solar panel fridge battery,,do the numbers add up

Submitted: Friday, May 23, 2008 at 12:56
ThreadID: 57914 Views:3862 Replies:6 FollowUps:22
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hi i am measureing the current draw and frequency of my 55ltr fridge and allowing for night time,im trying to decide what size solar panel i should buy
the fridge uses 60w for 1/3 of the time
does that mean a 20w panel will do[oops allow for 50% darkness,40w panel or do the numbers not add up so simply ie it may be able to produce 40w but will it recharge the battery at a rate of 40w
remembering that it has to overcome the batteries voltage to charge will it only charge at a fraction of 40w and will i need a biger panel to maintain charge
thanks
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Reply By: Mainey (wa) - Friday, May 23, 2008 at 13:49

Friday, May 23, 2008 at 13:49
First off you can't really do it mathamatically, because the numbers will *never* stack up as correct *every* single time, they will only give you a close idea.
Reason being, both the weather, and the accessories and the fridge draw, will never continiously be identical week after week.

Realistically, yes you need to replace what the fridge (and all accessories) will use, that is a reasonably simple equasion.
3ah x 24 = 72 Amps (Maximum)
BUT: fridge will use less in the evening when night is cooler and fridge is not being opened while your sleeping...
~66 Amps ?? (60/24 = 2.75ah, similar to claimed Engal draw)

So you need to replace in just 6 hours what you will use in 24 hours.

Therefore a solar system that can deliver 12 Amps (~200 Watt) would be ample for continious camping with no further charge required by the battery bank!!

However many guys are happy with a 4.85 Amp (80 watt) solar panel, because they are also prepared drive to recharge the battery too, it depends on what you enjoy doing when on holidays.

Then some panels are far more *efficient* than others!
Remember the 80 watt panel is rated @ 80 Watt "Maximum" - not average or realistic supply numbers!!

Mainey . . .
AnswerID: 305455

Follow Up By: Grungle - Friday, May 23, 2008 at 18:52

Friday, May 23, 2008 at 18:52
Quote "Remember the 80 watt panel is rated @ 80 Watt "Maximum" - not average or realistic supply numbers!!"

Not quite. It is rated at 80W under test conditions and you can usually get more power than they specify during spring & summer at around solar noon. I frequently get 5A to 5.1A from panels on remote repeater sites that we look after for various customers.

Standard Test Conditions (STC) specifies a temperature of 25°C and an irradiance of 1000 W/m² with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun.

Information sources:

BiPV Best Practice guidelines.pdf

Wikipedia - Solar radiation

Regards
David

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FollowupID: 571512

Follow Up By: Member - keith P (VIC) - Friday, May 23, 2008 at 19:15

Friday, May 23, 2008 at 19:15
???
Quote" 5=5.1A?
what size panel?
what regulated output voltage?

you will NEVER exceed the rated output POWER of the panel.!

dont expect to get 80W from the panel.
the best ive ever got is around 68W from my 80W panel.
i have a 80W bp panel and a power tech regulator.

12.6 V @ 5.1A = 64.26W
V x I = P

you need to understand how current and voltage are calculated to power

but as i previously said never expect full Power from a panel t think a figure of 80% in good conditions is about right and them you need to start calculating losses in the rest of the system
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FollowupID: 571517

Follow Up By: Nomadic Navara - Friday, May 23, 2008 at 20:07

Friday, May 23, 2008 at 20:07
Solar panels are rated at what is called the peak power point. For most panels this PPP occurs with an output voltage around 17 V. At the battery charging voltage of under 15 V the panel produces a little more current than the peak power point current - you will have to get the power curves for your panel to find the exact figure. A safe calculation for the power available to charge your battery is to multiply the PPP current by 14.5 V.

Solar panels regularly do produce more output than they are rated. The ratings are based on direct solar radiation only. During periods of broken cloud, extra radiation reaches the panel from reflection from the clouds, periods of near 150% of expected currents have been recorded. However, don't rely on these outputs.

PeterD
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Follow Up By: Grungle - Saturday, May 24, 2008 at 16:55

Saturday, May 24, 2008 at 16:55
???
Quote" 5=5.1A?
what size panel?
what regulated output voltage?

Hi Keith,

The panels are BP 80W units and yes I am fully aware of ohms law (been an radio and electronics tech now for nearly 20 years).

I don't use the power rating of a panel to calculate the size of a system but instead the current rating of a panel. Also at no time did I say you will get 80W from an 80W panel. I said I have gotten 5A to 5.1A from a single panel.

I know I can get 5A from an 80W panels at certain times as I see it all the time. I design and build solar powered remote repeater sites as part of my job as the manager of a radio communications business. I also maintain all emergency services sites here in CQ. I have access to engineers direct from the manufactures and have done the courses.

I am only correcting people who say that it is impossible to get more current from a panel that what is listed on the sticker. Just because they have never been able to get 5A from an 80W panel doesn't mean it can't happen.

Using ohms law to calculate the Amp output of a panel is of no use. There are too many variables and inefficencies. You yourself said the most you have got from an 80W panel is 68W. Well that would mean 5.62A @ 12.1V or 5.48A @ 12.4V or 5.39A @ 12.6V according to ohms law. That is an average 111% efficiency!

Regards
David
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FollowupID: 571672

Reply By: Crackles - Friday, May 23, 2008 at 14:15

Friday, May 23, 2008 at 14:15
If only it was that easy :-) Temperature, cloud cover etc makes it very difficult to be specific in calculating power use & charge. As a guide those I know using 2 x 40 watt panels to charge a 100ah battery running a fridge & a couple of camp lights seem to get around 4 to 5 days use in temps around 32 degrees. With more effort shifting the panels regularly to the ideal angle another day or 2 could be gained. With 4 x 40 watt fixed panels feeding a 300ah battery , a friends system on his van keeps up with all his power needs (fridge/lights/small inverter) & seems to be about the size required to ensure reliability.
I suppose you need to decide how long you intend to park in one spot for. Reputable solar shops will be able to calculate your power needs & recommend a system that will keep up.
Cheers Craig...............
AnswerID: 305456

Reply By: Topcat (WA) - Friday, May 23, 2008 at 16:04

Friday, May 23, 2008 at 16:04
When I started using solar energy over 25 years ago I was told that to run a fridge & some lighting where the average daily consumption was around 20 amps I would need a minimum of 120watts of solar power to keep the supply battery topped up. I use 160 watts of solar power to run my 60 ltr. Trailblaza & a couple of lights at night & found this was acceptable for extended power consumption & keeping the supply battery more then 75% fully charged. So what I was told was well within the ball park. Cheers.
AnswerID: 305471

Follow Up By: Mainey (wa) - Friday, May 23, 2008 at 18:29

Friday, May 23, 2008 at 18:29
Ummm, 25 years ago there was a fridge that used ONLY 20 Amps daily ??

Can I suggest if you can't maintain *100%* battery charge and also run the 60Lt Trailblaza and some lights, with 160 Watts, then you may have a problem with the Solar Regulator or Battery.

Mainey . . .
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Follow Up By: Topcat (WA) - Friday, May 23, 2008 at 19:26

Friday, May 23, 2008 at 19:26
Mainy, to clarify the matter - if you have read any of my previous posts on deep cycle batteries & solar power you will know that I endeavour not to discharge my batteries more then 75%. Most of the time they are fully charged. That is why I get good mileage out of my batteries. The last set (which I have just replaced this month) gave me 7 years usage. The fridge is the unit you refer to & that is the daily consumption - 20 amps. Not bad for one that has been trouble free for that long!!!!. Cheers.
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FollowupID: 571519

Follow Up By: Topcat (WA) - Friday, May 23, 2008 at 19:32

Friday, May 23, 2008 at 19:32
Correction to my last. I don't discharge more then 25% of the battery capacity, meaning they always have at least 75% capacity. My solar regulator is functioning correctly. Cheers.
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FollowupID: 571522

Follow Up By: dagwill - Friday, May 23, 2008 at 19:37

Friday, May 23, 2008 at 19:37
topcat do deep cycle batteries need higher voltage recharge system than standard batteries,or does one size fit all
thanks
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FollowupID: 571523

Follow Up By: Nomadic Navara - Friday, May 23, 2008 at 20:16

Friday, May 23, 2008 at 20:16
Dagwill

It is not whether the battery is a cranking battery or a deep cycle one that counts. It is the chemistry of the battery that you have to consider. You will find batteries of similar chemistry with different recommended charge voltages. The best voltage to charge a battery is the one recommended by the manufacturer.

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Follow Up By: Nomadic Navara - Friday, May 23, 2008 at 20:19

Friday, May 23, 2008 at 20:19
Mainey

Topcat said "over 25 years ago I was told" - He did not say that the figure was fact. He also used a larger panel than was recommended and found that it was the required size.

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Follow Up By: Topcat (WA) - Friday, May 23, 2008 at 21:38

Friday, May 23, 2008 at 21:38
Dagwill, from experience & as Mainey has mentioned with manufacturers recommendations, I have found that recharging of deep cycle batteries, it is better to trickle charge (usually no more then 5amps) then to fast charge as this tends to degenerate the plate structure. So if you can maintain a small battery discharge (no more then 50%) you will get more 'Cycles' out of them. The more cycles the longer the battery will last. I hope this clarifies the matter. There is much more subject matter to be found in the archives of this forum & on the net which will help you understand the differences between deep cycle & high discharge (cranking) batteries. The main cause of lead acid battery deteriorization of is heat & sulphation so correct charging methods are important. ie. a decent 3 stage charging system with a 'pulse' setup to reduce sulphation. Cheers.
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Follow Up By: Topcat (WA) - Friday, May 23, 2008 at 21:49

Friday, May 23, 2008 at 21:49
Dagwill have a look at Bill Darden's web site on batteries. It will answer a lot of your queries. You can find it here:

http://www.batteryfaq.org/

Cheers
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Follow Up By: Mainey (wa) - Sunday, May 25, 2008 at 09:36

Sunday, May 25, 2008 at 09:36
Peter, my point was not relative to being fact or fiction, because the 'sentence' was: "I was told that to run a fridge & some lighting where the AVERAGE daily consumption was around 20 amps"
My post was 'only' relevant to the information supplied, that being there was a fridge made 25 years ago that ONLY used 20 Amps daily !
That is a very low power draw, even by today's improved technological standards.

Yes, the larger 160 Watt solar capacity (~10 Amp) Topcat uses is a bonus, but as I said I believe it should be capable of maintaining the battery system 100% not ~75% charged.

If it is ONLY ~75% charged on day one, then obviously on day two it has to be *less* than 75% charged, because the solar system is in fact not maintaining it to 100% charged.

It obviously therefore has to be loosing some battery capacity daily, because it is stated as only being charged to ~75%, and it can't possibly be maintained at that level consistently, if it can, then I have to wonder why it can't be maintained at 100% charged ??

As I said, I believe there maybe a problem with the battery or the solar regulator, because to put in 10 Amps when your only using 4 Amps (simplified) the numbers just do not add up - as with-in reason they should.

By comparison: I use a fridge/freezer that runs @ ~10 Amps (2.8ah), solar system (200+ Watts) replaces 12 Amps Maximum, by 10am daily I have a fully charged battery system, yes 10am DAILY.

This is more fully explained and with confirming pictures clearly shown, if you view, "Member: My Profile" as shown below.

Mainey . . .
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FollowupID: 571782

Follow Up By: Topcat (WA) - Sunday, May 25, 2008 at 16:04

Sunday, May 25, 2008 at 16:04
Mainey, I think you are a bit confused with what I have said. True 160 watts of solar power will produce a 10amp charge current but only under ideal sunny conditions. I made a corrective statement saying that I try not to discharge my deep cycle batteries more then 25% of their total capacity (i.e. when the solar sytem is not operating at night). The batteries come up to the fully charged state during daylight hours so don't quote me as saying they only come up to 75% charged. I use a 3 stage solar 10amp solar charge system & there is no problem (R) with my batteries or the solar regulator. There will be times in overcast conditons where the charge rate is low & your battery may not get to be fully charged during that period that is a fact!!!. That is also why I have a large capacity deep cycle battery system (225 amps) for camping for extented periods of time. Cheers.
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FollowupID: 571842

Follow Up By: Mainey (wa) - Sunday, May 25, 2008 at 16:26

Sunday, May 25, 2008 at 16:26
Topcat
You now post:-> "I made a corrective statement saying that I try NOT to discharge my deep cycle batteries MORE THAN 25% of their total capacity"

The capacity is 225 Amps @ 12 Volts !!
So if it's discharged 25% of it's capacity, what is that in Voltage ??

Mainey . . .
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FollowupID: 571844

Follow Up By: Nomadic Navara - Sunday, May 25, 2008 at 17:11

Sunday, May 25, 2008 at 17:11
Mainey

Again you have your knickers in a twist. In FollowUp 11 you appear to be picking up something made as a correction in FollowUp 10. Topcat actually made that correction in FollowUp 3 which was correcting a statement he made in FollowUp 2. Again you are confusing readers of this thread instead of making any valuable contribution to it.

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Follow Up By: Mainey (wa) - Sunday, May 25, 2008 at 18:37

Sunday, May 25, 2008 at 18:37
Peter,
sorry if your confused, I thought it was just a simple question I asked, and directly relevant to followup #10 which Topcat made and states:
"I made a corrective statement saying that I try NOT to discharge my deep cycle batteries MORE THAN 25% of their total capacity"

The question remains:
"The capacity is 225 Amps @ 12 Volts !!
So if it's discharged 25% of it's capacity, what is that in Voltage ??"

Mainey . . .
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FollowupID: 571880

Follow Up By: Topcat (WA) - Sunday, May 25, 2008 at 21:22

Sunday, May 25, 2008 at 21:22
Ok Mainey, to answer your question the voltage@25% discharged is approximately 12.5 volts which is from the manufactures specs.& here is the table:
100% fully charged - 12.65 volts. (S.G. Electrolyte 1.265)
75% charged - 12.5 volts (S.G. Electrolyte 1.225)
50% Charged - 12.2 volts (S.G. Electrolyte 1.190)
25% Charged - 12.0 volts (S.G. Electrolyte 1.155)
Discharged - 11.9 volts (S.G. Electrolyte 1.120)

If you professed to know so much about batteries you would know this. Also note that voltages vary with gel cell & AGM batteries. Temperature of the electrolyte is another factor as well. The average temperature when taking readings should be 80deg.F (26.7degC). Cheers.
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FollowupID: 571918

Follow Up By: Mainey (wa) - Monday, May 26, 2008 at 00:21

Monday, May 26, 2008 at 00:21
Topcat, In your post you say:-> "I use 160 watts of solar power to run my 60 ltr Trailblaza & a couple of lights at night & found this was acceptable for extended power consumption & keeping the supply battery more then 75% fully charged"

So what I now understand from the information you have posted is with your 160 watt solar system, your Auxiliary battery system does not get below 12.5 Volts when running your fridge and lights.

No generator can compete with your solar system performance!

Thank you for being patient with me :-)

Mainey . . .
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FollowupID: 571944

Reply By: blue one - Friday, May 23, 2008 at 18:43

Friday, May 23, 2008 at 18:43
I concur Topcat.

I have two panels (128w total) and haven't had a problem to date with keeping a 32 ltr freezer & 40 ltr fridge in operation.

You have to have ice for the Missus G&T.

Cheers
AnswerID: 305498

Follow Up By: blue one - Friday, May 23, 2008 at 20:20

Friday, May 23, 2008 at 20:20
Great to be back in the deserts.

Cheers
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FollowupID: 571542

Reply By: dagwill - Friday, May 23, 2008 at 20:20

Friday, May 23, 2008 at 20:20
yeah a G&T just has to be COLD
ive done some googleing and didnt realise the fine voltage constraints for lead acid batteriese
ie if voltage drops below 11.8 its as good as dead flat and damage will have resulted
so perhaps there is not a huge voltage diff between a standard and a DC bat. at 50% charge
thanks for all input, please forgive my ignorance
AnswerID: 305516

Follow Up By: blue one - Friday, May 23, 2008 at 20:27

Friday, May 23, 2008 at 20:27
Dagwill,
Even a dead flat deep cycle will behave and re-charge around 15 -20 odd times.

It is going to happen when for some unknown reason you run out of battery.

Look at the benefits, a huge barby & smorgasboard for a week.

Cheers
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FollowupID: 571547

Reply By: dagwill - Friday, May 23, 2008 at 22:44

Friday, May 23, 2008 at 22:44
blue one
it took me a minute to apreciate your coment,,,,i get it
thanks
AnswerID: 305538

Follow Up By: Mainey (wa) - Sunday, May 25, 2008 at 11:03

Sunday, May 25, 2008 at 11:03
If any battery becomes "dead flat" when used in a Solar system, as we talking about in this thread, most often it will indicate a severe problem with the solar regulator.
Because all quality specified solar regulators disconnect the Auxiliary battery system from the fridge when the battery voltage becomes low.
The Solar system will continue to recharge the Aux battery via the Solar regulator, which will automatically reconnect the fridge when the Auxiliary battery is back at a realistic voltage.

The early "dead battery" (if it's a suitable battery for a Solar system anyway - most *efficient* is an AGM Deep Cycle) indicates the Solar system, via the solar regulator, is not maintaining the battery fully charged.

Mainey . . .
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FollowupID: 571799

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