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valhalla360

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Posted: 06/20/21 09:22am Link  |  Quote  |  Print  |  Notify Moderator

cliffy49 wrote:

I have read where folks suggest any where from 1 to 2 watts of solar per ah of battery. My question, is this based on battery rating (ie 215 AH) or is it based on useable amp hours(105)?

All replys are greatly appreciated.


A poorly thought out rule of thumb. As likely to give you an undersized system or an oversized system. Pretty good chance, you are left unhappy. If you are drawing negligible watt-hours from the battery bank each day, a very small solar array can handle a large battery bank but then why bother dragging around a huge battery bank.

The question is how many watt-hrs do you use per day (w-hr = amp-hr * voltage).

A solar panel will produce around 4-5 w-hr per name plate watt rating...so a 100w panel will generate around 400-500w-hr on a typical day.

Then how many overcast days with minimal solar do you want to be able to go before the usable battery capacity is gone.

Let's say your fridge uses 50w and is on 50% of the time, so you need 50w * 24 * 50% = 600w-hr to service the fridge. (repeat for the other items that will be run of battery power and add them up to get the total w-hr).

If you want to go 3 days with overcast days, that's 1800w-hr you need in usable battery storage. For lead acid, you will need an overall rating of 3600w-hr (limit usage to around 50% of the rating) which translates to 300amp-hr @ 12v. Lithium you can use around 80% of the rated capacity.

Assuming you want to recharge the bank completely the next sunny day (1800W-hr), you will need around 450w of solar panels.

If you are only off grid for a couple days at a time or you are willing to use a generator occasionally to top up the batteries, you can reduce the size of the battery bank and solar panels..

When you come up with your final sizing, it's not a bad idea to upsize by 10-20% as the systems aren't 100% efficient.


Tammy & Mike
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StirCrazy

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Posted: 06/20/21 09:39am Link  |  Quote  |  Print  |  Notify Moderator

I learned a new method of sizing yesterday while I was reading. for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank. now how you adjust this for other loads I havent figured yet but from a charging stand point for your batteries you want to get about 21 amps from your panels. for myself, I would probaby double this and call it good.

Steve


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2oldman

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Posted: 06/20/21 10:27am Link  |  Quote  |  Print  |  Notify Moderator

StirCrazy wrote:

for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank.
With Li's you can go way over that.

Dutch_12078

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Posted: 06/20/21 10:33am Link  |  Quote  |  Print  |  Notify Moderator

I think "too much solar" and "too much battery" are in the same category as "too much truck". The only time anyone complains about it is when they're paying the bill to buy it... [emoticon]


Dutch
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mr_andyj

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Posted: 06/20/21 10:46am Link  |  Quote  |  Print  |  Notify Moderator

Generally, RVers tend to do the same things, use the same appliances, the same amount of power needs etc, in general in a wide-range statement...
200 watts minimum I suggest. 300 is much better. If you do use a lot of DC power, or if you use an inverter to run a TV or other things then you will be on the high side of power users as opposed to the one LED light users who uses one gallon of water a day and charges a phone or two...

How many panels can you fit on the roof?
Put more than you need as panels are cheap at 75 cents per watt or so, and adding one later might be difficult depending how you run the panels - if you run two in series then later want to (and only have room for) one, then this requires more than just extra wire.

Panels will put out 18 volts. It is the amperage that varies. Max amps at noon pointed directly into the sun; minimal later in the day when panel is flat on the roof and getting the sun at an angle...

Your calculation seems to rely on ideal, maximum solar. You will have sub-maximum sun most of the time.

Boon Docker

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Posted: 06/20/21 11:46am Link  |  Quote  |  Print  |  Notify Moderator

cliffy49 wrote:

I have read where folks suggest any where from 1 to 2 watts of solar per ah of battery. My question, is this based on battery rating (ie 215 AH) or is it based on useable amp hours(105)?

All replys are greatly appreciated.


Do 1.5 watts of solar per AH of battery and you should be fine.
(300 watts of solar for 215 AH of battery)

Almot

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Posted: 06/20/21 05:18pm Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

StirCrazy wrote:

for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank.
With Li's you can go way over that.

With flooded you can go below 0.1C, it will just take longer to get to Absorption. AGM recommend 0.2C optimum current, and again, you can go way below 0.2C.

The OP never told what battery he has. Or how he is going to use his solar - daily energy needs, days of camping etc - IMO, this is what solar planning should start with.

pianotuna

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Posted: 06/20/21 06:03pm Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

StirCrazy wrote:

for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank.
With Li's you can go way over that.


Acceptance rate for a 100 amp-hour flooded lead acid at 85% state of charge is 12.5. That translates to 12.5%, not 10%. Further lead acid are self regulating--that's why it takes nearly the same time to get from 50% to 85% as it does from 90% to 100%

Battleborn recommends 0.5 c for longest life, but 1c is safe.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, soon to have SiO2 batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

2oldman

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Posted: 06/23/21 10:46am Link  |  Quote  |  Print  |  Notify Moderator

Almot wrote:

The OP never told what battery he has. Or how he is going to use his solar -
Appears to be MIA

cliffy49

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Posted: 06/23/21 11:49am Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

Almot wrote:

The OP never told what battery he has. Or how he is going to use his solar -
Appears to be MIA


I am still here. Been busy trying to get the trailer ready.

As far as batteries, I have 2 Deka GC2's wired for 12 volts. At this time I am just lookinig to keep the batteries charged when the trailer is in storage. Last year I just took the 1 12 volt out when ever I came back from a trip but have since purchased the 2 Dekas and they are jsut a little heavy to be taking out of the battery box after every trip.

At some point we may try boondocking but have not yet.


cliffy49
2016 F150 Ecoboost & max tow (Gone)
2021 Silverado Custom 2500HD
2018 Catalina TH26 Toy hauler


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