Why is 36 cell solar panel Better?

I currently have six 100 watt 12v Renogy Mono panels with the specs below: note 36 cells per panel.

I am looking at buying some 12v Rich Mono panels with the specs below: note 33 cells per panel.

I currently use a pair of Bogart PWM charge controllers.

I want to add more panels and upgrade to a Victron MPPT system and also want the flexibility of connecting the panels in various series-parallel configurations.

The Rich panels are the only panels I can find that match the existing Renogy volts and AH, and my physical size requirements. Moreover, the Rich panels are the only panels I have found that have 12 AWG wires rather than 14 AWG wires. My old Renogy panels have 10 AWG wires.

Question: Will the 33 cell Rich panels pose a problem?

PS: I still do not know if the Rich panels have any diodes. My research is ongoing.

Thank you for any input you can provide!

For MPPT controller, if you are placing panels in series so the Vmp is moderately above battery voltage there is no problem.

Do not use a 32 or 33 cell panel, single or multiple in parallel if you intend to use an MPPT charger on a four-cell stack 12v LFP battery array. You need at least a 36 cell panel for this, or run two or more 32/33 cell panels in series.

There is roughly 0.45v to 0.5v per cell Vmp contribution depending on panel temp. MPPT controller requires some overhead voltage above battery voltage to work properly. Some of the overhead is for the DC to DC converter, some of the overhead is to allow the controller to search for MPPT point. 32 or 33 cell panel is okay for a PWM controller on 12v battery.

Most MPPT controllers will just drop into PWM mode operation if they cannot establish a reliable MPPT point but panel Voc is still above battery voltage.

For MPPT controller, if you are placing panels in series so the Vmp is moderately above battery voltage there is no problem.

Do not use a 32 or 33 cell panel, single or multiple in parallel if you intend to use an MPPT charger on a four-cell stack 12v LFP battery array. You need at least a 36 cell panel for this, or run two or more 32/33 cell panels in series.

There is roughly 0.45v to 0.5v per cell Vmp contribution depending on panel temp. MPPT controller requires some overhead voltage above battery voltage to work properly. Some of the overhead is for the DC to DC converter, some of the overhead is to allow the controller to search for MPPT point. 32 or 33 cell panel is okay for a PWM controller on 12v battery.

Most MPPT controllers will just drop into PWM mode operation if they cannot establish a reliable MPPT point but panel Voc is still above battery voltage.

Normally I would agree with you, but the panel performance values are almost a perfect match.

Normally I would agree with you, but the panel performance values are almost a perfect match.

You are comparing apples and oranges specs. One is for 25 degs C panel, one is for 47 degs C panel.

33 x 0.564v = 18.6v Vmp for a silicon mono cell is not possible unless cells are held to 20 degs C. The Rich spec claims 25 degs which is not realistic with sun panel heating. Maybe in wintertime cold temps. At 47 degs C, which is still relatively low panel temp with sun panel heating it would be 0.516v x 33 = 17.0v Vmp, not the 18.6v listed in their spec.

Renogy lists 18.6v Vmp for 36 cell panel and their spec is for 47 degs C +/-2 degs. That will have about 0.516v Vmp at 47 degs C x 36 cells is 18.576v which matches their spec pretty well.

Monocrystaline cells
OK... sorry, I have not read all of the above because of time restrictions but I do have some comments.

All so called 12V panels that I am familiar with have an open circuit voltage of about 21-22 VDC and Vmp is usually around 18V.

There are good reasons to wire panels in series and that is to minimize the voltage drop in the wires between the panels and the controller. I strongly recommend the OP avoid the pulse width modulation style controllers. MPP controllers are now affordable and allow putting panels in series. The only caution one needs is to understand the so called safe voltages are those below aobut 45V. Think old systems with 48 VDC huge batteries.

I wired my panels (33v) 2 in parallel in series with another 2 in parallel for my boat. Sure, I can get voltages above 70 but I can also get by with almost no voltage drop in the wiring between my total W panels and my Victron controller with two strings of #10 wire. Thanks for all the helpful replies.

I have an existing lead acid battery bank and believe that my Bogart PWM system was the best system for my lead acid batteries. I am now building a 560 Ah Lifepo4 battery bank and will be installing Victron charging and monitoring components.

Given that Rich solar never replied to my inquiry about whether their 33 cell panels have diodes, and given the uncertainty respecting the actual specifications vs. what Rich Solar advertises, I am going to pursue other alternatives.



Thanks again!

Given that Rich solar never replied to my inquiry about whether their 33 cell panels have diodes, and given the uncertainty respecting the actual specifications vs. what Rich Solar advertises, I am going to pursue other alternatives.

I have 4 Rich 100W poly panels that perform very well. I&#;m not going to pull one off to pop open the diode/junction box but I&#;d be shocked if they didn&#;t have diodes. They play well with 4 WindyNation monocrystaline 100W panels

Can you explain more what you meant by this? Thank you.

The &#;let-go&#; voltage of DC maxes out a bit over 50V by &#;definition.&#; 40VDC can still kill you but not likely you&#;ll hold on long enough to do that, though it can burn you.
50VDC isn&#;t likely to immediately kill you either but it may and is probable it may immobilize you long enough to inflict severe injury- or kill you. 90VDC can shut vital systems down or un-time your heartbeat and may kill you dead on the spot or wack out your body that you collapse dead later or die that night in your sleep.

Is that true all the time? No. Maybe not even close- but the probabilities are a risk factor too great to ignore or take lightly. I can&#;t remember exactly now (not a joke, it&#;s just long ago) but 24VDC tingled me once. It was more than a tingle really but I suddenly became more respectful of DC is what I remember.

Registered User
Join Date: Aug Boat: Lagoon 400S2
Posts: 3,755 Images: 3 Re: Solar - more cells = better? More cells = higher Voltage. For the same Size it also less current (Amps). Same area produce usually the same power, there are more efficient and less efficient modules, but as a rule of thumb the Wattage is the same for the same area.

Is it good or bad depends on the needs and on the solar regulator / charger.

If you want to use a cheep PWM controller, you want the Voltage at maximum power point Ump around 15-16V, so 32 cell modules are optimal for 12V with PWM, the modules deliver almost Imp as charging current. The downside is, this current requires some cable thickness. The controller just switches the solar module through to the battery / switches off if the battery voltage goes to high. If you use a panel wit more cells, the voltage will be higher, but also the current is limited to maximum Isc (shortcut) and the module would be less efficient with a PWM controller, because there is no transformation to a lower voltage, its like a shortcut for the excess Volts..

If you use a MPPT controller, the 32 cell modules are "useles" when used in parallel. The controller works best if it can transform high Voltage, low current into low Voltage (14.4V) and high current. So modules with higher Voltage (more cells) are better. e.g. a 330Wp module with 96 cells 1m x 1.5m in size delivers 60V Ump and almost 5A. This is a good thing, you can use smaller cables to the controller, and after the controller your current will be 23A / 14V. You can add more modules in parallel, so with 5 of them you would reach 100A charging current, but only 30A solar current over the cable to the controller. Same module with a PWM would deliver only 6A (84 W), you would loose 246W, because of the wrong controller.

So there are different modules for a good reason. There are optimal sizes for different controller types. Moderator

Join Date: Jul Boat: Bestevaer.
Posts: 15,426 Re: Solar - more cells = better? I would recommend 36 cells as normally the optimum for a 12v battery system. 32 cells were popular in the early days of solar, when good regulators were hard to find. The 32 cell panels or "self regulating panels" could supposedly be used without a regulator, but they were not very successful, and 36 celłs has proven to be better for a 12v system.

There are some exceptions and limitations:

The first, is that the best, high efficiency, panels are not generally available in a 36 cell configuration. The 12v market is so small that most solar panel manufacturers only produce high efficiency solar panels in a configuration that is optimal for grid tie or home use. It is difficult to find the best high efficiency solar panels in anything other than 60+ cells at least if you are looking at rigid panels. There are a few of manufactures that produce high efficiency flexible solar panels with a lower cell count, perhaps because flexible panels are not commonly used on buildings.

The second exception is that some MPPT solar controllers require more than 36 cell panels (for a 12v battery bank). This only applies to very small number of controllers. Unfortunately, included in this category is the very popular Victron range. If you do want to use 36 cell panels with these controllers it is probably optimum to wire a couple of 36 cell panels in series, but this is less than ideal.

So given a choice of otherwise equal panels (this is unlikely), 36 cells are best for a 12v system unless you have one of the Victron controllers when around 40+ cells is optimal.

However, cell count (providing it enough) is not usually the dominant factor effecting purchase. Hence, I have just installed some 96 cell rigid panels (on a 24v system). These panels do not have the optimum cell count, but other factors such as the high efficiency are more important. Registered User
Join Date: Aug Boat: Lagoon 400S2
Posts: 3,755 Images: 3 Re: Solar - more cells = better? Quote: Originally Posted by guyrj33 CatNewBee, help me out here. What's an 'Imp' and what's a 'Ump'?
I couldn't make sense out of the numbers you used when trying to show MPPT controllers are better than PWM controllers.
Would you give it another try?
Thanks I stands for current,
U for voltage and
P for Power.

*mp stands for the maximum power, so Ump is the Voltage at max. power and Imp is current at maximum power

*sc stands for short circuit, makes only sense for I, Isc is the current, that the module produces if you short + and - of the connector. The voltage then is obviously U=0

*oc stands for open circuit, makes only sense for the Voltage, so Voc is the Voltage at the wires when they are not connected and no current is flowing, obviously I=0.

Imp is the current at maximum power point along the U/I curve of a panel.

This are the most important parameters of a solar panel. Uoc gives you an idea, what maximum input Voltage your regulator should be able to handle, Ump and Imp allows you to dimension your wires and calculate Pmax of a panel. Imp is also important for the selection of your controller (maximum Input current) According to your wiring (parallel panels / serial panels, strings of serial connected panels etc.) you can calculate the resulting array voltage and current.

There are some more parameters, like module efficiency and temperature coefficient, isolation resistance, maximum string voltage, maximum string current. This are important if you consider to create a larger array of serial or parallel modules.

Some use V instead of U. in their data sheets. Ohms law: R = U / I where R stands for Resistance, U for Voltage, I for Current. I guess, it depends on the country, what symbols are used.