greekgeek avatar image

75/15 mppt charge controller panel sizing

hi there, i am new to this forum and also a PV beginner, so my questions might sound silly, apologies.

two things:

1. i have a 75/15 mppt installed combined with 2 50w westech panels

panel specs:

Maximum power (W)
Voltage at maximum power (V)
18,6 V
Current at maximum power (A)
2,69 A
Open-circuit voltage (V)
22,7 V
Short-circuit current (A)
2,88 A
Length x Width x Height Frame/Socket
668 x 545 x 35 mm
Maximum voltage
1000 V
4,3 kg

how many more panels can i connect in parallel? as far as i can understand, i need to add the short circuit current figures and make sure that total amps don't exceed 15 amps.

2. i want to deliver ~15 amps 30-35 meters away (100-114 feet) from the battery bank. a dc cable calculator i found online, suggests that an AWG 00 (~9mm diameter) cable should be used. has anyone done anything similar?any suggestions?

thank you all in advance for your answers

charge controller compatibility
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Hi greekgeek,

You don't mention how cold your location gets, or the voltage of your battery bank.

The 75/15 should be located as close to the battery as possible.

EDIT: You might be able to wire 3 panels in series (temperature dependant)

the charge controller is near the batteries, when i say i want to deliver 12v current 35 meters away, i mean the controller's load output (which i want it to be 12v and not 24 or smthing else because i use 12v appliances without an inverter)

That's a very long distance to run 12V power and you will probably have a fair amount of voltage drop over that length at higher currents. So how long is the solar panel cables from panels to MPPT - I understood this to be 35m...

the distance between the panels and the controller is about 3-4 meters. the solar cable calculator that i mentioned above indicated a 3% loss when using 9 mm cable which is ok. i suppose another solution would be to use 12/24 v converters but i am not sure if this is a good idea cost wise.

I don't know what you mean by 9mm cable as it doesn't match up to 00 AWG.

00 AWG (2/0 AWG) has a cross sectional area of ~64mm & a conductor diameter of ~11.1mm.


Resistance of 00 AWG (2/0 AWG) is ~0.0003 Ohm/m.

With a 35m run, the 2 way length is 70m.

So 70 x 0.0003 = 0.021 Ohm resistance.

Voltage drop is V=IR, & you stated that the current / I = 15A.

So 15 x 0.021 = 0.315V voltage drop.

@ 12V the voltage drop % is 0.315 / 12 = 0.026 = 2.6%.

So yes, that cable would work OK over that 35m length with a 15A load, but its not really reasonable in my opinion & would cost you way too much...

Its probably cheaper to buy a cheap 240V inverter & run a 240V extension lead...

Agreed. Running 70 meters @ 12v requires a LOT of copper. Looks like about $600 USD. Inverting the load to AC right out of the charge controller will probably be far less expensive, if your loads can run on AC and are not requiring too much watts like a Refrigerator compressor.

The 500VA Victron inverter is a great value well under $200 USD if it can work. After that cost jumps up quite a bit.

Another idea would be to move the batteries and charge controller close to the 12v loads and then, yes put three panels in series and let their connecting cable to the charge controller cover the larger distance.. At 48 volts DC you could run the distance with a far thinner wire than at 12v. Also, larger voltage drops would not cause much problems with the MPPT adjusting to the charge. Some loss of charging power, especially early and late but no other penalty. Another 50 watt panel is going to be cheaper than 70 meters of 00.

Still, if AC is workable for the loads then the inverter is an easy solution.

why you mentioned temp dependant?

He means that if the temperature is very cold, then then the open circuit voltage with 3 panels in series could increase to the point that it exceeds the 75V max allowable for your MPPT. If that was the case, then 2 not 3 panels in series would be the acceptable limit.

ok i see, but what very cold means (cold like -5 or -15 C?) and what is the relation with voltage output? is there a way i could calculate the voltage rise?

Hi greekgeek.

These look like your panels?

Voltage rise is calculated from temp below 25c, say -15c.

That's 40c temp difference from 25c

40 x 0.35% (Voc temp coefficient) = 14%

Voltage rise will be 14% of 22.7v =25.88v

yes that's the one. wow! thanks a lot for your answer

6 Answers
Mark avatar image
Mark answered ·

Yes the limit of these panels or strings of these panels in parallel is 5 (15 / 2.9 = 5.2)

However I would aim to increase the voltage first by running panels in series. The limit of these panels in series is 3 (75 / 22.7 = 3.3). This also provides some buffer for cold temperatures since 22.7 x 3 = 68.1V (vs 75V max allowed). If it gets really cold where you live then the max open circuit voltage may need to be properly calculated at the minimum possible temperature and if required the number of panels in series limited to 2.

The higher voltage & low current will allow you to use thinner wire to transfer the same power. The MPPT will also wake up earlier each morning with a higher PV voltage.

Once the final configuration is confirmed, then wire size can be determined. Check wire size is capable of the max current (which shouldn't be an issue @ 15A max) and voltage drop % is acceptable (ideally <2%). If its suitable, I would try to use regular 6mm^2 PV cable with MC4 connectors.

PS. There is no 'limit' to MPPT input solar power, it will just limit the charge current to the maximum limit (and accordingly limit solar power). You just need to ensure the maximum input voltage and short circuit current is NOT exceeded when assesed individually.

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is it possible for a 24v panel setup to charge a 12v battery setup? it would save me some trouble (and money) if so, but i don't know if it's a good idea.

With a MPPT charge controller you should not consider the PV as 12V or 24V, that's more of a consideration/term used with old PWM type charge controllers.

You can use any PV voltage to safely charge a 12V battery bank with a MPPT, providing it does not exceed the MPPT voltage limit which is 75V max in your case (& providing that it's higher than your battery voltage). The MPPT will only charge with whatever voltage you program it to charge at & convert the higher PV voltage into a higher charge current.

In general, the higher the PV voltage the better the performance...

hey there,

when converting the panel voltage from 12 to 24v should i set my victron mppt controller into 24v or this is auto-detected?when "system voltage" is mentioned, are we talking about the panels voltage or the battery?

i was given a 275w 24v poly panel (imp 8.77) and i want to connect it to the 75/15 mppt controller, i believe that the controller can easily handle it

sorry for the silly question but i am a novice in solar stuff, cheers

All infos you need are already in this (very long) topic.

Your MPPT 75/15 can handle a PV voltage of up to 75V.
System voltage = your battery voltage

So you MPPT can easily handle the panel voltage but the 15A will maybe a limit.
Under good conditions the panel can give you enough power for more than 15A charging current.

greekgeek avatar image greekgeek Matthias Lange - DE ♦ ·

Thank you for your answer, i hate to waste your time guys and thank u all for your inputs.the reason i am asking again is to make sure i am not wasting money or building something dangerous. The victron mppt calculator page says that a 270w poly panel works with a 100/20 controller. When i change the system voltage to 24v, it indicates a 75/10 controller. Why is that? I apologise for asking maybe obvious stuff for you guys but for me these are invaluable infos.cheers

greekgeek avatar image greekgeek Matthias Lange - DE ♦ ·

Under good conditions the panel can give you enough power for more than 15A charging current.

can you please elaborate? how can a panel that barely provides 9 Amps van give more than 15 Amps? i suppose you mean when its cold the efficiency increases, but THAT much? i am really confused since the victron calculator page suggests a 100/20 charge controller and the victron 75/15 instruction manual says that it can handle up to 400w /24v. thank you for your time guys

JohnC avatar image
JohnC answered ·

I got distracted and Mark beat me by miles, but posted unaltered...

Hi G. Presuming 12V batts if the existing panels are paralleled.

The 15A seems to be your worry, and fair enough. But you don't have to parallel them, and in series the V may double but the A won't. Wired in parallel the A doubles but not the V. They will actually work better in series (just a light/sunshine thing).

You can series those panels safely up to 3x to stay below the 75/V (Klim said 4x, but qualified it as it's getting borderline).

So in theory 3x wired strings paralleled 5x times is about your max. So 750W, enough, ha? Over the nominal rating, but still ok. You can string em 2x or 3x to match your panel numbers.

Without checking your wire calcs, the higher the V the better, so 3x strings would be nice.. 2x of those strings = 300W nominal, to give the full treatment. Or 2S2P wired will give the same Amax (under 6A). Make sense?..

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greekgeek avatar image
greekgeek answered ·

thank you for your answers, i live in southern Greece (near Athens, average summer temp is ~30 C up to 40 C [about 86-104 F], winter temp is about 5-15 C [41 F-59 F], sometimes it can be as low as -1 or -2 C [28-30 F] which is rare.

i chose the parallel configuration because of shading, the cabin is surrounded with trees and i thought it would be a good idea.

"So in theory 3x wired strings paralleled 5x times is about your max. So 750W, enough, ha? Over the nominal rating, but still ok. You can string em 2x or 3x to match your panel numbers. "

sorry i didn't get this JohnC

so, the bottom line is, that i should add 1 more panel and connect the panels in series not parallel, right?

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~0°C as the min temp is not super cold so you will probably be OK with 3 panels in series. But the only way to know for sure would be to calculate the max open circuit voltage using the temperature correction factors for your panels.

Now you don't need to choose only series or parallel for the wiring - you can use a combination of both, as long as the final outcome is under the MPPT input limits.

For example you could wire 2 panels in series to form a 'string' and then use 4 strings in parallel (2S4P) - so 8 panels & 400W in total.

A full list of possible options/examples are;

2 Panels (100W): 2P or 2S

3 Panels (150W): 3P or 3S

4 Panels (200W): 4P or 2S2P

5 Panels (250W): 5P

6 Panels (300W): 2S3P or 3S2P

8 Panels (400W): 2S4P

9 Panels (450W): 3S3P

10 Panels (500W): 2S5P

12 Panels (600W): 3S4P

15 Panels (750W): 3S5P

As you can see - you have many options.

With the shading concerns & long cable run, you need to weigh up your options to determine what will work best overall.

But also consider that your MPPT has a maximum charge current of 15A. So with a 12V system charging at say 14.4V, the max charge power is 216W.

Now you can add more max solar power than this to ensure good performance in poor sunlight conditions, but realistically there is probably little gain going above 2x the max charge power, so ~400W max solar.

wow! thanks a lot for your answer, i appreciate your trouble. i think that 4 panels =>200 W is more than adequate for my needs. if i choose 2s2p it's better than 4p? i am asking this because you mentioned something that the mppt works better in a series configuration.

Yes I recommend to have at least 2 panels in series, not all panels in parallel if possible (particularly with your panels).

A MPPT charge controller really needs to have a PV array providing a voltage that is significantly above the battery voltage to work effectively.

This is because the PV voltage must be >5V above battery voltage to start charging each morning.

Extract from data sheet;

For example - if your 12V batteries are @ 13.5V in the morning, then there must be enough sun for the PV to provide >18.5V for the MPPT to simply turn on & start charging.

Now the Vmp of your individual panels is ONLY 18.6V & Voc is ONLY 22.7V. So there needs to be a fair amount of sun to get your PV to produce enough voltage to exceed the 'wake up' threshold. In a winter or overcast day it might not even start all day or until very late in the day...

But if you run some panels in series the voltage will increase (double for strings of 2 panels in series & triple for 3 panels in series) and things will work much better - starting earlier each morning & running until later each evening.

There is also a significant gain running higher voltage with the minimum solar panel cable required. Particularly with higher currents & longer cable lengths.

If you have the same PV power, but run it at 2x the parallel voltage, then the current is only 1/2. The current is what is used to calculate voltage drop in a particular cable length. So 1/2 the current means 1/2 the voltage drop. Then 1/2 the voltage drop divided by 2x the voltage (to calculate voltage drop %) means 1/4 the original/parallel voltage drop %, just by making this simple configuration decision / change...

So in effect, if you run 2x or 3x the voltage you could use a cable with 1/4 of the cross section area to achieve the SAME voltage drop % result. As your solar power is not very high in the scheme of things, this is probably not a big deal for your setup.

1557457655827.png (9.6 KiB)

hi there guys, i know it's been some time but i tried to add this question to the same thread so that you understand what i am talking about. I tried to buy the same panels in terms of characteristics but i couldn't find any. I ordered a 50w Westech panel but although the model is the same, the label indicates slightly different characteristics. I then stupidly bought another 50w panel (from another brand) that again is different. So i ended up having 2 identical 50w Westech panels, a slightly different 50w Westech panel and an Offgrid tech, 50w panel (which is also slightly different than the others).

Can i still connect all these in series? 2S-2P to be exact. i aim to make 2 strings of 50 w 24 v each. The differences between panels are minimal, for example Imp : 2.69- 2.78-2.69. Isc : 2.88 A- 3.00 A etc. All 4 panels are 50w mono.

Any response and help would be much appreciated, thank you all.

Yes, wired 2S2P as you propose would be the best way to go.

you don't think that these small differences between panels are playing any important role to the setup?

Small differences won't. In most cases the mppt production 'curve' will have just a slight Amps/Watts slope, and make little difference to the overall Vmp determined. It'll 'split the difference' when tracking, and if the panels aren't way apart, you'll unlikely see a detectable difference in output.

Nike sez "Just Do It". So do I. Go on.. :)

Here is an excellent explanation that should help you optimize with your mixed panel setup.

Shading changes things a little. But like Mark says, you have many options. You can have up to 5 parallel strings to stay under the max Isc, in strings of 1x, 2x or 3x.

Given you may not want a 'major' expansion, I'd maybe suggest 2x strings (so 2S2P) to give you a higher panel V that will work better in poor light conditions. And maybe give you the same shade split you have now..

greekgeek avatar image
greekgeek answered ·

sorry, i forgot to mention that my battery bank is only 100 Amps (5 Agm 20 amp batteries connected in parallel).

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It seems that you really like wiring things in parallel! :-)

There are some logevity / balance risks with wiring so many batteries in parallel but that is off topic...

Generally it is safe to charge most AGM batteries with a current of up to 20% of their C20 capacity/AH rating, so ~20A max for a 100AH battery bank.

Your MPPT charger having a 15A max charge current is a good size for a 100AH battery bank - at 15%.

As stated before, the MPPT will limit the solar power coming in as required - you just can't exceed the MPPT max open circuit voltage or short circuit current limits.

There are some logevity / balance risks with wiring so many batteries in parallel but that is off topic...

is that so? please elaborate. i connected my batteries like this:

i found this really good article that persuaded me about this particular setup:

anyway you guys are more solar savvy than me, so any thoughts or suggestions would be appreciated.

P.S. well, connecting things in parallel seemed to be easier for a beginner like me :)

1557418945282.png (4.3 KiB)

Firstly, you could have accomplished the same outcome with ONE 12v 100Ah battery & ONE 200 to 300W solar panel (with an open circuit voltage of ~40V). I expect that this setup would have been simpler, cheaper & more reliable/last longer.

But I do understand that things sometimes start out small & as you get to understand more/requirements change you want to expand & don't want to throw away your original equipment.

With batteries in parallel the general guidance/good practice is to avoid the configuration if possible, but if required then try to limit to 2 batteries/banks in parallel.

The issue is due to imbalance due to the interconnecting wiring (which can be worked around somewhat) and also differences within the batteries themselves - even if they were all manufactured in the same batch & have been exposed to the same usage conditions. It only takes very small differences to have a significant negative effect over time.

With any more than 2 batteries/banks in parallel you really need to A) use a good quality (thick copper) positive & negative bus bar, and B) use IDENTICAL length & type of cable leading from each battery to the bus bars. This will limit differences between batteries due to differences in resistance & accordingly voltage drop between each battery and the load or charge source.

Now even with this measure taken, particularly as the batteries age there will be larger discrepancies between performance/behavior and some batteries will be doing more/less of the work (providing more/less power & absorbing more/less charge). This can/will eventually lead to a premature failure of 1 battery, but as they are all in parallel - it can go unnoticed for some time. Commonly it will end up damaging either some or all the other batteries first, before the issue becomes apparent & action is taken.

Using a single battery, less batteries in parallel or batteries in series - either eliminates or mitigates these risks/issues.

Firstly, you could have accomplished the same outcome with ONE 12v 100Ah battery & ONE 200 to 300W solar panel (with an open circuit voltage of ~40V). I expect that this setup would have been simpler, cheaper & more reliable/last longer

yeap i totally agree, things turned out this way because i started with one battery and a 20w panel.. my "system" was not properly designed (well wasn't designed at all to be honest)and after a few months, i ended up with 5 small batteries and 2 panels. although i wasted some money i learned a few things about solar energy.

anyway, the "right" thing to do now would be to connect my panels in series (maybe adding a panel to make a 3s configuration, later on if i buy another one i could construct a 2p2s setup) and also connect my batteries in series and parallel to make a 24v 60 amp battery bank (that means i have to buy one more battery).

i believe that the controller can detect automatically the voltage so there is not a problem.but what about the load output? is it going to be 24 v ?

With the latest MPPT firmware the charge controller will only auto detect and store the battery voltage on the 1st start up.

But you can always manually update it using Victron Connect software and a Bluetooth interface or VE.Direct to USB cable. If your MPPT is the SmartSolar version then it already has Bluetooth capability built in & you can simply connect directly upto it with your phone & the Victron Connect app.

Yes the load output will always be simply equal to battery voltage.

It's also not a great idea to mix batteries of different age.

Going 24V would help a with your 35m cable run...

well my battery bank is about 15 days old so i suppose it's ok to add another battery. the thing i don't understand is this:

you mentioned that a 2s panel configuration is better than 2p because of the higher voltage that is given to the controller. however since the battery bank would be also set at 24v, the panel voltage should again be 5v higher than the battery bank isn't it? so what is the advantage?

Still not ideal but less of an issue if the existing batteries are still fairly new. You should fully charge all batteries separately before adding any new batteries to minimise the balance difference.

Yes the PV voltage should always be a fair amount higher than the battery voltage to make sure it has no issues starting up & charging in sub optimal solar conditions. So work this out for any combination before you proceed. Even 2 panels in series with a 24v battery bank has a higher voltage delta as compared to a parallel panel setup with a 12V battery bank.

The advantage of running 24v battery system is that for the same power, the current will be 1/2 that of a 12v system & the voltage drop % will be 1/4 that of a 12v system with the same cable. So basically you can get away with cable that has 1/4 of the cross sectional area by simply doubling the voltage.

esteban avatar image
esteban answered ·

@greekgeek: A panel's short circuit current figures are not actual current when used. You can derate it by 10% at least, maybe 15%. A 100 watt panel, for example, will give you 85 watts maybe at noon if pointed well. So take that 2.9 amp rating down to 2.6 and add that up to hit your 15 amps. And as stated, ocassionally exceeding your controller's amperage rating will not be a problem it will just be a little lost power when and if you have enough sunlight to go above the 15 amps. All considered, I would suggest that a maximum of 6 x 50 watt panels is a good use of your 15 amp controller.

Putting a pair of panels in series makes a lot of sense for reasons mentioned. Once you get two in series you are extending your charge day for most of what's possible. Then, while you could add a third to the series to get the voltage higher, you have another consideration. So running each pair in parallel will help with your shading, I suppose. 2S2P or 2S3P is what I would be thinking. BUT 3 in series (3S) is also a reasonable starting point.

300 watts (nominal rating) of panels is a reasonable limit for the 75/15, whatever configuration of series or parallel you choose.

While it doesn't seem like you need much power to charge 100 amp hours of battery, you have lead acid, which really wants to get through the bulk charge and have a lot of time in the remaining stages. I would start with at least 200 watts of panel and consider adding another pair of 50 watts panels later if you are not sure.

See this:

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I just want to clarify some of the advice given above;

There IS a problem if plan to use a PV array with a higher short circuit CURRENT than what the MPPT can handle & you shouldn't really 'de-rate' this figure (even though the panels will only provide this current in a short circuit condition & that isn't a typical state).

Also, the max PV short circuit current rating of an MPPT is independent of the rated/max charge current - it can be higher or lower. In case of the 75V/15A model it just happens to be the same @ 15A. On larger MPPT units like a 150V/100A it is actually lower @ 70A max.

See extracts from related MPPT data sheet;

Same logic goes for the PV open circuit VOLTAGE - that should be calculated at the lowest possible temperature for the installation site (as that is when it could be highest).

But you CAN safely add more POWER than the 'nominal' PV power required to provide the rated/max charge current output of 15A. It is also recommended/good practice in order to get the most out of your MPPT during sub-optimal conditions (which is probably >95% of the time...).

For example if you have 2x the PV power required to produce full charge current, then even when the PV is only providing 50% max power (such as an average day), you will still get 100% MPPT charge current/power. Hope you get the idea/logic.

1557454443805.png (103.5 KiB)

thank you for your input, great video by the way!

southernoceansolar avatar image
southernoceansolar answered ·

hey victron/forum. i want to use this module in 250w version on the roof of a van with the 75/15 MPPT to charge a 7S10P 18650 (29.4V) pack im building. i will use one of your phoenix inverters (what would you recommend for .5kWh lithium pack and this panel?) in the victron mppt configurator it seems to not like the configuration over 67 degrees at 24V. what do you think? ok to give it a run..........also it will go underneath the roof racks so 12 cells of this module will be covered a lot of the time.

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All very, very bad.

Open up a new question please. Batt pack ah? Inverter model?

hi klimate. not sure it needs new thread as i want to use this MPPT 75/15. the 18650s are all 2100mAh-2300mAh in a 7S10P arrangement with BMS and BMV12 (the cell isnt that important as i have a few options im thinking in 24V config) my key questions is can i use module above and will the MPPT ramp down over 67 degrees. i think i will use the 24V/500 phoenix but maybe only the 375 model. interested in your thoughts on this experiment--its my own

The 75/15 PV voltage needs battery volts + 5 volts, your panel wont charge the battery, even in full sun. Shade is bad and will reduce output further.

Your battery pack is a bit on the small side, just be aware of that.

a trina 300W panel works that is only .5V higher work with victron calculator up to 70 degrees. im happy to run a more suitable module or i could run it in 12V but the panel is free. so i want to know i it will work. it works up 67 degrees so it seems feasible. what does the mppt do? im assuming i can set the parameters that allow it to work with this module? what size pack do you think is more suited?