newbie help with maximum VOC please

[Hedges]

this is what mpp solar say...


Your LR4-72HPH-445M solar panel size (STC): 445W, 49.1 VOC, 11.53 ISC

It’s no problem to use 2 panels in series, the PV input: 890W, 98.2 VOC, 11.53 ISC.

What they said is true only if your ambient temperature never drops below 25C.
In this case, if you listen to the manufacturer it could kill the inverter.
While their engineers probably know what they are doing, the front-line people apparently can read some values on a data sheet and do simple math, but are unaware of normal issues using PV panels.

According to the data sheet I find for your panel:

ENF Ltd.

LONGi Solar Technology Co., Ltd. Solar Panel Series Hi-MO 4m LR4-72HPH 445-465M. Detailed profile including pictures, certification details and manufacturer PDF

www.enfsolar.com

Voc will increase 0.27% for each degree below 25C.

For my San Jose location, to be conservative I assume -15C which is record cold or a bit below. So 40 x 0.27%
Using that assumption, two panels in series would reach 109V

Gnubie noted that using the two panels in parallel rather than series would work for 2kVA model MPPT, staying above its minimum 30V input. So that is the way to go.

 

[charleschaime]

What they said is true only if your ambient temperature never drops below 25C.
In this case, if you listen to the manufacturer it could kill the inverter.
While their engineers probably know what they are doing, the front-line people apparently can read some values on a data sheet and do simple math, but are unaware of normal issues using PV panels.

According to the data sheet I find for your panel:

ENF Ltd.

LONGi Solar Technology Co., Ltd. Solar Panel Series Hi-MO 4m LR4-72HPH 445-465M. Detailed profile including pictures, certification details and manufacturer PDF

www.enfsolar.com

Voc will increase 0.27% for each degree below 25C.

For my San Jose location, to be conservative I assume -15C which is record cold or a bit below. So 40 x 0.27%
Using that assumption, two panels in series would reach 109V

Gnubie noted that using the two panels in parallel rather than series would work for 2kVA model MPPT, staying above its minimum 30V input. So that is the way to go.

and I don't lose any performance from not connecting in series? or should I just buy one that can handle higher voltages? thanks for your help

 

[Hedges]

and I don't lose any performance from not connecting in series? or should I just buy one that can handle higher voltages? thanks for your help

Out of 900W of panels, not enough to mean many $$ worth of power.

You have to study efficiency vs. voltage curves for the charge controller or hybrid to figure it out. Often, lower voltage input (less conversion being performed by inductors) is more efficient.
There is typically some power level in middle of range, maybe 30% of full load, where charge controller or inverter offers highest efficiency.
Higher voltage string with lower current through the same long wire improves efficiency. Power loss = I^2R. Current varies through the day as sun on panels changes.

If MPP solar offers another model that accepts higher Voc and does what you want, that is good.
You will probably add more panels later, and power more loads.

Up to two string (or single panels) in parallel, no fuses needed. More than two and each should have a fuse the size shown on panel label.
So a unit that accepts higher voltage PV input would let you double PV without adding fuses.

The more you ask, the more complicated the answers become. But you can design a more capable and expandable system.

 

[charleschaime]

Out of 900W of panels, not enough to mean many $$ worth of power.

You have to study efficiency vs. voltage curves for the charge controller or hybrid to figure it out. Often, lower voltage input (less conversion being performed by inductors) is more efficient.
There is typically some power level in middle of range, maybe 30% of full load, where charge controller or inverter offers highest efficiency.
Higher voltage string with lower current through the same long wire improves efficiency. Power loss = I^2R. Current varies through the day as sun on panels changes.

If MPP solar offers another model that accepts higher Voc and does what you want, that is good.
You will probably add more panels later, and power more loads.

Up to two string (or single panels) in parallel, no fuses needed. More than two and each should have a fuse the size shown on panel label.
So a unit that accepts higher voltage PV input would let you double PV without adding fuses.

The more you ask, the more complicated the answers become. But you can design a more capable and expandable system.

ok you convinced me. so the volts stay the same but the amps double. So I need cable and mc4 connectors and mc4 y connectors for 30 amps? and this all goes straight to the charge controller / inverter hybrid. what size cable please? and do I put a fuse or on/off switch before it gets to the box? many thanks for your help

 

[Hedges]

ok you convinced me. so the volts stay the same but the amps double. So I need cable and mc4 connectors and mc4 y connectors for 30 amps? and this all goes straight to the charge controller / inverter hybrid. what size cable please? and do I put a fuse or on/off switch before it gets to the box? many thanks for your help

Two panels in parallel is about 25A, so yes wiring for 30A is good. Using Y connector & cables that are 10 awg would be good (look up NEC ampacity tables). I buy a pre-made MC cable, cut it in half to make pigtails for the charge controller. (Double check voltage polarity before completing circuit)

If this was a residential install, we would be a bit more careful on wire sizes, make sure ampacity of conductor was 156% of Isc if no overcurrent protection or else 100% of fuse/breaker size.

No fuse required for panels with just two in parallel. Switch (or DC breaker rated at least 50% above expected PV current, e.g. 35A or higher) is convenient.

900W into 24V is about 40A, so 25% larger 50A breaker and at least 8 awg is good there.

 

[charleschaime]

thank you, would you also tell me what sort of bms i should buy for use with this inverter. I have 8x Eve LF90 cells so to get 24v I have to get an 8s bms? What amp?
Thanks for any help.

 

[sunshine]

From an online calculator, I determined my array in 24v would have 107.4799 Total Volts in Array but this inverter/mppt charger says maximum voc should be no more than 105v

This datasheet has the voc at 49.1v

98.2voc. 2 in series
There should be a label on the back of the panels to confirm.

 

[charleschaime]

This datasheet has the voc at 49.1v

98.2voc. 2 in series
There should be a label on the back of the panels to confirm.

thank you for your reply but i think you replied to a question at the start of this thread that has been long answered.

 

[charleschaime]

Two panels in parallel is about 25A, so yes wiring for 30A is good. Using Y connector & cables that are 10 awg would be good (look up NEC ampacity tables). I buy a pre-made MC cable, cut it in half to make pigtails for the charge controller. (Double check voltage polarity before completing circuit)

If this was a residential install, we would be a bit more careful on wire sizes, make sure ampacity of conductor was 156% of Isc if no overcurrent protection or else 100% of fuse/breaker size.

No fuse required for panels with just two in parallel. Switch (or DC breaker rated at least 50% above expected PV current, e.g. 35A or higher) is convenient.

900W into 24V is about 40A, so 25% larger 50A breaker and at least 8 awg is good there.

Am I right in thinking that in order to determine what size BMS I need for this, I simply divide the total maximum load of the inverter, say 2400 watts by 24 volts which equals 100 amps. So I need a 8s 100 amp bms? And what about the C rating of my cells, which says 2c max discharging and 1c charging. Should I oversize the bms? Should I get a bms with separate port for charging? Yes lots of questions....

 

[Hedges]

We size (slow blow) fuses and breakers according to continuous inverter output (+25% to avoid nuisance trips.)
But BMS will need to supply peak surge currents for motor starting loads. Some inverters can deliver double power for a few seconds.

Batteries and BMS are sometimes marketed with ratings in excess of what they can really handle. One of the posts on this forum had a link to an article from a marine shop which showed pictures inside cheap vs. quality batteries and BMS. Skimpy wires and more in the cheap ones hobbled their performance.

So read up here and elsewhere about various models, find something which can actually meet your needs.
I haven't used lithium or BMS but other members have. I use AGM which can supply high currents and needs no BMS. It was a good fit for my grid-backup application.

 

[Dzl]

Am I right in thinking that in order to determine what size BMS I need for this, I simply divide the total maximum load of the inverter, say 2400 watts by 24 volts which equals 100 amps. So I need a 8s 100 amp bms? And what about the C rating of my cells, which says 2c max discharging and 1c charging. Should I oversize the bms? Should I get a bms with separate port for charging? Yes lots of questions....

Sort of yes. You are conceptually on the right track. The BMS is sized relative to the maximum amount of current that will flow through it, as well as to the number of cells in series (4S, 8S, 16S, etc). If your inverter is your only load, you can use it to ballpark inverter size. But the specific math you are using leaves out some variables.

First, to estimate the max current flowing to your inverter:
[Max Power] / [Inverter Efficiency] / [Inverter LVD voltage] = [Max Input Current]
For example:
2400W / 0.85 / 24V = 118A

A note on max power, what you use for this value will depend on your inverter and/or use case. For a high quality, high surge low frequency inverter, such as the Victron Multiplus or Samlex Evo where surge is measured in 10's of seconds to over a minute, it may make more sense to use the surge rating as max power, but if you are using a cheaper high frequency inverter it may make more sense to use the continuous power rating as max power. Alternatively, if you know you will not need or exceed some number of Watts, you could size based on this value, this is non-optimal but an option. Some people would disagree with me on this last part.

As with many of the cheap off-brand / unbranded electronics coming out of China, there specs are often overstated, for this reason you do want to oversize the BMS. There are differing perspectives on how much is enough, some people say oversize by at least 1.25x, some people say as high as 2x. I suspect, that again the answer depends on both your use case, how conservative you are by nature, and also how likely and how often you will be near your inverters max power.

As to you specific questions:
1. Should I oversize the BMS: Definitely
2. Should I get a common or separate port BMS: All other things being equal, separate port, but since all other things are not equal, it depends
3. What about C rate: C rate can factor into the equation, but it doesn't have to, most people don't size the BMS based on that. It can be a useful factor to consider though, particularly for smaller battery banks.

 

[charleschaime]

Sort of yes. You are conceptually on the right track. The BMS is sized relative to the maximum amount of current that will flow through it, as well as to the number of cells in series (4S, 8S, 16S, etc). If your inverter is your only load, you can use it to ballpark inverter size. But the specific math you are using leaves out some variables.

First, to estimate the max current flowing to your inverter:
[Max Power] / [Inverter Efficiency] / [Inverter LVD voltage] = [Max Input Current]
For example:
2400W / 0.85 / 24V = 118A

A note on max power, what you use for this value will depend on your inverter and/or use case. For a high quality, high surge low frequency inverter, such as the Victron Multiplus or Samlex Evo where surge is measured in 10's of seconds to over a minute, it may make more sense to use the surge rating as max power, but if you are using a cheaper high frequency inverter it may make more sense to use the continuous power rating as max power. Alternatively, if you know you will not need or exceed some number of Watts, you could size based on this value, this is non-optimal but an option. Some people would disagree with me on this last part.

As with many of the cheap off-brand / unbranded electronics coming out of China, there specs are often overstated, for this reason you do want to oversize the BMS. There are differing perspectives on how much is enough, some people say oversize by at least 1.25x, some people say as high as 2x. I suspect, that again the answer depends on both your use case, how conservative you are by nature, and also how likely and how often you will be near your inverters max power.

As to you specific questions:
1. Should I oversize the BMS: Definitely
2. Should I get a common or separate port BMS: All other things being equal, separate port, but since all other things are not equal, it depends
3. What about C rate: C rate can factor into the equation, but it doesn't have to, most people don't size the BMS based on that. It can be a useful factor to consider though, particularly for smaller battery banks.

Thanks for your advice. What about micro-inverters? Should I be looking at them? Can I use one on an RV? They seem to be cheap on ebay...

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[Hedges]

Microinverters are generally grid-tie, will only produce AC if you already have AC (plugged in to shore power) and the sun is shining. That wouldn't do you any good off grid at night.
Some microinverters work with battery systems, but for the most part aren't what you want for an RV.

You need a way to produce AC from battery, with enough power to run whatever appliances. Propane/electric refrigerators use a heating element, hundreds of watts, which can be operated by PV/battery but are inefficient. Either 12V or AC refrigerators use a fraction as much power. AC ones draw about 5x their nameplate rating to start (same for air conditioners) so inverter needs to be sized accordingly.

Select a suitable sine wave inverter or else hybrid with PV charging included.
An RV (or marine) inverter which will be plugged in to shore power should allow shore power to provide neutral/ground bonding, then perform that bonding itself when off grid. Units built for that purpose have a relay. I'm setting up a vehicle mounted unit and will simply plug the power cord in to a dead outlet with neutral/ground wired together.

You need a charge PV controller if not built in to inverter. For small system with panels just a few volts above battery, PWM can be OK. Otherwise, MPPT allows higher voltage PV strings and accommodates voltage variation that occurs with temperature.

Battery needs to match charge controller and inverter as well. Charging needs to be set to match battery's preferred profile, and inverter needs to shut off at low state of charge. So tentatively select all parts and compare their specs before buying.

 

[charleschaime]

I ended up buying EPSolar MPPT XTRA4415N-XDS1 40A 12-48V, gives me more room with voltages.

 

[charleschaime]

Two panels in parallel is about 25A, so yes wiring for 30A is good. Using Y connector & cables that are 10 awg would be good (look up NEC ampacity tables). I buy a pre-made MC cable, cut it in half to make pigtails for the charge controller. (Double check voltage polarity before completing circuit)

If this was a residential install, we would be a bit more careful on wire sizes, make sure ampacity of conductor was 156% of Isc if no overcurrent protection or else 100% of fuse/breaker size.

No fuse required for panels with just two in parallel. Switch (or DC breaker rated at least 50% above expected PV current, e.g. 35A or higher) is convenient.

900W into 24V is about 40A, so 25% larger 50A breaker and at least 8 awg is good there.

Hi again, if I wanted to connect just one of these panels using 12v battery bank , what size cable i need? thanks!

 

[Hedges]

Hi again, if I wanted to connect just one of these panels using 12v battery bank , what size cable i need? thanks!

Is that the Longi PV panel, 11 ~ 12A Isc, 400 ~ 500W?

PV panel side of charge controller, 12 awg is good for the current from a single panel.
Battery side after an MPPT converter, if going to a discharged 10V battery, current could be 48A so 8 awg is good.
... but connection to battery should be fused. 50A is OK, but some chance of it tripping. 60A fuse/breaker and 6 awg might be better.
If charge controller limits its output to 40A, then 8 awg and 50A fuse is good.

https://www.usawire-cable.com/pdfs/nec%20ampacities.pdf

That's assuming wire isn't terribly long, so voltage drop due to IR isn't excessive.