17.2 volt Vmp panel and Victron MPPT controller

divingtactics

Hi All,

I have two 40-Watt solar panels on the roof of my boat and each one of them is fed to each battery (house and start) by a small cheap Chinese CMWD PWM controller that are sold as MPPT, but of course they are not.

My solar panels are Ozcharge 40-Watt Mono panels with the following specs:

Max. power (Pmax): 40W Rated Voltage (Vmp): 17.2V Rated Current (Imp): 2.33A Open Circuit Voltage (Voc): 21.5V Short Circuit Current (Isc): 2.5A

I was thinking of changing these cheap PWM controllers for the Victron SmartSolar MPPT 75/10 with built in Bluetooth.

https://energyconnections.net.au/product/victron-shop/454/1461/81328/victron-smartsolar-mppt-7510-charge-controller-with-built-in-bluetooth

My batteries are wet cell century marine batteries.

This controller needs a difference of + 5 volts to begin the bulk charging phase and after it starts, this 5 volts is reduced to 1 volt over the battery voltage.

I spoke to the supplier of the Victron controller and initially he said that the panel voltage was too low at 17.2 volts but when I told him the VOC was 21.5 volts, he said it would be fine. I have checked the panels disconnected from the controllers and they did have an OC voltage of 19-20 volts on a cloudy day but it was my understanding that when connected to a controller that the VMP was closer to what the actual output is.

So my question, if you kind folks can help is, do you think this MPPT controller is a good match for these panels or will it be governed too much by the VMP and not the VOC. I suppose I am not sure if I believe the supplier and don't want to spend $300 on new controllers just to have them only work during the middle of the day and not much during winter when the sun angle is lowest. I am located in Melbourne Australia, which is latitudinally similar to Tennessee I reckon.

I mainly want to get rid of the CMWD controllers because I don't expect them to last and if they fault, they may cook my batteries, which I don't really need at sea.

Maybe I should just buy some better quality PWM controllers. Money is not a governing factor anymore.

I hope you guys can help.

Cheers,

DT

Estragon

IMHO, the temperature of the batteries and panels would be a factor.  The panel voltage will be lower at high temps, and higher in breezy and cool conditions.  Charging voltage for optimal battery charging will be higher for cool/cold batteries.  For example, if the panels will typically operate in relatively low wind, hot-ish noon sun while batteries live in a moderate temp bilge area, staying with pwm may be a better choice.  Even decent ones are pretty cheap, so taking a spare or two to sea may work better.

For this purpose, (temp adjusted) Vmp is the relevant number, as this is what the controller needs when charging.  Voc is relevant for string sizing to ensure the string voltage stays under the controller maximum (often 100-150v) when the sun first hits the panels on a cold morning.  Wken the panels warm, Voc will drop, and when the controller starts bulk charging, voltage will drop further.  Mppt wants ~25-30% over battery voltage, so an absorb target of say 14.7 would want ~18v or more.  Vmp spec at 17.2 sound iffy unless the panels are pretty cool.

Mppt tend to work best at capturing higher voltages in cold conditions and/or to allow for higher voltage series strings to keep wire size reasonable over longer runs to pv.  Running both panels in a series string to a single controller and an ACR might be an option if there are no shade issues and the start/house batteries are compatible.

mike95490

I suspect you may not have high enough PV voltage for MPPT to function, the charger may fall back to PWM. 
 I'd start with 25Vmp as the low voltage needed to have MPPT be successful with a 12V bank. Lead Acid will want to occasionally hit nearly 16V for some EQ cycles (but not sealed batteries).   Does the controller manual indicate the minimum voltage for MPPT to work ?

divingtactics

mike95490 said:

   Does the controller manual indicate the minimum voltage for MPPT to work ?

No, not that I can see. Thanks, I'll get some better PWM controllers and stick with them as I don't want to put an ACR on the circuit at this stage. Thanks again for your help.

Cheers,

DT

divingtactics

Estragon said:

IMHO, the temperature of the batteries and panels would be a factor.  The panel voltage will be lower at high temps, and higher in breezy and cool conditions.  Charging voltage for optimal battery charging will be higher for cool/cold batteries.  For example, if the panels will typically operate in relatively low wind, hot-ish noon sun while batteries live in a moderate temp bilge area, staying with pwm may be a better choice.  Even decent ones are pretty cheap, so taking a spare or two to sea may work better.

For this purpose, (temp adjusted) Vmp is the relevant number, as this is what the controller needs when charging.  Voc is relevant for string sizing to ensure the string voltage stays under the controller maximum (often 100-150v) when the sun first hits the panels on a cold morning.  Wken the panels warm, Voc will drop, and when the controller starts bulk charging, voltage will drop further.  Mppt wants ~25-30% over battery voltage, so an absorb target of say 14.7 would want ~18v or more.  Vmp spec at 17.2 sound iffy unless the panels are pretty cool.

Mppt tend to work best at capturing higher voltages in cold conditions and/or to allow for higher voltage series strings to keep wire size reasonable over longer runs to pv.  Running both panels in a series string to a single controller and an ACR might be an option if there are no shade issues and the start/house batteries are compatible.

Thank you very much, this is great information and lets me know exactly what I want. I'll buy some better quality PWM controllers and stick with that technology for the boat batteries. I appreciate both of your input very much.

Cheers,

DT

kampf2000

Im using a button 30 amp controller with 2 sets of 3 17.5 volt panels. Each set of 3 is wired in series.  It works well and the bluetooth monitor is great.

Tecnodave

I just installed a very similar controller, a Victron smart solar 100/20 and read that same claim that a MPPT controller would work at battery voltage +5 volts. I gotta dought that so I tried it . System is 24 volt Surette S-550's and two unknown brand 250 watt 72 cell panels. I wired it up as 2P and it did produce a little power but switching panel to 2S quadrupled the power with much better performance,

I would not recommend that controller with a 36 cell 18 volt panel. Minimum would be a 60 cell grid tie type (30 volt @mpp)  or a 72 cell 24 volt system panel (36 volt @mpp)

please note that the solar on this system is only a backup for a 17.5 kW generator powered meral  sculpture shop needing standby power when not welding, cutting, etc. it is a work in progress and more panels will be added soon

David