Solar panels - output observation

[[2574]]

I thought that some folk hereabouts, who are wondering whether solar power might be viable for their boat, might be interested in a simple method of assessing what (real life) charging performance can be expected from a stated panel power output (in W).

By experience and observation I have found that the charging power delivered to the batteries (at somewhere between 12.4v and 13.8v depending upon state of battery charge) can be readily ascertained by dividing the stated panel output by the open circuit voltage of the panel. Most panels seem to have an open circuit voltage of about 22v so, by way of example, my 238W array produces about 10.8A in full sun. Interestingly, if there is thin cloud giving a hazy sunlight (and therefore a cooler panel) the charge increases slightly to over 11A.

Now the above might be stating the 'bleedin obvious (VxA=W etc) to some folk in the know, but I am aware of some folk who divide the stated output in W by the expected charging voltage (say13v). Doing this calculation would give an overstated and misleading 18A (using my example above) charging rate.

I should add that this observation is for an array managed by an MPPT controller connected to Solara M Series semi-flexible panels.

I hope this helps somebody, if not then consign this post to the dustbin in the ether......

Rob

 

[Hypocacculus]

This would be really useful information if I understood the implications of a 11A charge vs an 18A charge!

For the electrically challenged like me, what does that mean in terms of keeping my beer cold and my GPS talking to me?

 

[Mistroma]

I've found this to be useful: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

Just pick your location on map and enter panel size in W (it uses kWp but ignore that for now) and slope of 0 deg. (assuming panel is flat on deck). Click calculate and look at first column of results "Average daily electricity production from the given system (kWh)".

Read results as avg. Wh per day each month (because panel size was actually W not kW).
I find Ah per day can be more useful so usually paste the results into a spreadsheet and divide the first column by 13.

Results from this model have been close to output I've actually achieved in various locations over past couple of years.

More complex than OPs method but does give output for your location during whole year in terms of Ah generated each day.

 

[pmagowan]

This would be really useful information if I understood the implications of a 11A charge vs an 18A charge!

For the electrically challenged like me, what does that mean in terms of keeping my beer cold and my GPS talking to me?

18 amps can cool about 70% more beer than 11A.

 

[Hypocacculus]

18 amps can cool about 70% more beer than 11A.

Thanks! While I'm asking dunce level questions - Wouldn't an 18A charge fry my battery anyway? Or is that all taken care of by my charge controller? What would happen to the extra amps?

 

[pmagowan]

Thanks! While I'm asking dunce level questions - Wouldn't an 18A charge fry my battery anyway? Or is that all taken care of by my charge controller? What would happen to the extra amps?

The best way to fry your battery is in olive oil over a medium gas. The other questions are for experts!

 

[ShinyShoe]

How big is ur battery?

Let's assume it's 100Ah. You should probably only reduce it to 40%. So 11A will take roughly 6 hours to charge from 40% to full with no current drain happening. 18A would be more like 3.5hrs.

Would you have 6 hours of full sun ?

You want your charge controller to stop fast charging before 100%.

I'd guess there are some on here with at least 2 if not 3 120Ah units... With 2 batteries you'd need 144A to charge it so 15+ hours of full sun on 11A and under 10 on 18A...

 

[pampas]

gets dumped by the controller

 

[[2574]]

This would be really useful information if I understood the implications of a 11A charge vs an 18A charge!

For the electrically challenged like me, what does that mean in terms of keeping my beer cold and my GPS talking to me?

When we lay to the anchor in the summer we reckon to draw a constant 2.0A. That allows for a 30% cycling fridge compressor, the lights (LED) in the evening, the FM radio/CD, water pump, gas & CO alarms, Navtex etc...... So, over 24hrs we draw down a total of 48AH* (24hrs x 2.0A). Assuming the panels produce power at 50% of maximum for nine hours each summer day (only when the sun is near perpendicular to the panel will the rated output be achieved, at more acute angles of sunlight the solar performance decreases, nothing once the sun has set, obviously) so in a day the panels at 11A will produce 9 x (11x50%) = 50AH which makes us self sufficient in power consumption/generation. Do the same calculation at 18A and you'll convince yourself that you have loads of surplus power (9 x (18x50%) = 81AH.

Or more simply the more A that you can generate the better! With solar its important to tackle both generation and consumption; maximise generation and minimise consumption where you can. There I go again - statin' the 'bleedin obvious!

Rob

* AH is abbreviation of AmpHour - so a 3 A load running for one hour will have consumed 3AH.

 

[Hypocacculus]

Thanks all. that's very helpful. We have inherited a solar panel with the boat we bought but I've never really had much idea about what it could do.

 

[sailinglegend420]

... So 11A will take roughly 6 hours to charge from 40% to full with no current drain happening. 18A would be more like 3.5hrs.....

It will take a lot lot longer than you suggest because as the batteries get charged the current they will accept falls away. More like 24 hours!

....You want your charge controller to stop fast charging before 100%......

Your controller doesn't 'fast charge' till the batteries are full - it is a 'constant current' fast charger till the voltage reaches about 14.4v and then it becomes a 'constant voltage' charger at whatever current the batteries will accept. Yes you need to reduce the voltage to a much lower 'Float' voltage when the batteries are nearly full. The trouble with all charge controllers is they don't know the size of your battery bank so can never know when the bank is fully charged, so they have to work on algorithms based on time, voltage and current, or a combination of all three to switch to Float which will finish getting the batteries to a full charge.

So all those people who say their batteries are fully charged by lunchtime are all being duped by the 'Charging Gotcha'. Controllers are designed to 'charge' your batteries not 'over charge them'. They may well switch to 'Float' at 90% of fully charged. If you cycle your batteries from 50-90% that's 80% of the useable capacity re-charged, but at 90% SoC you still have 20% of your useable capacity not being replaced. There won't be enough hours in the day left when they have dropped to a much lower Float voltage to get enough Ah back in to fully charge the bank.

The real downside of all this is the batteries will sulfate and reduce their capacity, and unless you get your batteries REGULARLY back to 100% the Lead Sulfate crystals will harden and cannot be removed. A monthly equalization charge of 15.5v may remove some of the sulfation, but not all batteries can be equalised!

In answer to the OPs posting instead of all that maths with volts and amps and watts and time simply divide the panel wattage by 3 if in high summer in the Med, and by 4 if in the UK.

So Rob if you are still in Gospit then your panels should give about 63Ah per day!

Your calculations should have been 238/17 = 14 amps (17v is the panel output under load) - then 14 X 9 hours of sunshine X a 50% averaging figure =63 Ah.

 

[OthaWeis]

I thought that some folk hereabouts, who are wondering whether might be viable for their boat, might be interested in a simple method of assessing what (real life) charging performance can be expected from a stated panel power output (in W).

By experience and observation I have found that the charging power delivered to the batteries (at somewhere between 12.4v and 13.8v depending upon state of battery charge) can be readily ascertained by dividing the stated panel output by the open circuit voltage of the panel. Most panels seem to have an open circuit voltage of about 22v so, by way of example, my 238W array produces about 10.8A in full sun. Interestingly, if there is thin cloud giving a hazy sunlight (and therefore a cooler panel) the charge increases slightly to over 11A.

Now the above might be stating the 'bleedin obvious (VxA=W etc) to some folk in the know, but I am aware of some folk who divide the stated output in W by the expected charging voltage (say13v). Doing this calculation would give an overstated and misleading 18A (using my example above) charging rate.

I should add that this observation is for an array managed by an MPPT controller connected to Solara M Series semi-flexible panels.

I hope this helps somebody, if not then consign this post to the dustbin in the ether......

Rob

Thanks for sharing out this useful information..I am working on my new boat so will take those important points in consideration..