What wire size for 400W solar panels in parallel

[rolandka]

Hi there,

I just want to confirm my thought process with you guys... just to make sure everything is right...

I'm getting 4 x 100W solar panels for now but there will be a possibility to increase the pv array to 600w. So Ideally I want to account for this and get the cable thick enough to handle that load. These panels are 24V and will be charging the 12V LiFePo4 onboard batteries through an MPPT charger (Victron SmartSolar 100/50).

I would like to connect them in parallel so I was wondering what size cable wound I need from the modules to the charger. The modules are roughly about 2-3 meter away from the charger. The battery is no more than 1m away from the battery.

The parameter of the panels:
Power (Pmax): 100 watts
Voltage (Vmp): 36.6V
Max. Current (Imp): 2.81 A
Open circuit voltage (Voc): 42.7 V
Short circuit current (Isc): 3.03 A
Max. System voltage: 1000V
permissible ambient temperature: -40 ° C - + 85 ° C

I want to connect the solars in parallel which means that the amperage is additive. so that means roughly 18amps. I was planning to use 5-8mm2 cables from the pv array to the charger and 16mm2 between the charger and the battery.

Also, I'm thinking to make the parallel connections at the panels but where should I do that really? Should I do it closer to the battery?

Thanks guys!

 

[lpdsn]

In the end it will depend upon what voltage drop you consider acceptable. That is very likely to mean that you'd choose far thicker wire than the minimum that is safe with the maximum current, i.e. won't overheat.

There are lots of voltage drop calculators on the web - a quick search will find them.

With an MPPT controller any power lost in the cables will be lost (more basic controllers discard voltage so it matters less).

Calculation for 600W panels: Max theoretical current = 16.86A

8mm2 cables (AWG 8) has a resistance of 2.061 mOhms per m. So I'll use 3m from your post (so 6m total length of cable).

V=IR so 16.86A multiplied by 6 X 0.002061 gives 0.2085V drop. W = VI so a whole 3.5Watts in wasted energy between the panels and the controller.

 

[rolandka]

Yes, you are right. Less than 1% voltage drop might be a little bit overkill for such a system... I'm more than happy with having even 2% drop. I could save a bit of money and go with 6mm2 or even 4mm2.

Thank you for your help, really appreciated.

 

[lw395]

4mm2 might be a bit light, depending on the temperature rating of the cable, how it's going to be enclosed and the local ambient you're designing for.
Don't forget to allow that the sun can actually be a bit brighter than standard test conditions, new panels typically over-perform by several % ( in order to meet the minimum in later life) and bits of boats can get very hot in bright sunlight.
The % drop is usually not important in real life, as the charger will often be throttling back the power in peak sunshine.

 

[RichardS]

Yes, you are right. Less than 1% voltage drop might be a little bit overkill for such a system... I'm more than happy with having even 2% drop. I could save a bit of money and go with 6mm2 or even 4mm2.

Thank you for your help, really appreciated.

I use 4 mm sq for the long run from the 4 x 100W panel array to the controller. Mind you, I have 2 in parallel and 2 in series so can get away with lighter cable. This was the size recommended by the solar panel seller/installer.

Richard

 

[rolandka]

4mm2 might be a bit light, depending on the temperature rating of the cable, how it's going to be enclosed and the local ambient you're designing for.
Don't forget to allow that the sun can actually be a bit brighter than standard test conditions, new panels typically over-perform by several % ( in order to meet the minimum in later life) and bits of boats can get very hot in bright sunlight.
The % drop is usually not important in real life, as the charger will often be throttling back the power in peak sunshine.

I went with the size in-between. I've ordered 6mm2 cables just to be on the safer side. It says... temperature range: fixed installation: -40 ° C to + 100 ° C max. conductor temperature. I'll try to make it as short as possibe.

I use 4 mm sq for the long run from the 4 x 100W panel array to the controller. Mind you, I have 2 in parallel and 2 in series so can get away with lighter cable. This was the size recommended by the solar panel seller/installer.

Richard

I'm going to have 400W panel array to start with (I've just ordered them) but I would like to have the option to increase it if needed. I also thought about connecting them series/parallel. Is there any other benefits joining them such way? I would be worried about shading...

Also, what's the length of your long run?

 

[William_H]

Hi there,

I just want to confirm my thought process with you guys... just to make sure everything is right...

I'm getting 4 x 100W solar panels for now but there will be a possibility to increase the pv array to 600w. So Ideally I want to account for this and get the cable thick enough to handle that load. These panels are 24V and will be charging the 12V LiFePo4 onboard batteries through an MPPT charger (Victron SmartSolar 100/50).

I would like to connect them in parallel so I was wondering what size cable wound I need from the modules to the charger. The modules are roughly about 2-3 meter away from the charger. The battery is no more than 1m away from the battery.

The parameter of the panels:
Power (Pmax): 100 watts
Voltage (Vmp): 36.6V
Max. Current (Imp): 2.81 A
Open circuit voltage (Voc): 42.7 V
Short circuit current (Isc): 3.03 A
Max. System voltage: 1000V
permissible ambient temperature: -40 ° C - + 85 ° C

I want to connect the solars in parallel which means that the amperage is additive. so that means roughly 18amps. I was planning to use 5-8mm2 cables from the pv array to the charger and 16mm2 between the charger and the battery.

Also, I'm thinking to make the parallel connections at the panels but where should I do that really? Should I do it closer to the battery?

Thanks guys!

Just interested in your use of Victron MPPT controller to 12v Lithium Iron Phosphate battery. Are they compatible?
As I understand it Lithium cells run at voltages from about 3.4 to 4.1 volts so you have 3 cells in series to give voltage of 10.2 to 12.3 volts. (discharged to charged). These voltages while probably Ok for consumer items meant to run on 12v lead acid the charging voltage is vastly different to a lead acid battery. Lithium charge controllers all have sensors to compare voltage of each cell to ballance and stop charging if a inbalance occurs and when fully charged. They also usually have a low voltage cut off for discharge. Now probably you (OP) have taken all this into account and have a suitable lithium charge controller but if not do get one before using a MPPT contoller for charging lead acid battery on lithium battery. If so how does the MPPT smart charger feed the lithium charge controller? olewill

 

[RichardS]

I'm going to have 400W panel array to start with (I've just ordered them) but I would like to have the option to increase it if needed. I also thought about connecting them series/parallel. Is there any other benefits joining them such way? I would be worried about shading...

Also, what's the length of your long run?

My long run is 8 metres each way .... so 16m of cable.

The series /parallel question is an interesting one and there have been several recent threads about it .... although none of these involved lithium batteries so the discussion might not be totally relevant.

In general, the Victron works better with a higher voltage rather than a lower one so if you can have two panels in series and thereby generate 40V+ that is a "good thing" for the controller and a "good thing" for minimising cable losses. However, the downside is that panels in series are affected by shadows across one or more of the panels whereas the same panels in parallel would not be so affected.

You end up with a balance situation which depends on what voltages are being generated across the day, how much time the series panels spend in shadow and how deep those shadows are. Unless the parameters are towards the extreme end of the spectrum, it's difficult to determine which is the best configuration without trying out the alternatives and taking real measurements.

I ended up with the opposite 2 in series / 2 in parallel configuration to the one I initially thought would be the best.

Richard

 

[rolandka]

Just interested in your use of Victron MPPT controller to 12v Lithium Iron Phosphate battery. Are they compatible?
As I understand it Lithium cells run at voltages from about 3.4 to 4.1 volts so you have 3 cells in series to give voltage of 10.2 to 12.3 volts. (discharged to charged). These voltages while probably Ok for consumer items meant to run on 12v lead acid the charging voltage is vastly different to a lead acid battery. Lithium charge controllers all have sensors to compare voltage of each cell to ballance and stop charging if a inbalance occurs and when fully charged. They also usually have a low voltage cut off for discharge. Now probably you (OP) have taken all this into account and have a suitable lithium charge controller but if not do get one before using a MPPT contoller for charging lead acid battery on lithium battery. If so how does the MPPT smart charger feed the lithium charge controller? olewill

First, all of this is still on paper but I've already ordered everything I need to set up my new battery system. It's coming next week so wish me luck setting up. This is the main reason I cannot go into the very specifics right now.

Most of the recent Victron products are compatible with LiFePo4 batteries, what I really mean by this is that they are programmable. In the case of this MPPT (SmartSolar 100/50), there are around 8 preprogrammed charge algorithm (including for LiFePo4). The charge algorithm is also fully programmable if I don't like the preprogrammed ones. I should be able to set anything that the battery manufacturer recommends. I'll be also using Victron Multiplus Inverter/Charger in one which also programmable to my needs, it could stop charging at a specific voltage. This is all very good however as you said I need something to monitor the individual voltage of the cells... (although not everyone recommends this for such small systems, I do feel safer and I do think this is absolutely needed)

I think currently the LiFePo4 battery systems on the market are way to expensive. I could have got a Victron Smart battery with the BMS that would have taken care of everything... but I've chosen to go on a DIY route. This means that I'll be getting 4 x 3.2v/200Ah Winston cells and wiring them in series, this gives me a 12.8V/200Ah battery. Every cell will be top-balanced individually. A BMS will be used and this will take care of the balancing and has ATC (allow to charge) and ATD (allow to discharge) ports that can switch anything. These can be connected straight to the Victron Multiplus and could be used with a contractor that could swich the load off and on.

 

[rolandka]

My long run is 8 metres each way .... so 16m of cable.

The series /parallel question is an interesting one and there have been several recent threads about it .... although none of these involved lithium batteries so the discussion might not be totally relevant.

In general, the Victron works better with a higher voltage rather than a lower one so if you can have two panels in series and thereby generate 40V+ that is a "good thing" for the controller and a "good thing" for minimising cable losses. However, the downside is that panels in series are affected by shadows across one or more of the panels whereas the same panels in parallel would not be so affected.

You end up with a balance situation which depends on what voltages are being generated across the day, how much time the series panels spend in shadow and how deep those shadows are. Unless the parameters are towards the extreme end of the spectrum, it's difficult to determine which is the best configuration without trying out the alternatives and taking real measurements.

I ended up with the opposite 2 in series / 2 in parallel configuration to the one I initially thought would be the best.

Richard

I should be ok then with the cables I ordered. Is there any way to actually measure the voltage drop due to the cables?

In theory the MPPT charger would be able to handle the voltage increase due to the series/parallel connection. As you said, I would have to take real time measurements though to make sure it's working as it should.

 

[VicS]

I should be ok then with the cables I ordered. Is there any way to actually measure the voltage drop due to the cables?

In theory the MPPT charger would be able to handle the voltage increase due to the series/parallel connection. As you said, I would have to take real time measurements though to make sure it's working as it should.

A digital multi-meter on the DC 200mV range would measure voltage drops large enough to be of concern

 

[PaulRainbow]

Hi there,

I just want to confirm my thought process with you guys... just to make sure everything is right...

I'm getting 4 x 100W solar panels for now but there will be a possibility to increase the pv array to 600w. So Ideally I want to account for this and get the cable thick enough to handle that load. These panels are 24V and will be charging the 12V LiFePo4 onboard batteries through an MPPT charger (Victron SmartSolar 100/50).

I would like to connect them in parallel so I was wondering what size cable wound I need from the modules to the charger. The modules are roughly about 2-3 meter away from the charger. The battery is no more than 1m away from the battery.

The parameter of the panels:
Power (Pmax): 100 watts
Voltage (Vmp): 36.6V
Max. Current (Imp): 2.81 A
Open circuit voltage (Voc): 42.7 V
Short circuit current (Isc): 3.03 A
Max. System voltage: 1000V
permissible ambient temperature: -40 ° C - + 85 ° C

I want to connect the solars in parallel which means that the amperage is additive. so that means roughly 18amps. I was planning to use 5-8mm2 cables from the pv array to the charger and 16mm2 between the charger and the battery.

Also, I'm thinking to make the parallel connections at the panels but where should I do that really? Should I do it closer to the battery?

Thanks guys!

Make the parallel connections close to the controller, your 6mm cable will be more than up to the job.

If there is a way to connect any of the panels in series, not only will it effectively reduce voltage loss, the Victron controller will work all the better for it.

Keep the cable run from the controller to the batteries as short as possible, this is where you really don't want voltage drop. 16mm sq is vastly overrated, be surprised if it will fit the controller terminals.

 

[William_H]

First, all of this is still on paper but I've already ordered everything I need to set up my new battery system. It's coming next week so wish me luck setting up. This is the main reason I cannot go into the very specifics right now.

Most of the recent Victron products are compatible with LiFePo4 batteries, what I really mean by this is that they are programmable. In the case of this MPPT (SmartSolar 100/50), there are around 8 preprogrammed charge algorithm (including for LiFePo4). The charge algorithm is also fully programmable if I don't like the preprogrammed ones. I should be able to set anything that the battery manufacturer recommends. I'll be also using Victron Multiplus Inverter/Charger in one which also programmable to my needs, it could stop charging at a specific voltage. This is all very good however as you said I need something to monitor the individual voltage of the cells... (although not everyone recommends this for such small systems, I do feel safer and I do think this is absolutely needed)

I think currently the LiFePo4 battery systems on the market are way to expensive. I could have got a Victron Smart battery with the BMS that would have taken care of everything... but I've chosen to go on a DIY route. This means that I'll be getting 4 x 3.2v/200Ah Winston cells and wiring them in series, this gives me a 12.8V/200Ah battery. Every cell will be top-balanced individually. A BMS will be used and this will take care of the balancing and has ATC (allow to charge) and ATD (allow to discharge) ports that can switch anything. These can be connected straight to the Victron Multiplus and could be used with a contractor that could swich the load off and on.

Well I and I am sure many others will be interested to see how it goes. One concern I might suggest is that on charge the battery voltage is going to be well above 17v. A higher voltage is needed to push current in to the battery. You might want to check equipment that might be operated when batteries are being charged. For example I imagine a heater fan could go crazy on high voltage and then drop right back in speed and power as voltage declines if not on charge. You might want to consider buck voltage converter for items that could be damaged by high voltage. Many electronics devices are designed to cope with 11v up to 30 volts. But beware of cheap resistor current limited lED's. I think if I had a choice I would go for 3 cells and accept a lower operating voltage. Once it is up and running can you also give us an idea of whether the battery does seem to deliver the claimed AHours. My experience with Chinese lithium 18650 cells and also NIMH cells is that provide barely 10% of the claimed AH. I hope your experience is better. olewill

 

[rolandka]

Make the parallel connections close to the controller, your 6mm cable will be more than up to the job.

If there is a way to connect any of the panels in series, not only will it effectively reduce voltage loss, the Victron controller will work all the better for it.

Keep the cable run from the controller to the batteries as short as possible, this is where you really don't want voltage drop. 16mm sq is vastly overrated, be surprised if it will fit the controller terminals.

So my panels are already 24 volts, the vmp is 36.6V if I do 2 x 3 then that gives me 73.2V and 8.43A. The MPPT can handle 100V so that should be fine.

There is solar calculator I've just found which was made by Victron. It is an excel sheet but seems pretty good. You can add your own modules and chargers in the sub-sheets. I've just done quickly to see whether I should connect the panels parallel or series/parallel. (see pictures below) Saves me some time and it factors temperature in the calculation.

 

[rolandka]

Well I and I am sure many others will be interested to see how it goes. One concern I might suggest is that on charge the battery voltage is going to be well above 17v. A higher voltage is needed to push current in to the battery. You might want to check equipment that might be operated when batteries are being charged. For example I imagine a heater fan could go crazy on high voltage and then drop right back in speed and power as voltage declines if not on charge. You might want to consider buck voltage converter for items that could be damaged by high voltage. Many electronics devices are designed to cope with 11v up to 30 volts. But beware of cheap resistor current limited lED's. I think if I had a choice I would go for 3 cells and accept a lower operating voltage. Once it is up and running can you also give us an idea of whether the battery does seem to deliver the claimed AHours. My experience with Chinese lithium 18650 cells and also NIMH cells is that provide barely 10% of the claimed AH. I hope your experience is better. olewill

That is something I did not think about to be honest. So thank you for mentioning. I've double-checked and the charge voltage should be between 14V and 15V (<14,5V recommended) and float on 13,6V. That is similar to AGM battery.

The specification of the cell says the following:
Operating voltage max 3,8V - min 2.8V At 80% DOD
Deep discharge voltage 2.5 V - The cell is damaged if voltage drops bellow this level
Maximal charge voltage 4 V - The cell is damaged if voltage exceeds this level

The reason I went with Winston batteries, is that many of the bigger brands uses them in their smart batteries. You can see the Winston cells on this website that are used by Victron. The site is in German but google website translator gives you quite a good translation.

I really hope that it will give the claimed AH as I would be devastated to find out the opposite. All this money and hassle for nothing... :ambivalence: