Q's on Calculating Power Consumption Requirements

[dave_in_delaware]

Finally had an epiphany

OK, I might be a little slow (no comments!!) but I just realized something while looking at my Calculations spreadsheet and changing some numbers around....

I'm currently using about 2158 Wh (180 ah) daily (worst-case scenario in winter, would be less in summer). But I've been multiplying by 5 to account for boondocking that many days without running a generator. This would yield the need for 900 ah needed out of my batteries for those 5 days. Well, I'm not planning on having a generator, but from an earlier post in this thread I'll be able to just run the engine or drive around and let the alternator recharge my batteries and that will suffice for my recharging needs.

So that means instead of multiplying by 5, I could multiply by 1 if I make sure I have the alternator going EVERY DAY for long enough to assist in recharging. This could be accomplished by driving to a new destination every day. (It is an RV after all). Using the 1 multiplier brings a total ah need of 180.

So, if I wanted to boondock for two days, I'd only multiply my daily usage by 2, right? This would bring my usage numbers down significantly (from the 5 multiplier) to 360 ah, and make it so I wouldn't need five 198 Ah batteries but more like two as long as I ran the engine or drove around to let the alternator charge up the house batteries (plus get into the sun for solar panel help, too).

Is my logic correct? I could do this as long as I drive around every day or every other day?

[Naptown]

OK, I might be a little slow (no comments!!) but I just realized something while looking at my Calculations spreadsheet and changing some numbers around....

I'm currently using about 2158 Wh (180 ah) daily (worst-case scenario in winter, would be less in summer). But I've been multiplying by 5 to account for boondocking that many days without running a generator. This would yield the need for 900 ah needed out of my batteries for those 5 days. Well, I'm not planning on having a generator, but from an earlier post in this thread I'll be able to just run the engine or drive around and let the alternator recharge my batteries and that will suffice for my recharging needs.

So that means instead of multiplying by 5, I could multiply by 1 if I make sure I have the alternator going EVERY DAY for long enough to assist in recharging. This could be accomplished by driving to a new destination every day. (It is an RV after all). Using the 1 multiplier brings a total ah need of 180.

So, if I wanted to boondock for two days, I'd only multiply my daily usage by 2, right? This would bring my usage numbers down significantly (from the 5 multiplier) to 360 ah, and make it so I wouldn't need five 198 Ah batteries but more like two as long as I ran the engine or drove around to let the alternator charge up the house batteries (plus get into the sun for solar panel help, too).

Is my logic correct? I could do this as long as I drive around every day or every other day?

You are correct as long as you drive long enough to charge the batteries. I would suggest the largest alternator you can find to speed up the process and particularly if you are just idling to charge as the output on the alternator will be lower at lower RPMs
I had boats and had the same situation (however not the winter thing) and would run the engine a couple of hours a day to charge if shore power was not available. Generally this was when we were moving the bot anyway.

[dave_in_delaware]

You are correct as long as you drive long enough to charge the batteries. I would suggest the largest alternator you can find to speed up the process and particularly if you are just idling to charge as the output on the alternator will be lower at lower RPMs

Thanks Naptown. I was hoping I was simply overlooking a simple solution to my power replenishment problems, and not really needing a trailer with 5 AGM batteries and a not-so-portable solar panel farm....

I think the standard size for a Ford E-series van is 120 amps. They do have the option for a second alternator to be installed, as well as heavy duty (155 amps) and extra heavy duty (225 amps) alternators available as options. There are also aftermarket alternators available, such as a Nations brand 240 amp model. Obviously the larger the alternator the better off I'll be. I guess the trick would be finding an alternator that'll actually fit into the van I get.

And I'd say driving to the next destination (higher RPMs) will be the preferred method of charging, as opposed to just sitting and idling forever and sucking up gas.


I am still searching for lower amp-draw components for my RV. I'd like to find a more efficient refrigerator and a more efficient heating system, since they're both my largest power drainers.

[Naptown]

What about a gas/ electric refrigerator? Those are common in RV's or at least used to be

[dave_in_delaware]

What about a gas/ electric refrigerator? Those are common in RV's or at least used to be

Yeah, they're still pretty common. Most RVs I know of have a 3cf or 4cf fridge in them, and they're usually 3-way (propane, AC, DC) powered. I don't really have the room for a built-in refrigerator in my design layout so I've been looking at the portable top-load fridges with a DC compressor. So far, I've been eyeing up a few that are relatively low amp draw:

Engel MT17 = variable from 0.6 - 2.3 amps
Engel MT27 = variable from 0.7 - 2.5 amps
Engel MT35 = variable from 0.7 - 2.5 amps

There are Thermo-Electric coolers, but they tend to draw more power than the DC compressor fridges:

Koolatron P9 Traveller Cooler = 3 amps
Koolatron P20 Compact Cooler = 4 amps
Koolatron P75 Kool Kaddy = 4 amps

Another option for keeping things cold might be one of the Engel Deep Blue Coolers, some of which can hold ICE up to 10 days. I guess that might work as well. I'd have to buy ice, but that doesn't drain the batteries 24/7.

[Zillard]

3. To find panel wattage needed multiply daily wh by 2,

Sunking, i cant find an explanation why you multiply by 2? Please explain or link to relevant thread?

Thanks

[Naptown]

This gives a rough requirement for use and recharging taking losses in controllers wiring and batteries.

[Zillard]

This gives a rough requirement for use and recharging taking losses in controllers wiring and batteries.

Awesome, that makes sense.

Thanks.

[dave_in_delaware]

As was stated, it's for inefficiencies (wiring, batteries, connections, etc).

Multiply by 2.0 for PWM controllers
Multiply by 1.5 for MPPT controllers

And I think I read somewhere that if no inverter is used (DC only), multiply by 1.3 (I guess for MPPT controllers).

[dave_in_delaware]

To continue the refrigerator discussion:

I think I've found a winner for "low amp draw" refrigerators...

A company called ARB makes a ton of different off-road 4X4 stuff. One product is an improved portable refrigerator which can withstand rough terrain driven by 4x4 vehicles. The fridges also have better insulation and a thicker door (top-open to keep the coldness IN). But the heart of their fridges is the German Danfoss compressor. It's the most efficient compressor out there, and it's a two-stage compressor. When you first turn on an ARB it uses about 5 amps per hour in "turbo chill" mode to bring everything inside the fridge down to whatever temp you set. Then, the digital temperature monitor inside the fridge turns the fridge on and off 15 times per hour to keep the contents at the set temp, which only uses maybe 1 amp per hour. And while most portable fridges can only maintain a 40

[Naptown]

Well if you're home then wouldn't you be driving somewhere and have the alternator cool down the fridge?

[dave_in_delaware]

Well if you're home then wouldn't you be driving somewhere and have the alternator cool down the fridge?

Yeah, you're right. The initial cooling ("turbo chill") of the contents would be done at home before the trip where AC power would be used. Then the fridge would be moved to the RV and plugged into 12v power. While driving to my destination, the alternator would be providing/replenishing the 12v power. So only when we're finally parked at our destination will the batteries actually be doing all the work. Although at only 1 amp it's not really doing all that much work, compared to other portable refrigerators.

[afaagagghhat]

Yeah, but that was the OP's data. I figured it to be a wide range wall wart supply, and at 120V, it used the max amps, at 240vac, it would use less.