Amp Hours--I don't get it

[DanDweller]

Hi, can someone please help me make sense of calculating how many amp hours worth of battery I need, taking the Dometic cfx28 fridge as an example?  Here is the published info on the Dometic site, and it might as well be in Greek:

ELECTRICAL

[font=Gibson, Helvetica, Arial, sans-serif]Input voltage (AC) 100-240 V
Input voltage (DC) 12/24 V
Rated input current (AC) 0.75 A
Rated input current (DC) 6.5 A
Rated input power (AC) 40 W
Rated input power (DC) 40 W
Input frequency 50/60 Hz[/font]

[font=Gibson, Helvetica, Arial, sans-serif][size=small][font=Gibson, Helvetica, Arial, sans-serif]Average energy consumption at AC (EN 62552)[/font][font=Gibson, Helvetica, Arial, sans-serif]0.18 kWh/24h[/font]

[font=Gibson, Helvetica, Arial, sans-serif]Energy consumption (DC@5/32°C)[/font][font=Gibson, Helvetica, Arial, sans-serif]0.27 kWh/24h[/font][/font][/size]


So, with this information, and assuming I keep my fridge on 24/7 using DC, how do I calculate amp hours?  Would it be 6.5 * 24 = 156 amp hours?  I also will run a vent fan for the full 24 hours, and it is said to draw 5 amps (Maxxfan), so then 5 *24 = 120?  So then between my fridge and fan I would draw 120+156=276 amp hours in a day?  That seems like a crazy amount, requiring a few batteries.  I was hoping to get a 100 amp hour lithium iron phosphate battery, but so that won't cut it?

 

[MrNoodly]

All I can tell you is that I run my Dometic CF-25 just fine with 208Ah of battery, rarely getting below 70% of capacity. I also charge my electronics and run some lights on that 208Ah.

Remember, the fridge doesn't run constantly, only when the temperature rises above the setting.

 

[Ticklebellly]

The fridge - draws 6.5 Amps when the compressor is running, you know, making cold. The fridge may be switched on 24/7 but the compressor runs for part of the time. Manufacturer pick some ratio of time on and time off and says Average consumption is XXX. Reality is that the fridge thermostat is set by the user, and the fridge thermostat waits until the fridge interior gets a little bit above what is set then turns on to bring the temperature back down to what was set. Time compressor runs to time the compressor does not run is called Duty Cycle. For example, in 41 Degrees C, my Engel runs nearly 80% of the time. In 22 Degree C days it runs about 17% of the time. So, for you, the AH used by your fridge is 6.5 Amps multiplied by the number of hours the compressor was running.

 

[Ticklebellly]

The fan - Draws 5 Amps when the blades are going around. Multiply 5 Amps by the number of hours the fan was running to get the number of Ah consumed by the fan.

You already seem to know that Amps is an instantaneous measure of the current flowing as one point in time. You need to add the amount of time the current was flowing to get Ah.

 

[Ticklebellly]

I have questions around the 6.5 Amps on DC. That is a seriously big draw for such a small fridge. The AC input range of 100-240 suggests there is a Switch Mode power supply in there between the AC input and the compressor. That would also explain why such a high DC draw. There is some inverter magic in there somewhere. Small fridges running Danfoss compressors typically draw around 3.5 Amps and the Engel draws 2.5 Amps. If you want to use a 100 AH LiFePo4 for your house battery, might be good to rethink the model and brand of fridge you want to get.

 

[Ticklebellly]

I also question the Maxxfan numbers. My Google search says 0.16 Amps on low and 2.5 Amps on high. Qua?

 

[tx2sturgis]

We need a few more pieces of information:

Will you be recharging and powering this equipment with solar for many hours a day? I assume yes, and if so, the power to run those items (during daylight) will mainly come from the solar panels, more or less... (the battery will be involved, but it wont be draining, assuming you have sufficient solar input...about 200 watts minimum, to replace the power used the night before, and top-off the battery the next day)

So the battery only needs to be 'online' and powering the items for, let's say, 12 hours at night. (make it easy, mathematically)

For the fridge, this is THE important spec: "Energy consumption (DC@5/32°C)0.27 kWh/24h"

0.27 kwh is 270 watt hours, BUT, divide by half (12 nite-time hours). Lets round up to 150 to account for hot night-time temps.

Since we are already dealing in watts, lets stay there. 

A max fan pulling 5 amps on high will burn 780 watt hours per night. ( 5 amps x 13 volts x 12 hours) But I bet you can run it on low power most nights...lets assume 1 amp. 156 watt hours...much better.

A typical 100 ah LFP battery is rated at around 1200 watt hours. (more or less).

Your loads as I calculated will pull about 306 wh from a 1200 wh battery. Entirely doable.

You have to replace that energy the next day, one way or another. (solar, generator, etc) 

The vent fan is your big 'variable'...running it on high all night long, plus the fridge, will cause a large 'dent' in your battery overnight. Unless you have PLENTY of solar, this higher rate of usage wont work for long with that one battery.

 

[DanDweller]

The fridge - draws 6.5 Amps when the compressor is running, you know, making cold.   The fridge may be switched on 24/7 but the compressor runs for part of the time.   Manufacturer pick some ratio of time on and time off and says Average consumption is XXX.   Reality is that the fridge thermostat is set by the user, and the fridge thermostat waits until the fridge interior gets a little bit above what is set then turns on to bring the temperature back down to what was set.   Time compressor runs to time the compressor does not run is called Duty Cycle.   For example, in 41 Degrees C, my Engel runs nearly 80% of the time.   In 22 Degree C days it runs about 17% of the time.   So, for you, the AH used by your fridge is 6.5 Amps multiplied by the number of hours the compressor was running.

Thank you, this is very helpful.  Ok, so with the Engel fridge on the hottest day the compressor might be on 80% of the time, so I would be looking at: 2.5*(.8*24hrs)=48Ah.  As for the Maxxfan, the internet has conflicting information, some saying the highest setting draws 4.4 to 5 amps, others saying only 2.5.  If I assume I'll be ok with whatever speed setting corresponds to ~2amps on the hottest day, then: 2*24=48Ah.  Besides those two main appliances, the other amp draws would be a 3-axis wall-mount fan which is said to draw about .3amps, a Shurflo water pump which draws a max of 7.5amps, LED lights, electric hair clippers which draw no more than 1.25amps, Macbook, and whatever my 1,500 watt inverter draws.  

I'm just trying to think of the heaviest day as far as power usage and make sure I get enough battery, but the Lithium ones are damn expensive, so if I can get away with 100 Ah, that would be nice.  I do plan on having 200W of solar panels.  If it seems like it would be cutting it close, then it might be worth it to me to go ahead and fork out the money for the Renogy 170Ah battery to have a nice buffer.  I can imagine there will be plenty of times when it is stormy out for a couple days (so no solar charging) but plenty hot out and so I would want to use the vent fan all day, and of course keep the fridge going.  I tend toward hot climates and use the vent fan pretty non-stop during the summer.  Is the 170Ah battery overkill for my power usage?

 

[DanDweller]

We need a few more pieces of information:

Will you be recharging and powering this equipment with solar for many hours a day? I assume yes, and if so, the power to run those items (during daylight) will mainly come from the solar panels, more or less... (the battery will be involved, but it wont be draining, assuming you have sufficient solar input...about 200 watts minimum, to replace the power used the night before, and top-off the battery the next day)

So the battery only needs to be 'online' and powering the items for, let's say, 12 hours at night. (make it easy, mathematically)

For the fridge, this is THE important spec: "Energy consumption (DC@5/32°C)0.27 kWh/24h"

0.27 kwh is 270 watt hours, BUT, divide by half (12 nite-time hours). Lets round up to 150 to account for hot night-time temps.

Since we are already dealing in watts, lets stay there. 

A max fan pulling 5 amps on high will burn 780 watt hours per night. ( 5 amps x 13 volts x 12 hours) But I bet you can run it on low power most nights...lets assume 1 amp. 156 watt hours...much better.

A typical 100 ah LFP battery is rated at around 1200 watt hours. (more or less).

Your loads as I calculated will pull about 306 wh from a 1200 wh battery. Entirely doable.

You have to replace that energy the next day, one way or another. (solar, generator, etc) 

The vent fan is your big 'variable'...running it on high all night long, plus the fridge, will cause a large 'dent' in your battery overnight. Unless you have PLENTY of solar, this higher rate of usage wont work for long with that one battery.

Yes, I plan on having 200W of solar panels.  I tend toward hot climates and end up using the vent fan non-stop during the heat of summer, day and night.  I'm sure there will be plenty of times when it is overcast or stormy outside (so no or mininal solar panels charging) yet plenty hot enough to want the fan on (as well as a .3amp draw wall-mount 3-axis fan).  

Basically, I don't want to cut things so close that if the sun doesn't shine for a day I end up having to idle the engine to charge the battery or plug into shore power.  I'm kind of leaning toward just getting the Renogy 170Ah battery to have some wiggle room.  That should be more than plenty for me then, right?

 

[highdesertranger]

it seems like you got it now.

I am confused with Dometic's numbers they say 40 watts on DC but then say 6.5 amps. that's funny math. 6.5 x 12 = 78 or 40/12 = 3.3.......highdesertranger

 

[Trebor English]

There are many variables. One specification sums it up:
Energy consumption (DC@5/32°C)0.27 kWh/24h

What that means is that the fridge will use 0.27 kilowatt hours per day when running on DC, 12 or 24 volt, with the inside temperature of 5 degrees C (41 degrees F) and an outside temperature of 32 degrees C (89.6 degrees F). 0.27 kilo watt hours per day is 270 watt hours per day. At 12 volts (270 divided by 12) the energy use is 22.5 amp hours per day.

The purpose of this specification is to show how much energy will be used per day with a specified interior and ambient temperature. The unit will probably draw 22.5 amps never per day. The on/off duty cycle, ratio, percentage, fraction, is not specified. The compressor running period and off period, minutes, are not specified. If l the unit is full of bottles of cold water the time it stays off will be longer as will the time it stays on once it starts.

If the ambient temperature is hotter, 95 degrees F, 35 C, and if the interior temperature is colder, 35 degrees F, 1.667 C, then the energy use per day will be higher. Opening the door, holding it open, loading warm drinks, and anything else outside of the standard test condition will raise the energy use.

If the running power is 40 watts that is 3.3 amps. The startup surge is probably twice that, 6.6 amps for two seconds. The specified current, 6.5 amps, is likely the maximum, the compressor start surge.

 

[DanDweller]

Ok, I see, thank you.  So in the end it is comparable to the Engel?

 

[Trebor English]

All of these products are similar. They are very expensive per cubic foot compared to regular residential fridges. They use much less electricity. There is no magic technology that some have and others don't have. I don't have a Dometic cfx28 fridge but I suppose that they are similar to other such products. The 22.5 amp hour per day energy consumption seems about right. These small fridges are usually reported to use mid to upper twenties of amp hours per day by people in hot places.

If your actual energy use is 30 amp hours per day and you use it two days with no sun, that's 60. A 100 amp hour battery will do that.

According to a you tube video by slim potatohead his $240 Alpicool C20 uses 5.8 amp hours per day. It is only 20 liters, 0.7 cubic feet so smaller than the Dometic cfx28 26 liter, 0.9 cubic feet.

 

[Ticklebellly]

 Ok, so with the Engel fridge on the hottest day the compressor might be on 80% of the time, so I would be looking at: 2.5*(.8*24hrs)=48Ah.  

If the ambient temperature stays high enough so the duty cycle is 80% for day and night, 24 hours, then Yes.   Reality is that highest temps are through the day, and the ambient temp drops off at night.   So for the hottest part of the day the duty cycle might be 80% and the cooler night temps might see a duty cycle down in the 40%.   I regularly experience night time temps so that the duty cycle drops to 10% or less.

We are talking gross figure estimates here of what power is needed to run the fridge.   Discussions of what size battery you might need, needs to consider that during the day, the power to the fridge is likely to be totally supplied from the Solar panel.   A 200 watt panel in full sun will run the fridge and have something left over to recharge the power taken from the battery to run the fridge overnight.

 

[Ticklebellly]

As for the Maxxfan, the internet has conflicting information, some saying the highest setting draws 4.4 to 5 amps, others saying only 2.5.  If I assume I'll be ok with whatever speed setting corresponds to ~2amps on the hottest day, then: 2*24=48Ah. 

Yep!   Again, if you run the fan 24 hours a day at a setting that draws 2 amps, then 2*24.   Are you likely to run the fan 24 hours a day?   Right through the night?   Maybe you will run the fan on max in the hottest part of the day and drop the setting to low at night.   Do you have a multimeter and know how to measure current?   Internet derived info on current draw are ball park only.   Having an amp meter in circuit while switching between fan settings will provide actual information.

 

[Ticklebellly]

 Besides those two main appliances, the other amp draws would be a 3-axis wall-mount fan which is said to draw about .3amps, a Shurflo water pump which draws a max of 7.5amps, LED lights, electric hair clippers which draw no more than 1.25amps, Macbook, and whatever my 1,500 watt inverter draws.  

Fan is not a problem.   The water pump will run for only a short total number of hours (minutes) in a day.   The 1500 Watt inverter will be a problem.   The inverter running at full capacity will draw lots and lots of current.    My opinion only, I would want at least 200 Ah LiFePo4 and be careful about how long the inverter ran.   I would also try to run the inverter early on sunny days so the battery had plenty of Solar charging to get it back up as soon as possible.

 

[Ticklebellly]

...........then it might be worth it to me to go ahead and fork out the money for the Renogy 170Ah battery to have a nice buffer. 

If you afford the larger battery, I would encourage you to go for it.   I still have concerns about the the apparent high draw of that model fridge, compared to others of similar size.    I have already mentioned that the inverter use needs to be managed.

 

[DanDweller]

All of these products are similar.  They are very expensive per cubic foot compared to regular residential fridges.  They use much less electricity.  There is no magic technology that some have and others don't have.   I don't have a Dometic cfx28 fridge but I suppose that they are similar to other such products.  The 22.5 amp hour per day energy consumption seems about right.  These small fridges are usually reported to use mid to upper twenties of amp hours per day by people in hot places.  

If your actual energy use is 30 amp hours per day and you use it two days with no sun, that's 60.  A 100 amp hour battery will do that.    

According to a you tube video by slim potatohead his $240 Alpicool C20 uses 5.8 amp hours per day.  It is only 20 liters, 0.7 cubic feet so smaller than the Dometic cfx28 26 liter, 0.9 cubic feet.  

Thanks for the video link and the general ballpark idea of what the fridges consume.  I'm not set on getting the Dometic--maybe I'll try the one he reviewed.

 

[tx2sturgis]

I want to point out that the larger 170ah battery will provide more usable power, BUT...if it is depleted after a few days of heavy usage with little or no solar, then it may be a bit harder to bring it up to full charge (with 200 watts of solar and high power fan operation, lights, fridge, and other items).

Attaining full charge (at least occasionally) is crucial to LFP batteries because that is when 'top balancing' occurs.

If the OP depletes that 170ah battery because there seems to be a 'bottomless well' of power, then it may be difficult to bring it back to full charge with only 200 watts of solar, especially during a week of cloudy weather. If those panels are portable then they can be moved to track the sun and will perform much better, but if they are roof mounted and fixed in place, the overall daily output is going to be less.

200 watts of solar will make around 10-12 amps under full sun (and less when off-angle, shaded, hot, or on cloudy days) and a depleted 170 ah LFP will consume all of it, but it will take about 17-20 hours (maybe more) of full output from the panels to fully recharge and balance the cells.

For the $500 price difference, I would buy the 100ah LFP and then buy another solar panel or two.

Not a deal killer but something to consider.

 

[DanDweller]

Yep!   Again, if you run the fan 24 hours a day at a setting that draws 2 amps, then 2*24.   Are you likely to run the fan 24 hours a day?   Right through the night?   Maybe you will run the fan on max in the hottest part of the day and drop the setting to low at night.   Do you have a multimeter and know how to measure current?   Internet derived info on current draw are ball park only.   Having an amp meter in circuit while switching between fan settings will provide actual information.

Yes, there have been hot stretches when I ended up running the fan 24/7.  I just want to be prepared for the heaviest power consuming days.  My cat also lives in the van with me.  
I have a voltmeter, but I don't think I've measured current, just voltage and continuity.  Anyway, I think I'm getting a good rough idea and will probably get the 170Ah battery for peace of mind and room to grow.  As for the inverter, I rarely use the one in my current van (just a shaver and laptop pretty infrequently) so it's not really a big deal.