Finally, initial Electrocacus DSSR vs MPPT side-by-side/RW results ... and they're SHOCKING! lol

[Dhowman]

Sorry, couldn't resist the screaming tabloid headline, but this is VERY encouraging for DSSR users and starts to address, what until now, has been a heated, albeit purely theoretical, debate:

Would have been great to see the overcast test results ... that's where most of the questions still reside, but think I heard him say that DSSRs edged out slightly the MPPT there too.

 

[Craig]

I really want to know what happens if you hook a SSR to a 350 watt panel and the other end up to a 24V battery regardless of the panel voltage. The most it would produce is 15 amps So if you have a 100 amp hour battery you would only be charging at .15C obviously you would have to pay close attention that you do not overcharge the battery . I think this is basically what electrodaucus SBMS does. Seems to me you would have more efficiency cutting out the middleman so to speak.

 

[KohalaJim]

As Dacian said, "no surprise" (lol)

Makes sense that panels matched to batteries naturally achieve MPPT intrinsically, and with the ideal diode design of the DSSR's there's virtually no power drop, unlike the small (but expected) power consumption of an MPPT controller. An MPPT controller gets closer to the DSSR20 performance with a higher voltage panel string, but still has it's overhead and losses.

Each has it's place, and you can still make individual cases for the series/parallel discussion and all these applicable methodologies.

 

[Dhowman]

I really want to know what happens if you hook a SSR to a 350 watt panel and the other end up to a 24V battery regardless of the panel voltage.

It's the "regardless of the panel voltage" bit that's the key to using the DSSR. You have to regard it to get the the natural power point matching of pairing 60-cell panels in parallel with a 24V battery (and all the DSSR does is turn that power on/off). Of course, if you need higher voltage due to a long cable run, MPPT would be best, but if you don't, buy 60-cell panels and hook 'em up in parallel to yur battery and control it all via SBMS0 via (up to 30) DSSR20s and totally avoid all the internal losses an MPPT will introduce that the DSSR doesn't. But, of course, I'm sure I'm not telling you anything you don't already know.

 

[Dhowman]

So, went back and re-watched the vid. Without some summary table at the end I was left trying to remember what his numbers were across all his tests so wrote 'em down this time:

Two 24V/250W panels in ...​	EPEver MPPT​	DSSR20​
SERIES (68.4V measured) - w loads	333W​	n.a.​
PARALLEL (33.3V measured) - w loads	309W​	325W (5% more)​
PARALLEL - no loads	294W​	328W (12% more)​

So, what can one say just looking at these #s? With matched panel/battery voltages (i.e. panels in parallel) DSSR is eking out about 5% more power. Under his no load test, that number jumped to about 12%.

The series test is what's interesting. Assuming all things were equal before/after he re-wired the panels, he was able to get about 2% more out of the MPPT. Don't know if one can read too much into that given his power measurements were jumping around that much from one second to the next. Let's call it a draw. What doesn't appear to be a draw, though, is the parallel MPPT vs DSSR output. Def looks like the MPPT needs a bigger voltage difference to be able to do what it does best, so hooking 'em up in series is best, but then, of course, you're more susceptible to partial shading on one taking the output of both panels down. Seems to make absolutely no sense to use an MPPT with 60 cell panels in parallel on a 24V battery. The overhead of the MPPT's internal workings is greater than any power point it can find to deliver any extra amps, so you wind up with less output than if you had nothing between panels and battery.

Would be really nice to see his results from overcast testing w panels in series. He seems to indicate that the DSSR did about 2-3% better in his overcast testing but don't know if that included the MPPT w panels in series compared to DSSR w panels in parallel or just the MPPT vs DSSR parallel results.

This is just whetting my apatite to this test myself cuz it has me wondering how, on an overcast day, my 2 panels in series hooked to an MPPT would perform compared to a DSSR with both of 'em in parallel. If my last 4 cells would just arrive, I could do that.

 

[gnubie]

Application makes all the difference.

I've had my eye on Dacian's projects from pretty much day one and they are nice indeed but horses for courses. If it's practical to have the load near the panels and use high current cabling, that's fine but if your load (batteries for example) are further away high array voltage means lighter cabling to the MPPT SCC. Take with one hand, give with the other. The cost of heavy gauge cable for a system with the load away from the array would more than cover the purchase of the MPPT controller and another panel or two for that matter if one is worried about power loss (and it's practical to fit another panel, horses for courses!).

 

[Dhowman]

Application makes all the difference.
...
if your load (batteries for example) are further away high array voltage means lighter cabling to the MPPT SCC. Take with one hand, give with the other. The cost of heavy gauge cable for a system with the load away from the array would more than cover the purchase of the MPPT controller and another panel or two for that matter if one is worried about power loss (and it's practical to fit another panel, horses for courses!).

...eee-yup ...

... if you need higher voltage due to a long cable run, MPPT would be best, but if you don't, buy 60-cell panels and hook 'em up in parallel to yur battery and control it all via SBMS0 via (up to 30) DSSR20s and totally avoid all the internal losses an MPPT will introduce that the DSSR doesn't.

 

[KohalaJim]

The test results here are what the PhD colleagues in my sphere would cite as "an n of 1".

We see by the results in the video that running the MPPT controller at a lower input voltage nets out as the most inefficient topology. Where I think you'd potentially see even larger disparity and losses with MPPT would be with higher voltage PV strings and lower voltage batteries.

A great @Will Prowse video subject I'd say (hint, hint).

 

[Dhowman]

I think to get anything really meaningful, you'd need to run a controlled side-by-side setup over a long period of time. It's no small undertaking. But the initial full sun testing and even the anecdotal overcast testing sure does seem to indicate what you might expect the result to be.

 

[KohalaJim]

Totally agree @Dhowman .
The disparities would compound even more over time.

 

[gnubie]

We see by the results in the video that running the MPPT controller at a lower input voltage nets out as the most inefficient topology. Where I think you'd potentially see even larger disparity and losses with MPPT would be with higher voltage PV strings and lower voltage batteries.

MPPT controllers are typically buck converters, one or two stage. Best of breed will be around peak 98% efficient (Victron rates their's as 98%). Higher current on the PV side of a MPPT controller will mean higher losses due to the effective resistance of PCB tracks, FETs etc so as an overall factor I'd imagine a decent MPPT controller will be more efficient with higher voltage and lower current on its PV input, of course this is just musing, I've not done any tests in this regard.

Hopefully I have read what you said correctly, and not that you are saying an MPPT would be better with higher PV voltage and the losses being the advantages of MPPT over PWM-ish operation, ie 'We lost to MPPT on this point'.

 

[KohalaJim]

MPPT controllers are typically buck converters, one or two stage. Best of breed will be around peak 98% efficient (Victron rates their's as 98%). Higher current on the PV side of a MPPT controller will mean higher losses due to the effective resistance of PCB tracks, FETs etc so as an overall factor I'd imagine a decent MPPT controller will be more efficient with higher voltage and lower current on its PV input, of course this is just musing, I've not done any tests in this regard.

Hopefully I have read what you said correctly, and not that you are saying an MPPT would be better with higher PV voltage and the losses being the advantages of MPPT over PWM-ish operation, ie 'We lost to MPPT on this point'.

Agree with you in §1, bucking converter for sure and the losses (albeit low) therein. What I'm attempting to describe would be efficiencies when you push the limits of the controller (highest PV string voltage charging lowest battery bank voltage)

In §2, it's not a comparison of PWM to MPPT, rather it's the comparison of the DSSR ideal-diode SSR vs. the MPPT at disparate limits over time.

 

[Dhowman]

In §2, it's not a comparison of PWM to MPPT, rather it's the comparison of the DSSR ideal-diode SSR vs. the MPPT at disparate limits over time.

Correct, there's nothing PWM-ish about DSSR ... it's constant current charging not constant voltage charging.

 

[jstushnoff]

Correct, there's nothing PWM-ish about DSSR ... it's constant current charging not constant voltage charging.

So from this video and your comment, it's the first I realized that Dacian's DSSR is not the same as a PWM...been looking at his system and others for over a year now! So, forgive the holes in my knowledge, but does that mean if my 60 cell panel hits the 32v potential it's rated for, I'll be applying all that voltage to the battery? Is this OK with Lithium as long as you're not exceeding the C rating with the current? How safe is this when you approach a full charge? (ie, doesn't the controller know when it's reaching full charge when the current drops to almost zero at the batteries max voltage?) I've been studying all this and experimenting for a while now, but obviously a bit to learn yet. Hate to think my expensive Victron was a waste, but with only space for 3 panels I want to get set up right.
Thanks

 

[gnubie]

Correct, there's nothing PWM-ish about DSSR ... it's constant current charging not constant voltage charging.

How is the current regulated?

 

[Dhowman]

So from this video and your comment, it's the first I realized that Dacian's DSSR is not the same as a PWM...been looking at his system and others for over a year now! So, forgive the holes in my knowledge, but does that mean if my 60 cell panel hits the 32v potential it's rated for, I'll be applying all that voltage to the battery? Is this OK with Lithium as long as you're not exceeding the C rating with the current?

Panels are constant current sources so when you connect them to your battery the panel voltage will automatically be the same as battery voltage and the amount of current will be proportional to the amount of sunlight hitting your panel. This becomes a very efficient set up when you intentionally match the voltage of your panels to the voltage of your battery. DSSR20's are just low resistance electronic switches with an ideal diode controller. Transfer efficiency at max 20A is 99.7% but your overall efficiency is derived from how close your panel and battery voltages are. All you're doing with this is designing with the hardware you choose what an MPPT does with it's software (continuously) but without the losses inherent to converting the voltage down from your panels, especially when those voltages are much higher than your battery. PV panel temperature impacts efficiency as well but to less extent.

Regarding how to manage your C rate, you can accomplish this with the # of DSSR20s you use. If you have a 200Ah battery and want a .5C max charge rate, limit the # of DSSR20s you use to 5. If you have the real estate for more panels, SBMS0 provides you with "Dual PV array functionality and SOC charge limiting." See the 3rd to last paragraph of this post for more on that and how you can optimize your charging for different light conditions by adding more than the # calculated above.

How safe is this when you approach a full charge? (ie, doesn't the controller know when it's reaching full charge when the current drops to almost zero at the batteries max voltage?) I've been studying all this and experimenting for a while now, but obviously a bit to learn yet. Hate to think my expensive Victron was a waste, but with only space for 3 panels I want to get set up right.

SBMS0 handles all this (see the rest of the post linked above for the "how"). I suspect that your Victron is NOT a waste but you may want to consider (if your MPPT and panel specs allow) series connecting those 3 panels to allow your MPPT to deliver the efficiency it was designed to provide. If your panels are 60-cell (24V nominal) and your battery is 24V (nominal), you may be better off going with DSSR20s if you want 'em in parallel, per results above. FYI, they're pretty cheap and now have ability to divert to a heating source when your battery's full and your panels are just sitting there with nothing else to do.

 

[Dhowman]

How is the current regulated?

See last post.

 

[gnubie]

OK, so not CC

I was going to make a few more observations, but I see that Dacian has already made them in the comments in that youtube clip.

The only thing that I disagree with is his statement that a MPPT charger throws out 6% to 8% of the input power as heat. Unless you buy bottom shelf products that simply doesn't hold true. That could easily be over 80 watts. You'd need a really big heatsink to cope with that passively, but yet the heatsink on my passively cooled MPPT controllers does not become burning hot, which it most certainly would under that sort of power load.

Victron rates their SCCs as 98% efficient, ie dropping 2% as heat. Given the temperature rise I see with them, that is quite believable. The couple of Renogy (SRNE OEM) SCCs I have been testing lately fall into the same category.

Is it a revelation that MPPTs like a bit of headroom to charge a battery? No. It's stated in the manuals for decent ones, along with voltages required to start charging, and maintain charging once started. It's certainly something that has been mentioned on this forum several times. MPPT controllers really make the life of the system designer much easier and systems more flexible by removing the necessity to keep the array Vmp near battery voltage.

This is not to say there is anything wrong with Dacian's approach, it's just another option for people to consider, and that has to be a good thing.

 

[KohalaJim]

OK, so not CC

I was going to make a few more observations, but I see that Dacian has already made them in the comments in that youtube clip.

The only thing that I disagree with is his statement that a MPPT charger throws out 6% to 8% of the input power as heat. Unless you buy bottom shelf products that simply doesn't hold true. That could easily be over 80 watts. You'd need a really big heatsink to cope with that passively, but yet the heatsink on my passively cooled MPPT controllers does not become burning hot, which it most certainly would under that sort of power load.

Victron rates their SCCs as 98% efficient, ie dropping 2% as heat. Given the temperature rise I see with them, that is quite believable. The couple of Renogy (SRNE OEM) SCCs I have been testing lately fall into the same category.

Is it a revelation that MPPTs like a bit of headroom to charge a battery? No. It's stated in the manuals for decent ones, along with voltages required to start charging, and maintain charging once started. It's certainly something that has been mentioned on this forum several times. MPPT controllers really make the life of the system designer much easier and systems more flexible by removing the necessity to keep the array Vmp near battery voltage.

This is not to say there is anything wrong with Dacian's approach, it's just another option for people to consider, and that has to be a good thing.

Indeed @gnubie, not CC, and more like "TC" (target current) ?

98% is a great efficiency for a switcher and I totally concur with the "design simplification" one gets with MPPT charge controllers. I can see where a less efficient controller can consume that additional power mentioned (6-8%) and therein lies the proverbial questions...

Not all MPPT's are created equal, and are there cases where an MPPT input/output range on the edge might consume additional power? That's the question I keep coming back to.

Options are also a good thing as you mention, and I think those who want more capability, granularity and transparency with their power systems get all of that with the Electro Dacus BMS / DSSR approach.

 

[gnubie]

Not all MPPT's are created equal, and are there cases where an MPPT input/output range on the edge might consume additional power? That's the question I keep coming back to.

Yep, agree completely. There are definitely poor quality SCCs out there that shed more of the array's power as heat but if one is considering the cheapie market I doubt the Dacian's efforts would be in the mix of things under consideration.

I have a Victron 100/50 controller being fed from an array with Vmp approx 60v charging a 24v lifepo4 bank and have had the input current up to ~25 amps which is where the Victron goes into limiting due to the output side hitting 50 amps. Although I didn't actually measure the efficiency the heat output of the unit was in line with the 98% efficiency Victron states. I can see that a cheapie may not perform particularly well with edge cases, but a decent product will handle it without problem. Your concerns about performance, at least when it comes to a decent SCC, aren't warranted IMO.

If I was to use a ED BMS / DSSR approach I'd have to reconfigure the array to be a large set of single panels in parallel and add panel(s) to cope with the ED pulling the panels off Vmp. That'd mean heavier cabling required along with racking for the panel(s), more fusing and/or diodes for the panels to protect against backfeeding should one panel develop a fault etc. It adds a fair bit complexity and a fair bit of expense.

The ED has its niche, so do MPPTs.