solar,amps and watts??

Submitted: Sunday, Oct 13, 2013 at 21:18
ThreadID: 104705 Views:2900 Replies:10 FollowUps:12
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Please,I need some advice[and plenty of help!].I have 100ah battery in ute,100ah battery in van.I run Evakool 50l fridge; & Evakool 30l fridge run as a freezer.I need folding solar panel kit[s] to run each , or both.Sometimes,we camp without the van!.Firstly,how do I calculate what amps the freezer draws[at -13degrees];what size solar panels do I need;& anything else I need to know.
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Reply By: Bludge - Sunday, Oct 13, 2013 at 21:52

Sunday, Oct 13, 2013 at 21:52
Try Redarc the solar calculator will assist you.
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Reply By: Ross M - Sunday, Oct 13, 2013 at 23:10

Sunday, Oct 13, 2013 at 23:10
alan m6
Each fridges will use it's same amount while running. it is how much does each draw in amps and for how long does it/they run in a 24 hr period.
A freezer at -13 will use more in 24 hrs than a freezer at -7. Also the ambient temp will have a large bearing on run time of both fridge and freezer in that 24hr period.
If you put both units in a warm/hot environment.,after they stabilize/normalize, that will give you some idea of the run times for both and after close monitoring of each you will know what amp hours you have to replace with the solar.

The solar has many variable factors and my idea is to have twice the required solar ability to cater for the night time running of each unit and warm but overcast days and to keep up/replace the charge and also run the fridges during the day time.

Testing with an inline ammeter will tell you what each does draw while running.
As an example
My situation, 30 litre Waeco as freezer and 39L Engel as fridge uses more than what a 160 watt system can provide in 30 degree ambient temp with good sun each day.
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Reply By: Member - John and Val - Monday, Oct 14, 2013 at 07:54

Monday, Oct 14, 2013 at 07:54
Alan,

That's quite a question and to be helpful, needs a detailed answer.

Suggest you will find Electricity for Camping a useful read.

I fully support Ross's suggestions to find just what each of your fridges draws (amps) when running, and how many hours each compressor runs in a 24 hour period. This is essential data that determines much of your daily energy requirements. ( Lighting, charging torch, ipad, laptop, camera, etc etc need to be considered too, but the fridges will be the big item.)

From there the next round of questions starts - how many days do you want to be entirely dependent on solar? Do you intend also charging from the vehicle? Charging from the vehicle can substantially reduce your solar requirements, but depends on your travel patterns - stopping in the one place for more than a few days means more reliance on solar unless you run the engine just to charge the batteries.

Check out Electricity for Camping . It discusses lots of this stuff.

Cheers

John
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Reply By: Herbal - Monday, Oct 14, 2013 at 08:35

Monday, Oct 14, 2013 at 08:35
You can use a multi meter to measure the amps. There are several videos on Youtube showing how.

What type of batteries do you have? Lead acid or LiFePo4 ?

If they are LA (AGM) then 20% of the capacity is useable. If they are LiFePo4 then 80% of the capacity is useable.

If they are AGM, then your 100ah gives you 20ah to use when not charging, ie at night. If you want to be fussy, you can use a meter and time the usage to work out exactly what your total usage is...Or you can guestimate. Look on the panels of the Evakools for the ratings. It will have 2 figures for example it might say 3.1 - 5.2 amps. Use the higher figure for your guestimate.

Say it is 5amps. Then it will use 5ah. Most fridges will run about 15 mintues per hour on average. So 5ah per 4 hours. 12 hours of night divided by 4 of use times 5ah = 15ah per fridge. So to be really safe, you need 30ah of useable battery capacity.

Remember, that is a guestimate using maximum figures...

Size of solar panels ? You want to be able to put 60ah into the battery. Again err on the side of caution...Say you get 4 hours per day of useable sun - 60ah divided by 4 hours = 15... 15 times 12 volts = 180 watts. So if you have the space to carry it a 240 watt panel would be great.
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Follow Up By: Member - John and Val - Monday, Oct 14, 2013 at 09:17

Monday, Oct 14, 2013 at 09:17
Herbal,

Only 20% of a lead acid battery's capacity is usable????? No!!

To get a good life span out of a lead acid battery it should not be repeatedly discharged more than about 2/3 of it's rated capacity, and should be fully recharged sooner rather than later.

To verify, check out the curves shown on most manufacturer's sites.

Cheers

John
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Follow Up By: Russ n Sue - Monday, Oct 14, 2013 at 17:23

Monday, Oct 14, 2013 at 17:23
Just a follow-up to your follow-up John. For clarification, do you mean that the State of Charge can be as low as 66% (2/3 of capacity remaining), or did you mean the Depth of Discharge can be as low as 66% (1/3 of capacity remaining)?

I would counsel for the former, rather than the latter for battery longevity. I agree that 20% Depth of Discharge is very conservative and would result in very good battery life expectancy (all things considered), but would require one to carry much more weight in additional batteries to achieve - hardly practicle in most RV and vehicle installations.

Either way, it certainly illustrates the purported benefit of LiFePO4 batteries, where 70% - 80% Depth of Discharge is supposed to do minimal harm to the batteries.

Cheers

Russ
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Follow Up By: Member - John and Val - Monday, Oct 14, 2013 at 21:48

Monday, Oct 14, 2013 at 21:48
Hi Russ,

Manufacturers publish curves for lead/acid batteries showing the life span (number of charge/discharge cycles) v’s depth of discharge for their batteries. A typical set of curves shows that with 100% discharge a battery has lost 40% of its capacity after about 200 charge/discharge cycles. Cycled down to only 80% discharged (ie 20% remaining charge) it will last almost twice as long, and if cycled down only 30% (i.e. 70% charge remaining) it will last 6 times as long. (Different manufacturers curves differ a bit, but the pattern is pretty similar.)

On the one hand, to get a good life span from our battery we shouldn’t discharge it too far or we’ll have to pay for a replacement. On the other hand, if we don’t make good use of its capacity we’ll need to pay for an extra battery to supply more capacity! Carrying an extra battery costs weight and space as well as $$.

There is a broad economic optimum where limiting the depth of discharge will result in a decent lifespan while also making good use of the battery’s capacity. That optimum is at about the point where we use a maximum of 2/3 of the battery’s capacity. (So we use up to about 70 Ah from a 100 Ah battery.) No problem if we drain it further than that occasionally, but better if we don’t drain it further than that routinely. Of course we should recharge asap to minimise deterioration.

Pity about the cost of LiFePO4 batteries – they are certainly attractive from an engineering perspective, but lead/acid still wins easily on economic grounds. No doubt that will change with time.

Cheers

John
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Reply By: Racey - Monday, Oct 14, 2013 at 09:12

Monday, Oct 14, 2013 at 09:12
Running freezers does consume a lot of power. I run a 40ltr Engel @ approx -15 deg and it draws 4.5 amps and during the daylight hrs @ around 30deg ambient it runs all the time. When I first set up, I used a 75a/hr Thumper which I soon found it would not last during the night. I recently installed a 130a/hr agm and use a 140watt folding solar panel. This will hold the battery and freezer for a couple of days in sunny weather. I estimate the unit runs for around 20hrs a day consuming 90 a/hrs. During daylight hrs the solar panels needs to supply enough power to charge the battery and run run the freezer. Unfortunately the battery never gets full charged. So top up with the generator as required.

My advice is get the biggest panels you can carry and afford. It's a case of the bigger the better to keep your batteries fully charged before the sun goes down. I wished I'd opted for a larger panel. On e-bay you can get 240watt panels for $380-00.
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Reply By: The Bantam - Monday, Oct 14, 2013 at 09:57

Monday, Oct 14, 2013 at 09:57
Alan
Like so many before you, I think you are being optomistic in the least with your expectations.

Do not believe any of the published average demand figures for portable fridges...they are optomistic to say the least.

A 50 litre fridge is a large fridge and will draw a significant average demand due to its size.

The second fridge run as a freezer.....as soon as you say the word "freezer" the average demand increases massively.....a given fridge operated as a freezer may draw 6 times or more than the same fridge operated as a fridge....worst case the motor runs continuously 24 hours a day...with a total demand of around 150AH ish.

In both cases I believe your batteries to be grossly undersized.

I run a 100AH sealed wet cell battery with a 30 litre fridge.....run as a fridge, with good fridge discipline and with it and its contents already chilled down I can get 2 days (sometimes more) out of that battery before the low voltage cut out stops the fridge.

The 100 AH battery is barely adequate long term for a "drive every day" situation.

If I run the same fridge as a freezer, It will last overnight.....IF the vehicle has been driven right up to dusk and in fair weather.

I and others have done the calculations and run the tests.....the general agreement is.

For a 30 to 40 litre fridge, run as a fridge and with good fridge discipline.
For any sort of reliability and with no other charging source the minimum is a 100 Ah battery and 160 to 180 watts worth of panels.

less than that and all you are doing is delaying the inevitable and using the solar panels to extend the run time on the charge from another source.

for what you have and what you expect, think in terms of 200AH of batteries for each fridge MINimum.....AND a roof full of solar panels.

Now remember.
the maximum safe temperature for a fridge is +5C, but most people will tell you the beer is not cold till +2C or lower.
The maximum safe temperature for a freezer is -15C and that is a not a hard freeze most home freezers run at -18C to -20C.

Remember too many portable fridges do not hold a constant temperature and the display will not indicate that variation ..so its not wise to run close to the edge, because the actual temperature in parts of the fridge can be a couple of degrees higher than the set temperature.

cheers
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Reply By: Member - Chris_K - Monday, Oct 14, 2013 at 17:50

Monday, Oct 14, 2013 at 17:50
Hi all

I can't see it in this thread (apologies if it's been mentioned before) - but to do the calculations, you also need to know how to calculate Watts to Amps. For example - a 120w solar panel will (at best) generate 10amps...obviously during daylight hours...the calculation is Watts / Volts = Amps (120/12=10). So you would need to know the draw from the fridge/s (use the multimeter approach) then work out what is left over - say 6amps. You would then need to know how long it will take to recharge your battery at 6amps during the daylight hours. We have one of these things attached to our deep cycle batteries...they work really well, and give you all sorts of information about charge and discharge rates.

http://www.youtube.com/watch?v=NPKh31L0I-E

Lots of marine/caravan places sell them.

Chris

AnswerID: 519690

Follow Up By: Brian 01 - Tuesday, Oct 15, 2013 at 07:57

Tuesday, Oct 15, 2013 at 07:57
Just to clarify this a little, a 12v 120 watt solar panel will not generate 10 amps.
A typical 12v 120w solar panel will have a short circuit current of a little less than 7 amps.
That's the most that it can produce under ideal test conditions, and at that point its power output is zero.
If pointed directly at the sun on a clear, cool day, you may fleetingly see around 6.5 amps coming from the panel if it achieves its maximum power point.
Assuming the above conditions:-
If using a PWM type controller, then the maximum current that you could expect to your battery would be in the vicinity of 6.5 amps.
If using an MPPT type controller, then depending on battery Soc, you could see up to 10 amps or even a little more, but that would be a rare occurrence and the battery would have to be at a low Soc.
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Follow Up By: Nomadic Navara - Tuesday, Oct 15, 2013 at 17:04

Tuesday, Oct 15, 2013 at 17:04
Quote "the calculation is Watts / Volts = Amps (120/12=10)."

Solar panels do not work like that. They will be operating with an output voltage slightly higher than the battery charging voltage when working full chat. As the battery charge rises and the charge current decreases the operating voltage increases. The rough rule of thumb is to divide the power in watts by 17 (roughly the peak power point.) The panel will deliver a little more current under a short circuit condition but dividing by 17 takes into account the imperfect sun position on a mounted panel.

The power delivered by a panel is a complex calculation. It can only be calculated for the peak power point.

The red line in the above graph shows the power developed for different current/voltage outputs from a cell. Use the green line to find the current that would be produced by the cell at particular operating voltages. This would be from a typical cell used in a panel of 36 cells. The panel would have a peak power of around 48W (1.34 * 36.)

From the red curve you can see how the available power drops off when you are not operating the panel at its maximum power point. That's why maximum power point tracking controllers (MPPT) have been developed. They electronically transform the current/voltage relationship of the power being delivered by the panel to match the maximum power point (MPP) of the panel.
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Follow Up By: Member - Chris_K - Tuesday, Oct 15, 2013 at 17:40

Tuesday, Oct 15, 2013 at 17:40
Great set of numbers folks...the bottom line is, if you really need to run all those fridges/freezers, perhaps a generator is more useful! :)

At least you don't need to do any complex calculations and take into account position of the solar panels, length and type of lead from the panel to the battery, and other things not mentioned.

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Follow Up By: Brian 01 - Tuesday, Oct 15, 2013 at 19:06

Tuesday, Oct 15, 2013 at 19:06
Member - Chris_K posted:
"At least you don't need to do any complex calculations and take into account position of the solar panels, length and type of lead from the panel to the battery, and other things not mentioned."


No...But you do need to take into account the number and position of irate campers who are about ready to stick that generator where it's dark and where it won't make so much noise.
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Follow Up By: Member - Chris_K - Tuesday, Oct 15, 2013 at 19:22

Tuesday, Oct 15, 2013 at 19:22
ha ha ha - yeah of course Brian...I don't use one unless I'm by myself! Brings tears to your eyes thinking about it tho. :)

A lot of places have "no generator" rules (eg most national parks)...so what does everyone do? They idle their cars for an hour or so to top-up/recharge batteries....doesn't make any sense to me having rules like that, cause people will always try to get around them! And most generators now days make less noise than a V8 turbo Diesel. It's also kind of hard to stick a Patrol or a Landcruiser where the sun don't shine.
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Reply By: Member - Michael A (ACT) - Tuesday, Oct 15, 2013 at 18:25

Tuesday, Oct 15, 2013 at 18:25
try www.foldasolar.com.au they have a series of documents explaining solar and its power usage via solar cells etc. In the docos they have 1 listing a lot of common camping fridges with their draing powre and requirement under differnt conditions etc.

Just down load and have a lok it may save you some misinformation form the 'experts' on this forum

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Follow Up By: Member - Chris_K - Tuesday, Oct 15, 2013 at 21:50

Tuesday, Oct 15, 2013 at 21:50
Hi Michael - I suppose that makes you an expert on a forum too...
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Follow Up By: Member - Michael A (ACT) - Wednesday, Oct 16, 2013 at 22:42

Wednesday, Oct 16, 2013 at 22:42
No not an expert as I did not offer an opinion on the problem just advice for where to find a number of documents that may answer the question without any bias

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Reply By: oldtrack123 - Wednesday, Oct 16, 2013 at 15:40

Wednesday, Oct 16, 2013 at 15:40
Quote"Member - Chris_K replied:
Hi all

I can't see it in this thread (apologies if it's been mentioned before) - but to do the calculations, you also need to know how to calculate Watts to Amps. For example - a 120w solar panel will (at best) generate 10amps...obviously during daylight hours...the calculation is Watts / Volts = Amps (120/12=10). So you would need to know the draw from the fridge/s (use the multimeter approach) then work out what is left over - say 6amps. You would then need to know how long it will take to recharge your battery at 6amps during the daylight hours. We have one of these things attached to our deep cycle batteries...they work really well, and give you all sorts of information about charge and discharge [end quote]

Sorry Chris
but as Brian has pointed out ,your calculation is based on a a misundertstandin of solar panel outputs
One would could virtually NEVER get the amps that you suggest
As it exceed a 120W panel's SHORT circuit current!!

One would be lucky at most times to get any thing near 7A.
EVEN at that,using a PWM reg ,
with a battery with 12V SOC voltage that means only 84W are actually going into the battery
Using a QUALITY MMPT reg will inprove on that

PeterQ
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Follow Up By: Member - Chris_K - Wednesday, Oct 16, 2013 at 17:23

Wednesday, Oct 16, 2013 at 17:23
Hi Peter

No need to apologize...the forum is a good place to gather information...I reckon that people correcting me is a good thing...! I always double check any information received.

We do regularly get around 6.5 - 7.0A from our 120w panels using the BM1 monitor to check it - so we must be lucky. In hindsight, my statement probably needed more careful wording - so thanks to all that pointed it out. The statement about Watts / Volts = Amps is still correct - however with solar there are obviously many, many other variables to consider when getting to the actual amps!

To get back on topic - there is also a pretty good article here about all the variables and calculating demand/draw etc. for fridges, lights etc.

http://www.redarc.com.au/solar/about/solar-faqs/

Chris
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Reply By: Member - graeme W (WA) - Thursday, Oct 17, 2013 at 03:25

Thursday, Oct 17, 2013 at 03:25
Hi Alan. Buy the most efficient fridge you can. I have a waceo 80 litre used as a fridge at 2 degrees and a 60 litre trailblazer used as a freezer at minus 18 degrees and regret buying the waceo. Both use on average 60 amps a day each. The trailblazer is 15 years old but still way more effecient. On top of my canopy i have 280 watts of panels run through a mppt regulator to two AGM batteries of 65 and 120 amps that are connected as one. The batteries are now in their 6th year . During reasonable weather ie fairly sunny i can get by with this setup without using a gennie and a 40 amp smart charger for backup allthough i usually carry both for extended trips just in case. Using some smarts such as filling up with beer in the morning and not at night and freezing fish in the vans gas fridge before putting in the trailblazer etc all help and keeping voltage in the batteries as high as possible will extend their lifespan.Hope this helps.
cheers graeme.
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Follow Up By: Member - alan m6 - Monday, Oct 21, 2013 at 21:03

Monday, Oct 21, 2013 at 21:03
Thanks everyone for your imput, it has been most helpful(and insightful) and has created discussion.I think that solar is the way of the future camping.Electricity forcamping was most interesting, and I think I'mgetting a grasp of 12v.After much searching, Ibought 2x 100w Bosch systems from Solar Gods, and so far they have run fhe fridge and freezer beautifully.Will make an appointment with my auto elec.when homein Donald, to refine the works.
Heading to Licola for Melb Cup weekend, will test solar! !
Lucky punting and happy travels

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