What is the charge controllers actual outputs for various solar inputs?

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John asked Feb 11, '20

What is the charge controllers actual outputs for various solar inputs?

Hi, I am still trying to get confirmation as to what the output voltages of a charge controller is at what voltages inputs from a solar panel.

Like if a solar panel rated at 55vdc @ 100% light, what's actually the output Vdc of the charge controller . in graph form eg: below. such as the victron 150/35

panel @ 100% 55vdc = ?? Vdc @ charge controller output to 12v battery.

panel @ 75% 25vdc = ?? Vdc @ charge controller output to 12v battery.

panel @ 40% 15vdc = ?? Vdc @ charge controller output to 12v battery.

panel @ 10% 5vdc = ?? Vdc @ charge controller output to 12v battery.

As I see at this stage the charge controllers may not put out a usable charging voltage when the solar panels voltages are higher than about 20vdc.

Can someone please confirm if they output all the time if the solar panel voltages vary between say about 13 Vdc and about 55vdc.?

Thank you,

John

MPPT Controllers charger

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Guy Stewart (Victron Community Manager) answered · Feb 11, 2020 at 09:08 AM

Hi @John,

@M.Lange answer is correct, and I would like to use the opportunity of your question to explain even further.

Solar panels, unless heavily shaded have a remarkably high and consistent voltage output. It is the current output that decreases.

Have a look at these I-V (Current vs Voltage) and P-V (Power vs Voltage) charts for a 300W solar panel.

You can see in the P-V curve, that as the solar radiation decreases, from 1000W/m2 to 200W/m2 (20%), that while the power drops from 300W to 60W, the Voltage output range remains nearly constant, with the Maximum Power Point (MPP) voltage identical at 33V, and the maximum open circuit voltage only dropping from 43V to 38V.

If the voltage is pretty constant regardless of the intensity of the light, then the Current must be changing.

You can see this clearly in the I-V curve, where the output current is dropping from 9.8 A at 1000W/m2, to 2 A at 200W/m2.

What this means is that the input voltage in a correctly designed and installed system free of obstacles should nearly always be within the acceptable voltage window of the MPPT for optimum performance (even when its cloudy).

Where voltage DOES vary is during heavy shading, like a tree branch obstacle, and due to temperature.

Due to the nature of the semiconductive silicon in PV cells, the effect of shading on the solar panel is so severe that if a single cell (of which there can be 36 to 144 in a panel) is completely shaded, then it will completely restrict the flow of electricity through it.

Solar panels use built in bypass diodes to then skip that troublesome cell group (usually a several horizontal columns of cells) to allow the energy from the other unshaded cells to flow.

The Victron MPPT itself is a buck DC to DC converter. It can only reduce the higher PV side voltage to the lower battery side voltage. It cannot boost the voltage from a PV panel up to charge a battery.

The MPPT can accept, and with good efficiency (ie very little loss of total power) convert any PV side voltage up to its maximum PV input voltage limit. This varies with the Victron models between 75V and 250V and is clearly printed on the unit itself, and all associated documentation.

On the battery side, it is the battery that is setting the system voltage. The MPPT will be converting the panel voltage and outputting a higher than battery voltage, to get the current to flow into the battery. But the battery chemistry will be dragging that MPPT voltage down at the DC bus level, and that electrical work is going into the battery chemistry to charge it.

Once the battery is full, and reaches a target voltage, the MPPT will adjust the conversion to maintain the float voltage. Still higher than the battery voltage at rest, but not enough to allow for meaningful current to flow, or the battery would be overcharged.

I hope this helps you understand a bit more about the equipment and the process.

pv-voltage-curve.jpg (352.4 KiB)

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John commented · Feb 11, 2020 at 06:33 PM

Hi and thank you for your explanation.

So would the victron unit therefore basically NOT cut off the solar panel inputs at any voltage in its full voltage range ie: 1volt-150v ?

I ask as some cheaper type ones cut the panel inputs if say the panel voltage is above 15vdc. (when using 12v battery bank). which would mean they can't handle panels that are putting out more than about 18 voc.

Regards

John

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Matthias Lange - DE ♦ John commented · Feb 11, 2020 at 07:34 PM

What you are talking/thinking about are PWM charge controllers. They are working like that.

A MPPT charge controller can use the full voltage range of the panels as long as the PV voltage is higher than the battery voltage (Battery Voltage+5V to start & Battery Voltage+1V for charging)

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John Matthias Lange - DE ♦ commented · Feb 12, 2020 at 11:54 PM

Thank you M. Lange,

That now makes sense and I now feel more comfortable to be able to work out what I need to use in my new configuration.

John.

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Guy Stewart (Victron Community Manager) ♦♦ John commented · Feb 12, 2020 at 02:16 AM

Here is some of the information that is displayed in VictronConnect for a Victron MPPT showing the relationship between Solar Voltage, and the Battery Voltage.

Now here is the same solar array a short time later, when the sun has come out. Notice how the power has increased from ~350W to ~1000W, but the PV Solar Voltage is the same!

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John Guy Stewart (Victron Community Manager) ♦♦ commented · Feb 13, 2020 at 12:00 AM

Hi Guy,

Thank you also, it helps.

As I haven't experience with a "real" MPPT charge controller, I am uncertain if or which one I need with my current batteries (12v) bank and 55-60voc solar panels I now have.

I just want the panels to be able to charge or maintain battery charge while the sun shines and I can draw in the batteries through the day also and they can then be still at full charged for night time use when the sun stops shining .

John.

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Guy Stewart (Victron Community Manager) ♦♦ John commented · Feb 13, 2020 at 12:32 AM

Hi @John,

Please use the Custom Modules feature of the Victron MPPT sizing calculator, and it will suggest an MPPT from the range for your situation.

I would then get in touch with a Victron dealer and discuss the whole situation, including battery sizing, load expectation, sun hours and PV array configuration with them.

Success!

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Answer 1 · 2020-02-11T05:56:15Z

Matthias Lange - DE answered · Feb 11, 2020 at 05:56 AM

It is a charge controller, so the output voltage depends on the state of charge of the connected battery/the charging stage (bulk, absorption, float) of the MPPT and the settings for the bulk-/absorption-/float-voltage.

Let's say you have a nearly empty lead battery with 11.5V so the MPPT starts charging with bulk at 11.5V with as much power as it can get from the panel(or the current set in the settings) until it reaches the absorption voltage of 14.4V.

The MPPT starts charging if the PV-voltage is 5V higher than the battery voltage and than charges as long as the PV-voltage is at least 1V higher than the battery voltage(or the battery is full).
For the example above: it will start at around 16.5V and will need a minimum of 12.5-15.4V during charging.

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