solar panel charging current

[Quaysider]

now please don't shout me down folk - I've tried searching... but I don't really know how to phrase the question.

 

What I'm trying to get my head around is the relationship between the stated charge/output of panels and the amount going into the batteries via a mppt,

 

ie - say I buy a 300 watt panel quoted to be 35v and 8.5a

 

bung it on to a mppt and connect the 12v battery.... does the charging current end up somewhere near 25a or stay the initial 8.5?

 

If it stays the lower amount, then 3 x 100 w ones seem to give a greater charge don't they?

[Jess--]

under strong direct sunlight you can expect a maximum of around 24A (based on 300w into 12.5v)

[WotEver]

... say I buy a 300 watt panel quoted to be 35v and 8.5a, bung it on to a mppt and connect the 12v battery.... does the charging current end up somewhere near 25a...

Essentially, yes. Less losses of course, nothing is 100% efficient.

 

Tony

[Quaysider]

cheers - now it makes sense why the larger panels are higher voltage / lower currrents... which of course means you can use thinnner wires between the panels and mppt as until it does it's trickery the current is lower.

 

Thanks muchly.

[bottle]

cheers - now it makes sense why the larger panels are higher voltage / lower currrents... which of course means you can use thinnner wires between the panels and mppt as until it does it's trickery the current is lower.

 

Thanks muchly.

 

Yes but do not forget about volt drop.

[nicknorman]

Yes if you take the panels' total wattage, and divide by the battery voltage which of course is not 12v, more like 14v or more, knock off a small percentage for efficiency, you get the maximum current going into the batteries under ideal solar conditions.

[Jess--]

I always tend to use 12.5v for working out the max charging current on batteries because as the battery voltage comes up the current they will absorb tends to drop off

[Detling]

You can knock off about 10% from the calculated maximum because it is unlikely you will be correctly angled and tilted to the sun and there is usually a slight haze even on the clearest of days. Rate the cables and fuses for above the maximum though to allow for ageing of the copper.

[MtB]

now please don't shout me down folk - I've tried searching... but I don't really know how to phrase the question.

 

What I'm trying to get my head around is the relationship between the stated charge/output of panels and the amount going into the batteries via a mppt,

 

ie - say I buy a 300 watt panel quoted to be 35v and 8.5a

 

bung it on to a mppt and connect the 12v battery.... does the charging current end up somewhere near 25a or stay the initial 8.5?

 

If it stays the lower amount, then 3 x 100 w ones seem to give a greater charge don't they?

 

 

It does if you live on the equator, it's midday in summer and the sun is shining down from a cloudless sky.

 

On the other hand, if you live in the UK, its Jan 30th and middle of winter, expect the charging current to be closer to the 0.3a I get from my 360w of panels on a typical day around now.

[Phil Ambrose]

Mike is spot on, so often people expect their PV array to work at 100% all year with no regard to the amount or strength of the sun and disregarding the inclination of the sun which is very low this time of year.

Phil

[WotEver]

Whilst all these caveats about real world performance of PV panels in the UK are valid I think the thread is drifting away from the original question, which was "the relationship between panel current and charger current when using an MPPT controller".

[by'eck]

I always tend to use 12.5v for working out the max charging current on batteries because as the battery voltage comes up the current they will absorb tends to drop off

 

Well that will certainly (more than than) compensate for controller inefficiences but in reality the max charge current the batteries will draw given constant solar conditions, will be when volts rise over 14, and will only decrease when well above.

 

I remember seeing 18 amps going into batteries from 2 x 130 watt panels - work back from that

Edited January 31, 2016 by by'eck

[Sea Dog]

So, in the real world application of panels on a Narrowboat roof, tilting or not, including mid summer cloudless skies, does anyone ever actually see the stated output from their panels in the UK?

[Chewbacka]

Whilst all these caveats about real world performance of PV panels in the UK are valid I think the thread is drifting away from the original question, which was "the relationship between panel current and charger current when using an MPPT controller".

That was fully answered in post #6, so the op should be happy with the answer they have. If the people now wish to digress I don't see a problem and if the op wishes to follow they may learn some more interesting stuff.

[WotEver]

So, in the real world application of panels on a Narrowboat roof, tilting or not, including mid summer cloudless skies, does anyone ever actually see the stated output from their panels in the UK?

I have read on here on more than one occasion folks reporting more than the rated output for their panels on a bright but cool day.

 

It's not a frequent occurrence but it does happen.

 

Tony

[nicknorman]

Yes presumably the panel output is quoted at some specified solar radiation level and temperature. Nature being nature doesn't always comply with some manufacturers' rules.

[bottle]

I think also that the rating etc. is under quoted so that the guarantee of 80% output after 20 years (or whatever it is) can be given.

[Justme]

So, in the real world application of panels on a Narrowboat roof, tilting or not, including mid summer cloudless skies, does anyone ever actually see the stated output from their panels in the UK?

 

 

On my install, yes & more. Not a boat but it has similar install angles & locations as we are in a woodland so the horizon is much higher than normal.

 

The 1.2kWp array has hit over 1.3kWp.

 

However its not peaks that you really want to be interested in but total daily capture.

 

My best day has been 7 x kWp IE 8.4kWh (thats 672 ah at 12.5v).

 

It could have done slightly more (poss 8 x kWp) but the batts were full & we were not using enough.

 

In winter we are lucky to get any power at all due to the sun not getting over the trees.

[Sea Dog]

 

However its not peaks that you really want to be interested in but total daily capture.

Frequently, I agree, but in my case I'm not seeking to all (or even much) of my electricity this way. Rather, I'm looking to achieve a sufficient tail current to top off my batteries after the engine has restored them to 80%, either from occasions when I do only a short cruise or an alongside charging session.

 

I reckon something like 15a will be plenty for my needs, so less than a 200w panel in theory, but more is of course ok. However, when it comes to controller, the more it can handle the more it costs. A 15a Victron blue solar mppt is a cost effective choice and can hack higher currents which it limits to 15a output. Victron have a little Web tool which tells you whether the controllers can cope with your panel fit under such over-capacity circumstances, so I know some panels well over 200w can be run through it.

 

I was just pondering whether solar panels ever achieve their stated output in this country so that I can specify an appropriate panel to most effectively support the notional 15a charge I'm looking for. Sounds like they do.

[Justme]

Frequently, I agree, but in my case I'm not seeking to all (or even much) of my electricity this way. Rather, I'm looking to achieve a sufficient tail current to top off my batteries after the engine has restored them to 80%, either from occasions when I do only a short cruise or an alongside charging session.

 

I reckon something like 15a will be plenty for my needs, so less than a 200w panel in theory, but more is of course ok. However, when it comes to controller, the more it can handle the more it costs. A 15a Victron blue solar mppt is a cost effective choice and can hack higher currents which it limits to 15a output. Victron have a little Web tool which tells you whether the controllers can cope with your panel fit under such over-capacity circumstances, so I know some panels well over 200w can be run through it.

 

I was just pondering whether solar panels ever achieve their stated output in this country so that I can specify an appropriate panel to most effectively support the notional 15a charge I'm looking for. Sounds like they do.

 

 

Timing will be the issue. When will you want that 15 amps?

 

If you drive over night & cant top up via solar during the day then your golden. If your going to run the engine or genny during the peak solar day 10am-4pm say then the solar wont finish of that last 20%.

 

Most controllers can cope with excess amps but not excess volts.

[Sea Dog]

Timing will be the issue. When will you want that 15 amps?

 

If you drive over night & cant top up via solar during the day then your golden. If your going to run the engine or genny during the peak solar day 10am-4pm say then the solar wont finish of that last 20%.

 

Most controllers can cope with excess amps but not excess volts.

Thinks Justme. Mostly I like to move on early-ish, so on those rare days when I don't go far enough to get back to a 100% charge, the solar will have most of the rest of the day to assist with the last bit. I don't generally stay more than a pair of days, and my batteries cope with that but, if I do need a top up I like to charge as closely after 8 as is sociable depending where I am, so I get the bonus of hot water. I'm thinking solar will serve me quite well outside of winter.

[peterboat]

i have relied on solar and my whispergen for the last 5 years summertime has never been a problem its always winter when the issues start for me. the only time when my batteries are fully charged is after a cruise, however my full tractions seem to have survived well with little loss of capacity over the 10 years i have had one bank [i removed the batteries from my old boat when i sold it] i decided a while ago that i would do the best charging i could but wouldnt let it rule my life. maybe its because i have proper batteries it seems to work for me