PV panels in parallel

[Gordon Bland]

Hi, currently on our caravan we have one 350w panel using a mppt controller to charge a gel battery. I'm shortly going to swap out the gel battery for a lifepo4 battery. So as part of the upgrade I'm considering an Electrodacus bms as well as considering panel matching so that I can remove the mppt controller.

These are the panels I'm considering. Seem to be 33 cell panels? There would be 3 of these used in parallel to just stay under the 20a limit for dssr20.

https://www.trademe.co.nz/motors/caravans-motorhomes/parts-accessories/solar-panels-accessories/listing-2634026698.htm

Maximum Power:120W (Pmax)
Maximum Power Voltage: 18.7 V (Vm)
Maximum Power Current: 6.41 A (Im)
Open Circuit Voltage: 22.4V (Voc)
Short Circuit Current: 6.69A (Isc)

My concern is that I would lose too much capacity without using mppt? I've watched your mppt video and seems to apply more to 24v than 12v?

Ballpark figures 3 x 120w panels = 360w less mppt losses. I'm after most output in winter so 5-10 degree Celsius temperatures. Using these in parallel would give me roughly 14v * 6.41a * 3 panels = 269.22w.

So would I be loosing almost 100w (25%) of solar output or is there something I haven't considered?

Thanks, Gordon

[Dacian Todea]

Gordon,

Those max power point values are for PV panel at 25C and for that to happen ambient temperature will need to be at around -5 to -10C

It is fairly unlikely those are 120W panels likely 100 to 110W and where they cheated is the max power point voltage.

Here is a mono panel with a proper spec https://www.renogy.com/template/files/Specifications/100-Watt-12-Volt-Monocrystalline-Solar-Panel-Specifications.pdf

Notice same size panel rated 100W but it is 36 cells instead of 33 cells thus relatively high max power point voltage as that is proportional with the number of cells.

So if you take that 18.9V / 36 cells = 0.525V per cell at max power point so 33 cells * 0.525V = 17.325V will be realistic for that 33 cell panel

Now also take a look at the IV graph in the renogy panel spec and see that you can go quite a bit left or right and you are still very close to the same max power point as the voltage decreases the current will increase so power is still very close to max power point and it will be even more so with a 33 cell PV panel.

A 4s LiFePO4 will be most of the time in the 13 to 13.5V and so in your mild winters with +5 to +10C during the day max power point of that 33 cell PV panel will be around 16V thus with DSSR20 in those conditions you will get about 90% efficiency so likely 90W from each panel as those are in reality 100W panels maybe 105W while with a quality MPPT you may get a bit more as good ones will be 92 to 93% efficient but 2 or 3W extra per panel is not going to make any difference and that is in winter in summer MPPT will perform worse than DSSR20

An MPPT will work at the higer 16 to 17V (below freezing) max power point voltage and slightly lower current while the DSSR20 will work at battery voltage (no choice there) with about short circuit current rating. Keep in mind that with both MPPT and DSSR20 there will also be a half a volt to one volt loss on the PV cables fuses and contacts.

So yes those 33 cell panels or even 32 cell ones will work best in winter and decent enough in summer with a 12V battery.

[Gordon Bland]

Thanks Dacian. I've listed my current solar system for sale today so hopefully it sells soon so that I can order the smbs0 and dssr20's etc. I have a couple of other questions however I'll ask them when I'm able to place an order.

[Gordon Bland]

Hi Dacian, I'm actually going to have 2 parallel pv strings and liked the look of the pv1, pv2, pv1 + pv2 figures. However I've since found out on the sbms0 user manual post that you mention

1. For now the unused PV2 and PV1+PV2 will stay as I use the same firmware for all models and since I need to test all models it will be confusing for me to see different screens.

My 2 strings will be 3 panels mounted flat on the caravan roof and the other string will be 2 portable panels able to be faced directly to the sun. The reason being that over winter with the sun being so low on the horizon the flat panels don't generate so much energy. Whereas panels square on apparently give another 30-40% output. So it would've been nice to see that quite possibly the 2 panels could outperform the 3 flat panels? Therefore I'll have to guesstimate I.e 4 amps being generated before string 2 connected and after connection now 11 amps being generated.

Another thing as string 2 will be used on an as needed basis I have an external 50A Anderson plug to connect the panels to. With the dssr20 and smbm0 left live is there any issue just plugging/unplugging the panels as required? I'm guessing the anderson plug could spark when being plugged in however wouldn't that imitate what switch contacts can do anyway?

Thanks

[Dacian Todea]

There is no problem for SBMS0 or DSSR20 to plug or unplug panels but the connector will get damaged so not a good idea.

You can just install a switch on the Remote signal of the DSSR20 connected to those external panels that way you can disconnect the DSSR20 before plugging or unplugging the panels and you will get no sparks thus not damage the connectors.  

[Gordon Bland]

I possibly can't install a switch in the dssr20 remote signal as I'm going to have both dssr20's controlled by 1 extio. The other 3 will be inverter, charger and water heater. So I could put a waterproof rocker switch in the cable connecting to the anderson plug and switch it on after plugging in.

I may be able to combine water heater and inverter on the same extio however water heater will need ssr control and inverter will need remote wires soldering across main switch. If these work together then the panel strings can have their own extio and I could then just install a switch on the Remote signal of the DSSR20 connected to those external panels.

[Dacian Todea]

Is not relevant that both DSSR20 are controlled by one EXT IOx as you have the switch only on one DSSR20 so only that DSSR20 will be affected by the manual switch.

[Barry Timm]

Not sure why you can't install individual switches for the DSSRs.... should be very simple?

[Gordon Bland]

On page 3 the wiring diagram is simplified so sorry I don't 100% understand it. The dssr20 has both pvp and pvn (p being positive and n being negative) terminals which correlate to the sbms0 extio terminals. E.g. extio3 using green, green/white wire. Can the extio3+ and - terminals accept 2 cat5 cables? If so then I'd just put the switch in one of the green wires. Otherwise green wire would go from extio3+ to first dssr20 pvp and green/white extio- to pvn. Then each wire would piggyback off to pvp and pvn on second dssr20. Wouldn't the switch then need to be between first and second dssr20 so only 1 disabled?

[Dacian Todea]

Gordon,

That is a simplified diagram but you can can have a common point and then run wires to each DSSR20 remote thus you can intrerupt just that wire that is going to to a specific DSSR20 all remotes are in parallel there is no serial connection.

Those DSSR20 in the SBMS0 user manual is the old model current DSSR20 has his own separate manual and is diffrent from the first version that is no longer available.

See page 7 figure b on how the new one is connected  https://electrodacus.com/DSSR20/DSSR20.pdf so if there where multiple DSSR20 theneach will have is own wire form the EXT IO4- to their Batt remote connection and if you wanted to disconnect any of them you will just add a switch in series with the DSSR20 you want to disconnect.

Same applies for the one with diversion where in series with Heat remote signal you can inset a thermostat to stop the heating if you got to max temperature.

[Gordon Bland]

Hi Dacian, I also watched the video where someone installs a dssr20. As the extio4- wire is cat5 or similar gauge cable then is it alright to use a light gauge wire for the battery negative as it's just completing the extio4 circuit?

[Dacian Todea]

Gordon,

No Battery- is not just for the EXT IOx circuit it is also for protection as it will clamp the energy stored in the PV cables when battery charging is stopped. You will want at least an 18AWG for the battery negative connection as that pules can be up to around 20A maybe a bit more but is just for a few ms way less than a second so wire will sure not get hot but 18AWG will be a good idea.

EXT IOx is less than 10mA so that can easily be 24AWG CAT5 wire.