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Discussion Starter · #1 ·
There has been a lot of discussion about which type of solar charge controller to use and whether to wire panels in series or parallel. Victron Energy has a pretty good technical white paper discussing the pros and cons of each on their web site, http://www.victronenergy.com/upload...Which-solar-charge-controller-PWM-or-MPPT.pdf.

The conclusion in the paper is for most sailors in the tropics, an MPPT controller with panels wired in series to raise the voltage is the preferred solution.
 

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I would caution users to test installations both ways before committing to a series only install, on a sail boat. If the series panels can not be individually shaded series will work and yield outputs earlier in the day too. The gains can be very good. Sadly on sailboats this is very hard to achieve and series often looses to parallel even though land based data suggests series wins.

On boats that swing to an anchor the only way to know which way performs best is to physically track the data both ways for a few week period. In the tropics series panels that share the same attributes/shade/access to non shade often can benefit from their own controller and another set from its own controller, say port vs. starboard panels..

Myself and I know Nigel and Bruce Schwab have tested this at length. The answer is that "it depends".... I have a hand full of boats wired in series, mostly trawlers, where the panels can't be impacted by shade and in these applications series wins. I have many sailboats where I have tested series vs. parallel and found parallel out performs series but in Maine we don't cook our panels the way they do in the tropics. I have a number of customers who are amazed at how much better their MPPT performance is here in Maine than it is in the tropics...

If I am setting up a multi-panel installation, that I know is going to the tropics, I always try and track boat specific data before doing the final wiring.. Another trick not mentioned in that article is to simply use 24V nominal panels but they are often larger than folks can fit..

Folks also should remember that once the batteries hit absorption voltage there is no MPPT boost... MPPT can only work in bulk charging....

This is a data set I did last spring that shows the gains of MPPT over PWM into a LiFePO4 bank.. Not huge gains but if you are out of real estate then MPPT becomes a good value. Many MPPT controllers have many more control parameters than PWM too.

 

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Discussion Starter · #3 ·
In the tropics series panels that share the same attributes/shade/access to non shade often can benefit from their own controller and another set from its own controller, say port vs. starboard panels..
I agree plus it provides redundancy. I have two panels in series on one side of my bimini and another two panels in series on the other side, each string wired through its own MPPT controller. I am always surprised how low the sun can be in the sky and the panels still start putting out a reasonable amount of power.

I have a number of customers who are amazed at how much better their MPPT performance is here in Maine than it is in the tropics...
The temperature effect is much larger than most people think.

Folks also should remember that once the batteries hit absorption voltage there is no MPPT boost... MPPT can only work in bulk charging....
I'm not sure I understand this one. I'm assuming, maybe incorrectly, that your reasoning is the voltage difference between the array and the battery aren't far enough apart for the MPPT algorithm to provide a benefit. If so, wouldn't having the panels in series still be beneficial because the voltage is much higher, reducing the 'loss' due to controller efficiency?
 

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I'm not sure I understand this one. I'm assuming, maybe incorrectly, that your reasoning is the voltage difference between the array and the battery aren't far enough apart for the MPPT algorithm to provide a benefit. If so, wouldn't having the panels in series still be beneficial because the voltage is much higher, reducing the 'loss' due to controller efficiency?
It is simple. Once at absorption voltage (constant voltage mode) it is the battery bank that decides what it accepts in current at X% SOC and XX.XX voltage... All MPPT controllers essentially become PWM controllers when they begin limiting voltage. Prior to limiting voltage they can take excess array voltage and turn it into slightly more current. Boost can only happen in bulk before the batteries have attained the limiting or regulation voltage.

In short you can't force feed more current into a battery than it can accept once the limiting voltage has been attained. The only way for current to go, once absorption voltage is reached, is down.. If you want more current to flow into the battery, at X% SOC, you would need to increase the voltage to do so.

MPPT controllers can only boost when they have the ability to provide more current (constant current mode) and this can only happen before absorption voltage has been attained.

In constant current charging (bulk) it is the current source is dictating how much it can provide. In constant voltage mode the battery decides/dictates how much current it can accept.

A real simple way to look at it is like this:

Bulk/CC = Current source is in-charge of current supply

Absorption/CV = Battery is in charge of current acceptance
 

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Discussion Starter · #5 ·
It is simple. Once at absorption voltage (constant voltage mode) it is the battery bank that decides what it accepts in current at X% SOC and XX.XX voltage... All MPPT controllers essentially become PWM controllers when they begin limiting voltage. Prior to limiting voltage they can take excess array voltage and turn it into slightly more current. Boost can only happen in bulk before the batteries have attained the limiting or regulation voltage.
Ok, I didn't see it that way because my house bank is relatively large (1040AH) so even in absorption mode the bank can take all of the power my panels can put out for some time.
 

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Ok, I didn't see it that way because my house bank is relatively large (1040AH) so even in absorption mode the bank can take all of the power my panels can put out for some time.
With boats and big banks and relatively small solar panels you will be in bulk most of the time. This benefits the boost time you can get from MPPT. The larger the bank, in relation to the current source, the later in the SOC curve you will hit absorption/limiting voltage. With my LiFePO4 bank I don't hit absorption until about 99.7% SOC.... I have customers who don't hit absorption until the high 90's even with lead acid...
 

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bell ringer
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This is a data set I did last spring that shows the gains of MPPT over PWM into a LiFePO4 bank.. Not huge gains but if you are out of real estate then MPPT becomes a good value. Many MPPT controllers have many more control parameters than PWM too.
The thing with this and other studies I've seen is that they always are basically 12V panels charging 12V systems.

I currently have a single 290W 35.6Vmp panel for my 12V battery system and an thinking of adding another panel. I haven't found a boat test on whether it is better to have 2 of these in series or parallel.
 

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The thing with this and other studies I've seen is that they always are basically 12V panels charging 12V systems.

I currently have a single 290W 35.6Vmp panel for my 12V battery system and an thinking of adding another panel. I haven't found a boat test on whether it is better to have 2 of these in series or parallel.
The only testing that matters is done by you on your own vessel. With your panels and boat my shoot from the hip is to wire them in parallel seeing as you already have high voltage panels... On sailboats series often loses unless the boat ties at the same dock every time and the panels are placed appropriately or they are port starboard etc....

The real answer is you need to track the data yourself, over time, both ways... When you have gathered enough data to be happy, wire them the way that yields the best performance. I wish there were a simple answer but with boats swinging on anchor, and lots of potential for shade, there is not a single simple answer. Power boats are easier because the panels can be mounted with no shade interference, sailboats not so much.. Nigel basically says parallel but series can win. Small(ish) 72 cell panels used to be a bit more prevalent but now days they are all pretty big...
 
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