Solar System with DC Failover and Storage - Page 1
Forum section: electronics / general

author: Lukas52
author_rank: Contributor
timestamp: July 24, 2023, 09:09:58 am
content: Hey,I have already started a thread in a different subforum, but things turned a bit and now it is more fitting hereThe original topic:https://www.eevblog.com/forum/mechanical-engineering/dc-failover-for-solar-power/Short summery:I am building a Solar System to power some DC loads with (Server and 3D printers)For this i am using two Victron MPPT 250/70 Chargers and two 8S LiFePo4 batteries, each with there own Daly BMS.I chose this configuration because i need 24 Volt DC for my loads, so going 16S doesn't do me any good. While i would have saved some money on the chargers, i would have spend nearly as much in additional wiring and a DC to DC converter, so two 8S in parallel it is.As User rteodor pointed out my original plan of using relays to switch to some AC to DC power supplies if my battery gets low has some flaws, he suggested using diodes to prevent reverse charging and provide a smooth transition. Great idea imo.However im not sure what diodes to choose... A single big one comes with 600 mV drop, a bunch of smaller diodes in parallel could be done with only 400 mV...Given that i have two batteries in parallel and don't want to use a special control unit i think the smartest way to get a proper SoC is by using a something like the Victron SmartShunt, i talks to my Chargers and the chargers can switch a AC relay to power up my power supplies. Getting the SoC from the BMSs is possible but i don't think the chargers can deal with two inputs of that sort since they run effectively in parallel as well.If the SoC reaches ~30% start up the power supplies and gently switch over until the battery has recovered.Only two drawbacks i see with this method is that the battery would spend the about 4 months / year at that state since i will not be able to generate enough solar power from November till February. Also using diodes would mean i'm wasting power all the time.My usual load is around 100 - 150 A during the day and less than 50 during the night, with peaks into 250 A. Worst case Nominal load is about 200 A.I am using 280 Ah EVE Cells.What i already have are two power supplies (22-28 Volt) that support parallel operation and the two solar chargers. Battery cells have been ordered as well. Everything else is still in planning phaseAny tips regarding Diode choice/ things to watch out for when putting them in parallel?Is there maybe a brilliant third way of switching between Solar/Battery power and my mains PSUs that i haven't considered?Thank you

author: Lukas52
author_rank: Contributor
timestamp: July 28, 2023, 07:36:09 am
content: Just in case anybody else has a similar problem:I went with two VS-175BGQ030 in parallel.I did some searching on how to parallel lots of diodes to hopefully get an even lower voltage drop, but turns out the more you use the more matched they need to be, so two seemed like a good thing to do.They can handle my peak easily and usually run at less than half the specified current, so i should only drop about 400 mV instead of the 470 at rated current.Also this strange power tab package means i can screw them right into my bus bar, effectively using them as heat sinkRegarding the actual wiring:I did add a, lets call it contactor, that open the battery negative once the power supplies have mostly taken over to prevent further discharge of the battery during the winter months (its an RC car steering servo moving a physical switch, but i did add some checks to make sure it actually switched in the Arduino i used to make sure it actually switched over).Once everything is done and didn't burn my house down for a couple of months i will create a showcase type post for those interested

author: Siwastaja
author_rank: Super Contributor
timestamp: July 28, 2023, 10:59:04 am
content: Note that diodes work poorly in parallel, because they have negative tempco on Vf, which leads to thermal runaway: the diode which happens to have lower Vf takes most of the current, which makes it heat up more, which lowers its Vf even more, which makes it take even larger part of the current... and so on.This does not mean diodes could not be paralleled, obviously if you parallel two diodes when even one is enough then nothing bad can happen. Just don't expect 2x current rating, or even close, maybe 1.5x could be relatively safe assumption. Mounting the paralleled diodes as close as possible on shared heatsink helps, as does hand-picking the diodes for as close Vf (at the same temperature) as possible.At that power level, designing an "ideal diode" circuit would offer significant power savings, at the expense of more complicated design work. At 24V, I would probably just use diodes and accept the need for a heatsink.Note that at 8*3.65=29.2V, this diode's maximum reverse voltage rating, 30V, is awfully close. See if you could find a 40V rated, or more, diode with otherwise suitable specs. Also note this part has huge reverse current. At 30V and Tj=125degC, almost 0.5A! This is so much it's hardly a diode at all, I would call it "diode* *) terms and conditions apply". Now 30V*0.5A = 15W of dissipation from just reverse voltage so it will never start to block as it never cools down.

author: Lukas52
author_rank: Contributor
timestamp: July 28, 2023, 01:38:50 pm
content: The high reverse current is something i have noticed among all high current low voltage drop diodes. The higher the current rating the higher the reverse current rating... There seems to be some physical design limitations at play.The next step up in voltage would limit me to 60 A dual trench diodes, considering the mouser stock.If i understand those correctly they are effectively two diodes sharing a cathode. I would assume they are very closely matched within the package, but i would still need at least 4 in parallel to be on the save side... As you mentioned paralleling diodes ain't trivial.During the winter months my improvised contactor solves that for the most part i think, but it is a problem depending on the weather.I took another look at Solid State relays thinking that maybe they would be worth it over time, but they all have the same problems as diodes, at higher currents they dissipate a lot of power.My brother (car mechanic) pointed my towards the DCNLM Series of Contactors, apparantly they are used in cars and can handle starter currents of up to 400A, and at less than 100 Euro each they don't seem to be the worst option. Add some capacitors the the load side to keep the voltage up during switching and they should last quite some time. 10 Watts of holding power on the coil is by far the most efficient option yet i think.

author: Lukas52
author_rank: Contributor
timestamp: July 28, 2023, 01:58:53 pm
content: I made a quick doodle of how i should be able to do this with just one DC contactor. (The DC Contactor is the switch in between the to negative bus bars. The colored boxes are bus bars)Two things im not sure about:1. Negative potential. Can i assume they are the same if i go back from AC power to battery/solar?2. Will i damage my PSU by having the battery apply a voltage to it while not in use? I am trying to avoid having to buy two DC contactors.

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2023, 06:27:50 pm
content: Just based on price per performance, I would use this dual schottky diode module and wire both sections in parallel:https://www.mouser.com/ProductDetail/Vishay-Semiconductors/VS-QA250FA20?qs=5aG0NVq1C4wENjMzBcwwvQ%3D%3DUsing a higher current diode than necessary reduces the forward voltage drop.

author: Lukas52
author_rank: Contributor
timestamp: July 29, 2023, 09:22:58 am
content: Even at just 50 A each and a temperature somewhere between 25 and 125 °C this diodes would dissipate somewhere around 60 Watts... That would be more than 200 Euro in electricity a year... Not sure if that makes much senseThe Diodes i picked aren't that much better either to be honest... The idea with the contactor all of sudden looks like a much better way in the long run... 10 Watts of holding power will be hard to beat with conventional diodes.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2023, 02:37:56 am
content: An active diode using power MOSFETs with the source and drain reversed are feasible and would lower the forward voltage drop and power loss considerably, but require a drive circuit to control the gate.

author: Lukas52
author_rank: Contributor
timestamp: July 30, 2023, 08:37:46 am
content: I think i will go with a VS-400CNQ045 for know it seems to have the lowest Vfr of all diodes currently in stock and less reverse leakage than my previous suggestion.In the long run i will likely design my own ideal diode circuit to get better efficiency, since i haven't found anything of the shelf that would work, but given i have never designed a pcb before that will probably take some time i don't have right now

author: Siwastaja
author_rank: Super Contributor
timestamp: July 30, 2023, 09:25:33 am
content: Quote from: Lukas52 on July 30, 2023, 08:37:46 amThis part seems to have only very slightly negative Vf tempco, so paralleling works better, too. Going from 200A to 400A increases Vf by 0.17V, while going from 25degC to to 125degC decreases it by only 0.04V (average numbers of page 2 specifications). Therefore, the balancing effect of positive current->Vf coefficient swamps the negative tempco pretty well, and you get decent current sharing.Reverse current is still massive after about Tj=75degC so these parts require low ambient temperature and good heatsinking. These can thermally run away while in "blocking" state!For worst case 200A load and power dissipation of 200A*0.57V = 114W, assuming ambient temperature = 45degC and maximum Tj = 70degC, the heatsink should have Rth = 25K/114W - 0.095 K/W - 0.10 K/W = 0.024K/W, basically a nearly infinite heatsink. It would be a good idea to pay attention minimizing that Rth_case-to-heatsink value of "0.10 K/W typical". But maybe you could live with Ta=35degC and Tj=80degC or so.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2023, 02:45:08 pm
content: Quote from: Lukas52 on July 30, 2023, 08:37:46 amThe driver circuit is not complicated and could be done by hand. Most of the difficulty is layout and construction of the high current side, which at hundreds of amps becomes more like copper plumbing.

author: Lukas52
author_rank: Contributor
timestamp: July 31, 2023, 04:29:56 am
content: Quote from: Siwastaja on July 30, 2023, 09:25:33 amThank you for that! I never thought that the blocking state would cause thermal problems tbhMy ambient is lower (usually less than 30°C), so i might get a bit of a boost there, i also was going to use an old tower style cpu heatsink from a server (large enough base plate to cover two of those beasts) and just run a fan at low RPM. The heatsink is designed to deal with about 200 Watts. However CPU temps are usually below 100°C, so higher temperature -> more heat transfer -> more cooling ?With some proper thermal paste the case to heatsink value should be better than just screwing it down with a pad i would assume.The idea was to run two diodes (packages), with the legs in parallel, so each leg would only see ~100 A.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2023, 05:54:13 am
content: Fan cooling is optimal when you aim for low temperatures. Natural convection requires large temperature differences as the thermal resistance is not constant but gets better at higher temperature differences (in other words, is very poor with small differences).Fan-cooled heatsink is almost linear so easier to approximate: if the CPU runs at Tj = Ta+60degC at 200W, then it would be Tj = Ta+30degC at 100W. I would use the largest high-performance CPU cooler I can easily find, just in case.

author: f4eru
author_rank: Super Contributor
timestamp: August 02, 2023, 09:18:12 pm
content: why double a charger wit a PSU ?just make sure the charger can act as a PSU, and no need for a switch and other complication....