Coffee by the numbers - your results may vary!

[GuyT]

Some time ago there was a post about coffee makers and battery power usage. Some say to use a fuel stove to make coffee since it uses so much power. I recently purchased a Kill-a-Watt meter and tried it on my Mr. Coffee machine this morning.
Here are the results for 6 cups of coffee:

Power from coffee maker - 930 Watts.
Time for 6 cups to brew - 6 minutes.

Power usage was 90 Watt-Hours to brew the coffee for the 6 minutes and then as soon as the coffee was done brewing the internal thermostat kicked on and reduced the power consumption to 2 Watts (got to keep the power light on).
The power then cycled on and off to keep the coffee hot. Power levels to maintain the temperature of the coffee was very low!
After 15 minutes the total power consumption was 100 Watt-Hours which included the brew time.

So what that means for the battery if you have an inverter:

Brewing 6 cups will require about 7.5 Amp-Hours from your battery - probably 8 when you consider the efficiency from the inverter.
Maintaining the coffee hot will require about 1 Amp-Hour every 5 minutes.

So, go ahead and get that inverter for your coffee - the power aint so bad after all!

 

[Ross]

That also means that you need a minimum 1000 watt

inverter. How much do they loose in heat during operation? Or better said what is their efficiency?

 

[sailaway78]

I still prefer

There are drawbacks to any method. The Mr. Coffee or similar coffee makers use battery power, some more than others. The perked coffee uses stove fuel and heats up the boats interior. I prefer perked coffee, in an old blue camping pot. My morning coffee is an important part of my day. So for me it comes down to what you prefer. The hell with the drawbacks.

 

[Napa Ed]

electron disadvantaged

How large is your bank? I only have a single house battery (27) so I couldn't afford to spend 7.5 amp hours on coffee. That's more than I use all day for instruments. Amp hours are like gold on my boat.

 

[Dennis W.]

Batteries and Amp Draw

The part your missing is the max amp draw on your house bank. 7.5 amps at 120 volts from your inverter will require 75 amp draw at 12 volts from your house bank. In real world applications that will greatly reduce the effectiveness of your house bank unless you have bank of at least 1500 amps.

 

[Ross]

This load will be like cranking the engine for

6 minutes without let-up.

 

[agaliha]

Just made the same discovery this summer

I have a Beneteau 361 with two 6D batteries for the house and a Group 27 for starting the engine. I have a 2000 W inverter/charger. I bought a little 4- cup Mr. coffee so that I don't have to fire up the propane stove to boil water. The cabin stays cooler and the coffee is better! I use the little prepackaged filter packs. I'm amazed at how much juice I can draw through the inverter. I'm watching the consumption with a "Link 1000" and the coffee maker doesn't even put a dent in the battery power.

 

[Ross]

agaliha, try putting that load on just you grp 27

starting battery! ;D

 

[sinnettc]

Coffee maker blues . . . .

. . . I've got 'em . . .

We just bought a Catalina Capri 22 which we'll be overnighting on. A week ago this last Friday we spent our first night on the boat. Since the Admiral suffers from the coffee addiction her first purchase was a single-serving black & decker coffee maker. My immediate response was to tear open the box and check the wattage -> 800! Back in to K-mart to pick up the biggest inverter I could find. Luckily it was on sale.

Saturday morning out comes the coffee pot. I plugged in the inverter (750w) and crossed my fingers. A few low battery warning beeps and 5 minutes later she had her cup of coffee and I made a note to bring the marine deep cycle battery for the trolling motor and run parallel batteries for next time.

Watching the meter drop into the danger zone inspired me to stop by Camping World on the way down this weekend. Sure enough, they had a 4 cup 12 volt coffee maker! Yahoo! Or maybe not. I returned from a trip ashore and before I even stepped on the boat she said "it doesn't work - it's not popping the breaker but it starts and then stops." Sure enough, even with 2 batteries, the 12v coffee maker buzzes for about 2 seconds, apparently figures out it's not getting enough juice, and shuts down.

Maybe it will work in the truck . . . with the engine running . . . if not can you guess what my Dad is getting for his RV for Christmas?

Morale of the story? I obviously know nothing about 12v electrical systems 'cuz I thought 2 batteries in parallel would let me take over the world.

 

[GuyT]

Might be your cables

Sinnettc, make sure your battery cables are heavy enough to handle the current without alot of voltage drop.
Use at least 4 gauge and try to keep the length down.
Your batteries may have been fine but your losses (voltage drop)were all in your cables.
Now, go get working on that to keep the admiral happy

 

[sinnettc]

Cables? - Questions!

4 gauge definitely sounds larger than what's there - more like what's on the inverter if I read the info on the AWG table I found right. Runs are both probably in the 8 foot range - rear battery box to the breaker panel and just aft of the porta potty into a locker, then to the panel - presuming he didn't just do a long run from the new battery box in the cabin to the aft one. My multimeter cables arent' quite long enough to tell and I haven't actually traced the wires, though I suppose I should. The pot plugs into the cigarette lighter plug which I'm sure doesn't have 4 gauge wire.

Anybody know what size the standard wiring is on a Catalina Capri 22 and whether the panel will accept wire that large?

Chris

 

[jviss]

Clarification?

"Maintaining the coffee hot will require about 1 Amp-Hour every 5 minutes."

"After 15 minutes the total power consumption was 100 Watt-Hours...."

I don't understand the math here.

6 minutes of brewing time consuming 7.5 AH would be 75 Amps for that 6 minutes, neglecting (potentially significant) conversion losses (inefficiencies). That's a huge load, and it's doubtful your batts would actually put out their rated capacity at that discharge rate.

"Maintaining the coffee hot will require about 1 Amp-Hour every 5 minutes."

Does this mean 12 Amps average load, for 12 AH per hour?

 

[GuyT]

battery drain is in amp hours and amps

jviss, you need to know that battery capacity is rated in amp-hours.
If the battery you have is a group 27, it may be rated anywhere from 80 to 100 amp hours. The battery manufacturers do this rating at a 20 hour rate usually since it makes their numbers look better.
So if you have a load that is 5 amps and it runs for 20 hours you have used 100 amp hours.
Now if you have a 75 amp load and it runs for 6 minutes(which is 0.1 hours exactly) you have used 7.5 amp hours.
Batteries have different discharge capabilities at different discharge rates and I am going to ignore that for now since it makes it more complicated.
But in general if you have a group 27 that is 100 amp hours and it was completely charged, when you brew your coffee you use about 7.5 amp hours out of that 100 amp hour capability. Or about 7.5 percent of the complete capacity.
Again - this is a generalization and is not exact because of the different capacities at different loads but I hope you get the idea now.

BTW, I would not recommend using a group 27 battery for this application. At a minimum I would use 2 in parallel. Inverters like a low impedance source and the more batteries you have in parallel, the lower the impedance will be.

 

[jviss]

understood...

GuyT,

I know exactly how capacity is measured, I wanted to be clear about what you were saying.

I am concerned about the efficiencies, and Peukert's law (diminishing capacity at higher discharge rates).

I think the short answer to my original question is "yes."