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Old 09-28-2022, 07:41 PM   #1
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Battery and Inverter Knowledge

I have two 75 amp hour deep cycle wet acid batteries. I also have a 1000 watt inverter. For the life of me I am having a difficult time figuring out what I can run and for how long using the inverter and battery power. Lets assume I can find out how much wattage or amperage a particular 120V appliance uses? This is embarassing and for this reason in the 4 years I have owned my coach I have never used the inverter.....
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Old 09-28-2022, 08:10 PM   #2
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Two 75 AHr LA batteries give you about 75 AHrs total usable power at 12 volts. Generally best not to draw LA batteries down more than 50%. The inverter is converting 12 v to 120 volts. So if a 120v device draws 10 amps at 120 volts then that places a 100 amp load on the 12v battery. But you only have 75 AHrs. So you could run that 120v load for about 45 minutes before the battery is at the 50% mark. Leaving nothing for everything else running at 12 volts.



You would well off upgrading to a better battery pack. At least 2 or 4 6 volt golf cart type batteries. Many of them are rated at 220+/- Ahrs. So two will provide 230 Ahrs / 2 or about 115 AHrs of usable power. Four of them will double that number. If lithium is in your price range you can do better since lithium batteries can be drawn down to about 80% opposed to 50%. However the conversion to lithium may also require other upgrades to inverters, converters, etc. Also they have different temperature limits.


I had a Minnie Winnie 22R for a couple of years. So I know you have a space limit of two batteries without doing some major mods. I installed two 6 volt GC batteries and life was much better, but I did not have an inverter.
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Old 09-28-2022, 09:04 PM   #3
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The part I dont understand is if a 120v appliance draws 10 amps at 120v how does it apply a 100 amp load to the battery instead of a 10 amp load?
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Old 09-28-2022, 09:09 PM   #4
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It has to do with Watts law.

https://www.engineeringclicks.com/wa...mula-ohms-law/
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Old 09-28-2022, 10:59 PM   #5
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Volts x amps is watts. 12v x 100 amps is 1200 watts, 120v x 10 amps is 1200 watts.
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Old 09-28-2022, 11:05 PM   #6
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Quote:
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The part I dont understand is if a 120v appliance draws 10 amps at 120v how does it apply a 100 amp load to the battery instead of a 10 amp load?

Because watts = volts x amps. Watts is a measure of amount of work done. Thus 10 amps at 120v is 1200 watts. Thus to make 1200 watts at 12 volts require 100 amps. 100 amps x 12 volts = 1200 watts.


This is why the cable between a battery and inverter must be large gauge. The exact gauge also depends on the cable length.
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Old 09-28-2022, 11:10 PM   #7
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Originally Posted by MW22r View Post
I have two 75 amp hour deep cycle wet acid batteries. I also have a 1000 watt inverter. For the life of me I am having a difficult time figuring out what I can run and for how long using the inverter and battery power. Lets assume I can find out how much wattage or amperage a particular 120V appliance uses? This is embarassing and for this reason in the 4 years I have owned my coach I have never used the inverter.....
So if you use the full 1000 watts you would pull 83.3333 amps from your battery in an hour.

What's ų ac voltage = Amps
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Old 09-28-2022, 11:38 PM   #8
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In general, a 1000w inverter can run small items like TVs, Satellite receivers, CPAP machines, phone, tablet and computer chargers. The things it canít run are most microwaves, coffee makers, hair dryers, induction cooktops, portable heaters and other items that generate heat by electrical resistance. Those things need a lot of watts to operate.

Your current battery bank canít do all of its 12v chores AND have much power left over to do a lot of high wattage inverter powering, too.

You are just getting started in the education process when it comes to powering your RV. Keep asking questions and reading up on batteries, chargers, and RV electrical systems. Itís a big subject.
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Old 09-29-2022, 07:54 AM   #9
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Two 75 AHr LA batteries give you about 75 AHrs total usable power at 12 volts.
What makes the other 50% "unusable" - ?

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Old 09-29-2022, 07:58 AM   #10
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Lets assume I can find out how much wattage or amperage a particular 120V appliance uses?
You don't have to assume a thing. You measure and know. The most direct way would be to use a battery monitor that shows watts - they can be had very cheaply. Turn the device on, read the number. Alternatively you could use an AC power meter like a kill-a-watt and operate the device, and know. From there the watts consumed can be related to your battery capacity as indicated on the battery monitor. "Better" battery monitors will give your full to empty and/or remaining time/capacity on the fly.

It would be fair to point out that pulling a kilowatt out of a pair of light duty batteries won't last long but just because you have a kilowatt inverter doesn't mean you are always drawing a kilowatt. Just running a TV set, charging a laptop or other nominal draw appliance this can work out fine. Having a battery monitor will go a long way towards knowing what you can use and for how long.

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Old 09-29-2022, 08:29 AM   #11
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What makes the other 50% "unusable" - ?
It’s not unusable. You can use 100% of the available amps hours out of your battery bank. However, your batteries have a limited number of charge/discharge cycles. The deeper you discharge the batteries the shorter the number of cycles you can expect to get out of them. So, if you want your batteries to last and provide good service you should try to keep them at or above 50% state of charge.

This 50% number is not a hard and fast rule. But is a generally agreed upon best case. Using 70% of your available amp hours occasionally won’t necessarily harm your batteries but doing it daily will simply shorten your batteries life.

What really kills FLA batteries is to deeply discharge them and then not quickly recharge them. Letting them sit at a low state of charge causes the buildup of lead sulfate crystals on the plates. This sulfation can sometimes be reversed but is the number one cause of battery failure - the deeper you discharge your batteries the more you cause sulfated batteries.
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Old 09-29-2022, 09:44 AM   #12
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The deeper you discharge the batteries the shorter the number of cycles you can expect to get out of them.
But the deeper you discharge them the more energy they supply. Deep cycle battery delivered Ah is nearly flat from 20% DOD to 100% DOD.

Quote:
if you want your batteries to last and provide good service
"Last" is application specific. If you observe the "50% Rule" the typical cycle life for a quality battery is 1200. That's a cycle a day for over three years straight. You would be hard pressed to find anyone that ever does this. So if you'll never be able to use all those cycles, why economize to try and achieve it? Not saying you must discharge >50%, I'm in the camp of charging whenever it's convenient for me. In RV use rarely is cycle life a concern, so using the "bottom half" of the battery whenever you want is perfectly OK. The battery will die of other causes before you will wear it out, so what are you saving?. Shallow discharges not only assures you will take unused capacity to the recycler it also translates to extra generator run time as well.

Quote:
But is a generally agreed upon best case.
Perhaps by those living off grid with storage batteries, charging with wind or solar and literally cycling their batteries every day. That does not describe RV use.

Quote:
doing it daily will simply shorten your batteries life.
It doesn't materially shorten service life at all. You get all the Ah you paid for, the only question is how fast it happens. Even 100% discharge offers hundreds of cycles which for most people is years of use. There are some good reasons to use arbitrary DOD thresholds in operation, but battery longevity is not among them.

Quote:
What really kills FLA batteries is to deeply discharge them and then not quickly recharge them.
100%. Most batteries don't die, they're murdered. DOD often gets the blame but is rarely the cause.


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Old 09-29-2022, 10:43 AM   #13
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So, I am new to the motorhome lifestyle. We bought our first Class A a few months ago. This discussion has me wanting to get a comprehensive understanding of my RV electrical systems (inverters, convertors, generators, 12V DC, etc.). Could you recommend some on-line resources for such knowledge - Youtube, websites, e-books, etc?

Thanks!

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Old 09-29-2022, 11:55 AM   #14
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There is lots of good knowledge to be gained from online videos. The quality of information is highly variable. Some of it is professional and spot on, others are just plain wrong. How do you tell the difference? One thing I do is to review many different videos. Somewhere in there you'll start to see some patterns.


Much of the printed material, not manufacture published, is by nature generic and is good for overall reviews. You should obtain as detailed materials as possible for your specific make/model and year of RV. Even then manufactures may make changes anytime in the model year.


I have a decent electrical background but not on a professional basis. One of the early things I do on a new to me RV is start going through entire rig and understand how the electrical and water systems are installed/modified. Problems are going to arise and it is nice to already have a good understanding how the various systems work and where each component is located. Or just count on paying someone else to fix stuff.


In reality the electrical systems are generally not all that hard to figure out. There are some notable exceptions is highly customized rigs.
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