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Old 02-05-2018, 01:06 AM   #1
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Inverter and Battery Capacity

We are working on our build sheet. Just got our hatch date moved up a month as they have finished their capacity upgrades so we are now under the gun to get the details finished.

My question is on using a 1500W inverter with just dual 6V batteries. I have read on threads the stock 6V batteries Escape supplies are good for about 232 AH. Researching solar in general and inverters I see there are two issues you need to worry about, not depleting your batteries below reasonable levels like 50% but also not drawing, or charging for that matter, at too high of levels. I keep seeing the C/8 rules suggested for the maximum current draw where C is the AH capacity of the battery bank. So with a 232 AH battery bank that would only be 29 amps. For a 1500W inverter, assuming 1500W is the inverter output with a 85% efficiency that is 1,765W input which at 12V is 147.1A WAY over the recommended 29A. Applying the C/8 rule to the 147.1A and you need a 1,177AH battery bank, a trailer full.

This is probably a continuous rating but still we are talking about using 5 times more current than recommended for the battery. Am I looking at this right and does anyone have any additional info. Based on this info, it seems using a 1500W inverter is just signing your batteries up for a short life.

Thanks, Dave
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Old 02-05-2018, 05:19 AM   #2
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Go here for some guidance parts 1 and 2....The 12volt Side of Life (Part 1)
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Old 02-05-2018, 09:00 AM   #3
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My experience with an inverter of that size is: It does not function when the battery drops below 80%, this is using the Escape provided microwave which I believe is 700 watts. Your mileage may vary depending on the age of batteries and other simultaneous uses of power. Now, this is not all bad, as the microwave still functions, it is just a limitation of how much battery they draw at once. It does have an implication to what you can microwave, warming up soup is different than cooking a bag of mini potatoes.

We boondock extensively and use the microwave mainly for vegetables, those frozen packages, whole potatoes, packaged and prepared potatoes. It works fine if you have a full battery.

None of this has an effect on battery life, the inverter will not operate at low battery capacity,
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Old 02-05-2018, 10:05 AM   #4
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My experience with an inverter of that size is: It does not function when the battery drops below 80%, this is using the Escape provided microwave which I believe is 700 watts. Your mileage may vary depending on the age of batteries and other simultaneous uses of power. Now, this is not all bad, as the microwave still functions, it is just a limitation of how much battery they draw at once. It does have an implication to what you can microwave, warming up soup is different than cooking a bag of mini potatoes.

We boondock extensively and use the microwave mainly for vegetables, those frozen packages, whole potatoes, packaged and prepared potatoes. It works fine if you have a full battery.

None of this has an effect on battery life, the inverter will not operate at low battery capacity,
I need to reread my inverter/EMS book(s). There's plainly more I need to learn. But please expand on "not" if possible. We don't have a m'wave, so even with recharging our e bike along with some led's on and keeping our propane water heater, fridge, and heater instrumentation working, we can't envision using more than ~250 watts. Will that be a problem at 80%?
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Old 02-05-2018, 10:07 AM   #5
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I need to reread my inverter/EMS book(s). There's plainly more I need to learn. But please expand on "not" if possible. We don't have a m'wave, so even with recharging our e bike along with some led's on and keeping our propane water heater, fridge, and heater instrumentation working, we can't envision using more than ~250 watts. Will that be a problem at 80%?
All of those things you mention are DC appliances or propane/dc, so an inverter is not involved.
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Old 02-05-2018, 02:55 PM   #6
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Go here for some guidance parts 1 and 2....The 12volt Side of Life (Part 1)
There is a lot of good info in the two parts of that guide but he doesn't ever mention there being a maximum current draw from the battery bank. This is the C/8 rule I was asking about. Seems like everyone just talks about capacities to figure out amp hours but never mentions a limit on how much current you can draw.
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Old 02-05-2018, 03:00 PM   #7
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According to the chart, 11.9V is the lowest and at 10.5V they are dead. These ar the parameters mentioned and the chart in the article.
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Old 02-05-2018, 03:02 PM   #8
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My experience with an inverter of that size is: It does not function when the battery drops below 80%,

None of this has an effect on battery life, the inverter will not operate at low battery capacity,
Fudge Brownie - When you say it does not function what exactly do you mean. Does the inverter fault because it can't supply the requested power without dropping the voltage or does the microwave run but just doesn't heat much.

"None of this has an effect on battery life" This is what I am questioning. The C/8 rule I saw stated was supposedly from the battery manufacturer stating if you exceed this value your battery life will suffer. Seems like the solar house people are expecting to get 10 yrs out of batteries and us RVers don't typically get anywhere near that. Maybe this is part of the issue.

Can people comment on how long their batteries last, if they use an inverter and how big of one??

Thanks - Dave
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Old 02-05-2018, 03:09 PM   #9
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The way I under stand it Dave,if you abuse the battery bank they will not last. Meaning using something as an inverter to the point where the voltage goes below 11.9. Occasional use is okay but daily or routine dropping that low will shorten the life. Solar is a gentle charge whereas the converter is quicker and a charger is too quick. The ups and down extremes hurts the battery, keeping the use in the middle is good and the battery will last.
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Old 02-05-2018, 05:15 PM   #10
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Fudge Brownie - When you say it does not function what exactly do you mean. Does the inverter fault because it can't supply the requested power without dropping the voltage or does the microwave run but just doesn't heat much.

The inverter I have will alarm and shut down when the charge in the battery drops to a certain level. I am not sure what that level is on the inverter, but using the measurements from my Trimetric Battery Monitor, it appears to be at about 80% of battery capacity. Obviously the microwave shuts down as well. This applies to an Escape trailer equipped with the dual six volt battery system. I do not think the inverter can power the microwave when the trailer is equipped with a single 12 volt battery.

There are a number of variables present, some I mentioned already. Frequent repeated use of the microwave for short bursts will cause an alarm and shut down. So duration comes into play. Sometimes waiting for 15 to 30 minutes will allow the battery to recover enough and the microwave can be used again.

Using the microwave is the only time I have encountered a shut down. I also use an 120 volt portable vacuum cleaner without any issues. Those are the only non 12 volt devices I use. The microwave is a huge draw, no microwave and no problem.

While it is theoretically possible that you could over charge your batteries it would require a rogue controller/charger to do so. The built in WFCO charger is so lame that it does not even fully charge your batteries. See the posting I did on this subject. I have only found a half a dozen solar controllers that can charge at the recommended Interstate rate and zero built in RV chargers that can achieve the recommended rate. There may be some portable units that could charge at those rates, I am not familiar with those products, and they are not a practical solution. So over charging, while possible, is going to be an exceptional event.

I am confused about "the C/8 rule", could you explain what this is? I have not read about damaging a battery by drawing too much power, only by running the battery below 50%.
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Old 02-05-2018, 06:05 PM   #11
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While I would not want to go over the C/8 values for long time draws, for the short times most use the inverter for things like a coffee pot or toaster at 60 amps, or even the microwave at 95 amps, the damage to the batteries will be minimal.

Would it be better to have the battery capacity so you could draw 100 amps or so at C/8 rates? Yes, but many RVers have been doing short term draws at higher rates and still have the batteries last 7-8 years. To run a 100 amp load, technically you would need 800 amp hours worth of batteries, or around 8 6V batteries. That might fit in a large trailer or motorhome, but would not be practical in a small fiberglass trailer.

I ran a 1000 watt inverter, often drawing 60 - 80 amps for 5 years in my Escape 17, and a pair of 232 amp hour, 6V batteries were still in good shape when I sold it. If you are concerned, feel the case of the battery at the end of your heavy discharge. If it is hot, consider cutting back. If just warm, I wouldn't worry.
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Old 02-05-2018, 10:06 PM   #12
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I do not think the inverter can power the microwave when the trailer is equipped with a single 12 volt battery.
This should be a non-issue on new builds as the 1500W inverter option from ETI requires the inclusion of the dual 6V battery option.
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Old 02-06-2018, 12:13 AM   #13
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[I]Fudge Brownie - I am confused about "the C/8 rule", could you explain what this is? I have not read about damaging a battery by drawing too much power, only by running the battery below 50%.
I did a little more digging on battery discharge capacity. Northern Arizona Wind and Sun seems to have a very active solar forum with a lot of the techi details. Here is a link to the thread I was looking at. They list the following draw categories:
C/20 -- Average discharge rate (5 hours per night, 2 nights, 50% max discharge)
C/8 -- Maximum continuous discharge rate
C/5 -- Maximum short term discharge rate (run a microwave, power saw, minutes to an hour)
C/2.5 -- Maximum surge discharge rate (starting well pump, etc.)

Here the C is the Amp Hour capacity of the battery bank. Per Escape's build sheet the Dual 6V batteries are 225 AH. So for the microwave example, it would be short term, so you should only be drawing 225/5=45A out of the battery. For a 700W microwave being driven by a 85% efficient inverter that would be 824W coming into the inverter from the battery and at 12V that would be 69A. I think when you go above the C/5 rating you start loosing battery capacity and getting voltage drops.

I think this explains why people are having trouble running the microwave off the 1500W inverter. Taking the recommended 45A max out of the battery at 12V we have 540W capable of being supplied to an inverter without issue. With an 85% efficiency in the inverter that would provide 459W of AC power out of the inverter.

My original question was, isn't the 1500W inverter that Escape sells a bit to large for the dual 6 V battery and I think the answer is yes. From Fudge_Brownie's experiences it sounds like it can kind of work some of the time, probably most of the time if you learn the proper conditions as he has. For people that want a microwave it would be a resonable solution and probably why Escape pacakages the 1500W model. For me, I won't have a microwave so choosing a 600W inverter seems a smarter choice. I should be able to run anything within it's ratings without issues.

Dave
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Old 02-06-2018, 12:21 AM   #14
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Jon - Thanks for the actual operational experience. So at 80 amp you were running C/2.8 so pretty aggressive vs the recommendations. Of course the recommendations are always made to keep people out of trouble in all conditions, ie you can probably run a lot more current in a cold environment than a hot one. Your suggestion of back off when it is getting hot is probably the best solution if you want to push the window.

On a slightly different note, I have a nice 1000W Xantrex inverter/charger that I have installed in my current trailer. I am now considering just installing that in my new Escape 21 but it would/could replace the battery charger (converter??). I was curious what others thought of the stock charger. Might it be worth replacing with the inverter/charger with this combined unit or just stick with an inverter only.

Thanks - Dave
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Old 02-06-2018, 11:28 AM   #15
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I did a little more digging on battery discharge capacity. Northern Arizona Wind and Sun seems to have a very active solar forum with a lot of the techi details. Here is a link to the thread I was looking at. They list the following draw categories:
C/20 -- Average discharge rate (5 hours per night, 2 nights, 50% max discharge)
C/8 -- Maximum continuous discharge rate
C/5 -- Maximum short term discharge rate (run a microwave, power saw, minutes to an hour)
C/2.5 -- Maximum surge discharge rate (starting well pump, etc.)

Here the C is the Amp Hour capacity of the battery bank. Per Escape's build sheet the Dual 6V batteries are 225 AH. So for the microwave example, it would be short term, so you should only be drawing 225/5=45A out of the battery. For a 700W microwave being driven by a 85% efficient inverter that would be 824W coming into the inverter from the battery and at 12V that would be 69A. I think when you go above the C/5 rating you start loosing battery capacity and getting voltage drops.

I think this explains why people are having trouble running the microwave off the 1500W inverter. Taking the recommended 45A max out of the battery at 12V we have 540W capable of being supplied to an inverter without issue. With an 85% efficiency in the inverter that would provide 459W of AC power out of the inverter.


My original question was, isn't the 1500W inverter that Escape sells a bit to large for the dual 6 V battery and I think the answer is yes. From Fudge_Brownie's experiences it sounds like it can kind of work some of the time, probably most of the time if you learn the proper conditions as he has. For people that want a microwave it would be a resonable solution and probably why Escape pacakages the 1500W model. For me, I won't have a microwave so choosing a 600W inverter seems a smarter choice. I should be able to run anything within it's ratings without issues.

Dave
Just to add another real life comment - I have the Escape provided batteries, inverter, and microwave (as well as a pair of 160 watt solar panels). I also have a Bogart Trimetric battery monitor that, among other things, reads actual battery current (in & out). Running the microwave draws 95 amps on full power, or about 1 1/2 amp hours per minute. I have no low voltage alarm problem making a 2 minute microwave run in the morning when the batteries are already down by 20 - 25 amp hours.
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Old 02-06-2018, 02:03 PM   #16
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All of those things you mention are DC appliances or propane/dc, so an inverter is not involved.
Yes, thanks cp That was careless of me and I should not have included most of the draws in my question. But as far as the 250 watt total, 90% of it will come from my e bike charger (mentioned), which is 110 powered. It conditions 110v AC to 48 v DC for charging. So change 250 to ~225 an I'll ask my question again. Will my inverter "work", per Fudge Brownie's characterization for a 700 watt draw, if my battery is below 80%, but is providing only ~225 watts/efficiency factor of the inverter, to my inverter, and meets all of the other "C" rules. More generally, what, if anything, stops an inverter from "working", if it is getting enough battery voltage/current? How do it know?
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Old 02-06-2018, 02:13 PM   #17
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Have you looked into a dc charger, seems inefficient to convert DC to AC back to DC!
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Old 02-06-2018, 02:28 PM   #18
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Yes, thanks cp That was careless of me and I should not have included most of the draws in my question. But as far as the 250 watt total, 90% of it will come from my e bike charger (mentioned), which is 110 powered. It conditions 110v AC to 48 v DC for charging. So change 250 to ~225 an I'll ask my question again. Will my inverter "work", per Fudge Brownie's characterization for a 700 watt draw, if my battery is below 80%, but is providing only ~225 watts/efficiency factor of the inverter, to my inverter, and meets all of the other "C" rules. More generally, what, if anything, stops an inverter from "working", if it is getting enough battery voltage/current? How do it know?
Unfortunately, the answer is maybe. With a single battery, you are pushing it. Most inverters first alarm at around 11 volts, and shut down at 10.5V. Since this is measured at the input of the inverter, wire size and length has a big effect on the voltage at the inverter. Long or undersized wire may leave you with plenty of voltage at the battery, but not enough at the inverter.

As to what else stopped the inverter, most large inverters will shut down if there is an overload (more than their rated peak or continuous wattage), over voltage (often 15.1V) or overheating.
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Old 02-06-2018, 02:46 PM   #19
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Unfortunately, the answer is maybe. With a single battery, you are pushing it. Most inverters first alarm at around 11 volts, and shut down at 10.5V. Since this is measured at the input of the inverter, wire size and length has a big effect on the voltage at the inverter. Long or undersized wire may leave you with plenty of voltage at the battery, but not enough at the inverter.

As to what else stopped the inverter, most large inverters will shut down if there is an overload (more than their rated peak or continuous wattage), over voltage (often 15.1V) or overheating.
Agree that an inverter not getting enough voltage will shut down. Appreciate the actual voltage levels - I'll do my homework, but just for a second check.
Also thanks for mentioning wiring losses. But per inspection of my 1500 w inverter, 225 ah 6*2 battery set up, I'm not too worried. Short reaches of THICK 12 v cable, on both sides.

So, no matter the % charge on my batteries when using my inverter I need to watch out for low battery volts while drawing on them (which can proxy for low inverter volts for any possible duty I will ask of it in my case), at WHATEVER state of charge my battery pack has, right?

And thanks for the over voltage info as well. I don't plan to jack up my charging voltage with any gizmos that would lead to problems, but I'll definitely keep that in mind.
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Old 02-06-2018, 03:21 PM   #20
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Have you looked into a dc charger, seems inefficient to convert DC to AC back to DC!
Agree that it is as "inefficient" to do so. Probably 10+% lost each step. Suppose I could mount a switched mode converter next to my batteries (too many amps at 12 volts to plug into a cigarette lighter type12 volt receptacle), but I wouldn't have the charge control of my charger. And the additional loss is manageable. I think V answered my question and I'm just wasting your time...
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