Solar panels: 60 cell panels, 24V battery, and DSSR20

[Demosthien]

I've been trawling the posts regarding solar panels used in conjunction with SBMS0 and DSSR20. In many posts there is usually a mention of "matching the panels to the battery" and one such example is the advice to use solar panels with "60 cells" with a 24V battery.

Can someone explain what the "match the panels to the battery" comment actually means in terms I could apply to any panel/battery setup?

I'm in the midst of planning our 24V system and the solar panels I had previously selected had 60 cells... However, on going to purchase them the site has updated their listing with some new(er) solar panels, the spec sheet indicates these solar panels have 120 (6x20) cells.

Does this mean I'm out of luck here and these new(er) solar panels won't work with my 24V system?

[Dacian Todea]

Those 120 half cell panels are no different from a 60 cell panel and they will match a 24V LiFePO4 battery as good as the older panels.

What they did is cut the 60 cells in half and make two smaller 60 cell panels then connect those panels in parallel.

The advantage of this arrangement is that the busbars connecting the cells are shorter so lower voltage drop across them than o a simple 60 full size cells panel.

The other advantage is that since this are two smaller panels in one shade on one half of the panel will not affect the other half with is basically a separate panel in parallel.

  

[Demosthien]

Great, thanks for that info Dacian. That's one less thing to worry about.

Could you also elaborate on the magic that is the "match your panels to your battery" comment that I see around the forums?

[Dacian Todea]

There is no magic :)

About 80% of the world production of PV panels is made out of 60 cell PV panels (that may have changed a bit in recent years where some of that may have been taken by 72 cell PV panels ) and then a much smaller fraction is made of smaller 36 cell PV panels and even smaller fraction some strange non standard number of cells in series.

Each PV cell is like a diode but is an open diode very flat and large surface and so works as a generator in presence of light.

Since each cell is a diode the max voltage across will be just around 0.7V that is when the diode (PV cell) will short all the energy produced by light so all energy is lost in the diode (PV cell).

To be able to use the energy you will want to connect a load that reduces that voltage so the current instead of being sorted by the diode (PV cell) flows through your load.

And in our case the load is actually a battery and that is perfect as battery is basically at constant voltage.

If you look in the spec of your PV panel the 335W model you have 41.32V open circuit voltage so if you divide that by the 60 cells in series you get 0.688V almost that 0.7V rounded number that I mentioned before.

Now if you look at the last graph you see a current vs voltage based on panel temperature and in a sunny summer day panels will be at 60 to 70C so looking at that purple +70C curve you see you get max current only if voltage is somewhere around 27 to 28V  and LiFePO4 will be around 26.5 to 27V for most of the time thus you will be at or expremly close to max power point can get to 99 even 100% efficiency in some particular scenario (100% efficiency will ignore the voltage drop on wire) but anyway closer to max power point than an MPPT can get but that is for a particular panel temperature.

If you get an average over a year depending on climate that average will be likely similar or better than what an MPPT can do without the cost and complexity of an MPPT.

[Demosthien]

That is a great explanation. I am able to grasp the concept now. Thanks again.

[Dmitry Katsubo]

Thanks for verbose explanation Dacian,

I've got few newbies questions, if I may.

The spec from the 1st post actually says there are 120 calls on the panel (20×6). I would suggest that the panel consists of two 60 subpanels connected in series (so if I would see 2×60 in the spec I wouldn't be confused). Is it so?

The current vs voltage graph you refer to also says that with lower temperatures (let's take black line = sunny winter day) the voltage may be 32 or higher. Will it damage the batteries? How SBMS0 protects the 24V battery pack from overvoltage (which allows maximum 3.65×8 = 29.2V charge voltage)?

Thanks!

[Barry Timm]

The SBMS0 switches off charging as soon as the highest cell reaches 3.55v, thus 28.4v assuming perfect balance, but in reality a little below that if any unbalanced cells. My 24v battery cell variance remains at the default minimum for cell balancing, I think 11mV.

[Dacian Todea]

Dmitry,

All PV panels are split in groups of 20 to 24 cells in series each group has a bypass diode.

So  a typical 60 cell panel will be split in 3 equal parts of 20 cells and each of this groups will have one bypass diode so that if small part (even a single cell) of that one third of the panel is shaded the other two unaffected parts can still produce max current but this only works with larger series string panels and MPPT like you normally find in residential grid connected solar arrays.

Since that is made of two smaller 60 cell panels it will have a total of 6 diodes that is what they try to explain by 20x5

PV panels are constant current sources so when connected to a battery they will have the exact battery voltage no matter how many cells in series they have or what the ambient temperature is and the current will not be higher than short circuit current that is just slightly higher than max power point current. So no battery voltage will not be affected by the PV panel as PV panels are constant current sources.

Barry,

Yes in real life it will be less than 28.4V for an 8s LiFePO4 battery. Battery voltage will drop quite a bit as soon as the charging current is stopped to probably around 27V or less depending on Load. 

[Dmitry Katsubo]

Thank you Dacian, I am a step closer to the understanding why MPPT is not needed in the ideal setup.

Having said that PV is a constant current source, how SBMS0 protects the batteries from high current? The panels are pulling 10A, but SBMS0 is only capable to offload 0.5A during balancing. How the batteries are protected from not being overcharged?

[Dacian Todea]

Dimitry,

I guess you ask how are the individual cells not overcharged and that is easy as the SBMS will just stop the charging as soon as the highest cell gets to max set limit say 3.55V for LiFePO4 thus no cell will ever exceed 3.55V exactly.

[Dmitry Katsubo]

Thanks for the information, Dacian,

Again, sorry for dummy question. I have carefully inspected SBMS0 manual and tried to search in this group... but still I don't understand how SBMS disconnects the battery. Please point me to the appropriate resource where it is explained if you have one in mind. I thought that the magic is happening in DSSR20, but I am not sure, because if it happens in DSSR20 then it affects the current through the load / inverter.

Second if you say that charging stops if one of the cells reaches 3.55V, then there is a chance that another cell is undercharged so the pack is left unbalanced. Is that possible? If yes, does it make sense to use passive balancers that will dissolve current on the batteries which are already charged?

Thanks!

[Dacian Todea]

The SBMS0 will not disconnect the battery but it will need to be able to disconnect all loads and all chargers.

The DSSR20 is just a PV charger that can be turned ON or OFF by the SBMS0.  The SBMS0 needs to be able to control also the inverter and if you have DC loads it needs to be able to turn those OFF also with something like a Victron Battery Protect that has remote ON/OFF and so it can be controlled by the SBMS0

The cell balancing is done during charging and so it can be done for many hours each day if necessary and even if battery is unbalanced at initial connection it will become balanced withing a few days or weeks depending on how imbalanced it is and from there the battery will be kept in balance.

The so called passive balancers are completely useless and no it makes no sense to use those. 

[Dmitry Katsubo]

Dacian,

Am I right saying that SBMS40 = SBMS0 + DSSR20 + Battery Protect (e.g. Victron)? Or SBMS40 is also not able to disconnect the battery in case of overcharge?

[Demosthien]

On Tuesday, 30 June 2020 21:43:45 UTC+1, Dmitry Katsubo wrote:

Dacian,

Am I right saying that SBMS40 = SBMS0 + DSSR20 + Battery Protect (e.g. Victron)? Or SBMS40 is also not able to disconnect the battery in case of overcharge?

I'm not sure I understand your question properly but it looks like you're asking about the capabilities of the SBMS40 v those of the SBMS0?

[Dacian Todea]

Dmitry,

The Load+ on SBMS40 and other SBMSxxx models is designed for non capacitive DC loads so LED lights, water pumps other similar thing same as Victron Battery Protect.  If you have an inverter that will need to have remote ON/OFF and be directly connected to battery as both Load+ and Victron Battery protect will fail if connected to a large capacitive load like an inverter or large DC-DC converter.

But to also answer your question yes SBMS40 will be similar to a SBMS0 + 2x DSSR20 and something like a BP or just a SSR for non capacitive DC loads but the SBMS0 is more flexible as it can support up to 30kW PV array with multiple DSSR20 and it has the dual PV array option that SBMS40 can not support and also the charging being separated optically there is no chance of damaging the ISL94203 if you remove the cell balancing while PV panels are connected.

Also SBMS40 requires a heatsink to handle 40A PV charging where DSSR20 will not require any heatsink neither the SBMS0 making the SBMS0 front panel mount as all connectors are at the back.

[Jim Fowler]

Hi Dacian,

I am new to solar and trying to understand this myself as I build out a solar setup for my motorhome.  If what you said above about panels being constant current and therefore the voltage will match the battery, then why the need to match 60 cell panels to 24v battery?  Does that mean that I could use a 72/144 cell panel with my 24v battery?  If so, is there a disadvantage or advantage to doing that?  I have been debating between lower voltage and wattage 60 cell panels, or higher voltage and wattage 72 cell panels.  I have limited space on my small motorhome, so I was thinking the higher wattage 72 cell panels would allow me to get more charge from less panels.  Would I still be able to use DSSR20s with this setup, or do I need to go with an MPPT in this case?

I am also thinking about adding panels on top of a trailer that I often pull behind the motorhome.  This would allow me to have greatly expanded abilities as it is a 20ft trailer, but I don't always take it with me so I would need to be able to connect and disconnect it without difficulty or interfering with the system.  It seems like with the distance from where the batteries are compared to the trailer that I might be better going in series connection to get higher voltage to use smaller wire for this case?  Is that true, or would I still be better off using DSSR20s for this?

[Barry Timm]

Anything over 60 cell/120 half-cell panels is wasted as the higher voltage is never needed or used. Best to stick with 60 cell panels which provide sufficient voltage and around 20 amps per pair, unless you have REALLY long wires but in your RV application, it would be completely unnecessary. Best to maximize the number of 60 cell panels you can fit to the roof. FWIW, I replaced all the high fans and covers on my 5th wheel with much lower 4" high Maxxair models and made my own 5" tall panel mounts from C-section aluminum that has a 5" width on its side, cut into 3" sections. This allowed me to fit all my panels over the various vents and fans with zero shade and attain an easy 1890w of PV on my small 28.5 foot long 5th wheel. The system works PERFECTLY. I have so much spare power that I can provide power to a buddy's RV parked next to me. :-)

[Dacian Todea]

Jim,

As Barry already mentioned the 60 cell panels are just ideal and going to 72 cell panels you will have a panel that is 20% larger and 20% more expensive but the same amount of current put in to battery do no there will be no advantage in going for 72 cell panels.

Even if you go with an MPPT for the trailer it will be very inefficient to have more than two panels in series so you will still need to have multiple cables.

You can parallel two panels on a 10AWG PV wire and that will go to one DSSR20 so yes you will need to have multiple connections but is lower voltage so safer to handle.

It may be that the motorhome can fit enough panels for your needs depending on what those are.  

[Ben Marascalco]

I bought 4-72 cell panels before I knew this because they covered my trailer top perfectly and I thought the more the better. Just wondering if that extra unusable power will make up partially for loss on cloudy days maybe? 

[Dacian Todea]

Ben,

If you could only fit 4 panels either 60 or 72 cells then they will perform the same is just a bit of extra cost for the extra 12 cells in each panel.

There will be no advantage in any type of day but of course it will also not be worse.

[Barry Timm]

If you already have the 72 cell panels, I'd consider controlling them with two Victron MPPT 100/20 SCCs, with each SCC connected to two panels in series. It's around $356 for the two controllers, but at least you'd be using all of your PV potential with your limited roof space.

Debatable how much extra solar harvest you'd get compared to 4 x 60 cell panels managed with 2x DSSR20s for much less money, but I'd think you would see a few percent more harvest.....

[Ben Marascalco]

THanks.
I dont mind the extra money if they will collect what is available. I am using a Victron Multiplex and know that they are designed for marine use. If the mmpt's are the same the it probably better, because I will be spending a lot of time directly on the water with this system. I just want to use the new sbmso with this battery set up.