How Close Is Close Enough With Series Panel Specs?

[RonH]

I am looking to upgrade my RV rooftop solar system from 200 W to 400 W. I presently have two Renogy 100 W panels on the back half of the trailer, wired in series to reduce line losses as much as possible. The Renogy panels are 1.5 years old. Their listed specs are Vmp=18.6V, Imp= 5.38 A, Voc=22.3 V, and Isc= 5.86 A.

I am adding two new HQST 100 W panels to the front half of the trailer. These panels’ specs are Vmp= 20.3 V, Imp= 4.93 A, Voc=24.3 V, and Isc= 5.34 A. My plan is to create two parallel strings of two panels in series. Because it will be difficult to relocate the two existing panels on the roof (taped on), and because of the way the roof is typically partially shaded at a campsite, it would be best if each two panel series consisted of one Renogy and one HQST. Are these two panels close enough in spec and age for this series string to operate efficiently? Alternatively, I could series pair the two Renogys together and the two HQSTs together, but because they would be on either side (port and starboard) of the trailer each panel in the series might not get full sun.

Any thoughts from the group on best way to go?

Second question:

Based on Will’s reviews, I really like the EG4 LL Server Rack 400 AH battery. However, this setup is for a boondocked RV in remote areas, and I really need to try to build in as much redundancy as possible so I am not SOL if something goes wrong. As a result, I am most likely going to go with two 200 AH RoyPow Power Urus batteries. This gives me two BMS’s and at least 200 AH should a battery go bad.

Any thoughts on how batteries fail? I mean it may not be any more reliable to have a two battery system if the way things typically fail is that some other part of the system takes out both batteries at once. How reliable are BMS’s and individual batteries these days?

Third (and last) question:

I fully understand low temp charging protection and possibly battery heaters. What about low temp discharging? Would consuming power from a charged battery at below freezing temperatures cause damage? How about simply a charged battery sitting in a below freezing condition? I am planning on my battery compartment being within the living space of my RV, but it would be possible for the entire RV to go cold.


Thanks in advance for any and all assistance.



BTW, I love Will’s site and videos. As another poster noted, “I wish I was excited about ANYTHING in life as Will is about batteries”! He’s been a real help. I was particularly impressed by the fire he started with the punctured Chins solid state cell. What would happen with a LiFePO4 chemistry, say if you punctured it or shot a bullet through it?

 

[SeaGal]

...What about low temp discharging? Would consuming power from a charged battery at below freezing temperatures cause damage?

Welcome! From most specs I've seen, then discharging down to -20C will not cause damage to a LiFePO4 cell, but the energy you'll get out of it compared to if it was at +20C will be somewhat diminished. I have seen figures of 25% less capacity mentioned, but others on here will no doubt have a better real-life experience on that.

 

[OTRwSolar]

I fully understand low temp charging protection and possibly battery heaters. What about low temp discharging? Would consuming power from a charged battery at below freezing temperatures cause damage?

0c is typically used as a go/no-go threshold (BMS low temp cut-off) for charging. However, you can safely charge at <0c if you reduce charge current to a safe level relative to the temperature of the cells. Important to note though, at some point, if cell temperatures get low enough any amount of charge current could potentially damage the cells.

Similar theme applies to discharging. The only difference is the go/no-go threshold for discharging is lower, typically around -20c or so.

What defines a "safe" level is open to some debate.

 

[SeaGal]

However, you can safely charge at <0c if you reduce charge current to a safe level relative to the temperature of the cells...

I disagree (= starting the debate ). Or rather I prefer not to deviate from manufacturers specification. See, for example EVE's recommendation for their LF280K cells is as shown in the table below = no charge below 0C.

 

[OTRwSolar]

As I mentioned, what defines a "safe" charge current, at a given cell temperature, varies based on the wants/needs of the end-user and manufacturer alike. You'll notice I did not mention a specific charge current just for this reason.

In any event, my reply was primarily directed at answering the OP's specific question about discharging in subfreezing temps, not to start another cold temp charge debate

Added edit: Have to wonder why more BMS manufacturers don't provide the capability to limit charge and discharge current based on several different temperatures, similar to the sliding voltage scale the BMS uses to determine SOC. The typical go/no-go method, using just one temperature, isn't the most efficient approach.

 

[SeaGal]

Added edit: Have to wonder why more BMS manufacturers don't provide the capability to limit charge and discharge current based on several different temperatures, similar to the sliding voltage scale the BMS uses to determine SOC. The typical go/no-go method, using just one temperature, isn't the most efficient approach.

Agree with that! I implemented exactly that feature with my DIY control system - see also my posting here...

Science based discussion on charging LiFePO4 below 32°F

It wouldn't be hard for an engineer designing a BMS to scale charge current with temperature and state of charge. Seems like it could be done in firmware. For example: 10A@-10°C, 15A@-5°C, 20A@0°C, 30A@5°C, 40A@10°C, 100A@20°C. This would allow them to better protect the battery from damage at...

diysolarforum.com

 

[HRTKD]

Added edit: Have to wonder why more BMS manufacturers don't provide the capability to limit charge and discharge current based on several different temperatures, similar to the sliding voltage scale the BMS uses to determine SOC. The typical go/no-go method, using just one temperature, isn't the most efficient approach.

Because most BMS are either on or off. There is no in between. If they added throttling that would likely increase the cost so much that it wouldn't be affordable anymore.

To Ron's question about the new panels, I say connect them up and see what happens. They're close enough that while you may see a slight deoptimization, it won't be much. If you don't like the outcome, put in a second solar charge controller for the new panels.

 

[OTRwSolar]

If they added throttling that would likely increase the cost so much that it wouldn't be affordable anymore.

Disagree. IMO all it would take is some additional code using existing hardware to implement (assumes a BMS has a cell temperature sensor).

No need to "throttle". All that's needed is current and cell temperature data, and the ability to turn the mosfets on and off. All available in most BMS's these days. Coding would be roughly equivalent to that used by a BMS to determine SOC based on voltage. Instead of 90%, 80%, 70%, 60%, etc. SOC equating to a given voltage, you'd have temperatures (for example 60c, 45c, 25c, 10c, 0c, -10c, -20c, etc.) corresponding to an X amount charge current and a Y amount of discharge current. If charge or discharge current exceeds amount X or Y, respectively, at a given temperature, a fault occurs, the mosfet opens, current goes to zero.

 

[RonH]

Because most BMS are either on or off. There is no in between. If they added throttling that would likely increase the cost so much that it wouldn't be affordable anymore.

To Ron's question about the new panels, I say connect them up and see what happens. They're close enough that while you may see a slight deoptimization, it won't be much. If you don't like the outcome, put in a second solar charge controller for the new panels.

Interestingly, if you series/parallel the four panels, just based on the math it doesn't seem to matter if the two dissimilar panels are the series strings or the identical panels are in series. At the end of the day (if you take the lowest panel I for the series pairs and the lowest panel V for the parallel strings, you end up with basically the same wattage. What I don't know is that if in the real world there is some other factor that makes dissimilar series or dissimilar parallel strings more problematic.

In my case, the sun is more likely to strike more evenly along one longitudinal side of the Airstream (rather than more evenly across the full stern or the full bow). This would suggest that my two panels in series both be on the same side so as not to shade a partial series string. This necessitates that my series strings be comprised of the dissimilar panels since my two existing panels are permanently mounted on either side of the stern and my two new panels will be mounted on either side of the bow. My two parallel strings will each have the exact same theoretical output, however they will undoubtedly be different in real world situations since one side of the trailer won't be receiving the direct sunlight.

If I do it the other way around, theoretically, I'll also lose substantial output due to both of the partially shaded series strings.

Thanks for your feedback!
Ron

 

[SeaGal]

Disagree. IMO all it would take is some additional code using existing hardware to implement (assumes a BMS has a cell temperature sensor).

No need to "throttle".

IMHO there are two relatively distinct functions of an 'inverter-connected-BMS'. One is to provide absolute protection of the cells, so it will itself immediately shut off charge or discharge current when certain conditions are met - the most important being cell over or under voltage and over or under temperature.

In addition to that basic and essential functionality, an 'inverter-connected-BMS' also sends data to the inverter such as state of charge. One of the other parameters it sends is max charge or discharge current. So to add throttling would not ...

"likely increase the cost so much that it wouldn't be affordable anymore."

... as it is just a few extra lines of code.

It has been discussed on this forum several times that FOX-ESS battery racks tell the inverter to reduce charge current (having seen UK installations do exactly that below 15C and again below 10C) and IIRC Pylontech also do that below about 5C.

So, as @OTRwSolar observed above, it seems strange that most stand-alone BMS's that you can buy don't have some sort of configurable parameters which can be set to reduce charge current depending on temperature and SOC.