Multiple Batteries & Camping Cooler Short Duration

System

First let me say this is a great site I wish I had found weeks ago as it would have saved me going all over the place to get the info I have to this point. I have sifted though al the previous posts looking for my answer but didn't  see what I was looking for so...

I need to power a cooler for  7 day periods from time to time while camping in a remote but accessible by car location.
The equipment is
- P27 Koolatron which uses about 50 watts (12 V 4.1A) and would be run about 16 hours a day
-Twelve 12 volt 7 amp 20hr batteries (surplus UPS batteries that I have tested under load satisfactorily)
-Will be purchasing a 40 or 50 watt mono crystalline Solar Panel 2.3 A or 2.8

I plan on connecting all 12 batteries in parallel into a battery box I have made. I plan on connecting the solar panel to the battery pack to the cooler. Under this method will the solar panel provide the bulk of the energy to power the cooler, with the batteries topping up the requirements in less than optimal conditions? This is really my main question.

I am also considering putting a mechanical timer in between to shut the cooler of for ten minutes every hour to lessen the load and help maintain a charge. Would this be of value? I have tested and the temperature drop is acceptable

Alternatively I could split the batteries into two 6 packs and use the panel to charge one pack while the other pack runs the cooler, but this seems inferior, due to the fact I want to keep the charge level above 50% and the long charge time for the batteries.

Is the 50 watt the best choice or could I get away with the 40 watt or for that matter should I get 60 watt? I will have a charge controller, although under my preferred plan can’t see that it will be necessary, as the load would equal the charge.

Even though I will be able relocate the panel to full sun at will, is there much concern about the mono-crystalline panels not working well in limited sunshine such as dusk, dawn or rainy conditions?

Thanks for any comments.

niel

Re: Multiple Batteries & Camping Cooler Short Duration

"I need to power a cooler for  7 day periods from time to time while camping in a remote but accessible by car location.
The equipment is
- P27 Koolatron which uses about 50 watts (12 V 4.1A) and would be run about 16 hours a day
-Twelve 12 volt 7 amp 20hr batteries (surplus UPS batteries that I have tested under load satisfactorily)
-Will be purchasing a 40 or 50 watt mono crystalline Solar Panel 2.3 A or 2.8"


to put this simply it won't work for you. break this down into amphrs and you'll see why.
firstly, we'll list the load and it's 4.1 amps for 16hrs. 4.1x16=65.6amphrs. now just knowing this would say to me i'd need this and maybe another 20-30% for losses. going at 25% for losses this is 65.6x1.25=82amphrs. now this figure is very close to that of your battery capacity so you will pretty much deplete your batteries which should never go below about 50% depth of discharge. (especially used batteries as their capacity is more in question) ideally your minimum capacity should be 164amphrs.
finally we come to the pvs. it must supply 82amphrs/day to keep the cooler going. i'm assuming 4hrs per day of full sun even though it is the better seasons of the year you'll be using a cooler there will be some cloudy times. that means solar providing for 4hrs per day requires 82/4=20.5amps capacity in pvs to fully run it and this is needed so that it can go for 7 days like this. going by the ptc rating is better and would be about 90% of the stc rating of the pvs so this would be 20.5/.9=22.78amps stc rated. at an average of say 17v for the pvs this could represent 387.25w of stc rated pvs.
now i didn't account for the 10 minute factor of every hour, but you'll still come up short as this would only be 160 minutes over the 16hr period and save 2hrs and 40mins worth of drain. now if it were a 9hr period with 10 minute rests/hr it might work out for your batteries, but you still need much more in pvs than 40-50w and would be roughly half of the 387.25w figure i stated for about 193-194w. the w on pvs could vary somewhat depending upon the output voltage, but you are more interested in the amps being delivered as 4 of those 50w pvs you mentioned will not deliver the approx. 11.65amphrs you'll need as using 4 of the 50w pvs is 2.8x4=11.2amps in pvs or slightly shy of your needs.

BB.

Re: Multiple Batteries & Camping Cooler Short Duration

Well... To address what you asked:

watthrs=4.1a*12v*16hr= 787.2 watthrs/day... or, roughly 65 amphrs/day... You would want, at least, a 130 amphour rated battery for 1 day's of use (I think you have 140 amphour rated batteries at 20 hour rate???--And not a 7 amphr rated battery at 20 hour rate---These 140 amphr batteries should be somewhat larger than your standard car battery, a 7 amp hour battery would be the size of a motorcycle battery and you would need about 20 of them for one day storage).

You did not say where you are camping--but assuming you have sun, it is not too hot (desert) and such--I am getting about 15-18 kWhrs / 3.5 kWhrs of panels on my home Grid Tied system (for six months of the year--during good weather)... Implies:

panel size = ( 3.5 kwatt panels /15kWhr/day ) * 787 watthrs/day * 1 / 80% (battery charging eff) = 230 watt rated panel

Panel size is rough numbers... but should cover a reasonably sunny day for mid-April through Mid October (in my area south of San Francisco CA). There can be periods of clouds/bad weather where for 2-3 days you may only generate 1/2 the power needed... More batteries will help you ride through those periods (one 140 amp battery per day of no-sun--to 50% discharge to keep battery healthy).

Your batteries are free--but usually, when you place multiple batteries in a bank--you want the batteries to all be of the same age and capacity (and manufacturer). Otherwise, you battery bank will only be as good as the weakest battery.

Placing a timer in the circuit will probably not save any energy... Keeping the temperature higher will save energy.

A 40-50 watt panel will only supply about 1/5 to 1/4 of your daily power needs (i.e., almost can ignore its charging effects in your setup).

Splitting the batteries in two banks probably won't help anything.

The 230 watt panel is a bit on the large size to support your load. If you wanted to assume optimum operation (full sun, lots of hours, manually repositioning the panel a few times per day, using AGM batteries instead of flooded lead acid)... You could go down to about (6/8 is the rough, average, ratio between fixed and tracking panels in San Francisco, 20 kWhrs good day with cool panels):

http://rredc.nrel.gov/solar/pubs/redbook/ US solar radiation numbers for selected cities

min. panel size = 6/8 ( 3.5 kwatt panels /20kWhr/day ) * 787 watthrs/day * 1 / 98% (AGM battery charging eff) = 105 watt rated panel

So, if you assumed everything was optimum, you could get away with as small as a 105 watt solar panel. You might try purchasing one 105+ watt panel and see how that works--if you have issues, you could always purchase a second.

As to performance in limited sun--from the way the panels work, assuming no partial shading by trees and such, all panels output power in proportion to the amount of sun falling on them... So any panel exposed to 50% sun will output, approximately 1/2 the mount of current. Some panels are more efficient--but that just means their physical size is slightly smaller for the same rating.

Mono and poly crystalline panels will, typically, last the longest in full sun (you are not permanently installing these panels--so should not be a big issue). The thin film panels are typically more flexible and impact resistant. For your use, the packing and transportation of glass panels may be a problem.

http://store.solar-electric.com/unus64wathfi.html (thin film panel from the sponsor of this web forum)
http://store.solar-electric.com/instock.html (price of in-stock solar panels)

Now, you asked for comments... I would suggest that you look for a cooler than can keep ice for 5-7 days (larger coolers better at keeping things cool than smaller ice chests). You might even build an insulated box around the ice chest and keep it in shade to help keep it cool.

Or, look for a propane powered fridge. I don't know what your requirements are (cu.ft, freeze or just fridge), but, roughly it seems that all the standard propane cooler, from small to large RV style can run for several weeks on a 20 lb propane cylinder.

One of the smallest portable propane units I have seen is (also runs on 110vac and 12 vdc):

http://www.lpappliances.com/RC3000.html uses (I guess) about 1/4-1/2 lb of LP per day.

And the bigger RV style fridges use about the same amount.

The $500 price for a 1.2 cuft cooler sounds kind of high... Until you compare with two 64 watt UniSolar panels at $375 each. Not counting lugging the batteries and solar panels required to run the setup.

I hope that this helps...

-Bill

PS: Here is a site that lists quite a few different Gas Refrigerators for RV use and such:

http://www.gas-refrigerators.com/dometic.html

crewzer

Re: Multiple Batteries & Camping Cooler Short Duration

The Koolatron probably isn’t a good idea for your planned needs. I have one, and it took a 300 W PV array with a 12 V x 230 Ah battery bank to keep it running in the Rockies during the Summer.

Alternatives include a built-in propane fridge, a portable propane fridge, or an Engel electric fridge. All of these alternatives cost more than a Koolatron, but they require fewer batteries and/or a smaller PV array.

See:
http://www.norcold.com/portable.cfm
http://www.norcold.com/322.cfm
http://www.dometic.com/templates/ProductCategory.aspx?id=424
http://www.dometic.com/templates/fp.aspx?id=488
http://www.advrv.com/refrigerator.htm
http://www.advrv.com/engel/engel_65.htm


HTH,
Jim / crewzer

System2

Re: Multiple Batteries & Camping Cooler Short Duration

Thanks for all your replies. I appreciate all your input.

Hopefully I can do better than 4 hours per day sunlight here as the average is 9.3. If the forecast is for crappy weather, I just won’t be going up there anyway. It’s a bit up north but the average temp is 75 day and 55 at night summer time

That being said, it looks like I will be under prepared. I will consider all you suggestions, but purchasing cost and transportation to the site are an issue as well.

One thing I didn’t anticipate is the loss factor mentioned by Niel. Where does that occur, from the battery to the cooler or the pvs to the batteries? For what it is worth all wiring length will be kept to a bare minimum, hopefully inches to a few feet on #10.

Unfortunately this model cooler does not have a thermostat, so that leaves me with a few choices, supplement with ice and decrease the running time, get a thermostatic switch to cut the current, increase the battery store or the solar output.

I did plan on respecting the 50% discharge rule, and at least I know the history of the batteries and they are all the same making, model and vintage. I recycled the bad ones but still must face the fact they will “equalize” when in parallel.

Thanks for all your suggestions and I guess the first field test will give me more info. I’ll be bringing my meter and I also have a small solar panel and one of those portable power packs as well as the car itself. I won’t use the engine to run the cooler, but it gives me means to charge batteries while driving.

BB.

Re: Multiple Batteries & Camping Cooler Short Duration

One big issue is that the P27 appears to be a thermal electric type cooler... They just are not very energy efficient at cooling. If you have "unlimited" energy (like driving a car or plugged in at the hotel), they work pretty nice (I have an older, cheaper version and it works very well--as long as it is plugged in).

If you want efficiency, it is either getting a cooler with a refrigeration pump or a propane powered unit (by the way, multi-powered propane powered units on 110vac or 12vdc are very inefficient--go figure).

The other losses that Niel was talking about--there is the loss of charging a lead acid battery (75-85% efficient, the AGM's are reported to be close to 98% efficient). Other issues that Niel was discussing is that a PTC rated panel will never put out its rated energy because solar PV panels are very sensitive to their temperature. The panels are rated about about 70F (20C--STC) in full sun--but, if you have every put your hand on anything black in full sun, you know how hot the surface can become (and the panel output voltage falls accordingly). There are several other derating methods (PTC, CEC) that come closer to what you may find on a cool day with pole mounted panels (free air with a breeze).

I went the otherway and gave my numbers based on real output from a real set of panels (after all losses, on a grid tied MPPT type controller) and used them to calculate a panel size for you.

There are other losses, like wire resistance, charge controller, etc. that all tend to take another few percent here and there. Your 10 awg wire to the panel should not be an issue here on this small of setup.

You still did not tell us where you are at... but you can use the Solar Radiation link I gave you to see how much "full" sun you can expect in your area (by month and by year).

By the way, what is the "portable power pack" you are bringing along... Typically, those do not have very large batteries in them and will not supply a whole lot of extra power for your cooler.

-Bill

Brock

Re: Multiple Batteries & Camping Cooler Short Duration

As was already mentioned the two big problems are that the cooler is quite inefficient compared to a regular "compressor" type refrigeration.  Let’s look at a regular fridge for reference if you ran yours for 16 hours you have as Jim stated “watthrs=4.1a*12v*16hr= 787.2 watthrs/day... “ Our full size 22CF fridge consumes 1250 watthrs/day a day, just under twice what that small fridge your talking about.

The second thing that hits me is that I am not sure you understand what "hours of sun" means.  Even if you have sun light for 16 hours on a cloudless day you might only get 5 "hours of sun".  I am way oversimplifying this, but it is how many hours of sunlight you get at optimum for catching solar power.  For example even though you might in full sun from 5 pm to 6 pm and even have your solar panel facing the sun since the suns rays are at a steeper angle most of the light is actually bouncing off the atmosphere and not hitting your panel, in that hour you might get 20-30 minutes of sun.

My suggestion would be a small cooler in a larger one.  I have done this on trips and it is amazing how long the inner cooler stays cold.  Even if the inner one was thermoelectric and you ran it for 4 hours a day (removed from the larger cooler of course) preferable at night when it is the coolest outside.

You did mention the “car”.  Does it run daily?  Could you charge the batteries while it was running?

System2

Re: Multiple Batteries & Camping Cooler Short Duration

Thanks again for all your good advice.

I'll try and respond to most of your comments:

I checked out the PDF for sun radiation and if I am reading it right, based on Sault St Marie MI I could get 6 KWh/m2/day at 0% and up  to 8.3  for  a rotating axis which I will try to maximize measuring output with an accurate meter and rotating/angling the panel. Now if I am doing my math right I should get about 1/16 of this based on panel size or 375-500 watts. Or, 7.5 -10 hours running time on a 50 watt panel, and yes this is the theoretical amount and subject to weather and other factors. ( Actually my math may not have been good. I would have 2 sq ft of panel which is .185 M2 so wouldn't that be 6 x .185= 1.1 kWh - 8.3x.185=1.53 kWh but that seems high to me. Anyone?)

The other fridges suggested are a nice alternative to be sure, but I think I will lean towards increasing my solar generation capability as it offers me flexibility to power other things as I increase the capacity. Besides it’s a toss up; 40 lb fridge or 40 pounds of solar and battery.

As for the light bouncing off and not hitting the panel, as they say "I did not know that". That's why I am glad I came to this site.

The portable power pack is good for maybe 4 hours runtime but is easily and quickly charged when the car is driven. As for running the car, I won't use it for the sole purpose of running the cooler as it goes against my "green" attempts but I will be able to run the cooler off it while going into town to further reduce the runtime of the solar/batteries.
I also have an "extreme" double wall conventional cooler which works very well I can put critical items like meat in there and put ice in every 2-3- days or so and I monitor the temperature with those  portable indoor/outdoor thermometers with the sensor wire.. I also simulate the cooler in a cooler scenario by using bedding etc. to provide additional insulation for the coolers and keep them out of the direct light. Likewise I can put block ice in the Koolatron and block off the vent at the bottom with a piece of foam

Anyways the whole experience will be a primer for me, and as the saying goes "good judgement is the result of experience and experience is the result of bad judgement.”

BB.

Re: Multiple Batteries & Camping Cooler Short Duration

I will take a shot at answering...

Full sun on the earth is about 1kW/m^2... So, people talk about a 6kWhr/m^2*day as being a "6 hours of sun" day... Your math looks fine for conversion of sq.meters to sq.feet--however, you also have PV panel efficiency to contend with... Take a guess at 12% or so. That brings your 375-500 watts down to 45-60 watts at the panel pigtales.

Assuming you have a solar panel of 50 watts, its actual output (temperature, less than ideal conditions, etc.) will be closer to 85% or 42 watts peak. and, on hot days, I would not be supprised to see it about ~71% or 36 watts peak output... (These are real numbers that I have seen between early spring and late June with my 3.5kW array).

And, if you are using liquid lead acid batteries, you will loose another ~80% +/- ~20% +/- charging them with the solar panel... Giving you an effective output of 34-29 watts peak from the panel... [edit: that is for 80% +/- 5% charging efficiency]

So, a 50 watt panel on a clear day really would output somewhere around 6-8.3 hours of ~30-35 watts of energy, or 180 watt*hours to 290 watt*hours per day (low*low and high*high estmates). Or, enough power for about 4-6 hours of cooler operation--on a good day. Hence my estimate for minimum panel ratings of 2-5x+ larger that you are thinking of using now). And why I agree with Brock's recommendation of a 300 watt panel to account for those occasional days of overcast and other issues--if you need guaranteed cooling.

Assuming that you need to cook on a camp stove, you can probably already justify the 20 lb propane cylinder (or even several 5 lb cylinders). And the electricity you generate could be of "better use" for a computer, audio, radio, cameras, etc.

In any case, have a great time. I really miss camping when I was growing up many decades ago...
-Bill

kc8adu

Re: Multiple Batteries & Camping Cooler Short Duration

derate used ups batts by 50%
this is backed up by testing 2 vyear old pulls.we still save ones that pass a load test and give them to local ham operators to run things like ht's.
my advice is to get a good agm g 27 or 31 and use the ups batts to run lightweight stuff like tent lights or a radio.

Brock

Re: Multiple Batteries & Camping Cooler Short Duration

Here in Green Bay we are listed at 4 hours a day for sun, I can only imagine up there it is even less. I will be up in Garden though over the 4th

Getting back to the original idea, maybe running the cooler just at night (when it's a lot cooler, especially up there) and maybe a couple of hours in the middle of the day I bet it would work with a smaller panel and stay cool enough. Heck our thermoelectric cooler will stay cold for a day after it has run a day as long as it’s pretty full. If you do wrap it in blankets make sure they are light colored or not in the sun. I use the blanket trick to keep pizza hot on the way home, 25 minute drive. It makes a huge difference.

Hey if you are bringing bottled water freeze some first and throw them in.