I'm looking to get a 12V/35ah 'solar' battery from HF (which says it's good for repeated deep charging). I want to marry it to a fan via a power inverter. Swim season is here, and a fan and (and perhaps a phone charger) for the tent would be nice for a change.

The fan is about 84W at the highest speed setting--simple box fan from the depot.
The charger brick on the phone says 115VA and .15 amps (about 17.25W).

I've got a 300W power inverter I'd like to use.

1. Should I be using the battery's watts (35AH * 12V = 420W) divided by the watts of the fan (84) to get about 5 hours?
2. Or, should I be using the battery's AH of 35 divided by the fan's .73A (84W / 115V) which is far greater?
3. Or, is it the battery's 35A divided by the fan's amps of 7 (84W / 12v--from inverter input) to get about 5 hours?

For now, I'm ignoring charging states and depth.

I'm a little stumped where the inverter is concerned, but it feels like #3 where the watts demanded by the fan are 84, but provided at only 12V from the inverter input, thus 7 amps and not .7 amps AC.

Thanks for taking a look.

 

If you are starting from scratch and plan to buy all that is required, then it will be better to get a 12 Volt BLDC fan and a simple Mobile Phone car charger. You don't need an invertor.

12 V, 35 AH battery works out to be something like 420 Watt Hours. A 85 Watt fan would run for about 5 hours - in theory. Practically, it would be less. Assuming an Invertor and Battery efficiency of 75% and 80%, it would be about 3 to 3.5 hours. Using an Invertor would reduce it further, to maybe around 2.5 hours.

 

Yes, use a 12V fan, or a few if you need more airflow. And a car USB charger. Skip the inverter. There are lots of 12V LED lights available too.
And a solar panel with a MPPT charge controller Connected up correctly, this charge controller will prevent battery over charging and discharging. Otherwise you can kill your battery pretty quickly by over discharging it.

 

I also would go with a 12 VDC fan, very common in RV applications. One possible problem with using an inverter is the output waveform type. Your fan motor may want a sine wave and most lower cost inverters output a modified sine rather than true sine wave out. You also have loss in the inverter so I see a 12 Volt DC fan as a better choice along with a mobile cell phone charger. I would do it all as dendad mentions.

Ron

 

Thanks folks.

Why did the original 5 hours get reduced by using an inverter? is there a way to pin down an accurate loss?

 

Thanks folks.

Why did the original 5 hours get reduced by using an inverter? is there a way to pin down an accurate loss?

Inverters come with an efficiency rating but the efficiency will vary with the loads applied. The simple way to look at is power out / power in * 100 will yield a percent of efficiency. For example is the inverter is running at 12 Volts and drawing 10 Amps the power in is 120 Watts. Least I forget this would apply to a TSW (True Sine Wave) inverter as I mentioned earlier. We measure the output RMS voltage and current and multiply the voltage * the current to get the power out. If my output is 120 Volts and my current is 1 Amp I have 120 watts or 120 / 120 = 1 and 1 * 100 = 100% but nothing is 100% efficient. This is over simplified and the type of load matters for example an inductive load like an AC fan motor.

Typically inverters are less efficient at the low end of their rated output so keep in mind the efficiency for a given inverter is not a fixed value. There is no pinning down an accurate loss as it depends on the inverter, load and other variables.

Ron

 

The additive inverse of the course correction factor multiplied by the constant speed of light divided by the time of day (except on Tuesdays) - - - uh - - - ? Forget all that stuff. Simply stated, inverters are notoriously inefficient. They can do the job but there's a lot to be sacrificed. The calculations you came up with - 5 hours - I'd say at BEST, about 80% of that. At worst, 60%.

You don't need an invertor.

That's the first thing I thought of too. But I understand, you want to run a bigger fan. Here's the thing: Bigger fans draw more current. Finding a 12 volt fan that will move the kind of air you want to is going to require some power. First thing that comes to my mind is an old radiator fan from a junk car. But they'll be loud. But far far more efficient than running an inverter to power a box fan. And if you want to modulate the speed of the fan you can buy or build a PWM power supply (Pulse Width Modulation). Part of the time the power is on and part of the time the power is off. You get an average of power. If half the time power is on and half the time power is off - you get half power output. Of course a fan from an old radiator will probably draw more current than you want. But with PWM you can reduce the amount of power going to it and have a comfortable airflow, fully controllable. And if you're in a dry climate you can rig up a swamp cooler (evaporative cooler) to chill the air. Humid areas don't do well with swampers.

Now you got me thinking. I have a party every 4th of July (damned yankee). I set up a shade to block the sun, but the daytime it's pretty warm (hot even). Having a swamp cooler powered by a car battery would be a fairly easy addition to the rig I've already built for hauling all that stuff out into the field prior to fireworks. Project for next year. This year I'm doing some stuff for the Navy.

 

I also would go with a 12 VDC fan, very common in RV applications. One possible problem with using an inverter is the output waveform type. Your fan motor may want a sine wave and most lower cost inverters output a modified sine rather than true sine wave out. You also have loss in the inverter so I see a 12 Volt DC fan as a better choice along with a mobile cell phone charger. I would do it all as dendad mentions.

Ron

Ok, going with the BLDC. Having trouble picking one that isn't off the charts expensive. I'd like to modify an old box fan with the new motor. So, I'm needed something that can handle the torque of those plastic blades and still turn 100+ rpm.

And yeah, the draw from those makes the math much nicer.

 

A Google of RV Fans brings up several variations. The link being some offerings on Amazon.

Ron

 

So there are. I found this one, and I was curious why it's 13.x volts and not 12. I could see something like 24 or higher, but 13 is so close to 12. They aren't big on putting a lot of specs in there.

https://www.rvpartsnation.com/rv-el.../hengs-12-volt-replacement-vent-motor-jensen/



http://www.adventurerv.net/ventline...rtsfeed_ppc&utm_medium=cpc&utm_source=Froogle

 

Just because a battery can survive a deep discharge does not means you should discharge it! The battery must not fall under 11V and must not exceed 14.4V.

Also it would be better everything to be on 5 or 12V. If the inverter is good, the losses will be no more than 5%.

You still do not need an inverter, more likely a regulator.

 

I don't think the first link you posted is actually a BLDC motor. Looking through the dark hole I can see coil windings on an armature, which suggests to me it's not BLDC but instead it's commutated. And looking at the printed specs on the side of the motor it says 12 volts on it. The second link, I couldn't tell much from the picture and didn't delve into it to see if I could figure anything out on it.

You asked why a motor would be rated for 13 volts and not 12. Well, since that first motor is for automotive applications, cars typically run 13.8 volts when the engine is on. That someone would bother to identify it as a 13 volt motor ? ? ? Maybe it's a Chinese knockoff. I'm not knocking products from China, but there ARE a lot of counterfeit parts out there. Experts estimate about 60% of what's out there is potentially a knockoff of some sort. This is especially important in the military procurement field, as we don't want to put a chip in a missile and have it go astray because what was put in there was touted as being military grade but was a cheap knockoff. Even if chips cost a buck, with millions out there - - - . Get the point. Nuf bout dat.

Back to your issue: Moving air using a fan and a battery. A box fan sure would move a lot of air. But so would a smaller DC motored fan. The trick is to mount the fan blades securely to the armature of the motor. Small fans like the sort on the dash of an RV, those don't move enough air to replenish the cabin, they simply keep a breeze on the driver. A bigger fan, like one from a car radiator would be more apt to move sufficient volumes of air, as it's designed to do just that. And like I said before, you can PWM power it to control speed. Your only need would be to mount it in some sort of housing to keep little fingers out of the fan blades.

If 1.6 amps is sufficient for your desire then you can go that route. Off hand I don't know how many amps a window box fan draws on high. But I'd imagine since it's fusible linked internally it probably draws around 5 amps. But that's just pure guesswork on my part. Don't take that number to the bank. I bet someone here has a better and more accurate idea of the current drawn by a 22 inch box fan.

 

So there are. I found this one, and I was curious why it's 13.x volts and not 12. I could see something like 24 or higher, but 13 is so close to 12. They aren't big on putting a lot of specs in there.

https://www.rvpartsnation.com/rv-el.../hengs-12-volt-replacement-vent-motor-jensen/



http://www.adventurerv.net/ventline...rtsfeed_ppc&utm_medium=cpc&utm_source=Froogle

Both links took me to 12 VDC Vent Motors? Did I miss something along the way?

While we refer to a 12 Volt automotive system the reality is a 12 Volt SLA Battery is about 12.7 volts and while in use in an automotive system the running voltage is about 13.6 volts. Does it matter? Not really.

I see Tony already covered it as I typed, let the dogs out and opened a beer.

Ron

 

Sorry Ron. It's those darn dogs again. Otherwise I'd be the late one.

 

Why does everyone say that automotive batteries are 13.7V? This voltage is not used! The alternator charges at 14.4V, all you did was buy a new battery and measure it at 13.7V, that's only because the battery is sustained at 13.7, if it will not be used for a long time, but that voltage is otherwise useless!

 

Why does everyone say that automotive batteries are 13.7V?

I didn't see that said, where was that said? I did say: "a 12 volt SLA battery was about a 12 Volt SLA Battery is about 12.7 volts" and nothing about 13.7 volts. My trucks and bike all run around 13.7 volts when running but anywhere up to 14.4 volts would be fine.

Ron

 

I have to reiterate what Ron said. A nominal 12 volt battery is typically 12.7 volts when fully charged. Automotive systems typically run at 13.8 volts as a sustained float charge on the battery and power for the rest of the automotive electronics. After starting the engine it's common to see battery voltages go up to 14.4 volts but it doesn't stay there, it drops back down to around 13.6 to 13.8 volts. That's what we're saying about automotive electrics. The electrical system normally operates at 13.8 volts. Voltage will drop with demand, but in theory it should not drop below 13.6 volts. I said "IN THEORY!" Older batteries can drop voltages down to 13.4 volts. Happens all the time with my wife's car. The alternator doesn't put out the proper voltage. When she had an old battery and I turned on all the electrics I saw voltages drop down to 12.6 volts with the engine running.

 

I don't think the first link you posted is actually a BLDC motor. Looking through the dark hole I can see coil windings on an armature, which suggests to me it's not BLDC but instead it's commutated. And looking at the printed specs on the side of the motor it says 12 volts on it. The second link, I couldn't tell much from the picture and didn't delve into it to see if I could figure anything out on it.

You asked why a motor would be rated for 13 volts and not 12. Well, since that first motor is for automotive applications, cars typically run 13.8 volts when the engine is on. That someone would bother to identify it as a 13 volt motor ? ? ? Maybe it's a Chinese knockoff. I'm not knocking products from China, but there ARE a lot of counterfeit parts out there. Experts estimate about 60% of what's out there is potentially a knockoff of some sort. This is especially important in the military procurement field, as we don't want to put a chip in a missile and have it go astray because what was put in there was touted as being military grade but was a cheap knockoff. Even if chips cost a buck, with millions out there - - - . Get the point. Nuf bout dat.

Back to your issue: Moving air using a fan and a battery. A box fan sure would move a lot of air. But so would a smaller DC motored fan. The trick is to mount the fan blades securely to the armature of the motor. Small fans like the sort on the dash of an RV, those don't move enough air to replenish the cabin, they simply keep a breeze on the driver. A bigger fan, like one from a car radiator would be more apt to move sufficient volumes of air, as it's designed to do just that. And like I said before, you can PWM power it to control speed. Your only need would be to mount it in some sort of housing to keep little fingers out of the fan blades.

If 1.6 amps is sufficient for your desire then you can go that route. Off hand I don't know how many amps a window box fan draws on high. But I'd imagine since it's fusible linked internally it probably draws around 5 amps. But that's just pure guesswork on my part. Don't take that number to the bank. I bet someone here has a better and more accurate idea of the current drawn by a 22 inch box fan.

The box fan I was looking at was around 6 amps.

The reason I asked about 13V was this little bit in the notes of that first (commutated) motor:

"The Heng's 12V Replacement Vent Motor Jensen is also highly fuel efficient. At 13.6 volts of DC power, you will get 1.6 amps from this replacement vent motor."

It says 12V / 1.6amps on the casing. So, I was confused about what it would pull at 12V--if at all.

I've found some of these that fit the D shaft. Putting a housing together wouldn't be too tough.

https://www.amazon.com/Ventmate-65484-Replacement-Jensen-D-Bore/dp/B0085IJIPC
Oddly it says 6" fan but the dimensions include 8.9 x 6.9 x .9, so that's clear.

 

I was confused about what it would pull at 12V--if at all.

I got the impression you didn't quite understand. That's the reason for the long winded explanation of automotive voltages.

Here are some numbers on how things stack out: A 120 VAC motor drawing 6 amps uses 720 watts (120 x 6). Watts is watts. In the end the efficiency of your system comes down to watts. At 12 volts (we'll look at 13.8 too), 1.6 amps is (12 x 1.6) is 19.2 watts. Watts is the measurement of the work being done. 720 watts of air movement versus 19 watts is a huge difference. Even at 13.8 volts, (that'll also change the amperage since amps, volts and resistance are all related; and the resistance of the 12 v 1.6 a motor is not going to change) lets look at some more numbers:

12 ÷ 1.6 = 7.5 ohms. 7.5 ohms x 1.6 amps = 12 volts. Since these numbers are all related, changing the voltage changes the amperage, since the resistance doesn't change. We know that 7.5 ohms will draw 1.6 amps at 12 volts we can calculate how many amps will flow if we run 13.8 volts.

13.8 ÷ 7.5 = 1.8 amps.

Power is calculated by multiplying amps and volts. So 13.8 x 1.8 = 24.84 watts. Still, that's no where near the same amount of work being done. 25 watts versus 720 watts, the bigger fan will move nearly 30 times more air. So what I'm saying is that a 1.6 amp motor isn't going to give you the same results. In fact, it's going to be hardly worth it at all. And you're just wasting energy. Of course, setting up a bigger DC motor is going to draw many more amps, and therefore move more air, which in turn will drain your battery even faster.

Suppose you find a motor rated at 10 amps and 13.8 volts. That's 138 watts. Better, but still not moving the same amount of air. If you're thinking the bigger fan blades will move more air - think again. Larger blades will have greater wind resistance and drag the motor down and make it run a lot hotter. So there's no way around it. The amount of air you want to move will depend on how much wattage you use.

Assuming 720 watts in a perfect world moves X amount of air at 120 volts, (I say "Perfect" because things are anything BUT perfect) your inverter solution would draw (again, perfect world) 720 watts. That means a 12 volt battery powering an inverter is going to consume 720 watts (perfect world scenario). 720 watts divided by 12 is going to draw 60 amps. Given that things are not going to be perfect, you can expect to use more power than that.

In short, a 1.6 amp motor isn't going to produce satisfying results. Cars are much smaller environments than a bedroom, and moving smaller amounts of air is perfectly acceptable in a smaller environment, whereas in a larger environment they will prove to be less than satisfactory. So assuming you're using 60 amps on a battery that is rated at (and I couldn't find any specific details) 70 amp hour, your fan isn't going to run very long. You may have to get used to the heat.

 

I'm looking to get a 12V/35ah 'solar' battery from HF (which says it's good for repeated deep charging). I want to marry it to a fan via a power inverter. Swim season is here, and a fan and (and perhaps a phone charger) for the tent would be nice for a change.

The fan is about 84W at the highest speed setting--simple box fan from the depot.
The charger brick on the phone says 115VA and .15 amps (about 17.25W).

I've got a 300W power inverter I'd like to use.

1. Should I be using the battery's watts (35AH * 12V = 420W) divided by the watts of the fan (84) to get about 5 hours?
2. Or, should I be using the battery's AH of 35 divided by the fan's .73A (84W / 115V) which is far greater?
3. Or, is it the battery's 35A divided by the fan's amps of 7 (84W / 12v--from inverter input) to get about 5 hours?

For now, I'm ignoring charging states and depth.

I'm a little stumped where the inverter is concerned, but it feels like #3 where the watts demanded by the fan are 84, but provided at only 12V from the inverter input, thus 7 amps and not .7 amps AC.

Thanks for taking a look.

Just bought a low cost 300 watt pure sine wave inverter on Amazon for $45. It runs my Traeger Pellot smoker with a micro processor just fine. With 35ah battery I can get about 5hrs. run time before I get to about 11.5 volts. Note the inverter I purchased has a built in cooling fan which did not need to kick on, when its hot out it will draw a little more.