150/100 Mppt - Isc input limit of 70a

DIY
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1

I understand the Isc limit of 70a for the 150/100 mppt. This is never to be exceeded. Got it.

My question is, if I have a configured setup of 56a Isc on the panels when i add a safety factor of 1.25 to the 56a of Isc on the panel I end up at exactly 70a Isc on the 150/100 - is this acceptable?

Do I even need to add 1.25 safety factor to the 56a Isc rating of a 4p panel setup that I am considering ? I totally get the 1.25 safety factor on the volts side of things - but amps? If I do add a safety factor to the Isc input of the 150/100 what is acceptable or considered safe enough ?

So does the 1.25 safety factor absolutely need to be added on the Isc amps rating of the panel setup ?

2

Your panel specs should give a value for the Variation of Isc with temperature. Calculate the Isc at the maximum expected temperature. This should be less than 25% increase over the quoted standard value.

You could also check the recommended controller using the Victron MPPT Calculator.

3

Need to dig in a bit more please - are you saying that one has to apply the cold weather safety allowance that we use for panels to the Isc input limit of the 150/100 as well?

I get that the panels increase voltage with cold temps. We allow a safety factor for that based on the temp coefficient of the particular panel and the lowest known temps for the area the panel is installed in. Got it.

I know the amps increase SLIGHTLY with hot temps. Got it.

However I didn’t know that we have to use a cold temp safety factor with the Isc input limit of the Mppt - could you please explain this a bit more please.

4

You asked if you need to apply a straight forward 25% factor on Isc. I am saying if you are concerned do the higher temp calculations to see what the effect is.

5

Oh OK - got you - my poor explanation caused my confusion here on this.

Ok so what I meant was I have been applying a 1.25% safety factor to the Isc input limit because that’s what I have been using for the cold temp issue with panels. So I also applied the same 1.25% safety factor to the Isc input limit of the 150/100. But then I got thinking about and thought that might not be needed because the amps are not affected by cold but more by heat.

The heat increase is not much. So a few amps is more then enough headroom

What about cloud edge effects ? Does that really drive the amps up and so need headroom on the amps for Isc limit or is the cloud edge issue a voltage thing only ?

So given the Isc input limit of the 150/100 or the 250/100 is 70a, how close can i go to that Isc input limit when designing a string of panels?

6

Cloud edge effect is due to higher irradiance. Irradiance does not affect Voc and Vmp much but mainly affects Isc and Imp. See the typical graphs below. I have no idea how much headroom needs allowing for this. However, normally it is a moot point because the panel current is less than the battery current so you do not often hit Isc limit before you are over panelled.

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17778179123835463767694122433348382Ă—282 5.44 KB

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17778181566123063633097500010793382Ă—282 23.8 KB

7

Well thanks for your help - you have shed some light on the problem but its not really solved.

There must be some rule on this - if the Isc input limit is 70A then how close can we get to that figure while remaining safe ? I know you dont know, so not directed at you at all. But there must be some way to figure this. if I worked on a safety figure of 1.25% then the real world limit would be 56A - the rest - between 56A and 70A is a safety head room.

But 1.25% seems to be way too much safety headroom. I just dont know how to appropriately allow for this. Does Victron specifically teach anything on this subject in thier courses they offer ? The answer is out there somewhere.

8

Use the mppt calculator. If its allowed its allowed.
And its not like switchgear its component limitations which are not the same thing

Leave headroom with operation temperature of the panels, cloud edge effect, or use the calculator.

9

I am calculating a string for Jinko 580 Tiger Neo N-type 72HL4-BDV panels. I wanted to use a 150/100 Mppt but have now seen I must upgrade to a 250/100 to get the PV power I need. Seems easy enough.

I would like to push out to 4S4P. At this config the VOC is fine - well under with lots of headroom so no issue with VOC at all. The Isc seems to be the problem and I dont know why - the Isc is 14.37A. So 4 x 14.37 = 57.48A. The Isc Input limit of the 250/100 is 70A. So this leaves 12.54A of headroom but is rejected by the calculator. I am trying to understand the rules around this. There must be a formula that is controlling this.

Using the calculator ;

it allows 4S3P but not 4S4P, it allows 3S4P but not 4S4P.

Which seems strange - if it was an AMPS safety thing I can understand why it wont allow 4P. But why will it allow 3S4P if the Amp is the problem? Regardless of whether its 3S or 4S the amps stay the same - 57.48A - so why will the calculator allow 3S4P (57.48A) but not 4S4P (57.48A) ?

The calculator allows 4S3P - so should I just leave it at that and move on and never learn anything? I would like to understand it a bit better, so am trying to figure out what the safety factor that one should use for the Isc input on Victron Mppt’s.

Plus my original question still stands - if I use a 125% safety factor on the input Amps for the Victron 150/100 or 250/100 it means I can use 56A as input using 125% safety - however this does take the Amps input right up to the 70A hard limit. (56A x 125% = 70A) Is it OK to use 56A input when doing calcs - ie, use 56A as the limit not to exceed for the Isc input limit on the 150/100 and 250/100 Mppt’s ?

10

Probably because you are more than the 130% oversize.

(4s4p) 16 x 580 = 9280w. (9280w Ă· 48v = 193A)

Its a total waste using 1 x 100A mppt. You are now is the 200A territory

11

OH ok i see now. Makes sense.

Thing is I am trying to over panel for the dark wet months. So if the calculator rejects a certain layout because of being over 130% I wish it would just say that instead of just outright rejecting it.

As it stands, until I come into this community and ask a ton of questions I dont know why the proposed string is being rejected. As you point out it is most probably because of being over 130% - OK I accept that the calculator is rejecting a proposed string because a parameter is going over its limit.

But it still doesn’t answer my question about how many amps can I design up to when using the /100 amp rating on a Victron Mppt. How much headroom do I need to leave for Isc input when using a 150/100 or a 250/100? Is it just the heat rating I need to calculate because the amps RISE a little with heat? Is there no need to think about cold temps when thinking about how much headroom I should leave for the amp limit input?

Ie, should I design around 56A input for the 70A Isc limit thereby leaving a 25% margin for headroom ? Can I design for 65A input to the 250/100 ? Can I design right up to 70A of input to the /100 series mppt’s? I cant find any clear statement on this anywhere. I dont want to setup a string of panels that is unsafe for the Mppt.

Sure I can follow the calculator - but that seems to be limiting on the 130% oversizing - which is fine from a certain perspective - ie the waste angle you alluded too - but if I want to oversize beyond that for reasons that make sense to me I want to make sure I dont push too many amps into input side of the mppt. I know the hard limit is 70A but do i leave some headroom on Amps or not ? Using the calculator I cant find that amp numerical figure that i must not go beyond while leaving some headroom - should I just assume I can go up to the number of amps that the heat calcs allow while respecting the hard 70A input limit and leave it at that ?

Re your calc : (4s4p) 16 x 580 = 9280w. (9280w ÷ 48v = 193A) - 1st thing is we need to calculate for 51v not 48v - So 182A. The LFP battery would never be at 48v. So this reduces the amps a little - not a huge amount for sure but it all adds up. Also your figures are at STC which we wont hit for all the known reasons. I am in an area that is hot while it is also raining. So very heavy overcast weather in between rain events while ambient temps are in the 30’s. It may go several days without rain but always very overcast and hot. So never going to hit STC in those conditions so never going to see 182A at that time of the year. I am not interested in trying to harvest more all the time throughout the year , just interested in harvesting more in certain times of the year. So i have to over panel for that.

Getting a bit off topic but one thought I had was to go ahead and build out a 4S4Psetup. When building a combiner box for this array, set it up such that I can turn OFF the 4th string in the clear sky sunny times of the year so that the extreme over paneling does not come into play at that time of year and then turn that breaker to the 4th string ON in Dec/Jan/Feb when we are struggling to make decent PV. That would solve the problem - is this a way forward that would not break any rules etc? Sure it is a manual thing - having to turn a breaker on and off twice a year - but it would solve the problem.

So with regards to amps - the hard limit is 70A input. The outgoing battery charging amps is up to 100A. Is the over paneling case I am putting forward, which is causing the amps presented from the panels to the battery charger in the Mppt to be well over 100 amps, the problem ? Is pushing the charging amps side of things too hard going to cause the Mppt to overheat ?

Not trying to flog a dead horse here, just want to understand it better.

12

I think your config of 4s 4p is ok from the hard rules aspect. {Since it allows the lower string but same parallel (and therefore lower power that keeps the 130%)}

Its just the 180% overpanel aspect. If you are comfortable with that i guess for me its a bit overkill as with 8 of those exact panels we pin the 250/100s to their limit often.

13

The 30% used for over-panelling isn’t just a thumbstuck, it has been verified to offer the best price/performance. In miserable weather, there is no amount of over-panelling that will help, unless you want to spend money that is.
Always better to split across mppts as you will at least have the option to use it when the sun comes out. far better return on the investment and kinder to the mppt.

14

This may not answer all your questions. But if you are comfortable with over-paneling beyond 130% the online calculator allows for that? But you have to enable the setting. I don’t know about the offline Excel calculator. At any rate with this, you can verify if you 4s4p config is being rejected for over-paneling beyond 130%

According to a Victron blog, you can over-panel all you like provided the VOC and ISC rules are respected. This of course doesn’t mean it’s a great idea to do it.

To your main question, I don’t think you will get a straight answer regarding the accepted head room as this is likely to vary with local conditions and your risk tolerance. Some people even choose to ignore, at their risk, the short circuit current limit. I would guess that as long as your current limit won’t go beyond the limit and the calculator passes your config, you are ok.

I could, of course, be wrong.

15

Thanks LX. As always I appreciate your insights.

As explained the issue is we struggle in Dec/Jan/Feb and even into April sometimes as we did this year. In our clear sky months from May to End of October going into November we produce ample amounts of kWh’s of energy.

So in Jan when the rain is the heaviest we get low PV production as expected. We do have a grid connection but the issue is that in the very first instance our grid is unstable at the best of times and second, as soon as we get a drop of rain the grid fails…every single time. So we have the situation that when it rains the grid goes down and our solar goes south. For various reasons a genset is not an option.

I only need to find extra PV in Dec/Jan/Feb/March. Esp Jan.

So this all comes down to installing more PV. So I am trying to get into an over paneling situation but I want to engineer it correctly. I dont mind putting extra panels up for those few months of the year we need them and switching them off in the bright sunny months when we dont need thier output. This is about having our life back and being able to operate while its raining resulting in low PV and the grid is down for goodness knows how long this time around. It gets old.

So i want to make sure that whatever array I put together is not going to harm the 250/100 (already accepted I need to upgrade from a 150/100).

I am trying to understand the amps side of things vis-a-vis the Mppt. The volts side of it is easy enough to understand - no confusion or issue there whatsoever.

As stated I get the hard 70A input limit. I just want to well understand all the issues surrounding sending amps into the mppt and the 100 amp battery charging output aspects. It seems I can stay under the 70A Isc input side of things but may be pushing the 100 amps battery charging output side too hard. By this I mean I get that the charging is always limited to 100 amps output, but if I have 182A of potential charging power but the Mppt can only output 100A then I am guessing that the charging side is being hit with too much potential? Does this make sense ?

I have read many times that the SCC just clips the over paneled output. But are all these amps being produced by the 180% over paneling having a negative effect on the SCC? If so is Victron saying they will accept 130% over paneling but beyond the Mppt is being pushed beyond its design limits ? If so why is this not clearly communicated to the community ? It seems the answer to any question is “Use the calculator” which is great for a qiuick easy answer but what if we want to know more about the limits and why?

16

Difference between DIY and a oerson with either a bunch of practical experience or a well learned qualification

The simple answer is heat.
Anything that is not used is wasted and usually as heat. Heat as tou can imagine is an enemy of electronics. So longevity is at play here.

Your idea of having the array ready but disconnected in more productive months is a good work around if you don’t want to spring for another mppt.

17

Thanks Fideri !

Great insight. So it seems I could do a 4S4P - but I am liking the idea of just turning off the 4th string at the combiner box at those times of they year we dont need them. It doesn’t matter the extra install cost to put those 4 extra panels up -when we need that extra power we need it. Its either that or get a genset which we really really really dont want to do.

18

Understood. Its late in life to change professions but I am really leaning towards wanting to be an electrician. Deeply fascinated with this stuff and LFP batteries.

Would active cooling on the Mppt help here? Not hard to run a Noctura or similar under the Mppt. I do get its best avoided but if we have too, is this an effective way to mitigate this problem?

The problem for “springing” for another Mppt is not so much the Mppt - after all that approach offers redundancy so not a bad way to go. Its more about all the other balance of system items needed to go with the extra Mppt. Will have to see what the final setup looks like - if I can get that extra Mppt in there without having to expand any other items , like another Lynx Power in for example it might be the better way to go. Thing is, I dont need the extra production the extra Mppt would offer in the “dry” season - we are blessed with an abundance of sun here. So that extra Mppt would just sit turned off anyway. Seems way more efficient to just turn a breaker off (and even with the extra Mppt - I still need the extra breaker etc anyway)

19

Yeah. A lifelong learner is not a bad goal. And its great there are so many resources now (hopefully not too crowded by the utter nonsense). I have met many DiYers that actually outshine the professionals. So i don’t always think its about the paper.

There are alot of guys on this forum who do use fans. But also I do not think many have the 180A% over panel.

20

LX, again thank you for your input.

Yeah I do get the 180% over paneling. Maybe as Nic says, it is better to just split it up as a 4S3P on the 250/100 as that leaves loads of headroom on both sides of the inputs, runs cooler, lives longer and the second Mppt gets a 2S2P and serves as another backup. We have the roof space so that is not an issue. I still want to know a lot more about how the Mppt is processing amps but will have to figure that out somewhere else. I wonder how much Victron teach in their courses?

I have the Easy Solar with the 150/100 onboard - but the 150/100 on the Easy Solar is just not sufficient for this setup. So already “springing” for another Mppt. The 150/100 is going to get a smaller West facing array - again for the rainy season where we get the Holy Trifecta - Batteries low after supporting loads all night, we wake up to rain and the grid is down after 3 drops of rain have come down. The rain can last all day and the grid can go down for days at a time.The only answers are more battery, more PV and possibly a genset. The genset is plagued with all sorts of local issues so trying to stay away from that.

Its the usual thing - no different here. We get all the energy we need from May through November. But comes Dec and 101% in Jan/Feb we struggle. We get about a meter of rain between Dec and end Feb. This year was a bit drier - we only got 34 inches instead of our usual 39 inches of rain.