Electric Usage Questions - Fiberglass RV



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Old 06-22-2019, 05:25 PM   #1
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Electric Usage Questions

I want to see what my power needs really are. Anything hard wired into the camper has been pretty easy. Where I'm getting confused are with cell phone and laptop chargers, and anything with a USB plug, run through a cigarette lighter style charger.

They're all taking 12V power from the battery, but...The laptop charger says it converts everything to 16.5V. For amp hours, do I divide watts by 16.5, then, not 120? I'm running it through an inverter (still need to figure the draw of the inverter itself).

For things run through the cigarette charger: it says it has an output of 2.4 amps, 5V per slot. What does that mean? Do I ignore the amps/watts of the appliance, and just say that anything running through this charger is using 2.4 amps?
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Old 06-22-2019, 05:53 PM   #2
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What you actually use is wattage. If you are charging the computer with either a 12V power supply or with an inverter to make 120 volts & use the power cord that came with the computer, you are getting the power from your battery (or converter if plugged in), so it makes the most sense to use 12V to determine the current.

That will still not give you an accurate measurement of actual usage because the computer will draw different amperages depending on the state of charge of the battery, and even how hard you are making the processor work.

The only way to really know how much 12V current you are using is with an amp meter in series with the load or a battery monitor. The amp meter will tell you the instantaneous current; multiply that by time & you get the amp hours used. Amp hours is how your battery capacity is measured, so it is the most useful information to know. A battery monitor usually gives you both instantaneous amperage as well as accumulated amp hours.

The USB outlet that is rated at 2.4 amps is the most current it can supply. Most cell phones draw less than that, while some tablets and other loads can draw more. For example, if your cell draws 1 amp of charging current, even though the receptacle is rated at 2.4 amps, it will only produce 1 amp. If you have a tablet or other load that tries to draw 5 amps from that particular USB receptacle, it will only supply 2.4 amps.

Again, since power for the USB receptacle is supplied by your battery, if the cell phone charger example used earlier draws 1 amp, the wattage will be 5V X 1 amp for a total of 5 watts. The input to the receptacle is 12V, so the amperage at 12V will be a bit less than .5 amps. All this is ignoring the efficiency losses, which can be as much as 10% - 20%.
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Old 06-22-2019, 06:58 PM   #3
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Get a "Kill-A-Watt" meter. Put it between your AC supply and your trailer cord. Note the wattage used by the converter at idle when the trailer battery is fully charged. Turn on your loads one at a time to see the increase in watts used. The "Kill-A Watt" can even track usage over time. If your AC supply is from an inverter, divide watt-hours used by 10 to get an approximate idea of amp hours needed from your battery, which includes conversion losses.
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Old 06-22-2019, 07:17 PM   #4
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I don't see how someone could actually calculate the actual usage by simply looking at all of the loads and all of the time they ran. Loads are variable, efficiencies of inverters varies, standby losses on inverters adds up even when no loads are running, and batteries will produce less overall power as the loads get larger. Then there are phantom loads we may not realize, such as CO detectors.

Then, if the goal is to size the solar charging system, you have to factor in collector orientation, which will cost you more than 50% of rated output if the panels are sitting flat. And lost energy due to charging inefficiency This can be another 20% with flooded batteries.

Even if you have a battery management system that calculates and displays all watt/hours out and all watt hours in, it still may not factor in the charging losses. Or you might get fooled because the battery is partly sulfated.

If you just go about your normal usage, and you have a system that records all watt/hours out, you will know what your usage is during that period. Then, if needed, you can look for ways to reduce consumption, such as using a smaller inverter, or using DC when the generator is running, or charging the phone in the tow vehicle, or reducing the lighting, etc. Or you can increase your solar capacity in a practical way and meet the goal of keeping up. Then you have to decide where the panels go, what type are they, ground or roof system, etc. And as you try to bring the charging capacity and the loads into agreement, be sure you don't add more loads and still not have enough capacity.

If you have a new battery with a certain rating, you can start from a full charged state and see how long it takes to reach 50% of capacity. Then you have a pretty good idea of use over time. And it takes into account all loads, no matter how variable they are. and all of those loads at the discharge rate you were actually applying to the battery. And in reverse, if you can measure how long it takes your particular charger to return your battery to "float" by recording all the power "in" it took to do that, you'll see a discrepancy and that will be approximately the charging inefficiency of your battery. That loss must also be made up by your solar, or other charging system. In other words, the charging inefficiency is a load. The battery itself is a load.

Some of this is like trying to figure out how fast your car is going by how much fuel you're burning. It might work, but it's faster and easier to simply look at the speedometer. This is because of the large number of variables involved. We may guess that only about 20% of the energy in the fuel is producing useful propulsion, and that we are getting about X MPG. So we can figure our general fuel use over trips. We can also begin to see how may watts of solar collectors, installed in our particular way, will result in the lifestyle we are happy with. Watts of solar = Miles per gallon. Lifestyle = trip length in the car. Each day, we need X watts of power. Power, that is, in the form of theoretical panel watts. Just as we don't know what all of the inefficiencies are in the car, we don't know what all of the inefficiencies are in the solar either. So we have to just go for it and refine later.

This all sort of comes down to playing it by ear, getting the feel of the system, etc. And then a cloudy day will happen and you'll have less than you expected. I think usage will always tend to rise to meet capacity, which means we are always operating with less than we'd really like.

I tend to use a lot of power if I think we'll be leaving tomorrow or the next day. Why be careful if the tow vehicle will be charging the batteries for hours the next day? But if we have no generator, are parked in the trees, it's cold at night, and we'll be there for a week, I have to be very careful. I just got told at Yellowstone that I could not charge my trailer from my truck! I deliberately left the generator at home and we were in continuous shade.

My sliding scale is: I can use all I want for a day, or even two. If three days are predicted, I have to plan to be a bit economical. Meaning limited heater. A week and there will be no heater. But usually, I do charge the batteries from the truck and continue to use what I want.

So, I'm not sure knowing all the loads is as important than knowing how the total system generally matches the lifestyle and where cuts can be made to extend as needed without losing vital equipment. Or, add a percentage more of solar to give you an extra day. You'll never really know the real numbers, but you will get by just fine and can trend in one direction or another. But you can't just add (2) 100 watt panels and expect 200 watts of power for 8 hours.

I started out with one 50 watt panel and one Group 27 deep cycle battery. The wires were connected to the battery with simple alligator clips, no charge controller. I disconnected them at night. And I had an amp meter in series on the + line. I loved that system!

While living on my boat for many years, and going to Mexico on it, I found ways to minimize generator usage and get the same results. One was to only charge enough to get the battery through the bulk phase where the most amps were being pumped in. Another was to charge when motoring, or motor to charge, at the best times, instead of running the generator while at anchor. And running heavy loads, such as the water maker only when the engine was running. Another trick I discovered was to artificially increase the 60 cycle rate of the generator output to 62 cycles. This is done by adjusting the mechanical governor on a non-inverter style generator. This made the charger go to a much higher output than it would have been doing on a partially charged battery. Not more than the generator could handle, but more than it would normally be doing at that state of charge. In a sense, an artificial "bulk" charge. I also tapped into the alternator regulator on the house battery alternator, and was able to hot rod it by sending more exciter current in. This overrides the natural scaling back of the charge rate as the battery comes up. Since I had (4) 8D batteries, they could tolerate a higher than normal charge rate, which lowered my run time. Again, an artificial "bulk" rate, that significantly reduced my overall charging time. Then, when motoring for hours, the batteries could reach full charge and avoid sulfation by letting the charging system operate normally.
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Old 06-23-2019, 08:07 AM   #5
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Well that's a lot (of good info) to digest. Thanks! I understand it's a moving target. With my laptop, I saw that there's a range from around 120w to 300w or so, depending on how hard I'm making it work. I'm taking all the numbers I can find, and using close to "max" amounts. I figure having a number puts me closer to ballpark usage than not going through this exercise at all.

I'm currently using a small, 150w inverter which plugs in to my cigarette lighter socket in the camper. I'll read these posts closer later today and see if there's a better way to go about this than what I'm doing. I understand nothing is going to give me some "absolute" number of what I use per day. But right now I'm completely in the dark as to my power use. So yeah, getting a number, along with an understanding of the amount of error involved, is going to be important. I don't need this to be perfect, just ballpark. I also don't plan to spend much money doing it. I saw a battery monitoring kit online for something close to $300 when you include the monitor and display. It looked like an excellent device and I'd love to have one. But with how low my typical power use is, there's just no way I'm spending that kind of money. That's more than the solar system I'm installing.

I'd like to get an amp hour number that gives me some general idea of what kind of storage and charge I need to do what I do.
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Old 06-23-2019, 12:11 PM   #6
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Raspy offers some excellent advice on living with solar electricity. Nicely done.



And it can also be educational to start measuring voltages and currents and becoming familiar with the numbers you should be seeing. You can always use an analog or digital multi-meter to directly measure DC current by putting the amp meter section in series with the load. (You can also use a shunt to measure the amps indirectly by spending more money.)

Harbor Freight and Ebay and etc. sell really cheap multimeters (in fact you can get one free with purchase sometimes) you can fool around with without spending a lot of money. Be sure to find out how many DC amps the thing will measure before buying. 5A DC would be an absolute minimum, 20A DC would be great but the meter will cost more money. A cheap meter is good to start off with while you learn to use it.

The DC Amps measurement is usually a special set of meter lead connections and the maximum amps you can safely measure is printed on the meter face. When you are not sure what the actual amperages you want to measure are, put a little automotive blade fuse holder in front of the meter with a fuse sized to blow before the meter capacity is exceeded, and have some spare fuses on hand. Some meters have an internal fuse, and some just burn out if you put too many Amps through. It's easier and cheaper to have your own fuse holder set up and make sure the fuse is smaller or equal to the meter amp capacity.
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Old 06-23-2019, 12:31 PM   #7
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If finding a way to put the standard multimeter in series to measure amperage is too much trouble, an inexpensive DC clamp on meter is available from Sears.

It goes around ONE wire of the circuit you want to measure.
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Old 06-23-2019, 05:18 PM   #8
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I guess so far I’ve been doing this as though it was abstract. Like I didn’t already have a camper. If I were camper shopping and looking to have a solar setup ready for when I bought it, then doing calculations the way I’m doing them would be the only way get a reasonably sized system. But...yeah since I’ve got the camper and use it, I might as well monitor my actual use rather than add up draws based on the stated power use.

I’ll decide on the best way to do it. I’ve honestly never seen my charge controller switch from “charge” to “full”. Not sure if that’s the truth, or if it’s just a cheap controller. I ordered one of the battery monitors you linked in another post, so that will make testing my battery state easier. For whatever reason, sometimes pulling the front cover off the battery and taking a volt reading is more than I feel like doing!

I already get by with nothing but solar to charge my battery. But I figure it would be nice to go through this excercise.
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Old 06-23-2019, 05:49 PM   #9
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power supply watt ratings are the MAXIMUM, not the actual usage.

I've never seen a 300 watt laptop, that would need *HUGE* batteries. most are 30-50 watts when running, maybe max 90 watts when charging the laptop battery concurrently with running a cpu/graphics heavy workload. My wife's work-issued Core i7 based Dell Precision Workstation has a 135 watt power brick, but that powers the dual monitor port expander as well as the laptop itself.

The VICTRON BVM-712 battery monitor with bluetooth is what I'd likely get. I would hook its shunt up in series with my batteries so all power into and out of the battery goes through the monitor.
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Old 06-24-2019, 06:48 PM   #10
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You're right John. Looking (googling) again, I can't find those numbers. I was on the apple site, reading specs of all the different year MacBooks...and thought those were the numbers I saw. I can't seem to find that page now. What I'm seeing now is more like 19-53W depending on what the laptop is doing.

So Raspy (sorry, too many John's around here!) what would you tell someone who wants to calculate their use? That it's a worthless exercise? I understand use varies, sun varies, etc. Is the only way for a person to get into solar in an educated manner to spend time in their camper watching their battery level drop, calculating sun angles, figuring out how much charge their alternator will give them vs what if they don't move for a week and playing with inefficiency levels of different systems? Is it not a worthwhile process when determining battery size and solar power to calculate draws using the stated power draw, based on the maximum I might use, like a cold, dark day? The point here is to give me a ballpark number of how much energy I'll use if I'm running my heat as much as I ever have, and the lights, and maybe a fan and charging a couple things while I'm at it. From there I can say "well I better not use that much power then because I can't afford the system to supply it!" etc. But it gives me some benchmark. "This" is how much power you may be using, maximum.

In the end, I can fit a group 27 battery and I have a 100W solar panel ready to install. This is the system I'll be going with and won't be modifying my battery tray or adding more solar anytime soon. But it's still really tempting to go through the exercise. Is it completely useless?
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Old 06-24-2019, 08:44 PM   #11
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Zack,

Calculating one's usage is important. Calculating it how, is where it gets interesting.

Then, aside from what your loads are now, how might they change, through necessity or by choice.

How much surface area is available for collectors and how much power does that represent?

How does one deal with handling and transporting more collectors if needed?

What is the maximum size of the budget for buying more collectors.

Can the collectors be used effectively where we want to camp? (forest, fog, or open high desert)

So, maybe it's a good idea to make the best guess you can. Err on the side of higher than expected loads, don't be unduly optimistic.

Then calculate the theoretical output and cut it down by half or less to compensate for the inefficiencies.

Then put it together and see where you are.

And remember, usage will rise to meet output. So, in a perfect world, we'd just fill the roof up with collectors and use all we could. Eventually the electric stove and electric water heater will max the system out. But somewhere in the middle, with a small area of roof, we are usually able to find a very nice system that serves us very well. especially when running modern laptops and phones that give enormous value with little power draw.

Bottom line, I have no idea how to accurately design a real world system that will do a specific job reliably while camping in various locations. But that is what batteries are for. They can extend the use to the next charging event.

Put 1-200 watts of suitcase solar into at least two large deep cycle batteries. I prefer four batteries. Charge with the TV when driving. And see where you are.

Or build a roof system and realize it will only put out 1/2 of the ground system that can be tracked and tilted by hand. It's harder to build, so do the suitcase system first.

I know I sound like a broken record, but getting really close with calculations seems hard and then your needs will change. A 200 watt suitcase system is a pretty big system and should support a fair amount of use. This would be equal to at least 400 watts of flat rooftop panels. Extra power during the day will translate into more use from the batteries at night, or making it through a rainy day, or a camp day under a tree.

Solar started out more as a supplemental system, that reduced generator use. Now it's transitioning into the main power source and the transition is hard to design.

I'm still trying to understand why AGM batterines are only 100 amp/hour, while a roughly equivalent size flooded battery is 200-225 amp/hour.

We have to start somewhere.

With solar thermal systems that I designed and built for years, I did extensive testing to find out how many BTU's per sq ft was a realistic output number under various conditions, or the max available under the best of conditions. Then I identified lots of variables and factored them in as needed, Such as orientation, climate, elevation, storage size, average temperature (an important one), transfer media, heat exchanger design, shade, family use patterns, etc.

Eventually, I would come up with what I thought was a practical system for a given family. Then I broke that down into packages of various sized tanks and various size and design collectors. Then I could offer these packages and discuss the various merits of each. And get homeowners involved in the decision.
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Old 06-24-2019, 09:09 PM   #12
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Thanks again John. Not sure if you’re speaking generally or to me. I have one group 27 flooded cell battery, and have been using a suitcase solar setup of 90w for the last 3 years. Seems to work fine, but I don’t monitor my battery close enough to know.

What I was looking to do is add up all my appliances, at their max and my max use, and see how many amp hours I’m looking at. Then I’d decide what size battery I need to fill those needs (only drawn as far as 50%). How many days would be limited by the size battery I can fit, and the type I can afford. Given what that info tells me, I would need to make up the difference in solar and reduction of use. I know that’s not a perfect way, but is it a reasonable way?

My needs are small. I have no idea of my daily power use. Only that a single 90-ish amp hour battery and 90w of suitcase solar, out however often I can get it out, for however long it’s in the sun (sometimes I’m gone all day, sometimes I’m around to track the sun), has worked. Judged only by the fact that on my very inaccurate camper display, my battery only occasionally hits “fair”, and very rarely “weak”, and lasted me 3 seasons before I apparently fried it hooking it up backwards, and so I’ll never know how well I was really doing.

The volt monitor I bought will give me a much more accurate look at how my battery is doing, so that’s a good start.
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Old 06-24-2019, 10:31 PM   #13
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Zach,

Sounds like you are doing it right, and working your way into it, in a smart and cost effective way.

So much of accomplishing things is making your best guess, analyzing the result, and discovering a better way to look at the problem. Then making another best guess. Etc. Iteration. Solar electric and solar thermal are two excellent applications for that approach. I enjoy discovering what went right and what did not.

The tools we gain from that curiosity based, and hands on approach, are very valuable later and in other fields too. And camping, should never be approached as an exact science.

Sounds like you're having fun learning, and working out the best compromise.

My new HQ19 has 300 watts of solar on the roof (theoretical max and nowhere near the actual). I carry a 100 watt suitcase. Most of my method in the past has been to have a huge battery bank and try to make it to the next move, to another spot, where the bats get charged on the way. Now, I'm planning more leisurely stays, with the new trailer, and managing the solar to make that happen. The generator will stay at home most or all of the time.
Yosemite could challenge the theory, with it's dense forest, but the high desert will cause the system to laugh at small loads. The desert invites longer stays with its hot springs, captivating beauty, and mystery.
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Old 06-25-2019, 08:31 AM   #14
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That's the next thing I need to tackle...my truck doesn't charge the battery at all. It registers that it's "charging" on my camper display, but I can drive for 10 hours, and once I disconnect from the trailer, there's no increase in battery charge. Clearly the wires are undersized but I haven't taken the time to figure out where the weak link is, or how to run larger wire.

I'll keep adding my theoretical usage up, and go from there.
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