Struggling a Bit with Solar/Battery Numbers and Expectations

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Blompson
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Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Blompson » Tue Jul 16, 2019 10:57 pm

I'm new to all this stuff and have always struggled to grasp concepts relating to electrical currents, measurements, and how power is stored and discharged. But I've been reading and pushing myself and running some numbers, and I *think* I'm getting close to figuring this stuff out. I'd like to check in with folks here though and get some feedback because like I said I'm new to this and I struggle, and I want to make sure I'm being safe and getting my work double-checked by people who know what they're doing.

So to start off/ a little context: I bought a converted shuttle bus that I'm fixing up and getting playa-ready, and got a 140 watt solar panel in the deal. I'm building two bucket swampies to keep the roughly 700 cubic ft. space cool, and I'll be out on the playa for almost two weeks. Now, Realistically, I'm fairly sure this large 140 watt solar panel would easily power a couple of swampies, but I'll have more control over them if I run them with a battery and use the panel to recharge. Additionally, having a battery to run some lights and charge a few small devices would be pretty neat. So I'm thinking I want to do a deep cycle battery and charge it with my panel.

Something else worth noting which I didn't notice until I started really looking into the numbers: The components I bought for my coolers draw a bit more power than figjam's original setup, and since there's going to be two of them, the battery drain will be considerably faster than the "this should last you all week long without recharging" amount that it's supposed to be. But I'm hoping It'll be okay. So, on to those numbers...

4.8w pump (x2)
9.6 watts

36w fan (x2)
72 watts

81.6 watts total

81.6w / 12v = 6.8 amps needed

So say I get a 90Ah battery....
90 Ah / 6.8 amps = 13.24 Ah

So if I've got this right, on a single 100% charge, I'd be able to power *just this setup* at full power for about 13 total hours.
Is this first piece of math correct?


Next, something I am struggling to grasp is how much charging I can expect from the 140w solar panel (without directional adjustment) each day, and how to measure that. I read elsewhere on the eplaya that we get about 4.5 full sun hours a day. Is that true? If so, does the following set of calculations make sense?

140w[panel] / 12v = 11.667 amps output
11.667 x 4.5 hours = 52.5Ah recharged per day

52.5Ah / 6.8 amps = 7.72 hours of cooler usage recharged per day

If this is accurate, or nearly accurate, I believe it means I should be replenishing over 7 hours worth of battery drain from the coolers per day, and if I use the setup for less than that number I will never deplete my battery (not counting other power draws). But please correct me if I'm wrong.

This all makes loose sense in my head, but my guess is that at best I'm overlooking certain varying factors and at worst I'm completely misunderstanding fundamental concepts and this whole thing is way off. I'd be very grateful for a bit of guidance at this point.

Thanks for being patient with this newbie... I do love a good project and this is stuff I really want to learn, but I'm beginning to feel like I'm dangerously close (perhaps literally) to having bitten off more than I can chew.
History does not repeat itself, though it often rhymes.

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XPTom
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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by XPTom » Wed Jul 17, 2019 7:30 am

Solar math can get fuzzy and it is good you are learning to speak AmpHours.

Your expectation of solar panel amps is biggest issue I see. The panel puts out it's advertised "140" watts at 17.7V and 7.9A. Typical garden variety "PWM" charge controller simply passes the 7.9A to the battery at reduced voltage that is more battery friendly. You're lucky to get 65-70% of panel rating into a battery. A more expensive "MPPT" converts that same output to say 9.8 at 14ish volts and puts 95% of rated wattage into the battery. MPPT controllers are more expensive but compare well to more/bigger panels using a PWM controller.

Your solar day may be closer to 6-7 hours. A flat panel might get 4-5 hours of prime output in the middle of the day but the many hours of reduced output before and after that can equal 2-3 prime hours.... don't let the dust build up on it. I tilted my panel and would point it toward the morn-noon-evening sun and was getting max amps for nearly 12 hours..... give or take a dust storm..

Lead battery? ….can take a 70% charge in 6-7 hours, but as battery voltage comes up the charging slows down and it can take another 6+ to top off. Simple Amp Hour math says a 70AH battery at 100% charge is the same as a 100AH battery at 70% charge, but that smaller battery won't get there during the solar day we discussed. Err high on the battery capacity.
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Token
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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Token » Wed Jul 17, 2019 9:14 am

I’ll second Mr. XPTom on the recommendations.

You got the drift of things and you’re ‘rithmetic is sound.

MPPT controller is a must, and spend a bit extra for a reputable brand. Look for a model that lets you adjust the low voltage shutdown so you can manage the battery discharge and ensure longevity of the AGM. Also make sure the controller has a “load” port where you attach your bucket coolers and other loads so that the controller can disconnect them if it needs to protect the battery.

Get an AGM battery since it is sealed and will not off-gas toxics when in your van. Very important.

With a good size panel and a good MPPT controller, it’s quite likely your charging rate will be over 10A at the battery during the day. Most Pb-Acid batteries don’t like to be charged with anything more than 1/10 of capacity current. i.e. a 100Ah battery should be charged by no more than 10A of current at 13.8V DC.

For that panel size, the sweet spot for battery capacity would be in the 120 - 140 Ah range, either as a single large battery or two smaller 60-70 Ah batteries in parallel. If using two or more batteries, make sure the cables used to tie the batteries in parallel are thick as your fingers.

And last, but not least, if you can keep all your loads as 12V DC devices, and don’t need an inverter to create 120V AC, it will improve system reliability significantly. So all the chargers for gizmos, gadgets, lights and LEDs etc - get the 12V car-chargers for those.

You are on the right path.

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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Blompson » Wed Jul 17, 2019 3:58 pm

Thank you both for the help, I appreciate it greatly. AGM sounds good, albeit a little spendier but worth it. I'm having a hell of a time finding anything in 60-70 or 120-140Ah though. I *am* finding 12v 35Ah AGM's for ~$50 a pop.. would it be a poor choice for any reason to parallel four considerably smaller batteries like this to make a 140Ah system?

Something else I'd like to confirm: the panel's output voltage (charge voltage) is set by the charge controller, and that voltage doesn't have to be the same as the battery's discharge voltage/ load voltage (12v). The battery will only push out 12v but can receive more when recharging. Is that right?

Lastly, Token you mentioned using a car charger.. do you mean taking a cigarette lighter style charger and wiring that into the system? I'm not sure what other kind you might mean, and if that's what you mean I don't quite know how to go about that.

Further reading on the car charger thing, links/advice for finding the right batteries, or any other help would be awesome if y'all get a chance. Thanks again for the help so far. I'm feeling a lot more optimistic.
History does not repeat itself, though it often rhymes.

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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Token » Wed Jul 17, 2019 10:20 pm

Blompson wrote:
Wed Jul 17, 2019 3:58 pm
Thank you both for the help, I appreciate it greatly. AGM sounds good, albeit a little spendier but worth it. I'm having a hell of a time finding anything in 60-70 or 120-140Ah though. I *am* finding 12v 35Ah AGM's for ~$50 a pop.. would it be a poor choice for any reason to parallel four considerably smaller batteries like this to make a 140Ah system?

Something else I'd like to confirm: the panel's output voltage (charge voltage) is set by the charge controller, and that voltage doesn't have to be the same as the battery's discharge voltage/ load voltage (12v). The battery will only push out 12v but can receive more when recharging. Is that right?

Lastly, Token you mentioned using a car charger.. do you mean taking a cigarette lighter style charger and wiring that into the system? I'm not sure what other kind you might mean, and if that's what you mean I don't quite know how to go about that.

Further reading on the car charger thing, links/advice for finding the right batteries, or any other help would be awesome if y'all get a chance. Thanks again for the help so far. I'm feeling a lot more optimistic.
Batteries are both science and dark-art-magic ...

You want to look for Deep-Cycle AGM or Pure Gell batteries designed for solar use.

100Ah should be fine for your setup and you can always add a second one if you need to.

If this is a one-and done thing for this year and you will never use it again, sure you can get two or three 35Ah universal AGM batteries and parallel them. They are used for UPS systems, electric fences, gate openers and suchlike. They will last considerably shorter than a solar designed battery - Maybe ~ 300 discharge cycles, which is fine for a 9-day event.

For ~ the same money you can get a proper solar AGM in the 100Ah range. These are designed for ~ 600 discharge cycles.

Spend $100 more and you can get a Pure Gel design that is good for ~ 1200 discharge cycles. About double the standard solar AGM.

Up to you to decide how durable you need to go.

On the use of car chargers, what I was saying is if you need to charge your phone or run a laptop computer, make sure you have the power for those devices run from the 12V battery.

Any local autozone Napa pep boys type parts store will have the pigtails to cigarette lighter automotive device. You would wire them on the “load port” of your controller: + or Red to + or Red; - or Black to - or Black.

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Token
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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Token » Wed Jul 17, 2019 10:30 pm

The dark art of solar panel voltage and battery charging:

Most solar panels will produce anywhere from 18 - 23 V DC, depending on amount of sun and the current load.

The charge controller will normalize that voltage down to a steady value that the battery likes to receive.

There are two main voltage values used to charge a battery :

Cycle - the main charging voltage that gets the battery to 80% capacity - usually between 14.2 - 15 VDC

Float - the slow charge from 80-100% capacity - usually 13.5-13.8 VDC

The solar charge controller manages the selection of voltage that is applied to the battery based on the battery state. It can sense that based on the current it is receiving.

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Re: Struggling a Bit with Solar/Battery Numbers and Expectations

Post by Token » Wed Jul 17, 2019 11:02 pm

For an example of battery grades ...

https://www.amazon.com/s?k=100ah+solar+ ... 00Ah+solar

The Universal AGM at the top is the cheap entry level. 3-5 year design life.

Below are the two grades of Renogy - a mid-level brand.

Then the big-daddy VmaxTank - American made and bulletproof- 10 year service life design.

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