They are just relays. Each are rated for 20A and can be hooked up to any constant current source. There is no over current protection (and can not be protected externally). Whether you want to consider them as "effectively" PWM controllers is a matter of intense debate on this forum.
Electrodacus-based System Schematic (final version w detail)
- Thread starter Dhowman
- Start date
I'm still pondering the implications of the shift to the main battery fuse and battery shutoff switch being moved to the negative terminal of the battery.
As I understand it the best practice is always to fuse the + side as close to the source as possible. What protection do we lose by going to a negative main battery fuse, what new failure scenarios are introduced. How can they be mitigated?
I'm also thinking about chassis ground. I can't tell from your diagram where your chassis ground point is, I assume at the negative busbar, If you locate the fuse on the negative side of the battery, am I correct in assuming your ground point should be upstream (further from the battery) from the fuse.
Currently I'm thinking the next best thing to a fuse directly on the +battery post is a fuse on the -battery post, for the same reason it makes sense on the + battery post (it leaves no room for a short to bypass the fuse), but the major difference being that the fuse or switch opening on the negative end would is that it leaves the entire system in a state of 'high electrical potential' (probably not the correct term), which in naivety seems potentially dangerous
As I understand it the best practice is always to fuse the + side as close to the source as possible. What protection do we lose by going to a negative main battery fuse, what new failure scenarios are introduced. How can they be mitigated?
I'm also thinking about chassis ground. I can't tell from your diagram where your chassis ground point is, I assume at the negative busbar, If you locate the fuse on the negative side of the battery, am I correct in assuming your ground point should be upstream (further from the battery) from the fuse.
Currently I'm thinking the next best thing to a fuse directly on the +battery post is a fuse on the -battery post, for the same reason it makes sense on the + battery post (it leaves no room for a short to bypass the fuse), but the major difference being that the fuse or switch opening on the negative end would is that it leaves the entire system in a state of 'high electrical potential' (probably not the correct term), which in naivety seems potentially dangerous
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Changes applied (yellow-highlighted below). Click the pic in this link for a higher rez version of this update (w/o the change highlights).
1. Up-ed the PV breakers from 25 to 32A.
2. "Buck" to "Boost" change.
3. Put the shunt sense wires where they're supposed to go.
4. Spec'd the shunts themselves.
A little more on shunt-spec'g that I've learned in the last 24 hrs:
For the 60A I can expect coming over the PV Shunt on sunny days, a 100A:100mV shunt would allow me to use 67% of SBMS's 0-90mV measuring range. A 100A:75mV would only use 50%.
(90mV/100mV)*100A = 90A
i.e. current on this shunt that would use 100% of that 90mV range is 90A
60A/90A = 67%
i.e. current on this shunt that would use 100% of that 90mV range is 90A
60A/90A = 67%
That # for a 100A:75mV shunt is only 50%. For a 90mV sensing range, that's the best I'm going to get out of any commercially available shunt (that I can find).
Same math applies for the 200A Load shunt. For my expected 120A max load, 100mV will allow me to use, again, 67% (vs 50% for the 75mV shunt). That the #s are the same is due to the fact that both rated current of the shunt and my expected max load are just coincidentally twice the #s for the PV shunt.
I can get both shunts here and both in the same dimensions, which should make them easy to place/connect (& look nice too!).
Because these can get hot, especially if you're pushing their rated amp spec, I was curious how to derate them if you're starting off in a hot climate. I intend to spend not a little time in the desert. Death Valley is one of my favorite destinations.
Temperature Derating (source): If operated in ambient temps above +25°C, the maximum continuous current must be further derated from the 2/3 value previously noted.
To find the maximum permissible continuous current (Ie) at an elevated temperature (Te), one must first calculate the maximum allowable power dissipation (Pe) at Te, using the following formulas:
Pe = Pa x [1-(Te-25°C)/100°C]
Ie = SR(Pe/R)
Ie = SR(Pe/R)
Where:
SR = Square Root
Ie = Maximum permissible continuous current at elevated temperature Te
Pe = Maximum power dissipation at the elevated temperature Te
R = Shunt’s resistance
Pa = 0.667 x shunt’s rated power at 25°C ambient (W=V*A)
Te = Elevated temperature
At 100F ambient, my 100A shunt is derated to 77A. My 200A shunt is derated to 154A. So, good to go.
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Maybe the fuse takes longer to blow? Failure scenarios covered here would also be covered having it on the NEG side, me thinks. Better than no fuse at all, no? Which I've seen some do, but I'd prefer both fuse and switch in a vehicle mounted system. I know in some jurisdictions, an exterior disconnect is mandatory on vehicles. Ditto for panel disconnect if output meets a certain threshold.
Decided not to ground to truck or camper chassis when I jettisoned notion of an earth ground. Everything you see in that diagram is isolated from truck and camper chassis.
As long as it stays away from battery NEG/GRND. I plan to fuse AT the battery terminals. Again, better than no fuse anywhere (except maybe between cells in the battery bank, which I've also seen ... Dacian informs me it will throw off ability of SBMS to accurately monitor/balance cell voltage).
Changes applied (yellow-highlighted below). Click the pic in this link for a higher rez version of this update (w/o the change highlights).
View attachment 10357
1. Up-ed the PV breakers from 25 to 32A.
2. "Buck" to "Boost" change.
3. Put the shunt sense wires where they're supposed to go.
4. Spec'd the shunts themselves.
A little more on shunt-spec'g that I've learned in the last 24 hrs:
For the 60A I can expect coming over the PV Shunt on sunny days, a 100A:100mV shunt would allow me to use 67% of SBMS's 0-90mV measuring range. A 100A:75mV would only use 50%.
(90mV/100mV)*100A = 90A
i.e. current on this shunt that would use 100% of that 90mV range is 90A
60A/90A = 67%
That # for a 100A:75mV shunt is only 50%. For a 90mV sensing range, that's the best I'm going to get out of any commercially available shunt (that I can find).
Same math applies for the 200A Load shunt. For my expected 120A max load, 100mV will allow me to use, again, 67% (vs 50% for the 75mV shunt). That the #s are the same is due to the fact that both rated current of the shunt and my expected max load are just coincidentally twice the #s for the PV shunt.
I can get both shunts here and both in the same dimensions, which should make them easy to place/connect (& look nice too!).
Because these can get hot, especially if you're pushing their rated amp spec, I was curious how to derate them if you're starting off in a hot climate. I intend to spend not a little time in the desert. Death Valley is one of my favorite destinations.
Temperature Derating (source): If operated in ambient temps above +25°C, the maximum continuous current must be further derated from the 2/3 value previously noted.
To find the maximum permissible continuous current (Ie) at an elevated temperature (Te), one must first calculate the maximum allowable power dissipation (Pe) at Te, using the following formulas:
Pe = Pa x [1-(Te-25°C)/100°C]
Ie = SR(Pe/R)
Where:
SR = Square RootIe = Maximum permissible continuous current at elevated temperature TePe = Maximum power dissipation at the elevated temperature TeR = Shunt’s resistancePa = 0.667 x shunt’s rated power at 25°C ambient (W=V*A)Te = Elevated temperature
My 100A shunt is derated to 77A. My 200A shunt is derated to 154A. So, good to go.
Looks like you made a good amount of progress towards understanding the workings of shunts.
I have one pretty basic question that I don't think you covered. What is the advantage of using more of a shunts measuring range? My assumption is that the more the range you use the more granular/precise the measurement?
Not sure if this will be helpful for you or not, but I figured I post it here for you or for others building with the SBMS.
Since learning that no disconnect devices (switches, fuses, breakers) should be between the positive terminal of the battery and the shunts, I've been pondering how best to change my design to meet this limitation while still meeting my goals and sketching out different options. My goals are pretty similar to yours I think, off the top of my head they are:
Below are 3 options I've sketched out:
Since learning that no disconnect devices (switches, fuses, breakers) should be between the positive terminal of the battery and the shunts, I've been pondering how best to change my design to meet this limitation while still meeting my goals and sketching out different options. My goals are pretty similar to yours I think, off the top of my head they are:
- Circuit protection inline with best practices for my application
- Each main circuit should have a manual resetting circuit breaker
- High interrupt, high current main battery fuse located as close to the positive battery terminal as possible
- Ability to quickly and manually/mechanically isolate the battery from the rest of the system
- Ability to quickly isolate the PV array from the rest of the system.
Below are 3 options I've sketched out:
Thanks. Yes, I've got all 3 incorporated into my design if you include the breakers. Gonna see if I can design (in my CAD drawing) putting the shunts right on the POS battery terminal w fuse and disconnect immediately downstream. Now that I've got dims for shunts, that's do-able. Just don't want some Rube Goldberg looking arrangement hanging off the POS battery terminal that might pose more problems than they solve.Not sure if this will be helpful for you or not, but I figured I post it here for you or for others building with the SBMS.
Since learning that no disconnect devices (switches, fuses, breakers) should be between the positive terminal of the battery and the shunts, I've been pondering how best to change my design to meet this limitation while still meeting my goals and sketching out different options. My goals are pretty similar to yours I think, off the top of my head they are:
I'm still a bit perturbed by point 3 (and by extension point 1)
- Circuit protection inline with best practices for my application
- Each main circuit should have a manual resetting circuit breaker
- High interrupt, high current main battery fuse located as close to the positive battery terminal as possible
- Ability to quickly and manually/mechanically isolate the battery from the rest of the system
- Ability to quickly isolate the PV array from the rest of the system.
Below are 3 options I've sketched out:
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Well the main difference between your design (and option A in the above post), and option B and C is that option B and C keep the main manual shutoff switch(es) on the positive side may offer some safety advantages.
I like this idea, I'm excited to see what you come up with.
SamCanadaRVsolarguy
New Member
- Joined
- Apr 7, 2020
- Messages
- 1
I'm new here, and your diagram is perfect for me. is there any chance you can describe the changes you made to make more of your system and accessories DC.that video was pretty valuable insofar as it's helped me to move about half of all my stuff to DC.
Unfortunately, Will took the vid down that described how best to do this. And it's all my fault. AND, did a quick search and can't find anything that's nearly as good (@Will Prowse ?). Soooo ... here goes:
- If it's a device that relies on an AC adapter (usually referred to as a "brick", cuz it kind of looks like one and is about as convenient to lug around as one) that has DC specs for Output printed on the back, then the part that plugs into your appliance is just delivering DC current and you can just cut that DC cord off the adapter (e.g. all laptops and some TVs and computer monitors just take DC at the back of 'em ... if they don't, it's because the manufacturer converts the AC to DC INSIDE the device, so unless you want to dig into your appliance to do this, you may be out of luck).
- Attach something like an XT90 connector to the 2 fresh ends you just created, making sure that POS wire on one still connects to POS wire on the other when you plug them back together. XT90s are keyed, so any time you plug them back together (to use if you need/want to go back to powering from an outlet), you're sure to be connecting POS to POS and NEG to NEG.
- Now plug that brick into an outlet and measure the DC voltage at the XT90 that's connected to the brick. This is the DC voltage you'll need to supply at the XT90 on the DC line that goes to your appliance when you hook it up to a DC source (battery).
- If that voltage matches the voltage of your battery, you're done. Just connect that XT90 to a similarly terminated wire coming directly from the battery (again, XT90 terminating that other wire to insure POS to POS and NEG to NEG).
- If that voltage is higher or lower, you'll need to get a cheap/small boost or buck (respectively) converter to step up or down (respectively) the voltage you supply to the DC line you cut off your brick (some converters allow you to go up ANd down ... "buck/boost" converters).
- There are LOTS of videos on those converters that explain how to pick the right one and adjust the output voltage on the converter. Find one that will work for you, attach a pair of XT90-terminated wires to the input and output of the converter (again, making sure that you attach them so that the POS input and output on the converter match the POS wire from your battery and the POS wire to your appliance.
- Connect the converter to the wire from your battery and measure/adjust the voltage on so that its output side matches what you measured at the XT90 attached to your brick when it was plugged into AC.
- Once you've done that, you can plug your device into the output of the converter and you're done. No more AC.
Ugh. Anything in that red box shorts and I'm toast. Just no way to protect those shunts and they're huge, with big posts and lots of exposed conductive surfaces. And they're suspended right over the battery box. Anytime I had that acrylic top off would be a disaster waiting to happen (e.g. connecting/disconnecting a NEG bus bar on a cell and the back end of my socket wrench bumps one of those?). Plus, this is in a vehicle, so damage scenarios that might cause that are not hard to imagine. IMO, just not worth the tradeoff. Would rather fuse at the NEG (left) side right at the posts and under that acrylic top with the disconnect right above that on the wall.
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Because it seems like you have devoted more time than most to thinking through and designing safety into your system, I'm wondering if you have any thoughts on ground fault protection. Its something I've been intending to devote some time to learning about, but so far haven't found the time.
Apparently since 2008, NEC code has required PV arrays to incorporate a ground fault protection device (GFPD) such as this one from midnite solar, but I haven't heard much talk about it, and I haven't come across anyone here building GFP into their systems (other than GFCI outlets on the AC side).
Do you have any thoughts on this class of safety device? Did you consider adding GFP to your system at any point in your planning?
Apparently since 2008, NEC code has required PV arrays to incorporate a ground fault protection device (GFPD) such as this one from midnite solar, but I haven't heard much talk about it, and I haven't come across anyone here building GFP into their systems (other than GFCI outlets on the AC side).
Do you have any thoughts on this class of safety device? Did you consider adding GFP to your system at any point in your planning?
Wiring Unlimited has a section (7.3) on this (mostly geared towards AC) but not entirely, it has some good conceptual info
ghostwriter66
"Here - Hold my Beer"
ghostwriter66
"Here - Hold my Beer"
Seriously -- I would give my left arm if our designers could remotely be as succinct and detailed as you are -- there are so many times we are out here in the middle of nowhere trying to install something looking at a diagram some corporate person has developed and the most often used phrase is "what the hell do you think this square is suppose to be?" .. followed by "maybe we have the diagram upside down" .. LOL
Hah!
Seriously though, this was all pretty new to me when I joined the forum (w only distant memories of Physics 101), but having to put this together on paper has really helped me understand the concepts y'all talk about so matter-of-fact-ly. Don't think I could have done one without the other.
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Try turning off your ad blocker. Went from grey canvas to the Dhowman's image after shutting off uBlock and refreshing.
mapguy525
Solar Enthusiast
What fuse type is being used. Mega, ANL, or Class T?
Mega and ANL fuses are "slow reacting". Class T is "fast" reacting.
In installations with inverters that can draw large amounts of current, a catastrophic event fuse should be on the main positive battery connection. This way it protects everything from the fuse output point to the negative battery post. Fuses, only on the negative side work but don't protect the entire wiring/components system, as completely in a catastrophic event.
Also, I only use name brand fuses due to the plethora of inexpensive unrated/unknown origin fuses in the marketplace that are cheap. Fuses (over current protection), IMO, is a questionable place to try "saving" money due to risk.
ANL fuses put under extreme duress can have the window blow out. I have only seen it happen once. Once is enough for me to avoid ANL fuses for catastrophic protection. This issue is also identified on the Marine How To LFP web pages.
The following is text pulled from a Magnum Inverter owners manual regarding fuses:
"If using a fuse, we recommend using a Class-T type or equivalent. This fuse type is rated for DC operation, can handle the high short circuit currents, and allows for momentary current surges from the inverter without opening."
One thing I have noticed over the years, Low end and low-mid level inverter owners manuals are very slim on over current protection guidance.
