Solar panel voltages and current ?

[ajpepe72]

Currently got a 100w flexible panel on my van which has a max output of 5.5amps (panel has open circuit voltage of 22v)

I’m thinking of switching to a bigger rigid or two small rigid panels wired in series.

My charge controller is a victron 75/15 mppt so want to get the best bang for buck from the new panels.

I’ve seen big 270w 36v panels but why do these only have a max output current of around 8.8amps when my 100w flexible does 5.5amps ?

Thanks.

[sshortcircuit]

W = I*V. Different voltage. When the voltage goes up the current comes down.

 

[ajpepe72]

So with an mppt, would the increased voltage of say a 200w 36v panel give similar or above two 22v 100 panels wired in either series or parralel ?

[spirou]

Victron has a handy calculator to match solar panels to regulators. Simply use the presets or adjust parameters to whatever panels you're looking at.

 

https://www.victronenergy.com/upload/software/VE-MPPT-Calc-2_7.xlsx

[aandncaravan]

ajpepe72 - 2018-09-23 8:53 PM

 

So with an mppt, would the increased voltage of say a 200w 36v panel give similar or above two 22v 100 panels wired in either series or parralel ?

 

 

2 x 22v will be more efficient in typical UK low light conditions, with a greater overall year round harvest.

 

When the regulator drops a 36v panel down to 14.4v, the current will rise accordingly, so at 14.4v a 270W could deliver about 13amps, in optimum conditions with the Sun overhead, on the South Cornwall Coast, etc.

 

To charge a 12.7v battery requires a voltage higher than 12.7v ideally, about 14.4v.

A battery will charge a lot more slowly, not always making the most of the available current, if the voltage is only just a smidgen above that of the battery.

 

As the angle of the sun in the sky drops and as the cloud cover increases, etc. then both the voltage and current from the panel reduce.

 

So towards the the end of an Autumn day you might have a situation when the battery is now back up to 12.5v but the Solar regulator output (even on a good MPPT) is also only 12.5v, so no charge will take place even though the Solar panel might have a potential 2 amps to give.

 

Larger panels usually start off with a higher voltage, like 36v, so more likely to be able to deliver a minimum 14.4v in typical UK poor light conditions, so still give good charge rate.

But big panels are often far from ideal on motorhomes and usually designed for House roofs where they won't encounter the Road Salt corrosion of a Motorhome installation.

 

 

One trick that some employ when adding a second 22v panel is to connect them together so the output is 44v as even in very poor light you should still get a minimum of 14v into the Solar Regulator and make the most of even the smallest bit of current.

Obviously you need a high quality Solar Regulator/Charger that will handle the much higher voltage.

Our favourite Votronic MPPT 250 handles 50v input.

 

But the panels should be pretty well matched.

 

 

 

 

[ajpepe72]

That was my first thought, two 100w 22v panels wired in series to 44v (the victron has a max input voltage of 75v so we’ll within that)

Then I saw a 36v panel which was 270w so was confused/wondering which set up would produce the more current/power... 2 100w 22v panels in series or a single 36v 270w panel.

[aandncaravan]

ajpepe72 - 2018-09-23 9:28 PM

 

That was my first thought, two 100w 22v panels wired in series to 44v (the victron has a max input voltage of 75v so we’ll within that)

Then I saw a 36v panel which was 270w so was confused/wondering which set up would produce the more current/power... 2 100w 22v panels in series or a single 36v 270w panel.

 

 

It further depends on where you will be harvesting the Solar, if it is in Morocco in Summer then all panels, even an 80w, will pretty much be working at high capacity so always providing a good voltage.

 

But in the North of Scotland in Winter, you really need those extra volts as a 100w/22v panel will struggle to deliver 12.1v let alone 14.4v.

 

 

Some will tell you that a single 270watt panel will deliver exactly the same as 2 x 100w + a 70w and in terms of pure 'watts output' they are generally right.

But they forget there is a battery that is being 'fussy' about how it chooses to take that potential power, and voltage is critical to battery charging, changing the rules completely.

 

If the battery doesn't like the charging profile it won't take it up efficiently and why the battery you select will be as important as the Solar set-up. For example don't install old fashioned Trojans and the like, or you could be discarding up to to 30% of what your Solar panels are working hard to collect.

 

 

 

 

[ajpepe72]

I live in Bristol and our trips mainly are down south, Cornwall etc.

Batteries are 2 yuasa L36-EFB’s as recommended by your good self .

I have set the victron up to suit the yuasa’s recommended charge profile.

[aandncaravan]

Then I would suggest that if using Solar in that area of the UK, which can harvest 33% more than northern Scotland, your approach and new set-up will work really, really well, see charts here : http://www.aandncaravanservices.co.uk/solar-panels.php

 

 

 

Don't forget that if you struggle for power in Winter, it is 'acceptable' to raise a quality batteries charge rate by a little, like for example from 14.4v to 14.8v.

When a battery is cold, say below 20 degrees, a higher Solar charge rate doesn't promote the internal corrosion it would at room temperature, so battery degradation isn't such an issue.

Especially with the gentle charge current usually associated with Solar.

 

 

I am not going to say that battery life will be totally unaffected, but the laboratory testing of the Yuasa L36-EFB where it achieved it's outstanding 233 cycle figure, was done at a higher than usual voltages, over 15v, so it should shrug off a 'winter only' 14.8v AGM charge rate and still achieve 233 cycles.

But please don't use those charge rates outside late Autumn/Winter/early Spring.

 

 

 

If anyone runs a 12v Compressor Fridge from Solar, which absolutely hammers the battery, then the above 'raised voltage' approach in winter can result in improvements.

Firstly, because the higher charge rate itself charges the battery more fully and faster.

Secondly, because the higher voltage tends to make the Fridge more efficient with shorter run times.

Thirdly because the higher charge voltage helps combat voltage drop that might otherwise be experienced when the Fridge is active.

 

However, expect any battery powering a 12v compressor Fridge to have a shorter life solely because of the continuous charge/discharge that a Compressor Fridge creates.

 

 

[spirou]

Not sure about votronic but victron has temperature compensation (preset or custom) to handle summer/winter conditions. The sensor is in the regulator but with larger installations a remote battery temperature sensor would be sensible optional extra as it may deliver quite a charge and warm up.

[ajpepe72]

Spirou, I’ve seen the victron has internal temp sensor, the controller is in the cupboard above the batteries(separated by a shelf) would you still think the add on victron smart sense temp/voltage module would be worth fitting ?

 

During the summer months when van is used most my compressor fridge uses about 36ah a day and my current 100w panel on decent days puts in around 25ah a day so really does help a lot. There were really sunny days this year when by around 1pm my battery voltage as back up to 14.4v which is when my controller would switch to float so I’m pretty confident that an extra 100w panel or a 260w panel would see me be pretty self sufficient.

[spirou]

Using the calculator mentioned above I checked the 100W panel preset (victron SPM100) wired in series and it says you would be regulator limited to 15A output at low temperatures (and perfect weather). Whether that worst/best case scenario would possibly overheat the Yuasa batteries is something I will let Allan answer. I would be inclined to saying a battery temperature sensor is not absolutely necessary in your case.

 

You'd be within regulator limits with 2x80W panels (victron SPM80), anything above that hits a power/voltage limit at the extremes. That said, max power I've seen from a 120W panel in full sun in June was 112W with only a few other instances when it went over 100W, so I'm guessing the likelihood of dual 100W in series reaching the limits is minimal and well worth the extra power other times of the year.

 

One thing I'd watch out for is the system voltage setting. I had it on auto and one day I got a high voltage alarm with 16V+. The regulator somehow switched to 24V setting as I was doing some battery swaps at the time, trying to revive an old car battery and it got confused. I think I didn't disconnect the panel from the regulator when swapping the battery and that somehow triggered the events. So to prevent boiling the battery just set it to 12V manually. My old car battery got an unintended equalization charge for an hour or so, luckily it was still quite early in the morning and not much charge going in ;-)

[aandncaravan]

Ajpepe72, If it was me I wouldn't activate temperature compensation as it will work against what you are trying to achieve.

The sensor will pick up the higher temperatures that you suggest you use the vehicle under and back DOWN the voltage when you want every millivolt upwards.

 

 

I note you say a 'extra' 100w panel, suggesting you are thinking of adding a proper flat 100w panel alongside the existing low out put flexible panel?

 

If so can I suggest you have a re-think as flexible panels are not the best for high output?

Flexible panels use a less transparent plastic for the top, not Glass as obviously Glass doesn't bend, resulting not only in a less efficient panel initially with lower light transmission, but faster degradation as the light transmission is restricted as the panel is used.

You may have seen how Car Plastic headlights discolour with time reducing light output by up to 50%?

 

 

I would expect a good 100w Solar setup in June to put back close on 55Ah a day, that yours is only managing half that is testament to how poor some of the Flexi panels can be.

 

If you are going to 'series' two panels they need to be a really good match to work best.

 

Suggest you follow the link to the Solar Panel page I post above, and the other pages on Solar, before you decide your next move as running a compressor Fridge on 12v Solar is really challenging to even the very best batteries?

Maybe also see our discussion document on 12v Compressor Fridges - "Are they fit for purpose?" : http://www.aandncaravanservices.co.uk/compressor-fridge-12v.php as that may help you optimise it's use?

 

 

I would also suggest you think about selling the Victron on eBay and acquiring a Votronic MPPT250 also wired to the Starter battery for the reasons given on the web pages.

 

The Victron is ok, but it isn't designed for motorhomes, it wouldn't have a 24v option for one and, secondly, would have Dual battery output, not single.

 

 

The Victron is also usually installed with the inefficient Vanbitz Battery Master Style device which further saps power from both batteries and the the power it is transferring (it obviously isn't going to be 100% efficient).

Just moving to a Votronic without a Battery master can result in about double the power getting to the batteries in mid Winter.

 

 

 

 

 

 

 

 

[Paul-]

With my Morningstar controllers which are 12 or 24v the controller needs to know what voltage it should charge at, so it must be connected to battery before connecting to the solar panel.

 

It also has auto temp control so is fitted behind the battery box, which is handy as there are gas drop holes and means the controllers heat sink is well ventilated :).

 

The only downside was that it cost that me a couple of ££s for the extra for 6mm cable *-)

[ajpepe72]

I’d be swapping the flexi for two 100w rigid panels.

To be fair the flexi has been fantastic. It has a max output of around 5.5amps but I’ve seen 7.4 amps coming out of the controller on really nice days. People say mppt isn’t any benefit on smaller single panels but it definately is from how mine has performed previously.

I’m guessing at the ah yield of my panel, it seems to start off at around 2 amps on a good day then is normally up around 4.5 - 5.5 amps during the peak hours. I’ve just guessed at an average of 3amps per hour for 8 hours. Now I have the victron I can more closely monitor the actual yield.

I’ve only just bought the victron controller and am actually very happy with it, all monitoring is done on my phone via Bluetooth.

Although only single output, I have a cbe csb2 installed in the engine bay which diverts a small current to the starter battery once the leisure’s are at a suitable voltage .

 

The van gets tucked away from October to April so is only really a fair weather vehicle. This summer I came away from a weekend away and forgot to turn the fridge off, it was on for two weeks before I realised and van had been sat there and batteries were still at 12.6v at night when I checked, this was an exceptional two weeks weather wise though and no doubt the fact the fridge hadn’t been opened during this two weeks helped a lot.

[plwsm2000]

ajpepe72 - 2018-09-23 7:58 PM

I’ve seen big 270w 36v panels but why do these only have a max output current of around 8.8amps when my 100w flexible does 5.5amps ?

Thanks.

You probably already know that a solar PANEL is made up from a number of solar CELLS.

The current generated by each cell is directly related to its area (and of course the solar energy it is seeing) and if you were to double its area, the current will also double. The voltage on each cell is about 0.6V which is just down to semiconductor physics and is not dependant on its size.

A panel is wired to connect the cells in series to increase the voltage.

Therefore, panels can be made with more cells to increase voltage or larger cells to increase current.

 

 

 

?

aandncaravan - 2018-09-23 9:19 PM

...

As the angle of the sun in the sky drops and as the cloud cover increases, etc. then both the voltage and current from the panel reduce.

 

So towards the the end of an Autumn day you might have a situation when the battery is now back up to 12.5v but the Solar regulator output (even on a good MPPT) is also only 12.5v, so no charge will take place even though the Solar panel might have a potential 2 amps to give.

...

Actually, the voltage on the panel doesn't reduce anywhere near as much as the current does. As the sun comes up in the morning, the voltage rises sharply and then levels out as the amount of solar energy increases. The current however, is totally dependent on the amount of solar energy.

At low levels, the voltage can still be quite high but as the current will be low, the power output is low.

Even at levels as low as 0.2W falling on a panel with 32 cells in series, you will still get an open circuit voltage of around 15 Volts but at very low currents of around 10 milliamps.

 

The physics is explained here together with an open circuit voltage calculator.

https://www.pveducation.org/pvcdrom/open-circuit-voltage

(this is for a single cell and if you type in different levels of light generated current, you need to multiply the voltage output by the number of cells) 0.01A gives a voltage of 0.476V per cell

 

So with a mppt regulator, you can still get a useful amount of solar current late in the day with just a single panel.

 

 

If you do connect the panels in series, make sure you use bypass diodes (not blocking diodes) as if one panel gets even partially shaded, you will have a major loss in power from BOTH panels.

 

 

 

 

 

 

 

[ajpepe72]

Helpful info.

I’m tempted to just get a bigger 260w 36v panel for ease of connection/installation and they seem to be cheaper than 2 100w panels would cost.

As a guesstimate in unbroken sun at the best part of the day what would you expect from a 36v 260w panel run through an mppt controller ?

[BruceM]

I went through a similar investigation process when I decided to replace my aging solar system (single panel) – full story here https://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/200W-Solar-capacity-one-panel-or-two-/48350/ and here https://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Solar-Panel-Power/47274/.

 

Whilst using a single large 200W panel had its attractions, locating a shadow free space on my van roof for the 200W panel footprint defeated me (I have roof bars etc). I was also worried about maintaining the structural integrity of the larger solar panel given its size and the vibrations and jolts it would receive being on a van. I’ve no evidence to justify the concern, but it occurred to me that a large panel supported just around the edges might be subject to a greater amplitude of flexing than a smaller panel. Ultimately I opted for two 100W panels wired in parallel, the Votronic duo 250, and two of the now familiar Varta LFD90 batteries. We have led lighting and no interest in using a TV, so other than mobile/laptops and heating our power consumption is low. Going off grid this time of year even during torrential rain does not seem to be an issue though I’ve yet to put it to the test in December.

[aandncaravan]

plwsm2000 - 2018-09-24 11:22 AM

 

Even at levels as low as 0.2W falling on a panel with 32 cells in series, you will still get an open circuit voltage of around 15 Volts but at very low currents of around 10 milliamps.

 

The physics is explained here together with an open circuit voltage calculator.

https://www.pveducation.org/pvcdrom/open-circuit-voltage

(this is for a single cell and if you type in different levels of light generated current, you need to multiply the voltage output by the number of cells) 0.01A gives a voltage of 0.476V per cell

 

So with a mppt regulator, you can still get a useful amount of solar current late in the day with just a single panel.

 

 

 

 

But we are not interested in Open Circuit voltages that are only relevant when the Panel isn't connected.

We are interested in the voltage under load as that is how the panel will be used.

 

A Solar panel delivers absolutely nothing of any value until a load is placed on it and that is what a Solar regulator does. It loads up the Solar panel as much as it can to draw the energy from the panel, which it then converts into a decent charge profile to charge the battery.

 

 

An MPPT regulator works more effectively in low light conditions EXACTLY because it reduces it's load on the Solar panel to raise the voltage back into the charging zone.

Reducing that load so that the voltage recovers back up to a decent level to get upto 14.8v for an AGM battery is counter productive.

 

Far better to start with an higher voltage than you ever need and load the Solar panel to the max get the the maximum current.

 

 

When the light drops and the current falls to about 25%, the low light, end of day Autumn conditions I spoke of, the voltage under under load can drop dramatically below the charging voltage.

 

If you look at this spec below for a 80w Solar panel, you will see it drops nearly 5 volts when in typical use from the Open Circuit voltage :

 

Power (Wp) 80watts

Nominal Voltage (Vmp) 17.5V DC

Nominal Current (Imp) 4.65A

Open Circuit Voltage (Voc) 22.0V

Short Circuit Current (Isc) 5A

 

 

At medium to low light around 1.5 amps load, the voltage is likely to be considerably less. Obviously a 60w panel or 40w panel is going to behave significantly worse.

 

You are making assumptions based on what you have seen, I am basing mine on what I have seen.

 

 

 

Like the Solar problem on a 2018 model Hymer with 2 x 95Ah AGM batteries I saw at 11:00 today.

 

The Dealer had fitted a 100W Solar panel on the roof, which subsequently failed and was replaced under warranty, but in replacing that they have short circuited the Schaudt LR1218 Solar Regulator (the incorrect none AGM version).

 

The Solar panel is working fine and the owner who is good at DIY will replace the Schaudt LR1218 with a Votronic MPPT250.

 

Worthy of note is that despite the vehicle being a 2018 spec Euro 6 model, the Alternater charge rate didn't meet the AGM batteries required 14.8v but was a better than average 14.3v.

 

See photo with engine running, and Solar regulator (Blue unit top right) inactive.

 

Despite the young age the twin AGM's were looking tired, not surprisingly without the correct Alternator and Solar charge.

 

 

 

 

 

 

[aandncaravan]

A follow up to the above 2018 model Hymer that had Solar power charging issues :

 

We went to see the van again today because the owner had concerns the Solar still wasn't working correctly following his replacement of the burnt out LR1218 with a Votronic MPPT 165.

 

He had wired it up correctly and it seemed to give higher charging voltages, but he had noticed some anomalies that he wanted us to check.

 

I took with me a £7.50 Wattmeter prewired into a Schaudt cable so we could just plug it straight into the Elektroblock EBL 30 and see exactly what Amps, Volts, Amphours, Watts, etc were going into the batteries.

 

His concern was based around the 'wildly' varying voltages he was seeing coming out of the Solar panel into the Votronic as they rolled erratically from 20v down to 13v and back again.

I told him that was normal in low light conditions (which was very, very heavy thick cloud, North Wales, 11:30 on 2/10/2018).

 

I connected the new Wattmeter.

Straight away the Wattmeter showed 0.3amp charge rate, which although seems low, is consistent with the very thick Cloud and time of year.

 

Happy that everything was working, I explained why the voltage between the Solar Panel and the Votronic rose and fell like it did because:

"The Votronic draws the maximum power it can from Solar Panel by placing a big load on the panel. In low light conditions this can result in the voltage dropping below 14v and lower than the ideal charge voltage.

So the Votronic reduces it's load on the Solar Panel until the voltage rises back up to above 14v to always give the ideal charge rate, hence seeing the voltage vary up and down between the Solar Panel and the regulator as the Votronic tries to find the optimum point.

In the bright sun of June these rolling voltages won't be seen as the Solar Panel will be producing more than enough power to keep the voltage high".

 

As expected, there was a constant, decent voltage between the Votronic and the Batteries/Power Distribution unit.

 

 

By 12:00 the cloud had thinned very slightly and we were seeing up to 1.1amps from the 100watt panel.

 

 

The Wattmeter is a really easy device to install, just two screw block connectors into the existing cable run between the regulator and the Power Controller/battery and at £7 isn't expensive, search ebay.

It shows current going into the battery plus volts, watts, Amp hours, etc.

 

I supplied and installed the Wattmeter and did a general check for free, not because they were such nice people, but because they said I could have one of the old AGM batteries that Mr B. removed to fit Yuasa L36-EFB's.

 

The plan is to cut open the AGM's to see why they were delivering such little capacity when only just over a year old with light use. The voltage has already dropped to 12.79v from 12.98v in just over 24 hours, so it isn't looking healthy.

 

Obviously with the vehicle being delivered without an AGM compliant Alternator and the Dealer fitting a none AGM Solar regulator that is going to take a toll on AGM batteries.

 

 

When we were cutting open the Banner AGM's a few years ago there was always carnage inside the battery, this will be the first Varta AGM we have opened in a while. The first so young as well, so can't wait to discharge it fully and see inside it. See it below awaiting surgery.