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Old 10-17-2021, 07:54 PM   #1
Bus Nut
 
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another battery question

Hey people, please educate me...

I read the excellent articles the PNW Steve linked to about power converter size(older post)...max inverter for 12v = 1000 watts. But in the 2nd link it said that a much smaller inverter was appropriate and in that equation, they used a 220ah battery. So, if I have 2 Renogy 200ah deep cycle batteries it looks like I should run a 500 watt inverter.

Now my question, There seems to be a contradiction here, I'd like to install a 1000 watt inverter, be able to make coffee or run a small micro wave occasionally. But by doing the math from the above referenced articles it looks like my inverter should only be 480 watts?? (I'll call it 500 watts)

What did I miss?

TIA

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Old 10-17-2021, 08:11 PM   #2
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Really the **power** Watts your bank puts out can be pretty high with AGM.

problem is the **Energy** side Ah

at a high C-rate, and only using half the capacity to get decent lifespan

More than a few minutes out of time you just run out of juice, lots of stress as voltage drops amps can't keep up.

But it all depends on your Ah draw per minute, and how many minutes per session

as well as of course Ah per 24hours total.

Best to get a coulomb counting wattmeter and actually measure. Figure 15-20% added from inverter conversion waste.

Or, cook with propane and then you don't need such a big bank.
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Old 10-17-2021, 08:43 PM   #3
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ok, so if I get a 1000 watt inverter, and we have a high energy draw, IE microwave, but only run it 10 minutes at a time, I'm not likely to overtax the system. (small- 900 watt microwave).

We will most likely camp where we will have access to shore power. But, no doubt, there will be times when we boondock...on those occasions, we are ok with not having hot water or fresh cooked food. Microwave to heat up left overs.

I really plan to keep this bus as uncomplicated as possible...
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Old 10-21-2021, 12:44 AM   #4
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Originally Posted by BarnYardCamp View Post
ok, so if I get a 1000 watt inverter, and we have a high energy draw, IE microwave, but only run it 10 minutes at a time, I'm not likely to overtax the system. (small- 900 watt microwave).

I may be misremembering but I believe the power stated by microwaves refers to the power they direct at heating food, not the power they need as an input. In other words a 900W microwave will consume more than 900W to deliver 900W of heat to whatever is inside it.


In either case (whether im correct or misguided on this) you want probably want more overhead than ~10% of your expected max load, particularly if you will be buying a cheap-ish inverter or may have multiple small loads consuming power at the same time as the microwave.


There is no downside to a somewhat larger inverter, other than a bit higher idle power consumption in some cases, The article/equation you alluded to is probably really looking at the ratio between batter bank size, and loads (power consumers), the inverter is just a middleman, just because you have a 2000w inverter doesn't mean you will ever draw 2000W unless you have a load that pulls that. I suspect that you may have misinterpreted the article (or possibly the article has misinterpreted/misrepresented something) but I cant say for sure without knowing what the article is.
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Old 10-21-2021, 12:58 AM   #5
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Originally Posted by BarnYardCamp View Post
ok...We will most likely...boondock...keep this bus...uncomplicated...
.
I took a long time to realize this:
* SIMPLER IS BETTER.
I think the final shove was delivering RecreationVehicles manufacturer-to-dealer and dealer-to-shows.
The things are enormously complicated.
.
.
We acquired our 750-Watt inverters from a local-owned family-operated auto-parts store.
How many?
I think we might carry a half-dozen in the rig.
.
Pretty handy, we keep a couple more in the Dodge.
.
We cook on portable induction hot-plates.
On 'low' at around 600-Watts, we realize we are pushing it, but so far, so good.
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Old 10-21-2021, 01:52 PM   #6
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Originally Posted by dzl_ View Post
I may be misremembering but I believe the power stated by microwaves refers to the power they direct at heating food, not the power they need as an input. In other words a 900W microwave will consume more than 900W to deliver 900W of heat to whatever is inside it.


In either case (whether im correct or misguided on this) you want probably want more overhead than ~10% of your expected max load, particularly if you will be buying a cheap-ish inverter or may have multiple small loads consuming power at the same time as the microwave.


There is no downside to a somewhat larger inverter, other than a bit higher idle power consumption in some cases, The article/equation you alluded to is probably really looking at the ratio between batter bank size, and loads (power consumers), the inverter is just a middleman, just because you have a 2000w inverter doesn't mean you will ever draw 2000W unless you have a load that pulls that. I suspect that you may have misinterpreted the article (or possibly the article has misinterpreted/misrepresented something) but I cant say for sure without knowing what the article is.
Reference the power rating on the microwave...yet another point that I hadn't thought of is that the power rating was power output. I found that the Hamilton-Beach 900 watt microwave uses 1450 watts of power, that's a
significant difference.

As far as using a "cheap-ish" inverter. Somethings it's best to pay up for quality. I think the power inverter is such an appliance. One of the hi-tech guru's on this forum recommended Xantrex and/or Samlex. The Xantrex 1000w model is just under $400. Not cheap but not ridiculously expensive either.

I've read a lot of build threads and have nothing but respect for all the techno wizards on this forum.

So maybe I need to re think my wants. Wants being able to keep it basic with a small battery bank and being able to enjoy some convenience things such as a microwave, make it so my wife can make herself some coffee and maybe watch some TV if we can't be outside, and run a CPAP over night. Most of our camping will be where there is shore power. There will be times when we boondock for a night maybe two.

I really appreciate any guidance given as I recognize that techy stuff is not my forte.
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Old 10-21-2021, 02:26 PM   #7
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Quote:
Originally Posted by dzl_ View Post
I may be misremembering but I believe the power stated by microwaves refers to the power they direct at heating food, not the power they need as an input. In other words a 900W microwave will consume more than 900W to deliver 900W of heat to whatever is inside it.


In either case (whether im correct or misguided on this) you want probably want more overhead than ~10% of your expected max load, particularly if you will be buying a cheap-ish inverter or may have multiple small loads consuming power at the same time as the microwave.


There is no downside to a somewhat larger inverter, other than a bit higher idle power consumption in some cases, The article/equation you alluded to is probably really looking at the ratio between batter bank size, and loads (power consumers), the inverter is just a middleman, just because you have a 2000w inverter doesn't mean you will ever draw 2000W unless you have a load that pulls that. I suspect that you may have misinterpreted the article (or possibly the article has misinterpreted/misrepresented something) but I cant say for sure without knowing what the article is.
I have a Renogy 2000 watt inverter and a 280 AH LiFePo battery. I recently bought a 3000 watt inverter because I think it will last longer, being used at a lower percentage of its maximum capacity.

I like Marge's idea to have multiple inverters. My instant propane water heater runs on 110AC and I have a small inverter dedicated to that device. My fridge currently runs on the Renogy inverter and so far my battery capacity is pretty much able to handle the fridge for 4-5 days at a time without recharge, so I've met the design requirements for that.

I have a smaller inverter and components to dedicate it to the fridge so that it only runs when the fridge needs to run. I wanted to do that back when I was really paranoid about having enough battery power, based on my planned usage.

Actual usage doesn't approach my capacity limits, so I was somewhat smart in procrastinating on building that cool 'only on when fridge is on' feature. I've not yet built that into the bus, primarily because I have plenty of battery to run the fridge with the inverter always on and the parasitic draw is negligible.
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Old 10-21-2021, 02:40 PM   #8
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As far as using a "cheap-ish" inverter. Somethings it's best to pay up for quality. I think the power inverter is such an appliance.
Agreed. I think an inverter is definitely one such appliance if you will be relying on it often (if its only for occasional use and non-critical things, there is more of an argument to be made for compromises in price/quality). For a reputable inverter you pay more upfront, but in exchange you should get a quality and well built product, from a reputable company with decent documentation and support, and ratings that are specific and realistic (cheap inverters are notorious for inflating the specs, sometimes outrageously), that should last many years/decades.

Outback, Victron, Samlex, Magnum, Xantrex, have good reputations as top tear or close to top tier brands. There are some others that straddle the line between mid range and budget, and a few budget options that many people like but have more hit and miss QC and shouldn't be relied upon for near 100% of spec.

Quote:
So maybe I need to re think my wants. Wants being able to keep it basic with a small battery bank and being able to enjoy some convenience things such as a microwave, make it so my wife can make herself some coffee and maybe watch some TV if we can't be outside, and run a CPAP over night.
Possibly, but possibly you just need to reorient or reprioritize those wants, not give up on them.
  • Coffee -- can be made in many ways, a simple cone filter, french press, stovetop espresso maker, turkish/greek coffee maker, or the dreaded instant coffee as a last resort, cut your need for electricity so long as you have a way to heat water or a propane/gas range or camp stove.
  • Fresh Food -- you mentioned earlier, being willing to give up on fresh food when you boondock but hoping to at least be able to heat leftovers. In my experience this is backwards (from a practicality standpoint). My experience is that its quite easy to cook fresh when off grid (propane, camp stove, campfire, etc) its the microwave that will be less practical (though totally doable if you plan for it). The exception to this would be if you have all electric appliances, but then a portable campstove and some small disposable propane tanks solves this.
  • TV -- A few hours of TV shouldn't be a huge problem, so long as your battery bank can support it. A reasonable sized LCD TV isn't super power hungry.
  • CPAP -- No idea, but I know that many people accomodate this in their energy budgets without too much trouble.
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Old 10-21-2021, 02:52 PM   #9
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I like Marge's idea to have multiple inverters.

Me to, partial or full redundancy is always nice, and if well implemented multiple inverters can also save power or money or both in certain contexts (but can also do the opposite, the devil is in the details of implementation).


A small efficient 'always on' inverter with a low idle power consumption for a few outlets or a fridge or other small always on devices, paired with a big heavy large inverter for big loads that is only switched on as needed, can result in a net power savings since the big boys (the quality low frequency ones) consume a lot of power just sitting their idling.


However multiple inverters could be less efficient also, if you have a half dozen just sitting idle all the time running one or two devices each that could be a lot of unnecessary idle power consumption, particularly if they are cheap/inefficient inverters.
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Old 10-21-2021, 06:00 PM   #10
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Maybe having a inverter dedicated to just the CPAP is a good idea. However, having one inverter or ten, if I draw from the same battery bank how does this change my situation?
Unless of course I'm only drawing from one inverter at a time or from more than one with low draw items and/or maybe the smaller inverters have a smaller parasitic draw. (edit;oops...I re-read DZL's post...already covered this.)

French press, she uses that now so, yes, we only need a way to heat the water.

I intend to have a electric cook top in the bus. The cook top and the hot water tank will only be usable with shore power as the power demands of just those two appliances would demand a sizeable battery bank along with all sorts of techno gadgetry.

We'll carry a small propane camp stove for outdoor use...the type that uses the little green bottles available about everywhere, other than that, no propane appliances are planned. If, at some time in the future we find that the all electric set up is insufficient then we could change to propane with out having to dismantle the bus(I hope)

I'm getting close to actually building walls so I need to get this figured out.

I appreciate the education!
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Old 10-21-2021, 06:45 PM   #11
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Quote:
Originally Posted by BarnYardCamp View Post
Maybe having a converter dedicated to just the CPAP is a good idea. However, having one inverter or ten, if I draw from the same battery bank how does this change my situation?
Unless of course I'm only drawing from one inverter at a time or from more than one with low draw items and/or the smaller inverters having a smaller parasitic draw.
Whatever the inverter(s) require for current, they are additive. That means if you have three inverters, you add together the draw of all three inverters, and that's what the battery will require. As long as the battery can take the discharge, and the wiring to the inverters is sized to handle the current, you're good to go.

Example: if you have an AC fan that draws two amps at 120 volts AC, the DC draw will be 20 amps (because watts equals volts times amps, so 2 amps drawn on the output side of the inverter x 120 volts = 240 watts, which divided by 12 volts equals a 20 amp draw on the 12 volt lines from the battery to the input of the inverter), plus a conversion factor due to the inefficiency of the inverter (maybe 10% but it varies), so the theoretical draw on the battery could be 22 amps. You can measure all this with an AC amp clamp and a DC amp clamp to get the real time efficiency of your inverter at different draws.

Each inverter is a device. It has a baseline draw when on but not inverting, usually .5 to 1.5 amps. So that can add up, but is usually offset by the fact that those inverters may not always be on.

Each inverter will draw what it needs, so if all are being used, they will all draw together, raising the overall battery drain.

I wanted multiple inverters for a couple of reasons. First, I didn't want to run the water heater on the main inverter if I could avoid it. The water heater has a small 110vac draw to run the electronics. I wanted to have hot water and not need the main inverter if I'm boondocking since everything else is 12vDC or propane, so I could just turn off the main inverter and avoid the parasitic draw to extend battery life. Same with the water heater when not in use. I also get some redundancy so if the main inverter failed we'd still have hot water.

Same with the fridge, but as mentioned our battery holds up pretty well so I haven't put the 1200 watt inverter in just yet.

In your situation, you have to decide if there's merit in a small wattage inverter for those specific needs, or if it is just easier to set up a larger inverter and be done with it.

Quote:
French press, she uses that now so, yes, we only need a way to heat the water.
One less electric appliance to drive battery and inverter requirements...
Quote:
I intend to have a electric cook top in the bus. The cook top and the hot water tank will only be usable with shore power as the power demands of just those two appliances would demand a sizeable battery bank along with all sorts of techno gadgetry.
Might be worth considering propane for both. They have a variety of propane tankless water heaters. Everyone approaches this differently but in my mind there's nothing like hot water in abundance.
Quote:
We'll carry a small propane camp stove for outdoor use...the type that uses the little green bottles available about everywhere, other than that, no propane appliances are planned. If, at some time in the future we find that the all electric set up is insufficient then we could change to propane with out having to dismantle the bus(I hope)
My kitchen has a camp stove and 1 lb green bottle-it's not connected to the 20 lb barbecue tank that powers the water heater. I kept them separate to simplify the installation, even though it means I need to stock green propane tanks for longer treks.

Some people get jittery about propane tanks, so you have to make your own peace with the propane setup. Note that if you do have 20 lb tanks in the bus they need to be installed in a cabinet that is sealed from the interior of the bus, with vents at top and bottom to the outside. Search this forum for more detailed information on those requirements.

The green propane tanks are notorious for leaking when you disconnect them from the camp stove or appliance. My solution is to leave them connected.
Quote:

I'm getting close to actually building walls so I need to get this figured out.

I appreciate the education!
You're asking all the right questions.
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