Wyoming B.S. Thread

OK, we need to back up. I think there's some confusion around how electricity works Or maybe not and we're just talking past each other.

First, some equations and abbreviations: V (voltage, V), i (current, A), R (resistance, O), P (power, W), t (time, H)

V = iR and P = i^2/R so
P = iV and
i = P/V

So, if something is pulling P = 120W from a V = 12V source, then i = P/V, so i = 10A. That's an instantaneous measurement. So, when I say that my fridge pulls 6-7A when going at full tilt, that means that when it's on full blast, if you put an ammeter in series with the fridge, you'd see that there was 6-7A of current flowing through the (12V) circuit. That means the fridge would be using 72-84W of power at that instant in time.

Now, if you want to talk about power consumption over time, then you start talking about Ah (Amp-hours) or (Watt-hours). So, when a battery says that it has a capacity of 76Ah (like the Northstar 24F that Mike and I run a pair of), that means you can (theoretically) draw 1 Amp over a period of 76 hours before the battery runs out. Likewise, since it's just math, you can draw 76A for 1 hour.

Now, most electrical devices don't have a constant power draw. The fridge is no different. Most of the time it is in "off" mode - just sitting there, with the insides warming up. In that case, it's drawing 0A at 12V, so P = 0*12 = 0W. When it turns on though, assume it turns on to "full blast" all the time, pulling 7A.

It could run like that for 76Ah / 7A = 10.86h before the Northstar 24F battery ran out. But, instead it runs for an hour, then is off for some period of time, then runs again, then is off again, etc.

Based on ARBs documentation - where it averages .85Ah, and a 7A draw when it is on, that would suggest the following:

7A * H = .85Ah, which means H = .12h. So, ARB assumes that the fridge is on for 12% of the time, and off for 88% of the time. I'd say that their being generous on that .

Make sense or no?

 

Yeah it makes sense, I've read the textbook lol But saying you fridge pulls 6-7amps is like saying your truck gets 70mpg but when it's a moment in time it's meaningless. Battery consumption should be talked about over time, per hour as fuel consumption is measured over distance. Now, there's no standard like that but there should be. The manufacturer of my fridge says it'll use 400w a day on average. What does that even mean? Gotta do the math to make it meaningful. 400W/24hr/12V=1.38ah. So I think we both are aware of what's at play but thinking in different units. Mine obviously being the better unit to use.

 

There are way, way too many variables to be able to say what the amp draw should be for a given fridge/situation. All that's left is to find out what your situation produces.

I would add the fact that newer fridges built within the last 5 years, by design, are more efficient than a 20 year old unit. This might be supported by what your fridge mfg says yours should be. FWIW, Dometic says mine is .77 Ah/h. They do not state what the circumstances were when that was calculated.

 

While my van is 21yrs old all my appliances are all new within the last few years. So 1.38ah draw is reasonable for a modern 12V fridge. I know someone locally that got one that is pulling 4ah which is pretty inefficient for a modern fridge IMO. ARB is pretty good about how they give the circumstances within which they determine their average draw with temps internal, ambient, and 66hour of testing for an average. Isotherm like I said was a smoke and mirror answer. I guess the numbers are good for averages to create estimates but not much more. It is interesting that the original owners did have an ARB fridge in the back AND the isotherm at the same time .

 

Obviously .

I'll just add this - the fridge actually needs to draw 7A (or whatever) when it is running. It can't draw .85 or 1.38 or whatever the "average over some longer duration" is. This doesn't matter all that much when the battery is fully charged, but it matters a LOT when you start to lose voltage on the battery.

That's because P = iV, so as you voltage drops, the amount of current you need to power the fridge actually increases. When you needed 7A at 12V for your fridge to get the 84W it needs to run (in that instant), you need 8.4A when you have 10V at the battery. When coupled with the fact that a battery gets less efficient at sending those amps as the voltage drops (because it's running out of the 76Ah), you start running into issues. You notice this with AA and AAA in headlamps and such too, as they drop from 1.5V to being "dead" at ~1.0V.

Anyway, that's why it's important to know average and instantaneous. Plus, instantaneous is what things like fuses are based on. It's why we have 10A fuses, rather than 2A fuses, on our fridges.

Edit: While your 1.38Ah fridge might half as efficient as what Mike's or mine is, I don't think it's all that bad, esp. given that you have twice the battery capacity we do. You could probably get something more efficient, but again, it's at a cost. Is it really worth $1K for a more efficient fridge? You could buy a couple more batteries for that same price.

 

It's the issue I have run into with my stove. It needs 12v to fire up but consumption is much much less once running. So I will agree that peak amperage and hourly amperage are important, I think we are not consistent on how we discuss such things though (not us, but everyone). I still think most conversation are about consumptions and not "start up" voltages, so ah is a better measurement for discussion in most cases.

The fridge at 1.38ah is great, not the best but far from the worst. I wish I had a baseline for how those specs were determined like I alluded to with ARB. Unfortunately, I don't so my math says round up to 2ah and that gives me 50hr of run time in an average environment. Consider it a 50% duty cycle and you're easily getting days out of it; before the solar assist. That is what I expect and far from what I was experiencing.

All in all I think we are very much in the same book but different chapters. Took me a while to refresh on electrical theory.

 

what is all this. Math?

https://youtu.be/supUN-J5s2g?t=3

40w equivalent light output. It's....weird and annoying how they do that.

 

I realize that this is a little bit of , but I'm going to do it anyway because it's the internet, hahahaha.


The issue you're running into with your stove startup is a little different than what we're talking about. The initial voltage check to get the fridge compressor (or in the case of your stove, igniter/fans/whatever) is actually because it needs a certain amount of Power (P[W] = Vi) to get everything moving, which is more than the power necessary to keep it moving. A lot like your MPG example, where it takes less fuel to cruise at 70 than it does to accelerate to 70mph.

Anyway, the reason that there is a V check and not a Power(W) or Current(A) check is that you can measure voltage without sending (any real amount of) power to the device, by putting a really big resistor into the circuit, but you can't measure current without actually flowing all the power through the circuit (thereby actually starting the device!). So, your stove (or our fridges) assume that if you have >12V at the battery, that it will be able to pull enough current to start up. Usually this is an OK assumption, until your battery starts to degrade, at which point it can't actually maintain those 12V when current is being pulled.

The thing we've been talking about - draw when on vs. avg. draw - is actually this chart. I've also included the startup spike that is what I think you were alluding to in your last post. That startup spike is one of the things that determines the fuse size for the circuit; the other is the number of Amps drawn when the battery gets to the lowest voltage the device can run on.

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if we aren't gonna debate/talk about stuff like this what else is this thread for? not much Tacoma stuff to talk about these days.

Thanks to you and Mike with helping get my mind right on the house battery stuff. One of those things a learned a decade ago and really have not needed to think about since.

 

Chiming in on the power talk!

Although I'm not new to electronics, I am somewhat new to solar and house-power.
I've been trying to manually monitor my house battery usage though the Renogy BT controller. Usually with my CSI fridge in eco-power mode, the draw is 2.5A-3A 30W-40W when is running full-bore.
I've also been trying to monitor the diesel heater startup and running-on-low power consumption.

As of now, I've got a single X2 27f (100ah total) dedicated house battery charged by a 175w panel through an MPPT controller.

Fridge and heater running all night in freezing temps outside (65-70+ inside, depending on ventilation) keeps me around 55% capacity in the morning.

Fridge only:

 

That's a nice app, similar to what my parents can has. Really can see what is going on with your system.

New $12 breaker and I've got 14.5v charge at idle

Next question for the group here is fridge maintenance. I intend to shit it off between trips so the fridge will "thaw" and I noted a smell because of it. Are there any tricks y'all do to keep the fridge fresh between trips? Dessicant? Leave the door open? Or is it something to just leave on, the life span won't be effected greatly running 24/7?

 

Definitely leave the door open - it's like a washing machine that way; mildew will grow if you let the moisture stew in there. Then, once it's at room temperature, mist it with a water-bleach spray bottle and then wipe/dry that off. Mine is as fresh as the day I got it.

 

Ok great! Never used bleach before, what ratio to mix it at?

 

LOL, I dunno. Probably a tablespoon of bleach to 4 cups of water. Probably says on the bleach bottle. Doesn't matter all that much as long as you don't get it on anything you don't want to get bleached. I've been known to just dump a bit of bleach in my fridge and wipe it around with a paper towel.

 

I just let mine sit with the lid propped open for a few days. Then wipe it out with spray cleaner.

 

Hope y'all had a good weekend!

The photos are an hour apart. Couldn't catch the 60mph wind on camera

 

Was that Ice/hail?

 

Yes and yes and rain too! We would get a storm every other hour. The last two were pretty violent and necessitated dropping the top from the crazy wind blasts. Then it would clear and the sun popped and and was incredible.

More importantly the electric setup is running at full steam. Never and issues and stayed between 12.7v and 13v the whole time, and we weren't being conservative with the use.

 

Similar weather out in the Owyhee's. Luckily, the mud didn't get toooooo sticky and gross.







So nice and green out there right now!

 

Looks fun.

Went to Powell and watched my sister band. They shredded.