Is my solar plan correct?

dune16

dune16

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Just wanted to run this past the experts here please. I'm planning to install 2 x 400W mono panels on my new boat. To try to minimise the risk of shading I am going to put each panel on it's own Victron MPPT controller. Is my plan below correct regarding how it's all going to connect up?

Solar panel 1 > Breaker > MPPT controller > Batteries
Solar panel 2 > Breaker > MPPT controller > Batteries

Should I connect both arrays after the MPPT controller to a single bus bar and have 1 set of + and - to the batteries or should I not have a bus and just have 2 sets of + and - cables (16mm2?) to the batteries?

Specs of the 2 panels I plan to use are:

Peak power 400W
Vmpp 41V
Voc 49.5V
Impp 9.76A
Isc 10.12A

I plan to use 6mm2 solar cable from panels to MPPT and then I'm unsure what size cable I need from MPPT to batteries as this will depend if I merge the 2 MPPT's outputs to a bus and then on to the batteries?


Thanks.
 
dune16 said:
Solar panel 1 > Breaker > MPPT controller > Batteries

Solar panel 2 > Breaker > MPPT controller > Batteries
Click to expand...
The breakers should be after the controller and as close to the batteries as practical In your case.
So:

Solar panel 1 > MPPT controller > Breaker > Batteries
Solar panel 2> MPPT controller > Breaker > Batteries


dune16 said:
Should I connect both arrays after the MPPT controller to a single bus bar and have 1 set of + and - to the batteries or should I not have a bus and just have 2 sets of + and - cables (16mm2?) to the batteries?
Click to expand...

Either is fine. Combining the output will need larger cables but half the length for the same voltage drop. The net effect is the same.

dune16 said:
I plan to use 6mm2 solar cable from panels to MPPT and then I'm unsure what size cable I need from MPPT to batteries as this will depend if I merge the 2 MPPT's outputs to a bus and then on to the batteries?
Click to expand...

The required cable size is dependent not just on the current but on the length of the run. You will need to estimate this before an appropriate size can be calculated. The current before and after the MPPT controller will be different so the appropriate cables need to calculated separately.
 
noelex said:
The breakers should be after the controller and as close to the batteries as practical In your case.
So:

Solar panel 1 > MPPT controller > Breaker > Batteries
Solar panel 2> MPPT controller > Breaker > Batteries




Either is fine. Combining the output will need larger cables but half the length for the same voltage drop. The net effect is the same.



The required cable size is dependent not just on the current but on the length of the run. You will need to estimate this before an appropriate size can be calculated. The current before and after the MPPT controller will be different so the appropriate cables need to calculated separately.
Click to expand...

Thanks for the assistance. I thought the idea of the breakers was to be able to isolate the panels from the controller (and therefore also from the batteries)? I think I'll perhaps just avoid using the bus then and go with 2 sets of cables from the controllers to batteries.

I estimate the length of each cable from panel to starboard engine room (where the controllers and batteries will be located) to be around 5 metres. Looking at the spec sheet of the panels I plan to purchase it says it comes with the leads in 4mm2, does this mean I can just continue this on for the 5m run in 4mm2 or should I up this to 6mm2 to try and mitigate the loss due to the run?
 
dune16 said:
Thanks for the assistance. I thought the idea of the breakers was to be able to isolate the panels from the controller (and therefore also from the batteries)?
Click to expand...

Circuit protection between the batteries and the controller is essential. Therefore a circuit breaker or a fuse is mandatory in this location. Isolating the solar input is not essential, but is nice to have. This can be switch (providing the string size is less than three panels) but switches rated for 50v DC are not common, so often a DC circuit breaker is used.
So:

Solar panel 1 > Breaker > MPPT controller > Breaker > Batteries
Solar panel 2> Breaker > MPPT controller > Breaker > Batteries

Is slightly better and more convenient, but the breaker before the controller does not eliminate the need for the breaker close to the batteries.

dune16 said:
I estimate the length of each cable from panel to starboard engine room (where the controllers and batteries will be located) to be around 5 metres. Looking at the spec sheet of the panels I plan to purchase it says it comes with the leads in 4mm2, does this mean I can just continue this on for the 5m run in 4mm2 or should I up this to 6mm2 to try and mitigate the loss due to the run?
Click to expand...

If the panel to controller wire run (note this can be much longer than the physical separation) is 5m for possitive and 5m for negative @ 9.8 A maximium current arround a 6-10 sq mm cable would be a good choice.

The cable run from the controllers to battery will carry a higher current (around 25-30A) , but the appropriate cable cannot be estimated without knowing the distance between the controller and the batteries.
 
noelex said:
Circuit protection between the batteries and the controller is essential. Therefore a circuit breaker or a fuse is mandatory in this location. Isolating the solar input is not essential, but is nice to have. This can be switch (providing the string size is less than three panels) but switches rated for 50v DC are not common, so often a DC circuit breaker is used.
So:

Solar panel 1 > Breaker > MPPT controller > Breaker > Batteries
Solar panel 2> Breaker > MPPT controller > Breaker > Batteries

Is slightly better and more convenient, but the breaker before the controller does not eliminate the need for the breaker close to the batteries.



If the panel to controller wire run (note this can be much longer than the physical separation) is 5m for possitive and 5m for negative @ 9.8 A maximium current arround a 6-10 sq mm cable would be a good choice.

The cable run from the controllers to battery will carry a higher current (around 25-30A) , but the appropriate cable cannot be estimated without knowing the distance between the controller and the batteries.
Click to expand...

Thanks again, I missed the detail from my old solar setup. I did indeed have an inline fuse after the controller to the batteries, a breaker between panels and controller. If the panels are about 9.8a and allowing 25% margin I'd want a breaker between panel and controller of about 12a?

The distance between the controller and the batteries will be 2 metres at most, they're in the same compartment. So just need to work out cable thickness and fuse needed for each controller to battery.
 
dune16 said:
If the panels are about 9.8a and allowing 25% margin I'd want a breaker between panel and controller of about 12a?
Click to expand...

12A is fine. The breaker size between the panels and the controller is uncritical in your case (this is not true if you three or more panels in the string). The solar panel is self limited to around a 10A output. The breaker between the panel and the controller is functioning just as switch.

dune16 said:
The distance between the controller and the batteries will be 2 metres at most, they're in the same compartment. So just need to work out cable thickness and fuse needed for each controller to battery.
Click to expand...

2m for the positive and negative cables carrying a maximum of 25-30 A @12v between the controller and batteries needs around a 10-16 mm sq cable. If you combine the solar panel outputs the current will be double (50-60A) so 16-25 mm sq cable will be needed.
 
noelex said:
12A is fine. The breaker size between the panels and the controller is uncritical in your case (this is not true if you three or more panels in the string). The solar panel is self limited to around a 10A output. The breaker between the panel and the controller is functioning just as switch.



2m for the positive and negative cables carrying a maximum of 25-30 A @12v between the controller and batteries needs around a 10-16 mm sq cable. If you combine the solar panel outputs the current will be double (50-60A) so 16-25 mm sq cable will be needed.
Click to expand...


Thanks very much again. I think I know what I need now. Nice winter project!
 
noelex said:
12A is fine. The breaker size between the panels and the controller is uncritical in your case (this is not true if you three or more panels in the string). The solar panel is self limited to around a 10A output. The breaker between the panel and the controller is functioning just as switch.



2m for the positive and negative cables carrying a maximum of 25-30 A @12v between the controller and batteries needs around a 10-16 mm sq cable. If you combine the solar panel outputs the current will be double (50-60A) so 16-25 mm sq cable will be needed.
Click to expand...

One other quick question, I'll be using 2 x Victron SmartSolar MPPT 100/50 controllers. I have read that they need a decent voltage from the panels to start doing the magic, as each panel will be on it's own controller I assume I'll still be fine as the voltage coming out of a single panel is sufficiently high?

Thanks.
 
noelex said:
Circuit protection between the batteries and the controller is essential. Therefore a circuit breaker or a fuse is mandatory in this location. Isolating the solar input is not essential, but is nice to have. This can be switch (providing the string size is less than three panels) but switches rated for 50v DC are not common, so often a DC circuit breaker is used.
Click to expand...
Not doubting you, but can you possibly explain why the circuit protection is necessary?
 
I recall about 5v above battery for the controller to start, I’ve never had any issue with this and it’s seems normal operation
 
dune16 said:
One other quick question, I'll be using 2 x Victron SmartSolar MPPT 100/50 controllers. I have read that they need a decent voltage from the panels to start doing the magic, as each panel will be on it's own controller I assume I'll still be fine as the voltage coming out of a single panel is sufficiently high?
Click to expand...

Victron controllers have a relatively high wake up voltage (5v above battery voltage). There is some concern that “12 v” panels wired in parallel may start up late. In practice this does not seem to be a practical concern except perhaps “12v” panels with lower than average Voc in hot climates with other charge sources such as wind.

As you are using solar panels with a Voc of almost 50v, you are well above the battery voltage so this is not a concern.
 
KompetentKrew said:
Not doubting you, but can you possibly explain why the circuit protection is necessary?
Click to expand...

Circuit protection between the battery and the solar controller is always needed, as the battery can potentially deliver a very high current. In a fault condition this could exceed the rating of the cable between the controller and the battery perhaps melting the wire and causing a fire.

The maximum current that can be delivered by the solar panel is limited to Isc. The cable size between the solar panel and the controller and the cable size inside the solar panel will always be designed to accommodate this current. So with one solar panel there is no danger of exceeding the wire capacity. The current is self limited and lower than the wire capacity so no circuit protection is needed on the solar side of the controller.

However, once three or more solar panels are connected in a string, in a fault condition the potential current can exceed the wire capacity, especially the wires imbedded within the solar panel. So current protection is needed on the solar panel side of the controller in addition to the always necessary circuit protection between the controller and the battery.
 
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KompetentKrew said:
Ok, thank you. So is an inline fuse acceptable as protection?
Click to expand...
Yes. A fuse or circuit breaker is OK for overcurrent protection, providing the specifications are suitable.

Breakers have the advantage of also acting as a switch, which can be handy.

The caution is that most marine fuse holders or even simple switches are limited to around 32v DC. This can be insufficient for the solar panel input. For example, the OP has an input of almost 50v DC.

DC circuit breakers are available with a much higher voltage rating so they are often the preferred approach.
 
dune16 said:
So something like the below would work between panel and controller? 2 pole means I'd only need the 1 for both panels?

https://www.amazon.co.uk/Low-voltage-Miniature-Circuit-Breaker-Energy/dp/B07D28C9CQ/ref=mp_s_a_1_3?crid=394OHHOWZLQ0D&keywords=solar+circuit+breaker&qid=1665434235&qu=eyJxc2MiOiI0LjY3IiwicXNhIjoiNC4yNCIsInFzcCI6IjMuMDYifQ==&sprefix=solar+cir,aps,98&sr=8-3
Click to expand...
Personally, I prefer to use name brand products such as Schneider, ABB etc for these important applications. The linked product specifies 250v DC suitability, but the rated voltage is listed as 63A ? Obviously a typo, but these errors do not inspire confidence.

Single pole circuit breakers tend to be much cheaper than dual pole breakers. So 2x single pole breakers will be better value and enable switching of each panel separately.

Dual pole circuit breakers when just connected to a single panel (so two dual pole breakers are required for two panels) enable , switching both the positive and negative supply. This provides better safety and isolation, but it is a more expensive and complex solution . As I have an aluminium boat I always use dual pole breakers for each panel, but these are overkill on a fibreglass boat.
 
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KompetentKrew said:
Thank you. I think my boat was supplied with no such protection, so I'll have to take a look.
Click to expand...
A few years ago a boat caught fire and was totally destroyed at Leros. It transpired that the owner had fitted solar panels the previous day without including fuses or a breaker. He went ashore for lunch leaving the boat at anchor. We watched it burn to the water (and the utterly inept attempts by the Greek services to extinguish it)
 
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