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ekarlson avatar image
ekarlson asked

Is this real? No charging until PV > BAT +5V?

Just got my SmartSolar MPPT 75/15 SCC today. Reading through the manual I have found something that makes no sense to me. It claims that the unit will not even turn on until the PV voltage exceeds that battery voltage by 5v?!?! Isn't the whole point of an MPPT charger that it will boost the PV voltage as necessary to charge the battery? Furthermore, since a lithium ion battery has a nominal voltage of at least 14V and a standard PV has a maximum voltage output of around 17V to 18V, doesn't that make the SmarSolar charger useless for charging a lithium ion battery (the solar panel won't reach the required 19V to even turn the silly MPPT on).


Am I reading this correctly? And if so, how the heck do I turn this *OFF* so that the charger actually works for lithium batteries?

MPPT SmartSolar
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seb71 avatar image
seb71 answered ·
Isn't the whole point of an MPPT charger that it will boost the PV voltage as necessary to charge the battery?


You got this wrong. The MPPT can bring the output voltage lower than the PV voltage, not higher. The advantage (over PWM chargers) is that you can use a PV array with higher voltage to charge a battery with much lower voltage.


Furthermore, since a lithium ion battery has a nominal voltage of at least 14V and a standard PV has a maximum voltage output of around 17V to 18V, doesn't that make the SmarSolar charger useless for charging a lithium ion battery (the solar panel won't reach the required 19V to even turn the silly MPPT on).


There are panels with Voc>30V. Pretty standard, too.

Also, you can wire more panels in series and so you get higher voltage on the charger input.


Edit to add:

It claims that the unit will not even turn on until the PV voltage exceeds that battery voltage by 5v?!?!

If it's connected to a battery, it will turn on even without PV connected. That Vbatt+5V is required for the battery charging to start.


Victron made a short video about it here - https://youtu.be/EVAvRs32TiE

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Kevin Windrem avatar image
Kevin Windrem answered ·

True but it does work! If you take a generic "12 volt" panel, it's operating voltage will be around 17- 18 volts as you say. But the open circuit voltage is much higher, say 20-22 volts. That's enough to start the controller. After it's started a 1 volt margin is all that is necessary.

The purpose of an MPPT controller is to find the optimum voltage/current point for maximum power transfer. I've never read where they would step up the panel's voltage to the battery voltage.

You have the option of connecting multiple panels in series which helps get over the startup threshold. Even two panels in series will most likely start the controller as soon as any light falls on the panels even if it's before they can produce any power.

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nickotron avatar image nickotron commented ·
Your video describes a typical 36 cell panel as having an open circuit voltage of 18v - mine happens to be 17.7 (after cable loss). The problem is my LifePo4 battery is typically 13.1, meaning I would never pass the turn on threshold so no value in Victron MPTT for me as I understand it. What I'm struggling to understand is why MPTT can keep charging down to +1V usefully, but only turn on at +5V?
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hjohnson avatar image hjohnson nickotron commented ·
@nickotron You shouldn't be seeing any cable losses initially at the startup. Prior to startup, the panel is essentially going to be showing its Voc, with no current flowing. Without current, resistance in your wiring is meaningless and both ends should read the same.

Once the buck converter in the MPPT is engated, it can then pull down the voltage to wherever it wants it. But it needs to start high. part of this is because it allows the MPPT to figure out where the maximum power point of your panel is. (That changes based on temperature and age).

On my boat, I went with 24v nominal panels, just to avoid this whole problem. I run a pair of 24v 60W panels in parallel, sitting on top of my dodger.

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nickotron avatar image nickotron hjohnson commented ·
I'm measuring 17.7 Voc at my panel plug, not connected to anything, and from the above have concluded there is no point in upgrading to a Victron MPPT controller from my current PWM, given it will only see Vbat+4.6 (less on a grey day) so will never start charging. Correct? Whereas my PWM controller does charge the batt. Yes a 24v panel or two 18's in series would solve this problem (but is not what I have). Thanks!
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kevgermany avatar image kevgermany ♦♦ nickotron commented ·
Correct.
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rslifkin avatar image
rslifkin answered ·

The solution is to use either higher voltage panels or multiple panels in series.

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ekarlson avatar image
ekarlson answered ·

I have seen discussions of MPPT that involve voltage boosting - but perhaps there was a DC-DC booster interposed in those scenarios?

It also sounds like when the MPPT is not engaged, the PV sees this as an open circuit? That would improve the situation greatly.

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rslifkin avatar image rslifkin commented ·

Correct, when the MPPT isn't drawing any power from the PV panels, it's equivalent to an open circuit, as there's no load on the PV panels to pull the voltage down.

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perstr avatar image
perstr answered ·

I have started replacing my Victrons with Genasun chargers since noticing that the Victrons never kicked in. On smaller sailboats (1x 120W panel) you seldom see +5V until the batteries are *dead*. The Genasun are superior. Dont, know why Victron added this limit in one of its updates. Possibly they are targeting larger installs and not boaters anymore.

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nickdb avatar image nickdb ♦♦ commented ·
It is not a limitation it is how MPPTs and electricity work as the panel voltage will be drawn down under load.

For small systems you will be better off with a PWM charge controller which will perform better in that environment

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Craig Chamberlain avatar image Craig Chamberlain commented ·
It's a non-issue in a well designed system. Solar panels usually come with a spec that includes the voltage/current curves and temperature compensation. So you can easily select PV panels and MPPT devices to suit pretty much any situation. Victron even created the MPPT calculator where you can input the PV panels spec params and it will tell you which MPPTs are suitable.

In my own garage roof mounted solution, I have a pair of 250/60 MPPTs running at 48V and serving a series-parallel array of 8 panels each (16 total panels). Each sub-array consists of two strings of four panels so the string outputs up to 4x the Voc of a single panel and up to 2x the Isc of a single panel. My panels have a Voc of 50.2V so just over 200V for the string at 25 Celsius (higher at lower temps) and with a battery voltage of 48V I never have a situation during the hours of daylight when the panels can't charge the batteries.

Yes, perhaps Victron gear is designed for larger arrays than just a single 120w panel but perhaps there are 120w panels out there which can generate Vbat + 5V given that the MPPTs can run at multiple battery voltages from 12-48V

So I think it's simplistic and misleading to say that "Genasun are superior" - it might be for your single panel if it is not well matched to a Victron MPPT, but for anyone designing a system, it's all about selecting components that work optimally together.

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kevgermany avatar image kevgermany ♦♦ Craig Chamberlain commented ·
It's not that Victron gear is only designed for larger systems, it's a simple mismatch between panel, battery and solar controller selection. See @nickdb reply.
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