approx 4 amps.

Your statement is confusing the watts with the amps.
FOR THE SAME wattage, the current at 12V will be 10 times the current at 120V, plus a little for inverter losses.
You COULD have said "5 amps at 120 volts equals 50 amps at 12 volts", which is correct.

My apologies. I over simplified. I'm aware that voltage times amperage equals wattage. Since most people get confused about watts I tried to keep it simple, and that's why I implied the 10X conversion rate was an easy way to figure power usage. Most appliances are rated in watts, not amperage. So, if the owner looked at a coffee maker's wattage and wanted to power it with his inverter, the draw would be 10X the indicated watts from his battery. As for inefficiencies of the inverter and voltage loss from wiring, I just left those confusing factors out - again, to keep it simple.

[tenbear]

1. Think Ampere Hours. One ampere hour, AH, is a current flow of one amp for one hour. Figure the AH capacity of your battery. 2 GC batteries will have a capacity of ~225 AH, one Marine type 29 will be about 110AH. You can draw the battery down to about 50% of it's AH, so you have half of the above available

The refrig will use about 15AH per day. Each incandescent light bulb about 1.5 AH each hour, LED lights about .3 AH each hour. The water pump about 7 AH each hour.

Figure it out.

[wa8yxm]

Answers by the numbers

1: How long will battry last.. Non sufficent informaiton, I have used 12 volt rechargable batteies that range from very small to very large, a small one, a few hours, a 4,000 pound one, several days to a week. Generally, if you are using the furnace, "Overnight" unless you have expanded the battery bank, I would not count on more than one day with a standard bank even without the furnace.

2: yes
3a: Yes
3B: How much draw on the battery.. There is a fairly complex math formula that gives you the precise answer.. Watts/10 is very close

So a 50 watt device sucks about 5 amps from a 12 volt battery.

[kaydeejay]

Fishinghat wrote:
..........As a rule of thumb, you can figure a draw rate of 10 times, or 50 watts at 120 volts equals 500 watts at 12 volts. Most of us don't use our inverters to run any high wattage 120 volt appliances, like coffee pots, crock pots, hot plates, etc. TV sets, radios, lights, etc. should work just fine........

NO NO NO NO!
50 watts is 50 watts!
At 120 volts that is approx 0.4 amps, at 12 volts that is approx 4 amps.

Your statement is confusing the watts with the amps.
FOR THE SAME wattage, the current at 12V will be 10 times the current at 120V, plus a little for inverter losses.
You COULD have said "5 amps at 120 volts equals 50 amps at 12 volts", which is correct.

[pianotuna]

Hi Fishinghat,

Watts remain the same. Amps do go up by a factor of ten

60 watts @ 120 volts = 0.5 amps

60 watts @ 12 volts = 5 amps.

To the OP. What matters most is the total number of amp-hours of storage. Most RV's have woefully small battery banks.

Many folks who boondock have solar systems to help them extend the time between recharging.

Fishinghat wrote:


As a rule of thumb, you can figure a draw rate of 10 times, or 50 watts at 120 volts equals 500 watts at 12 volts. Most of us don't use our inverters to run any high wattage 120 volt appliances, like coffee pots, crock pots, hot plates, etc. TV sets, radios, lights, etc. should work just fine.

[2oldman]

As sh said, watts are the same regardless of the power source.

Using an inverter, the amperage required from the batteries is roughly 10x the amperage at 120vac, because the voltage of a battery is roughly 0.10 of house voltage.

These are good reads:
Battery FAQ
This tutorial may be of some help. 12v side of life.

[MrWizard]

50 watts is 50 watts..
When the voltage changes, the amps change accordingly, but it is still 50 watts

50 watts computer? 0.42 amps at 120vac, roughly equals 4.5 amps at 12vdc input to inverter
It's not directly a ten times conversion of amps , because there is approx 10% increase use/ loss during the conversion
55 watts D.C. input to get 50 watts 120vac output/use

[sh410]

Watts are watts. 50 watts at 120v is a little more that 0.4 amps. 50 watts from the battery at 12v is about 4.2 amps.

[RoyB]

I increased my batteries to 255AHs (three each GP24 85AH Standard Issue Interstate batteries) and installed a PURE SINE WAVE 600WATT INVERTER as close to the battery terminals as I could get it and still be inside the trailer.

We camp out off the power grid alot and this usually gets us from 6PM to 8AM the next morning doing pretty much what we want to do in the evening. I watch the DC VOLTS like a hawk and when it drops down to 12.0VDC I will want to connect my shore power cable to my 2KW Generator and run it for about three hours time span. Using smart mode technology I can recharge my 255AH batteries back up to around 90% state of charge in this three hour generator run time period.

Now we can do this all over again for the next day/night run off the batteries.

Been doing this since 2009 and we are pretty much successful doing it now. Its all second nature what we can run and not run...

Watch HDTV till 11PM, do DVDs, charge all the cell phones and computers, light up the trailer with LED lights, run fans, Momabear likes to have her small electric lap blanket when its cool, light up outside LED lights, actaull about all the things we normally do at an Electric site except NO AIR CONDITIONER or high powered Microwave use.

Once you get the routine down it becomes all second nature...

I even mix in some Ham Radio operations doing this as well...

Got to plan it out however. You just can't show in the woods and think it will work. It got dark on us around ten PM at night a few times until we figured out what we could run and not run off the batteries.

The big deal for us was watching the DC VOLTS on the battery bank. If you keep the batteries up above 50% discharge rate and re-charge the next day then they will not have any harm done to them. We can do 12-14 discharge cycles from 90% to 50% and then we start seeing the batteries not performing like they should. Then we have to do a full 100% charge period which takes around 12-hours to complete.

We are just now getting interested in adding 360WATTS of SOLAR power to use the high sun during the day to get our batteries better than 90% charge state before it gets dark. This should cut down on the generator run time somewhat for the way we are doing it. Looking forward to getting some panels installed to see how it works with normal battery draw we are accustomed to each day.

I said we have a 600WATT PSW Inverter setup but our normal draw is around 300WATTs from 6PM to 11PM and then of course the normal parasitic drain for the whole period. The other 300WATTS is just available if we need it.

I wouldnt go below a 300WATT INVERTER and also would not even consider a MSW type INVERTER when running electronics off the INVERTER. The PURE SIONE WAVE Inverters used to be high dollar but the cost has come down big time on them now. With PURE SINE POWER you don't ever have to worry about burning up some of your high dollar electronics using it. My AIMS 600WATT PSW Inverter is available from AMAZON for $149 the last time I checked. Alot of folks will tell you they do just fine using the MSW INVERTERS but I just never saw the need to take any chances.

Just takes some planning

Roy Ken

[Fishinghat]

I'll try and answer your questions as I understand them.

1. When dry camping, your battery will power everything that is not 120 volt. You may be able to go several days on battery power, or maybe just one night. It all depends on your use level, how many batteries, how new they are, etc.

2. When plugged in, your battery may not be automatically charged unless you have a system to do that. You probably do with the converter. An inverter is normally combined with the converter so it operates automatically.

3. Yes, you won't have 120 volt unless you run your generator or have an inverter.

As a rule of thumb, you can figure a draw rate of 10 times, or 50 watts at 120 volts equals 500 watts at 12 volts. Most of us don't use our inverters to run any high wattage 120 volt appliances, like coffee pots, crock pots, hot plates, etc. TV sets, radios, lights, etc. should work just fine.

On edit, this was not included in your question(s), but two 6 volt deep cycle batteries for the house are preferred over one 12 volt. Also, your converter probably doesn't charge the engine battery. You would have to install an automatic trickle charger to do that. I didn't cover your question about the electrical draw of an inverter, but it does draw some power even while not powering anything. You can shut if off when not in use. (I have never turned mine off in over 10 years, but I have four 6 volt house batteries and I'm connected to shore power except when on the road and dry camping.)