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Forum Index : Solar : is there a formula for....

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pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 26
Posted: 11:34am 31 Oct 2020
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The maximum size solar panel you can connect directly to any given battery size (with blocking diode of course)?

Just out of curiosity.

Google reveals lots of examples of people doing this but I'm looking for a formula or a rule of thumb to mathematically determine the largest safe panel size to maintain a battery without a regulator.
Off grid man caver
 
nickskethisniks
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Joined: 17/10/2017
Location: Belgium
Posts: 252
Posted: 12:47pm 31 Oct 2020
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If I would do it, I would measure the current the battery is taking under float charge and go from there. I think a good battery has a datasheet that mention those currents.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3680
Posted: 09:10pm 31 Oct 2020
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This is not a good idea if you want the battery to last any length of time.

What you can get away with, and for how long depends on the battery type and chemistry, as well as just its relative size.

The main problem is not just excess current possibly overheating a very small battery, but without a charge regulator to limit charging voltage, the voltage may eventually rise high enough to cause permanent battery damage.

The best resource on the internet is "battery university" and learn about the characteristics of different battery types, and what can happen if any of the battery ratings are exceeded. https://batteryuniversity.com/learn/

Go ahead and have some fun, its an excellent way to learn.

There is a very big difference between messing about with an old junked car battery, and working with several thousand dollars worth of large Lithium cells.
Cheers,  Tony.
 
pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 26
Posted: 11:20am 02 Nov 2020
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  Warpspeed said  
The main problem is not just excess current possibly overheating a very small battery, but without a charge regulator to limit charging voltage, the voltage may eventually rise high enough to cause permanent battery damage.


Obviously there is potential for damage if one is using too large of a panel and too small of a battery, I'm not talking about putting a 100w panel on a 20ah battery.

What I'm interested in is determining the maximum panel size for any given battery size (or type) mathematically to achieve a desired "safe all day" voltage (whatever that may be). Obviously as battery voltage increases it also increases the internal resistance and stops accepting current from the panel.
Off grid man caver
 
Revlac

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Joined: 31/12/2016
Location: Australia
Posts: 429
Posted: 12:48pm 02 Nov 2020
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I really doubt any formula would work for this....too many variable's to deal with, the batteries deteriorate over time and change the resistance.

I have small panels charging batteries without regulators, 3w for a car battery and 3-5w for 4wd and truck batteries, also use a 20w panel on the old tractor if it won't start in the morning, give it an hour or 2 or 3 then kick it in the guts.
leave it on for 2 days or more and it will boil the truck battery.

Now I had a 12v 100ah deep cycle battery with a reg (running inverter and accessories) with a 150w 12v panel for some time, eventually the battery had degraded, I bypassed the regulator to give the battery an equalize, it would get to about 13v and thats about it, it had a low resistance by the end of the day the battery was quite warm to hot.
Other batteries and various different chemistry would likely go high resistance and therefore would be up around 16v or so even with a small panel.

Ok I'm just rambling.


Another idea that could be done easily with a secondhand panel, would be to bypass some of the cells on the solar panel so as it cannot exceed the maximum safe voltage for your battery.
This is probably the easiest thing to do...any other ideas?
Cheers Aaron
Off The Grid
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3680
Posted: 08:14pm 02 Nov 2020
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  pollenface said  
Obviously there is potential for damage if one is using too large of a panel and too small of a battery, I'm not talking about putting a 100w panel on a 20ah battery.

What I'm interested in is determining the maximum panel size for any given battery size (or type) mathematically to achieve a desired "safe all day" voltage (whatever that may be). Obviously as battery voltage increases it also increases the internal resistance and stops accepting current from the panel.


Its not a case of a 224 watt rated panel being perfect and a 225 watt rated panel causing total instantaneous battery destruction.  
Its a value judgement based on specifications, expectations, and experience.

For instance, panels are rated at 1Kw per square metre solar insolation, and you are not going to reach that at sea level in oZ, typically expect to reach about 80% of that in a clear blue sky, and less than that most of the time, and panels age.

Battery ratings may be at 20 Celsius, but at freezing temperatures or mid summer in a tin shed actual performance will vary a lot. And batteries age too.

So there is no EXACT precise mathematical solution possible.
The best you can hope for is some expert advice.
Cheers,  Tony.
 
Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 727
Posted: 08:03am 03 Nov 2020
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  Warpspeed said  

For instance, panels are rated at 1Kw per square metre solar isolation, and you are not going to reach that at sea level in oZ, typically expect to reach about 80% of that in a clear blue sky, and less than that most of the time, and panels age.


I didn't know that but makes perfect sense with what I have seen and know.
Panels are rated at 25oC and in clear Winter sun in Sydney they are are way above that so there is going to be a fall off right there.

Only ( short) time I have seen panels making full power is with cloud edge.
That happens Minutes a day not hours and certainly not reliably.

I also agree with the obvious, the variables are virtually endless. The thing that's also not mentioned is the load on the batteries. Unless these are endlessly float charged there must be some load.
One only has to look at the difference a solar array can make from one day to the other.  Not talking perfect sun to storms, I'm talking one nice sunny day to the other. I'm often intrigued by that.

With the low cost of controllers these days, My question would be why not have a controller?  I have put several on panels for my fathers machinery. Just the little voltage limiting boards. I run several off a single panel just with a Diode before each controller. Its a far from perfect setup but keeps the batteries Charged, a LOT cheaper than those AC float chargers and they don't let the batteries cook.

These things are cheap and easy to wire up and just solve so many problems I am not sure why anyone would not want to use one?
 
pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 26
Posted: 10:53am 04 Nov 2020
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  Warpspeed said  
So there is no EXACT precise mathematical solution possible.
The best you can hope for is some expert advice.


I thought you guys were experts, that's why I signed up :)

I currently have a 10w 14.2voc panel on a 120ah lead-calcium car battery. It gets to 13.3v in good sun. I might just get a few 5w panels and a carton of beer and make a day of it and become the expert (within my own test parameters/variables).

  Davo99 said  My question would be why not have a controller?


I have a stack of them in my shed, but I'm a curious guy.
Edited 2020-11-04 21:02 by pollenface
Off grid man caver
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3680
Posted: 08:32pm 04 Nov 2020
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  pollenface said  The maximum size solar panel you can connect directly to any given battery size

  pollenface said  
I currently have a 10w 14.2voc panel on a 120ah lead-calcium car battery.

I think you can be fairly certain that a 10 watt solar panel is not going to be dangerously oversized for your 120Ah battery  
Cheers,  Tony.
 
pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 26
Posted: 11:18pm 05 Nov 2020
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  Warpspeed said  I think you can be fairly certain that a 10 watt solar panel is not going to be dangerously oversized for your 120Ah battery  


Oh believe me, I am.
Off grid man caver
 
pollenface

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Joined: 01/09/2020
Location: Australia
Posts: 26
Posted: 10:59pm 12 Nov 2020
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Over the course of 3wks, the little 10 watter has boosted my 120ah up to a steady 14.2v during full sun hours.

The 120ah battery actually runs the cooling fans for my MPPTs when the ambient temp is above 30c.
Off grid man caver
 
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