Limit to the number of 12V solar panels in series?

[mcarthur]

Is there any disadvantage to having one MPPT per panel? (apart from cost - MPPT's and longer wiring?)

Or another way - is there any advantage to putting multiple panels onto one appropriately sized MPPT?

[Bycrick]

MPPT controllers are cheap and getting cheaper. Both Texas Instruments and Maxim, big semiconductor manufacturers, have got what are essentially single-board designs and chips, that would make it cost-effective to build the controller into a bigger panel. But I don’t know of any commercial products yet.

Running series-connected panels increases the supply voltage, that means smaller wires and less system losses. The controller then serves the additional purpose of reducing the higher panel voltage to the desired battery voltage. This helps harvest more power under some conditions. The trade off is that panel shading becomes more critical.

So, the answer to your question, as it often is: It depends on your exact requirements.

[barnakiel]

Originally Posted by mcarthur

Is there any disadvantage to having one MPPT per panel? (apart from cost - MPPT's and longer wiring?)

Or another way - is there any advantage to putting multiple panels onto one appropriately sized MPPT?

?


The one we have is happier with high input voltage (a Victron mppt).


If you use panel 12V > Victron mppt > 12V battery you gain nothing from its mppt as it will kick up only very late in the morning.


Imho, mppt wire 24V panels onto 12V bat. And 48V panels onto 24V batt. etc.


b.

[noelex 77]

Originally Posted by mcarthur

Is there any disadvantage to having one MPPT per panel? (apart from cost - MPPT's and longer wiring?)

Or another way - is there any advantage to putting multiple panels onto one appropriately sized MPPT?

A seperate MPPT controller for each panel is often a good solution, at least for larger panels. It ensures the power point of each panel is optimium . This maximises the output from each panel, especially under challenging conditions such as variable shade. As these conditions are common on a boat, it is a popular option.

This option also provides valuable redundancy.

Parallel connection of the panels tends to be next best option on a boat, followed by series connection, which usually produces the lowest output.

Combinations are possible, such as one controller for each pair of panels.

There are some drawbacks of multiple MPPT controllers:

1. It can be more expensive, although multiple smaller controllers can sometimes be cheaper than a single larger controller of the same total capacity, so this depends on the specifics of the installation.

2. There are more wires that need to fed back to the controller. The wire size will be smaller than parallel connection so the extra cost and difficulty is often minor, but once again this varies from installation to installation.

3. Smaller cheaper controllers sometimes have poorer performance, and less adjustability than the larger more expensive options. This issue has been largely solved by the release of good quality small inexpensive controllers, especially the Victron range.

4. Self consumption: MPPT controllers can have relatively high self consumption, the power just to run the sophisticated electronics used to track the maximium power point of the panels and battery charging algorithms can consume significant power. Some controllers use around 5-6w just to function. If mated with a small solar panel the loss of power from multiple controllers can add up especially on poor solar days when power output needs to maximised. Fortunately some of the smaller, modern MPPT controllers have circuits with low self consumption and if matched with reasonable sized panels the self consumption of multiple controllers is still acceptable.

5. Charging coordination: ideally multiple controllers would be synchronised to work together. Without this feature conflict is still generally very minor and the better tracking of multiple controllers outweighs the occasional slight charging anomalies. This has traditionally needed large, expensive controllers to resolve, but in modern times some even small controllers can be made to coordinate well for little cost, thereby solving this concern.

[BlackHeron]

One more disadvantage of multiple controllers is that many newer controllers are dual-voltage 12/24v depending in what battery system voltage you are running, and are auto-sensing whennfirst powered up and connected to the battery.

At startup it is necessary to power up the battery connection FIRST, before connecting the solar panel(s) so the controller can automatically determine and set its own battery system voltage parameter. If this is done in reverse the controller may fumble and set its battery voltage incorrectly, such as when the battery is disconnected for some reason and the panels are still connected.

This could cause trouble and possibly damage your batteries. Every time the batteries are disconnected for troubleshooting or testing each and every solar panel must also be disconnected before powering up the controllers to the battery again. Only then can the panels be reconnected once the controller boots up and matches to battery system voltage of 12v or 24'v

With multiple controllers fed by multiple panels this may become tedious and errors/omissions are more likely to occur. Having a DC breaker on both sides of each solar controller is a good idea to make this procedure easier. Pulling wires off of the terminals every time is not ideal. With only one controller this only has to be done once and in one place..