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 > Amp draw vs battery bank & solar array = totally confused!

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Sarahps33

Winchester, VA

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Posted: 12/23/21 11:53am Link  |  Quote  |  Print  |  Notify Moderator

Hey there, so I'm hoping some of yall have already dealt with this and could shed a little light.

Things aren't making since to me. I have checked each appliance's output power that I plan on bringing on the road.

For example my laptop says 20v/2.25a on the charger.
For each device like this I went to this website for a quick calculation: https://www.batterystuff.com/kb/tools/dc-to-ac-amperage-conversion-run-through-an-inverter.html

And I'd turn it into 12v DC - giving me the amp draw.

In the case of my laptop it was converted to 4.14a
I took those amps and multiplied it by how many hours a day I would use it.
The most I'd ever use it in one day would likely be 7hrs.
4.14a x 7hrs = 28.98a

So that tells me how many amps I'd use from that laptop a day, right? Am I doing it right so far? Lol.

I did this for my coffee maker, phone charger, flashlights, printer, and so on and came up with 348 amps that we could "potentially" use in a day if we used and charged everything we owned in a single day (I think that's overkill, but just in case right.)

And that's without a fridge. We want a fridge with a freezer so we can be off grid for longer periods of time. I have one now and if it ran 12hrs outta 24 it would be an additional 192 amps.

So the total with a fridge is 539

That seems like an awful lot to me. I dont understand how people have whole gaming systems, fridges, and microwaves and can run off a small batt bank.

Did I mess up somewhere?

I'm reading that AGM's have a depth of discharge rate of 80-50%. If we used 80% of each battĀ then we'd need seven 100ah batts just to run for one day! (539a/80a=6.7 batts at 100ah). If we wanted enough back up power for 2 days (say it was cloudy out) than we need 7x3 = twenty one 100ah batteries. That's what I dont understand....

But! I also understand that this is where solar charging rates come in. I've only seen 60a being one of the highest charge controllers. So does that mean I can charge with 60 amps an hour with my solar panels? (Assuming I have full sun light?) And say I have 4 hrs of full sunlight a day 4hrs x 60 amps = 240 amps charged per day.

With an amp draw of 539 it would take roughly 2 days and 1 hour to put that back into the battery bank. And that would still only be replenishing our daily draw but not as fast as we use it and wouldnt provide those 2 days of cloudy weather cushion.

I'm so in need of some help here, lol. I cant wrap my head around it.

Thank you so much in advance for any info you can give me.

2oldman

NM

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Posted: 12/23/21 12:11pm Link  |  Quote  |  Print  |  Notify Moderator

Sarahps33 wrote:

For example my laptop says 20v/2.25a on the charger.

And I'd turn it into 12v DC - giving me the amp draw.

In the case of my laptop it was converted to 4.14a
I took those amps and multiplied it by how many hours a day I would use it.
The most I'd ever use it in one day would likely be 7hrs.
4.14a x 7hrs = 28.98a

So that tells me how many amps I'd use from that laptop a day, right? Am I doing it right so far? Lol.
Not quite.

To do a direct comparison you need to calculate watts. For example, the laptop is 20v/2.25a = 45 watts.

45w/12.5v= 3.6 amps. Not sure where the 4.14 came from.

Try doing it that way. And if you use 3.6 amps for one hour, that's called "3.6 amp hours", not "amps" and not "amps per hour." If your laptop takes 3.6 amps, it's taking 3.6 amps *continuously* to run.

A 60a solar controller would allow a charge rate of 60a, however, having enough panels to do that is another story.

time2roll

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Posted: 12/23/21 01:51pm Link  |  Quote  |  Print  |  Notify Moderator

Sarahps33 wrote:

For example my laptop says 20v/2.25a on the charger.
For each device like this I went to this website for a quick calculation: https://www.batterystuff.com/kb/tools/dc-to-ac-amperage-conversion-run-through-an-inverter.html

And I'd turn it into 12v DC - giving me the amp draw.

In the case of my laptop it was converted to 4.14a
I took those amps and multiplied it by how many hours a day I would use it.
The most I'd ever use it in one day would likely be 7hrs.
4.14a x 7hrs = 28.98a

So that tells me how many amps I'd use from that laptop a day, right? Am I doing it right so far? Lol.
The rating is the maximum. That would be high usage such as streaming a movie and charging the battery at the same time.

For the 7hrs use it may only take that power listed for 2 hours to re-charge.

A compressor fridge will be a challenge. May need 500 to 700+ watts solar to be off-grid continuous.

Propane fridge uses very little battery power (~500 miliamps) to run the controls while the cooling energy comes from the propane.


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MitchF150

Puyallup, WA

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Posted: 12/23/21 02:30pm Link  |  Quote  |  Print  |  Notify Moderator

Hi. Have you checked out those "Battle Born" lithium battery and solar setups?

That's what I hear is the 'best bang', but they are pretty expensive, but if off the grid is your goal and you can hang around where the sun does shine a good part of every day, I've seen videos where they will run a 13.5k RV AC unit from them and all the other components you mention.. (no direct experience with any of it myself)

But, they also have rather larger solar panel setups and lots of watts or amps or whatever it takes to keep them charged and lots of sun..

Good luck! Mitch


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wa8yxm

Davison Michigan (East of Flint)

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Posted: 12/23/21 03:09pm Link  |  Quote  |  Print  |  Notify Moderator

The first thing you need to know is that 2.5 amps at 20 volts is the MAXIMUM draw, it might average as little as 0.5 amps at 20 volts.

Only one way to be sure.. Meter it.. In this case I recommend using a Kill-a-watt meter on the wall plug side.. This is about a thousand dollars worth of meters in a 30 dollar box (A quick google search found it ranged 10-40)
Measures
Volts
Amps
Frequency
Volt-Amps (Not the same as watts)
Watts
Power factor (The relationship between the last two items)
And Think one more thing (Running time)
Oh KWH (Killowatt watt hours or watt hours that's the other thing)

To figure out what the battery needs to provide assume 10 volts after losses
So, if the computer draws 50 watts.. or 50 volt amps (use the larger) the batteries need to push 5 amps to the inverter.


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pianotuna

Regina, SK, Canada

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Posted: 12/23/21 04:24pm Link  |  Quote  |  Print  |  Notify Moderator

Sarahps33 wrote:



For example my laptop says 20v/2.25a on the charger.
For each device like this I went to this website for a quick calculation: https://www.batterystuff.com/kb/tools/dc-to-ac-amperage-conversion-run-through-an-inverter.html

....


I'm so in need of some help here, lol. I cant wrap my head around it.

Thank you so much in advance for any info you can give me.



There is a spread sheet here:

https://freecampsites.net/adding-solar/

Perhaps the best "real way" is to plug the RV into a kill-a-watt meter and measure the consumption for 6 days. You won't be able to exceed a 15 amp (1800 watt) draw.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

theoldwizard1

SE MI

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Posted: 12/23/21 04:55pm Link  |  Quote  |  Print  |  Notify Moderator

Sarahps33 wrote:


For example my laptop says 20v/2.25a on the charger.


You choose a bad example ! That is the MAXIMUM amount that charger can OUTPUT ! As 2oldmen said that is 45 watts. There is some power lost inside the charger so 50-60 watts is a reasonable expectation. Now if that charger was plugged into a 120V AC outlet it would draw about 1/2A.

If you have a charger that plugs into 12V DC, it would draw about 5A.

Also, once your laptop is fully charged and turned off, it will only draw a fraction of what I stated.

Sarahps33 wrote:


periods of time. I have one now and if it ran 12hrs outta 24 it would be an additional 192 amps.

Is that a 12VDC or 120VAC ? In either case, I would expect a fridge to run a most 8hrs per day.

Sarahps33 wrote:


I'm reading that AGM's have a depth of discharge rate of 80-50%. If we used 80% of each battery ...

You are reading that wrong ! For maximum life, you should only discharge them until there is 80% of the energy left in the battery.

Simply put, you can't get there from here ! Not without a lot more solar power or running a generator and chargers for several hours per day.

theoldwizard1

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Posted: 12/23/21 04:57pm Link  |  Quote  |  Print  |  Notify Moderator

pianotuna wrote:


Perhaps the best "real way" is to plug the RV into a kill-a-watt meter and measure the consumption for 6 days. You won't be able to exceed a 15 amp (1800 watt) draw.

Concur !

CA Traveler

The Western States

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Posted: 12/23/21 05:26pm Link  |  Quote  |  Print  |  Notify Moderator

The calculator adds about 15% due to loss of efficiency so 14.1A is reasonable for the calculator. However you need average input not maximum. AC appliances draw a lot of Ah (amp hours). The high efficiency Samsung 18 cu ft refer uses 100+ Ah per day.

Basically if you are going to use significant AC usage for devices then you'll need a very large battery bank, a large solar array AND a generator for those cloudy days. This is the reason most RVs to use propane for HW, MW, stove, refer and NO ACs for boondocking. Even propane for coffee!


.


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Bob


dieseltruckdriver

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Posted: 12/23/21 05:45pm Link  |  Quote  |  Print  |  Notify Moderator

I agree with the other posters here. You seem to be taking the loads that are listed as maximum and trying to correlate that to a 24 hour draw. Almost nothing works that way. Your laptop might use that much power for two hours a day while you are recharging the battery.

When you are doing your calculations, convert everything to watts. Then understand that in my experience my wattage charge numbers come in slightly lower than what 2oldman posted because when the sun is out, you are getting 13.6 volts or higher and that drops the amps needed even more. The 12.5 amps is a safe voltage number with no solar input. I am assuming some solar input during the day based on my experience.

On top of that you have to remember that almost everything has a duty cycle. That means maybe 2 hours a day to recharge your laptop. It might mean (I have no idea) 45% of the time your fridge needs peak power but not 100% of the time. Taking that into consideration you have to do some more homework before you get some true useable numbers.

Now that I have put that out there our personal experience is we can get by on probably 300 watts of solar. We know from experience that 200 is the bare minimum, and the 420 we have now can last us a long time, unless we get several cloudy days. That's why I am adding at least a couple hundred more watts. That gets us as much tv as we want to watch with the fridge, coffee maker and water heater on propane.

For some background, when we bought our current fifth wheel my wife told me she wants to be able to live a normal life without having to worry about power. I bought what I thought would work and over time found out I undersized the inverter which is 375 watts. Last summer she thought she would like to be able to use the microwave once in a while without the generator also. I had a 1200 watt inverter picked out but I might upgrade to an even 2000 watts after she said that. I reminded her that we would also need some more battery capacity to do that. We have two group 31 AGMs currently. If I buy more batteries I am getting lithium batteries for the duty cycle.

* This post was edited 12/23/21 05:53pm by dieseltruckdriver *


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