Solar panel fuse/breaker question :)

[MikeVan]

A question/advice requested re: solar panels ...

 

I've got a question re: fusing/breaker for a solar panel setup I'm installing on a van conversion - just want to sense-check my thoughts against someone else's brains, maybe someone with more experience of solar than me :) Thanks in advance!

 

So, my panels are 2x 270w Perlight Delta All Black ones… These: https://midsummerwholesale.co.uk/pdfs/perlight-delta-black-54-270.pdf

 

As I understand it, it's good practise to fuse any power source as early as possible, be that a battery or solar panel.

 

I obviously don't want a fuse on the outside, that'd be a pain! So am thinking that as the solar cable will come in via an entry gland that's above a top side cupboard (cupboard yet to be built!), then I could pop a circuit breaker in that cupboard - nice & convenient.

 

My calcs, then some questions:

 

- Panels state open circuit voltage of 36.07v, so I guess that's the max volts.

- They seem to say (on the PDF linked above) current at Pmax is 9.06a per panel, so that’s 18.12 amps max.

- I'll be connecting the panels in parallel (argued with myself on this... series would increase the voltage and this may be good in low light UK conditions to make sure the voltage is higher than the battery, and so enable the batts to charge... But then again if then ONE panel is shaded, I think it'd mess up the other if in series... Parallel won't increase the volts but will work better if one panel in partial shade, if I'm correct? Plus, doesn't the MPPT serve to increase the volts if needed, provided it has sufficient amps being delivered to it, to ensure the volts are sufficient for the battery to accept a charge? Of course the MPPT will be very close to the batteries to avoid any further V drop, nice thick cables too).

 

So with all of the above in mind, fuse/breaker size I think is:

 

9.06a per panel, parallel so doubled as above would be 18.12amps. Could I get away with a 20a breaker? (keen on using a breaker as I'm a bit crap at losing fuses!).

 

 

QUESTIONS

 

1 - Anyone sense-check my thoughts & calcs above? All opinions welcome... If I've messed up somewhere or am wrong, please do say so! Getting this right is more important than my pride for sure ;)

 

2 - With cable thickness/amps capacity calcs I believe you need to add 25% as a safety margin, I'm imagining the same is NOT true of a fuse, as the fuse is intended to protect the cable (not the appliance) from overload/overheating - am I right about this? If so, what are the thoughts about a 20a breaker when the max amps is 18.12a? All good? (I doubt the MPPT would complain if somehow 20a was delivered to it, it's a 50a MPPT - plus the cable I used will be rated above that anyway)... (?)

 

3 - What are thoughts on fusing both the + and - cables from the solar to the MPPT? Overkill/not needed or advisable? Been told both by solar folks in the past! Opinions appreciated.

 

4 - If I did go with a breaker of 20a, do you think a car audio one would work, like: https://www.amazon.co.uk/Buwico%C2%AE-Protection-Inline-Circuit-Breaker/dp/B0175T00AS?th=1 ? (NOT an affiliate link! genuine question)

 

Hmmmmm that's it, I think!

 

Cheers in advance :)

 

 

 

 

[colin]

A short on the panel to battery cable re output from panel will be unfortunate, a short re output from battery could be catastrophic, for this reason the cable is usually fused near the battery.

[MikeVan]

colin - 2020-01-02 5:03 PM

 

A short on the panel to battery cable re output from panel will be unfortunate, a short re output from battery could be catastrophic, for this reason the cable is usually fused near the battery.

 

Thanks for your reply Colin :)

 

Would definitely fuse as close to the battery as possible, especially as they are located under the bed! So yup, 100% will fuse there. Just wondering about a fuse for the solar too as from what I understand, sparkies say any power source should be fused as early as poss...

 

Not sure if my calcs & assumptions are correct above though, so keen to hear if I'm wrong with any of the calcs etc

[colin]

I may be wrong, but if the panel can only give a maximum output of 18amp, then putting a 20amp fuse will achieve nothing to stop the panel output.

[MikeVan]

colin - 2020-01-02 5:09 PM

 

I may be wrong, but if the panel can only give a maximum output of 18amp, then putting a 20amp fuse will achieve nothing to stop the panel output.

 

Understood. I guess it's just if the panel somehow sent more than 18a due to a fault (series short would give 36a if I'm right?). Not sure if/how this could happen and even if it did, MPPT is 50a max so maybe I'm being overfly cautious...

 

 

[MikeVan]

Also ... Issue is that the breaker linked in my OP is a 12v to 24v breaker, from what I understand this MAY be fine, the only risk being the plastic isn't rated to the (max) voltage of the panels, so a risk of arcing through the plastic... But surely there's a tolerance here?) Or would this maybe be better? https://www.amazon.co.uk/Circuit-Breaker-Electric-protector-switch-40A/dp/B082Y3XKWZ/ref=sr_1_4?keywords=48v%2Bdc%2Bbreaker%2B20a&qid=1577985615&sr=8-4&th=1

[plwsm2000]

Personally, I would just use a switch that is rated at 20A DC or more. You don't need to switch both + and - leads - Just one will do as this is only useful for maintenance work i.e. if you want to disconnect your battery.

 

Fusing requirements for Solar panels is not the same for batteries, mains supplies etc. as they naturally limit the amount of current they can provide. Even is strong sunlight, you can short out a solar panel and it will just generate up to its short circuit current and no more. Obviously shorting out a battery can create 1000's of amps so has to be treated differently.

 

If you really want to use a breaker, you should use a DC type. They also have a letter (B,C or D) to specify how long they take to trip. In your case a 20A "B" will be fine as it only has to cope with the Isc current (x2 for two panels) so about 19.2A. It is unlikely you will ever see this current in normal use though unless you go very far south.

 

 

 

MikeVan - 2020-01-02 4:47 PM

... Plus, doesn't the MPPT serve to increase the volts if needed, provided it has sufficient amps being delivered to it, to ensure the volts are sufficient for the battery to accept a charge? ...

Most, if not all MPPT controllers only reduce (step down) the input voltage to match the battery voltage and do not increase it (step-up). If the solar panel voltage is too low, the controller usually just shuts down and goes into standby.

 

[John Allen]

Hi,

 

Allan of AandN Caravans http://www.aandncaravanservices.co.uk always strongly recommended that an isolation device (he recommended a removeable fuse) was inserted between the solar panel and the regulator. This was in addition to any fuse between the regulator and battery. He also offered a label that could be printed out and placed by the battery warning anyone working on the system to remove the fuse on the solar to regulator wiring first.

 

Lots of advice on his website about this subject.

 

Cheers

 

John

[MikeVan]

Great! Thanks John, will have a good look around his site :) Think I've seen that site before, reading re: split charging relays & regulating the charge, forgot about this site though!

 

 

John Allen - 2020-01-02 8:28 PM

 

Hi,

 

Allan of AandN Caravans http://www.aandncaravanservices.co.uk always strongly recommended that an isolation device (he recommended a removeable fuse) was inserted between the solar panel and the regulator. This was in addition to any fuse between the regulator and battery. He also offered a label that could be printed out and placed by the battery warning anyone working on the system to remove the fuse on the solar to regulator wiring first.

 

Lots of advice on his website about this subject.

 

Cheers

 

John

[spirou]

1) when you really need the sun (anything but summer) you're probably not parking in the (partial) shade. Series wired panels will increase the effective lenght of day and make that MPPT do its job when it counts. In the summer heat you probably need much less electricity and even partial shading parked under a tree or something likely won't matter as much as getting any charge at all in other seasons. We're just on our way back from Finland where 2x160W perlight poly panels (series) produced 15W max, on the only 2 sunny days out of 10 or so. Mostly the max was less than 5W, many days even 0. I wasn't specifically looking for a sunny parking spot on those nice days (family visits and errands) so production was 10-20Wh but I doubt it would go over 40Wh if I did. On all other days we got 0-10Wh in a day. In other words...useless even in series. On the way through baltics and Poland max power gradually increased from 20 to 70W yesterday (20 to 190Wh per day). Looking at a sunny day ahead of us parked in Slovakian lower Tatra mountains so I'm wondering what we get today. Already down 40Ah and there's no way solar can recharge all of it.

 

2) Impp and Vmpp ratings of panels are made at STC. Voltage will be quite a bit higher at low temperatures so make sure your regulator can handle it. Input current is much less problematic as a) it's unlikely you will ever see the max value an b) as someone else said, volts push, amps are pulled. The regulator won't pull more current from panels than it can take. But volts above max can damage it.

Use the victron mppt calculator to see what you could be getting in different conditions. I'm using a 30A fuse before the regulator (under rear bench) but that's mostly for ease of maintenance.

 

3) just the + is enough

 

4) any fuse or switch will do

 

[Paul-]

I have my solar panel fuse below the regulator, I was going to fit a switch but its just as easy to remove the blade fuse. My fuse is used for maintenance work and when changing the battery, as I have to couple the battery to the controller before coupling the panels, I found it to be the most practical position.

 

My controller is within a couple of feet to the leisure battery.

[spirou]

94W max and 290Wh was the output for today. No idea how long it was in the sun as I was skiing and this was on the northern side of the mountain. But as expected, nowhere near enough to recharge the battery.

[MikeVan]

spirou - 2020-01-03 5:11 AM

 

1) when you really need the sun (anything but summer) you're probably not parking in the (partial) shade. Series wired panels will increase the effective lenght of day and make that MPPT do its job when it counts. In the summer heat you probably need much less electricity and even partial shading parked under a tree or something likely won't matter as much as getting any charge at all in other seasons. We're just on our way back from Finland where 2x160W perlight poly panels (series) produced 15W max, on the only 2 sunny days out of 10 or so. Mostly the max was less than 5W, many days even 0. I wasn't specifically looking for a sunny parking spot on those nice days (family visits and errands) so production was 10-20Wh but I doubt it would go over 40Wh if I did. On all other days we got 0-10Wh in a day. In other words...useless even in series. On the way through baltics and Poland max power gradually increased from 20 to 70W yesterday (20 to 190Wh per day). Looking at a sunny day ahead of us parked in Slovakian lower Tatra mountains so I'm wondering what we get today. Already down 40Ah and there's no way solar can recharge all of it.

 

2) Impp and Vmpp ratings of panels are made at STC. Voltage will be quite a bit higher at low temperatures so make sure your regulator can handle it. Input current is much less problematic as a) it's unlikely you will ever see the max value an b) as someone else said, volts push, amps are pulled. The regulator won't pull more current from panels than it can take. But volts above max can damage it.

Use the victron mppt calculator to see what you could be getting in different conditions. I'm using a 30A fuse before the regulator (under rear bench) but that's mostly for ease of maintenance.

 

3) just the + is enough

 

4) any fuse or switch will do

 

Interesting, thanks for the reply. My panels are 2 x 270w perlites, and this MPPT - https://www.photonicuniverse.com/en/catalog/full/526-High-efficiency-50A-MPPT-solar-charge-controller-for-solar-panels-up-to-625W-12V--1250W-24V--1875W-36V--2500W-48V-up-to-150V.html It says up to 150v, so I'd hope that'd be fine volts wise?

 

Interesting thoughts re: series vs parallel. I keep seeing arguments in both directions! If I had 4 panels I do a combo as that seems best. I get that volts are the force & amps are pulled, re: fuse, it was more for risk of a short I guess, but that's unlikely. I'd want to fit a DC isolator at least (as you say, for maintenance etc), but I figure a breaker or fuse is just as easy to use to isolate, and gives a bit of a safety bonus too in the event of a short etc (unlikely to get a short before the MPPT, but unlikely things do happen sometimes huh!).

 

A fair bit to consider this weekend!

[spirou]

If it can handle 150V input you're good to go in any configuration. Which one depends on when and how you use the MH. I know in my case I really don't care much for partial shading. We use it in all seasons and winters are by far the heaviest hit on the batteries with heating etc. As there is little sun we want to maximise production and are also much more likely to park in wide open spots. If clouds cooperate even better.

 

In the summer you'll see me run for the shade any chance I get. But as consumption is lower and days much longer with fewer cloudy days...I just don't worry about it. I prefer a cool(ish) vehicle than extra power. I'm not selling to grid and if I happen to need more power I can always move a bit.

[MikeVan]

spirou - 2020-01-04 8:13 PM

 

If it can handle 150V input you're good to go in any configuration. Which one depends on when and how you use the MH. I know in my case I really don't care much for partial shading. We use it in all seasons and winters are by far the heaviest hit on the batteries with heating etc. As there is little sun we want to maximise production and are also much more likely to park in wide open spots. If clouds cooperate even better.

 

In the summer you'll see me run for the shade any chance I get. But as consumption is lower and days much longer with fewer cloudy days...I just don't worry about it. I prefer a cool(ish) vehicle than extra power. I'm not selling to grid and if I happen to need more power I can always move a bit.

 

Good point re: moving a bit for extra power if needed. I intend to charge when engine is running anyhow, so I'm probably over thinking it hahaha! Still, better that than not I guess sometimes.

 

 

[MikeVan]

spirou - 2020-01-04 8:13 PM

 

If it can handle 150V input you're good to go in any configuration. Which one depends on when and how you use the MH. I know in my case I really don't care much for partial shading. We use it in all seasons and winters are by far the heaviest hit on the batteries with heating etc. As there is little sun we want to maximise production and are also much more likely to park in wide open spots. If clouds cooperate even better.

 

In the summer you'll see me run for the shade any chance I get. But as consumption is lower and days much longer with fewer cloudy days...I just don't worry about it. I prefer a cool(ish) vehicle than extra power. I'm not selling to grid and if I happen to need more power I can always move a bit.

 

You've almost convinced me :D Am now thinking of going series... If I were smarter I'd look at wiring in such a way I can switch between the two lol, but need a close first 3 way switch I think - had a diagram somewhere, but suspect not worth the hassle :D

[spirou]

There are MC4 Y juctions/combiners that might help you do both but I'd have to draw to figure this out for myself, just how my brain works.

 

Another + for series: like I said it extends the lenght of day by keeping the voltage higher for (bit) longer. As charging involves sometimes a leghty stay at bulk and absorption phases to fully charge a battery, and winter isn't the most generous with solar day lenght, any minute helps.

[plwsm2000]

spirou - 2020-01-05 6:42 AM

...

Another + for series: like I said it extends the lenght of day by keeping the voltage higher for (bit) longer. As charging involves sometimes a leghty stay at bulk and absorption phases to fully charge a battery, and winter isn't the most generous with solar day lenght, any minute helps.

spirou,

 

Just out of curiosity, have you seen any references that support the claim about series connections extending the solar day?. I have done some analysis myself, and although I am open to the possiblitity that I have done something wrong, I do not get the same result and it would seem to me that parallel connections give a slightly better overall result at very low light conditions when you also include the solar controller operation.

 

I am guessing the rationale is that as series connections give a higher voltage (twice as high than parallel), the controller can make use of this voltage to generate usable power? The problem I have with this is that the generated current at low light is tiny and not even enough to run the solar controller.

 

From my calculations (see first plot of Vos vs. photocurrent), with no load the solar panel output voltage rises very quickly. Just 4 milliamps of photocurrent gives around 14 volts output which is enough to start up a Votronic mppt duo controller. If the panels were wired in series, you just need about 1mA to give 7V (so 2 panels = 14V). These currents are otherwise significant except for the behaviour of the solar controller.

 

I can only speak about the Votronic mppt duo controller but I think it is likely that most other controllers do something similar.

 

In complete darkness, the controller draws about 5mA from the hab. battery for its standby operation. When the controller detects a solar voltage of greater than about 14V, the controller comes out of standby and tries to generate output power from the solar panels. When the controller is in this "running" state, it takes almost 1W of additional power (~80mA at 12V) for its own internal operation. I assume the controller must be running extra algorithms and driving the hardware which accounts for the extra power requirement in this mode. The problem is a very low light levels, this power can exceed the amount of solar power available and so the extra current is drawn from the battery (i.e it is discharging the battery rather than charging it!). Obviously when the sun rises a bit more, it then has sufficient power to start charging.

 

This behaviour can be seen in the second plot which I recorded back in the summer - the detail highlighted by the green circle shows this extra current taken from the battery early in morning and at sun set. With series wired panels, the 14V switching point will occur slightly sooner than with parallel panels so you would expect a slightly larger battery drain with series panels. ideally, you want to delay this start up point until there is more than 1W of solar energy available. I have sent several emails to Votronic about this but yet to receive a reply.

 

Don't be too concerned about this because at the end of the day (excuse the pun!), this effect is very small and not noticeable in actual use.

[spirou]

Haven't measured this yet but I'm up for it when I get back home.

 

You mention votronic needs +2 Vbat to start up while Victron does +5 Vbat. I'm guessing Votronic went with +2 because most MH sized panels are quite low voltage compared to larger residential ones. I've seen 18V panels which don't give you many options at low light. I remember reading a forum thread where conclusion was to continue using a PWM or add more panels in series for an MPPT to even work properly as voltage was too low to get out of standby many days in winter.

 

Further, I'm guessing the higher +5V start up limit in Victrons (+1 thereafter) might eliminate/minimize what you're seeing? In any case, the OP has high(er) voltage panels to start with so might not matter much either way.

 

Also interesting to compare is self consumption. Victron spec sheets says 30 mA but I don't know if that is standby or constant.

[Charles]

Got a 60a trip on our 150w solar panel. It obviously won't trip unless maybe a lightning strike ? *-)

[spirou]

Finally got a chance to see what happens when the MPPT is on yet the panel voltage/power are too low to actually do any charging. As suspected there's no dip in current going in reverse. At least not high enough to measure.

 

So when I was looking at the output this morning, with panel voltage barely over battery voltage (15-16V), it was enough to compensate for self consumption, even with MPPT saying it was producing 0W/0A, with occasional high of 1W during the 5min I was observing. Guess I should mention again, Victron MPPT turns on at +5Vbat, then continues to work as long as there is +1Vbat. In my case today I was seeing +2 to +4Vbat.

 

That said, when I have everything turned off (CBE panel) I have a constant standby drain of about 0.1A at night. Over entire night it typically measures -1Ah. I haven't bothered to check exactly what and how much contributes but this will be the MPPT, battery monitor, gas alarm and maybe some other things each drawing a tiny amount.

 

[plwsm2000]

spirou - 2020-01-30 7:58 AM

 

Finally got a chance to see what happens when the MPPT is on yet the panel voltage/power are too low to actually do any charging. As suspected there's no dip in current going in reverse. At least not high enough to measure.

 

So when I was looking at the output this morning, with panel voltage barely over battery voltage (15-16V), it was enough to compensate for self consumption, even with MPPT saying it was producing 0W/0A, with occasional high of 1W during the 5min I was observing. Guess I should mention again, Victron MPPT turns on at +5Vbat, then continues to work as long as there is +1Vbat. In my case today I was seeing +2 to +4Vbat.

 

That said, when I have everything turned off (CBE panel) I have a constant standby drain of about 0.1A at night. Over entire night it typically measures -1Ah. I haven't bothered to check exactly what and how much contributes but this will be the MPPT, battery monitor, gas alarm and maybe some other things each drawing a tiny amount.

I have a Votronic MPPT Duo controller that switches on at a lower voltage which helps explain the difference in our findings. The running currents of the different controllers may also be different (and most likely will be).

Also the current is relatively small and may not register on the normal display. My Schaudt display can only resolve to 100mA (0.1A) so the 80mA that I recorded is less than this resolution so still may display 0.0A.

My datalogger resolves to 1mA (0.001A) so can see these small currents (and others such as my light dimmers each drawing 5mA even with the lights off !)