computing

Solar Cluster: Considering options for over-discharge protection

Right now, the cluster is running happily with a Redarc BCDC-1225 solar controller, a Meanwell HEP-600C-12 acting as back-up supply, a small custom-made ATTiny24A-based power controller which manages the Meanwell charger.

The earlier purchased controller, a Powertech MP-3735 now is relegated to the function of over-discharge protection relay.  The device is many times the physical size of a VSR, and isn’t a particularly attractive device for that purpose.  I had tried it recently as a solar controller, but it’s fair to say, it’s rubbish at it.  On a good day, it struggles to keep the battery above “rock bottom” and by about 2PM, I’ll have Grafana pestering me about the battery slipping below the 12V minimum voltage threshold.

Actually, I’d dearly love t rip that Powertech controller apart and see what makes it tick (or not in this case).  It’d be an interesting study in what they did wrong to give such terrible results.

So, if I pull that out, the question is, what will prevent an over-discharge event from taking place?  First, I wish to set some criteria, namely:

  1. it must be able to sustain a continuous load of 30A
  2. it should not induce back-EMF into either the upstream supply or the downstream load when activated or activated
  3. it must disconnect before the battery reaches 10.5V (ideally it should cut off somewhere around 11-11.5V)
  4. it must not draw excessive power whilst in operation at the full load

With that in mind, I started looking at options.  One of the first places I looked was of course, Redarc.  They do have a VSR product, the VS12 which has a small relay in it, rated for 10A, so fails on (1).  I asked on their forums though, and it was suggested that for this task, a contactor, the SBI12, be used to do the actual load shedding.

Now, deep inside the heart of the SBI12 is a big electromechanical contactor.  Many moons ago, working on an electric harvester platform out at Laidley for Mulgowie Farming Company, I recall we were using these to switch the 48V supply to the traction motors in the harvester platform.  The contactors there could switch 400A and the coils were driven from a 12V 7Ah battery, which in the initial phases, were connected using spade lugs.

One day I was a little slow getting the spade lug on, so I was making-breaking-making-breaking contact.  *WHACK*… the contactor told me in no uncertain terms it was not happy with my hesitation and hit me with a nice big back-EMF spike!  I had a tingling arm for about 10 minutes.  Who knows how high that spike was… but it probably is higher than the 20V absolute maximum rating of the MIC29712s used for power regulation.  In fact, there’s a real risk they’ll happily let such a rapidly rising spike straight through to the motherboards, frying about $12000 worth of computers in the process!

Hence why I’m keen to avoid a high back-EMF.  Supposedly the SBI12 “neutralises” this … not sure how, maybe there’s a flywheel diode or MOV in there (like this), or maybe instead of just removing power in a step function, they ramp the current down over a few seconds so that the back-EMF is reduced.  So this isn’t an issue for the SBI12, but may be for other electromechanical contactors.

The other concern is the power consumption needed to keep such a beast activated.  The other factor was how much power these things need to stay actuated.  There’s an initial spike as the magnetic field ramps up and starts drawing the armature of the contactor closed, then it can drop down once contact has been made.  The figures on the SBI12 are ~600mA initially, then ~160mA when holding… give or take a bit.

I don’t expect this to be turned on frequently… my nodes currently have up-times around 172 days.  So while 600mA (7~8W at 12V nominal) is high, that’ll only be for a second at most.  Much of the current will be holding current at, let’s call it 200mA to be safe, so about 2~3W.

That 2-3W is going to be the same, whether my nodes collectively draw 10mA, 10A or 100A.

It seemed like a lot, but then I thought, what about a SSR?  You can buy a 100A DC SSR like this for a lot less money than the big contactors.  Whack a nice big heat-sink on it, and you’re set.  Well, why the heat-sink?  These things have a voltage drop and on resistance.  In the case of the Jaycar one, it’s about 350mV and the on resistance is about 7mΩ.

Suppose we were running flat chat at our predicted 30A maximum…

  • MOSFET switch voltage drop: 30A × 350mV = 10.5W
  • Ron resistance voltage drop: (30A)² × 7mΩ = 6.3W
  • Total power dissipation: 10.5W + 6.3W = 16.8W OUCH!

16.8W is basically the power of an idle compute node.  The 3W of the SBI12 isn’t looking so bad now!  But can we do better?

The function of a solid-state relay, amongst other things, is to provide electrical isolation between the control and switching components.  The two are usually galvanically isolated.  This is a feature I really don’t need, so I could reduce costs by just using a bare MOSFET.

The earlier issues I had with the body diode won’t be a problem here as there’s a definite “source” and “load”, there’ll be no current to flow out of the load back to the source to confuse some sensing circuit on the source side.  This same body diode might be an issue for dual-battery systems, as the auxiliary battery can effectively supply current to a starter motor via this body diode, but in my case, it’s strictly switching a load.

I also don’t have inductive loads on my system, so a P-channel MOSFET is an option.  One candidate for this is the Infineon AUIRFS3004-7P.  The Ron on these is supposedly in the realm of 900µΩ-1.25mΩ, and of course, being that it’s a bare MOSFET and not a SSR, there’s no voltage drop.  Thus my power dissipation at 30A is predicted to be a little over 1W.

There are others too with even smaller Ron values, but they are in teeny tiny 5mm square surface-mount packages.  The AUIRFS3004-7P looks dead-buggable, just bend up the gate pin so I can solder direct to it, and treat the others as single “pins”, then strap the sucker to a big heatsink (maybe an old PIII heatsink will do the trick).

I can either drive this MOSFET with something of my own creation, or with the aforementioned Redarc VS12.  The VS12 still does contain a (much smaller) electromechanical relay, but at 30mA (~400mW), it’s bugger all.

The question though was what else could be done?  @WIRING_SOLUTIONS suggested some units made by Victron Energy.  These do have a nice feature in that they also have over-voltage protection, and conveniently, it’s 16V, which is the maximum recommended for the MIC29712s I’m using.  They’re not badly priced, and are solid-state.

However, what’s the Ron, what’s the voltage drop?  Victron don’t know.  They tell me it’s “minimal”, but is that 100nV, 100mV, 1V?  At 30A, 100mV drop equates to 3W, on par with the SBI12.  A 500mV drop would equate to a whopping 15W!

I had a look at the suppliers for Victron Energy products, and via those, found a few other contenders such as this one by Baintech and the Projecta LVD30.  I haven’t asked about these, but again, like the Victron BatteryProtect, neither of these list a voltage drop or Ron.

There’s also this one from Jaycar, but given this is the same place that sold me the Powertech MP-3735, and sold me the original Powertech MP-3089, provided a replacement for that first one, then also replaced the replacement under RMA.  The Jaycar VSR also has practically no specs… yeah, I think I’ll pass!

Whitworths marine sell this, it might be worth looking at but the cut-out voltage is a little high, and they don’t actually give the holding current (330mA “engage” current sounds like it’s electromechanical), so no idea how much power this would dissipate either.

The power controller isn’t doing a job dissimilar to a VSR… in fact it could be repurposed as one, although I note its voltage readings seem to drift quite a lot.  I suspect this is due to the choice of 5% tolerance resistors on the voltage sensing circuit and my use of the ~1.1V internal voltage reference.  The resistors will drift a little bit, and the voltage reference can be anywhere from 1.0 to 1.2V.

Would a LM311N with good quality 1% resistors and a quality voltage reference be “better”?  Who knows?  Maybe I should try an experiment, see if I can get minimal drift out of a LM311N.  It’s either the resistors, the voltage reference, or a combination of the two that’s responsible for the power controller’s drift.

Perhaps I need to investigate which is causing the problem and see what can be done in the design to reduce it.  If I can get acceptable results, then maybe the VS12 can be dispensed with.  I may be able to do it with another ATTiny24A, or even just a simple LM311N.

2018/11/10 by Computing Projects Public Syndication Solar-powered Cloud Computing Thinktank 0

Solar Cluster: Reverting back to the Powertech MP-3735

So, for the past few weeks I’ve been running a Redarc BCDC-1225 solar controller to keep the batteries charged.  I initially found I had to make my little power controller back off on the mains charger a bit, but was finally able to prove conclusively that the Redarc was able to operate in both boost and float modes.

In the interests of science, I have plugged the Powertech back in.  I have changed nothing else.  What I’m interested to see, is if the Powertech in fact behaves itself, or whether it will go back to its usual tricks.

The following is the last 6 hours.

Next week, particularly Thursday and Friday, are predicted to have similar weather patterns to today. Today’s not a good test, since the battery started at a much higher voltage, so I expect that the solar controller will be doing little more than keeping the battery voltage up to the float set-point.

For reference, the settings on the MP-3735 are: Boost voltage 14.6V, Float voltage 13.8V. These are the recommended settings according to Century’s datasheets for the batteries concerned.

Interestingly, no sooner do I wire this up, but the power controller reaches for the mains. The MP-3735 definitely likes to flip-flop. Here’s a video of its behaviour shortly after connecting up the solar (and after I turned off the mains charger at the wall).

Now looking, it’s producing about 10A, much better than the 2A it was doing whilst filming.  So it can charge properly, when it wants to, but it’s intermittent, and inside you can sometimes hear a quiet clicking noise as if it’s switching a relay.  At 2A it’s wasting time, as the cluster draws nearly 5× that.

The hesitation was so bad, the power controller kicked the mains charger in for about 30 minutes, after that, the MP-3735 seems to be behaving itself.  I guess the answer is, see what it does tomorrow, and later this week without me intervening.

If it behaves itself, I’m happy to leave it there, otherwise I’ll be ordering a VSR, pulling out the Powertech MP-3735 and re-instating the Redarc BCDC-1225 with the VSR to protect against over-discharge.

Update 2018-10-28… okay, overcast for a few hours this morning, but by 11AM it had fined up.  The solar performance however was abysmal.

Let’s see how it goes this week… but I think I might be ordering that VSR and installing the Redarc permanently now.

Today’s effort:

Each one of those vertical lines was accompanied by a warning email.

2018/10/27 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Rescuing a btrfs partition from a failed “move”

Today it seems, the IT gremlins have been out to get me.  At my work I have a desktop computer (personal hardware) consisting of a Rysen 7 1700, 16GB RAM, a 240GB Intel M.2 SATA SSD (540 series) and a 4TB Western Digital HDD.

The machine has been, pretty reliable, not rock-solid, in particular, compiling gcc sometimes segfaulted for reasons unknown (the RAM checks out okay according to memtest86), but for what I was doing, it mostly ran fine.  I put up with the minor niggles with the view of solving those another day.  Today though, I come in and find X has crashed.

Okay, no big deal, re-start the display manager, except that crashed too.

Hmm, okay, log in under my regular user account and try startx:  No dice, there’s no room on /.

Ahh, that might explain a few things, we clean up some log files, truncate a 500MB file, manage to free up 50GB (!).

The machine dual-boots two OSes: Debian 9 and Gentoo.  It’s been running the latter for about 12 months now, I used Debian 9 to get things rolling so I could use the machine at work (did try Ubuntu 16.04, but it didn’t like my machine), and later, used that to get Gentoo running before switching over.  So there was a 40GB partition on the SSD that had a year-old install of Debian that wasn’t being used.  I figured I’d ditch it, and re-locate my Gentoo partition to occupy that space.

So I pull out an Ubuntu 18.04 disc, boot that up, and get gparted going.  It’s happily copying, until WHAM, I was hit with an I/O error:

Failed re-location of partition (click to enlarge)

Clicking any of the three buttons resulted in the same message.  Brilliant.  I had just copied over the first 15GB of the partition, so the Debian install would be hosed (I was deleting it anyway), but my Gentoo root partition should still be there intact at its old location.  Of course the partition table was updated, so no rolling back there.  At this point, I couldn’t do anything with the SSD, it had completely stalled, and I just had to cut my losses and kill gparted.

I managed to make some room on the 4TB drive shuffling some partitions around so I could install Ubuntu 18.04 there.  My /home partition was btrfs on the 4TB drive (first partition), the rest of that drive was LVM.  I just shrank my /home down by 40GB and slipped it in there.  The boot-loader didn’t install (no EFI partition), but who cares, I know enough of grub to boot from the DVD and bootstrap the machine that way.  At first it wouldn’t boot because in their wisdom, they created the root partition with a @ subvolume.  I worked around that by making the @ subvolume the default.

Then there was momentary panic when the /home partition I had specified lacked my usual files.  Turned out, they had created a @home subvolume on my existing /home partition.  Why? Who knows?  Debian/Ubuntu seem to do strange things with btrfs which do nothing but complicate matters and I do not understand the reasoning.  Editing /etc/fstab to remove the subvolume argument for /home and re-booting fixed that.

I set up a LVM volume that would receive a DD dump of the mangled partition to see what could be saved.  GNU’s ddrescue managed to recover most of the raw partition, and so now I just had to find where the start was.  If I had the output of fdisk -l before I started, I’d be right, but I didn’t have that foresight.  (Maybe if I had just formatted a LVM volume and DD’d the root fs before involving gparted?  Never mind!)

I figured there’d be some kind of magic bytes I could “grep” for.  Something that would tell me “BTRFS was here”.  Sure enough, the information is stashed in the superblock.  At 0x00010040 from the start of the partition, I should see the magic bytes 5f 42 47 52 66 53 5f 4d.  I just needed to grep for these.  To speed things up I made an educated guess on the start-location.  The screenshot says the old partition was about 37.25GB in size, so that was a hint to maybe try skipping that bit and see what could be seen.

Sure enough, I found what looked to be the superblock:

root@vk4msl-ws:~# dd if=/dev/mapper/scratch-rootbackup skip=38100 count=200 bs=1M | hexdump -C | grep '5f 42 48 52 66 53 5f 4d'
02e10040  5f 42 48 52 66 53 5f 4d  9d 30 0d 02 00 00 00 00  |_BHRfS_M.0......|
06e00040  5f 42 48 52 66 53 5f 4d  9d 30 0d 02 00 00 00 00  |_BHRfS_M.0......|
200+0 records in
200+0 records out

Some other probes seem to confirm this, my quarry seemed to start 38146MB into the now-merged partition.  I start copying that to a new LVM volume with the hope of being able to mount it:

root@vk4msl-ws:~# dd if=/dev/mapper/scratch-rootbackup of=/dev/mapper/scratch-gentoo--root bs=1M skip=38146

Whilst waiting for this to complete, I double-checked my findings, by inspecting the other fields. From the screenshot, I know my filesystem UUID was 6513-682e-7182-4474-89e6-c0d1c71866ad. Looking at the superblock, sure enough I see that listed:

root@vk4msl-ws:~# dd if=/dev/scratch/gentoo-root bs=$(( 0x10000 )) skip=1 count=1 | hexdump -C
1+0 records in
1+0 records out
00000000  5f f9 98 90 00 00 00 00  00 00 00 00 00 00 00 00  |_...............|
65536 bytes (66 kB, 64 KiB) copied, 0.000116268 s, 564 MB/s
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  65 13 68 2e 71 82 44 74  89 e6 c0 d1 c7 19 66 ad  |e.h.q.Dt......f.|
00000030  00 00 01 00 00 00 00 00  01 00 00 00 00 00 00 00  |................|
00000040  5f 42 48 52 66 53 5f 4d  9d 30 0d 02 00 00 00 00  |_BHRfS_M.0......|
00000050  00 00 32 da 32 00 00 00  00 00 02 00 00 00 00 00  |..2.2...........|
00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

Looks promising! After an agonising wait, the dd finishes. I can check the filesystem:

root@vk4msl-ws:~# btrfsck /dev/scratch/gentoo-root 
Checking filesystem on /dev/scratch/gentoo-root
UUID: 6513682e-7182-4474-89e6-c0d1c71966ad
checking extents
checking free space cache
block group 111690121216 has wrong amount of free space
failed to load free space cache for block group 111690121216
block group 161082245120 has wrong amount of free space
failed to load free space cache for block group 161082245120
checking fs roots
checking csums
checking root refs
found 107544387643 bytes used, no error found
total csum bytes: 99132872
total tree bytes: 6008504320
total fs tree bytes: 5592694784
total extent tree bytes: 271663104
btree space waste bytes: 1142962475
file data blocks allocated: 195274670080
 referenced 162067775488

Okay, it complained that the free space was wrong (which I’ll blame on gparted prematurely growing the partition), but the data is there!  This is confirmed by mounting the volume and doing a ls:

root@vk4msl-ws:~# mount /dev/scratch/gentoo-root /mnt/
root@vk4msl-ws:~# ls /mnt/ -l
total 4
drwxr-xr-x 1 root root 1020 Oct  7 14:13 bin
drwxr-xr-x 1 root root   18 Jul 21  2017 boot
drwxr-xr-x 1 root root   16 May 28 10:29 dbus-1
drwxr-xr-x 1 root root 1686 May 31  2017 dev
drwxr-xr-x 1 root root 3620 Oct 19 18:53 etc
drwxr-xr-x 1 root root    0 Jul 14  2017 home
lrwxrwxrwx 1 root root    5 Sep 17 09:20 lib -> lib64
drwxr-xr-x 1 root root 1156 Oct  7 13:59 lib32
drwxr-xr-x 1 root root 4926 Oct 13 05:13 lib64
drwxr-xr-x 1 root root   70 Oct 19 11:52 media
drwxr-xr-x 1 root root   28 Apr 23 13:18 mnt
drwxr-xr-x 1 root root  336 Oct  9 07:27 opt
drwxr-xr-x 1 root root    0 May 31  2017 proc
drwx------ 1 root root  390 Oct 22 06:07 root
drwxr-xr-x 1 root root   10 Jul  6  2017 run
drwxr-xr-x 1 root root 4170 Oct  9 07:57 sbin
drwxr-xr-x 1 root root   10 May 31  2017 sys
drwxrwxrwt 1 root root 6140 Oct 22 06:07 tmp
drwxr-xr-x 1 root root  304 Oct 19 18:20 usr
drwxr-xr-x 1 root root  142 May 17 12:36 var
root@vk4msl-ws:~# cat /mnt/etc/gentoo-release 
Gentoo Base System release 2.4.1

Yes, I’ll be backing this up properly RIGHT NOW. But, my data is back, and I’ll be storing this little data recovery technique for next time.

The real lesson here is:

  1. KEEP RELIABLE BACKUPS! You never know when something will fail.
  2. Catch the copy process before it starts overwriting your source data! If there’s no overlap between the old and new locations, you’re fine, but if there is and it starts overwriting the start of your original volume, it’s all over red rover! You might be lucky with a superblock back-up, but don’t bet on it!
  3. Make note of the filesystem type and its approximate location. The fact that I knew roughly where to look, and what sort of filesystem I was looking for meant I could look for magic bytes that say “I’m a BTRFS filesystem”. The magic bytes for EXT4, XFS, etc will differ, but the same concepts are there, you just have to look up the documentation on how your particular filesystem structures its data.
2018/10/22 by Computing Linux Development Public Syndication 0

Mapping call-signs to “hardware” addresses

This is another brain dump of ideas.

So, part of me wants to consider the idea of using amateur radio as a transmission mechanism for 6LoWPAN.  The idea being that we use NET/ROM and AX.25 or similar schemes as a transport mechanism for delivering shortened IPv6 packets.  Over this, we can use standard TCP/IP programming to write applications.

Protocols designed for low-bandwidth constrained networks are ideal here, so things like CoAP where emphasis is placed on compact representation.  6LoWPAN normally runs over IEEE 802.15.4 which has a payload limit of 128 bytes.  AX.25 has a limit of 256 bytes, so is already doing better.

The thinking is that I “encode” the call-sign into a “hardware” address.  MAC addresses are nominally 48-bits, although the IEEE is trying to phase that out in favour of 64-bit EUIs.  Officially the IEEE looks after this, so we want to avoid doing things that might clash with their system.

A EUI-48 (MAC) address is 6-bytes long, where the first 3 bytes identify the type of address and the organisation, and the latter 3 bytes identify an individual device.  The least significant two bits of the first byte are flags that decide whether the address is unicast or local, and whether it is globally administered (by the IEEE) or locally administered.

To avoid complications, we should probably keep the unicast bit cleared to indicate that these addresses are unicast addresses.

Some might argue that the ITU assigns prefixes to countries, and these countries have national bodies that hand out callsigns, thus we could consider callsigns as “globally administered”.  Truth is, the IEEE has nothing to do with the process, and could very legitimately assign the EUI-48 prefix 56-4b-34 to a company… in that hypothetical scenario, there goes all the addresses that might represent amateur operators stationed in Queensland.  So let’s call these “locally administered”, since there are suffixes the user may choose (e.g. “/P”).

That gives us 46-bits to play with.  7-bit ASCII just fits 6 characters, which would just fit the callsigns used in AX.25 with enough room for a 4-bit SSID.  We don’t need all 128 characters though, and a scheme based on DEC’s Radix50 can pack in far more.

We can get 8 arbitrary Radix50 characters into 43 bits, which gives us 3 left over which can be used as the user wishes.  We’ll probably call it the SSID, but unlike AX.25, will be limited from 0-7.  The user can always use the least significant character in their callsign field for an additional 6 bits, which gives them 9 bits to play with.  (i.e. “VK4MSL-1″#0 to encode the AX.25 SSID “VK4MSL-10”)

Flip the multicast bit, and we’ve got a group address.

SLAAC derives the IPv6 address from the EUI-48, so the IPv6 address will effectively encode the callsigns of the two communicating stations.  If both are on the same “mesh”, then we can probably borrow ideas from 6LoWPAN for shortening that address.

2018/10/10 by 6LoWHAM Amateur Radio Computing Emergency Communications Projects Public Syndication Thinktank WICEN 2

Solar Cluster: Further power bill reductions

So, since the last power bill, our energy usage has gone down even further.

No idea what the month-on-month usage is (I haven’t spotted it), but this is a scan from our last bill:

GreenPower? We need no stinkin’ GreenPower!

This won’t take into consideration my tweaks to the controller where I now just bring the mains power in to do top-ups of the battery. These other changes should see yet further reductions in the power bill.

2018/10/06 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Solar Cluster: Making the BCDC1225 get up and boogy!

So, I’ve been running the Redarc controller for a little while now, and we’ve had some good days of sunshine to really test it out.

Recall in an earlier posting with the Powertech solar controller I was getting this in broad daylight:

Note the high amount of “noise”, this is the Powertech solar controller PWMing its output. I’m guessing output filtering is one of the corners they cut, I expect to see empty footprints for juicy big capacitors that would have been in the “gold” model sent for emissions testing. It’ll be interesting to tear that down some day.

I’ve had to do some further tweaks to the power controller firmware, so this isn’t an apples-to-apples comparison, maybe next week we’ll try switching back and see what happens, but this was Tuesday, on the Redarc controller:

You can see that overnight, the Meanwell 240V charger was active until a little after 5AM, when my power controller decided the sun should take over. There’s a bit of discharging, until the sun crept up over the roof of our back-fence-neighbour’s house at about 8AM. The Redarc basically started in “float” mode, because the Meanwell had done all the hard work overnight. It remains so until the sun drops down over the horizon around 4PM, and the power controller kicks the mains back on around 6PM.

I figured that, if the Redarc controller saw the battery get below the float voltage at around sunrise, it should boost the voltage.

The SSR controlling the Meanwell was “powered” by the solar, meaning that by default, the charge controller would not be able to inhibit the mains charger at night as there was nothing to power the SSR. I changed that last night, powering it from the battery. Now, the power controller only brings in the mains charger when the battery is below about 12.75V. It’ll remain on until it’s been at above 14.4V for 30 minutes, then turn off.

In the last 24 hours, this is what the battery voltage looks like.

I made the change at around 8PM (can you tell?), and so the battery immediately started discharging, then the charge-discharge cycles began. I’m gambling on the power being always available to give the battery a boost here, but I think the gamble is a safe one. You can see what happened 12 hours later when the sun started hitting the panels: the Redarc sprang into action and is on a nice steady trend to a boost voltage of 14.6V.

We’re predicted to get rain and storms tomorrow and Saturday, but maybe Monday, I might try swapping back to the Powertech controller for a few days and we’ll be able to compare the two side-by-side with the same set-up.

It’s switched to float mode now having reached a peak boost voltage of 14.46V.  As Con the fruiterer would say … BEEEAAUUTIFUUUL!

2018/10/04 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Solar Cluster: Jury still out on solar controller, thinking of PSU designs

So, the last few days it’s been overcast.  Monday I had a firmware glitch that caused the mains supply to be brought in almost constantly, so I’d disregard that result.

Basically, the moment the battery dropped below ~12.8V for even a brief second, the mains got brought in.  We were just teetering on the edge of 12.8V all day.  I realised that I really did need a delay on firing off the timer, so I’ve re-worked the logic:

  • If battery drops below V_L, start a 1-hour timer
  • If battery rises above V_L, reset the 1-hour timer
  • If the battery drops below V_CL or the timer expires, turn on the mains charger

That got me better results.  It means V_CL can be quite low, without endangering the battery supply, and V_L can be at 12.8V where it basically ensures that the battery is at a good level for everything to operate.

I managed to get through most of Tuesday until about 4PM, there was a bit of a hump which I think was the solar controller trying to extract some power from the panels.  I really need a good sunny day like the previous week to test properly.

This is leading me to consider my monitoring device.  At the moment, it just monitors voltage (crudely) and controls the logic-level enable input on the mains charger.  Nothing more.  It has done that well.

A thought is that maybe I should re-build this as a Modbus-enabled energy meter with control.  This idea has evolved a bit, enough to be its own project actually.  The thought I have now is a more modular design.

If I take the INA219B and a surface-mount current shunt, I have a means to accurately measure input voltage and current.  Two of these, and I can measure the board’s output too.  Stick a small microcontroller in between, some MOSFETs and other parts, and I can have a switchmode power supply module which can report on its input and output power and vary the PWM of the power supply to achieve any desired input or output voltage or current.

The MCU could be the ATTiny24As I’m using, or a ATTiny861.  The latter is attractive as it can do high-speed PWM, but I’m not sure that’s necessary in this application, and I have loads of SOIC ATTiny24As.  (Then again, I also have loads of PDIP ATTiny861s.)

The board would expose the ICSP pins plus two more for interrupt and chip select, allowing for a simple jig for reprogramming.  I haven’t decided on a topology yet, but the split-pi is looking attractive.  I might start with a buck converter first though.

This would talk to a “master” microcontroller which would provide the UI and Modbus interface.  If the brains of the PSU MCU aren’t sufficient, this could do the more grunty calculations too.

This would allow me to swap out the PSU boards to try out different designs.

2018/09/27 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Solar Cluster: Return of the Redarc BCDC1225

Well, I’ve now had the controller working for a week or so now… the solar output has never been quite what I’d call, “great”, but it seems it’s really been on the underwhelming side.

One of the problems I had earlier before moving to this particular charger was that the Redarc wouldn’t reliably switch between boosting from 12V to MPPT from solar.  It would get “stuck” and not do anything.  Coupled with the fact that there’s no discharge protection, and well, the results were not a delight to the olfactory nerves at 2AM on a Sunday morning!

It did okay as a MPPT charger, but I needed both functions.  Since the thinking was I could put a SSR between the 12V PSU and the Redarc charger, we tried going the route of buying the Powertech MP3735 solar charge controller to handle the solar side.

When it wants to work, it can put over 14A in.  The system can run on solar exclusively.  But it’s as if the solar controller “hesitates”.

I thought maybe the other charger was confusing it, but having now set up a little controller to “turn off” the other charger, I think I can safely put that theory to bed.  This was the battery voltage yesterday, where there was pretty decent sunshine.

There’s an odd blip at about 5:40AM, I don’t know what that is, but the mains charger drops its output by a fraction for about 50 seconds.  At 6:37AM, the solar voltage rises above 14V and the little ATTiny24A decides to turn off the mains charger.

The spikes indicate that something is active, but it’s intermittent.  Ultimately, the voltage winds up slipping below the low voltage threshold at 11:29AM and the mains charger is brought in to give the batteries a boost.  I actually made a decision to tweak the thresholds to make things a little less fussy and to reduce the boost time to 30 minutes.

The charge controller re-booted and turned off the mains charger at that point, and left it off until sunset, but the solar controller really didn’t get off its butt to keep the voltages up.

At the moment, the single 120W panel and 20A controller on my father’s car is outperforming my 3-panel set-up by a big margin!

Today, no changes to the hardware or firmware, but still a similar story:

The battery must’ve been sitting just on the threshold, which tripped the charger for the 30 minutes I configured yesterday.  It was pretty much sunny all day, but just look at that moving average trend!  It’s barely keeping up.

A bit of searching suggests this is not a reliable piece of kit, with one thread in particular suggesting that this is not MPPT at all, and many people having problems.

Now, I could roll the dice and buy another.

I could throw another panel on the roof and see if that helps, we’re considering doing that actually, and may do so regardless of whether I fix this problem or not.

There’s several MPPT charger projects on this very site.  DIY is a real possibility.  A thought in the back of my mind is to rip the Powertech MP3735 apart and re-purpose its guts, and make it a real MPPT charger.

Perhaps one with Modbus RTU/RS-485 reporting so that I can poll it from the battery monitor computer and plot graphs up like I’m doing now for the battery voltage itself.  There’s a real empty spot for 12V DC energy meters that speak Modbus.

If I want a 240V mains energy meter, I only have to poke my head into the office of one of my colleagues (who works for the sister company selling this sort of kit) and I could pick up a little CET PMC-220 which with the addition of some terminating resistors (or just run comms at 4800 baud), work just fine.  Soon as you want DC, yeah, sure there’s some for solar set-ups that do 300V DC, but not humble 12V DC.

Mains energy meters often have extra features like digital inputs/outputs, so this could replace my little charge controller too.  This would be a separate project.

But that would leave me without a solar controller, which is not ideal, and I need to shut everything down before I can extract the existing one.  So for now, I’ve left the Powertech one in-place, disconnected its solar input so that now it just works as a glorified VSR and voltmeter/ammeter, as that bit works fine.

The Redarc is now hooked up to solar, with its output going into a spare socket going to the batteries.  This will cost me nothing to see if it’s the solar controller or not.  If it is, then I think some money on a VSR to provide the low-voltage protection, and re-instating the Redarc charger for solar duty will be the next step.  Then I can tear down the Powertech one at my leisure and figure out what they did wrong, or if it can be re-programmed.

The Meanwell charger is taking care of things as I type this, but tomorrow morning, we should hopefully see the solar set-up actually do some work…

… maybe. 🙂

2018/09/23 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Solar Cluster: Thank goodness for good monitoring

A few months back now, I had the misfortune of overshooting my Internet quota, and winding up with a AU$380 bill for the month (and that was capped… in truth it was more like AU$3000).  In fact, it happened a couple of times until I finally nailed down the cause.

Part of it was NTP traffic (seems lots of cowboys write SNTP clients now and point them at pool.ntp.org), some was the #Hackaday.io Spambot Hunter Project and related activity.  In short, I invested some money into upping the quota, and some time into better monitoring.

I wanted to do the monitoring anyway to keep an eye on operations, as well as things like the solar panel voltages, etc.  Since I got it in place, I’ve been able to get much faster notifications of when things go awry.  Much sooner than the 120% quota usage alarm that Internode sends you.

I’m glad I did that now, last night I left a few tabs open on the Hackaday.io site.  I noticed this evening they were still trying to load something and got suspicious… then I saw this:

Double checking, sure enough, something on one of those pages made Chromium get its knickers into a twist, and chew through all that data.

It took me a bit of tinkering to get the right query to extract the above chart.  Essentially there was a sustained 1.5MB/sec download for over 21 hours which would account for the 113.1GB that Internode recorded.

It’s a bit co-incidental that the usage dropped the moment I re-started Chromium.  Not sure why it was continually re-loading pages, but never mind.

The above data is collected using a combination of collectd and InfluxDB, with Grafana doing the dashboarding and alarms, and a small Perl script pulling the usage data off Internode’s API.

2018/09/13 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

Solar Cluster: Return of the power controller

Well, I’ve been tossing up how to control the mains charger for a while now.

When I first started the project, my thinking was to use an old Xantrex charger we had kicking around, and just electrically disconnect it from the batteries when we wanted to use the solar power.  I designed a 4-layer PCB which sported a ATTiny24A microcontroller, MOSFETs (which I messed up) and some LEDs.

This was going to be a combined fan controller and power management unit.  It had the ability (theoretically) to choose a supply based on the input voltage, and to switch if needed between supplies.

It didn’t work out, the charger got really confused by the behaviour of the controller.  I was looking to re-instate it using the Redarc solar controller, but I never got there.  In the end, it was found that the Redarc controller had problems switching sources and would do nothing whilst the batteries went flat.

We’ve now replaced both ends of the system.  The solar controller is a Powertech MP3735 and integrates over-discharge protection.  The mains charger is now a MeanWell HEP-600C-12 (which has not missed a beat since the day it was put in).

Unlike my earlier set-up, this actually has a 5V logic signal to disable it, and my earlier controller could theoretically generate that directly.

Looking at the PCB of my earlier power controller attempt, it looks like this could still work.

Above is the PCB artwork.  I’ve coloured in the sections and faded out the parts I can omit.

In green up the top-left we have the mains control/monitoring circuitry.  We no longer see the mains voltage, so no point in monitoring it, so we can drop the resistor divider that fed the ADC.  This also means we no longer need the input socket P2.

Q2 and Q7 were the footprints of the two P-channel MOSFETs.  We don’t need the MOSFETs themselves, but the signals we need can be found on pin 1 of Q2.  This is actually the open-drain output of Q1, which we may be able to hook directly to the REMOTE+ pin on the charger.  A pull-up between there and the charger’s 5V rail, and we should be in business.

In yellow, bottom left is the solar monitoring interface.  This is still useful, but we won’t be connecting solar to the battery ourselves, so we just keep the monitoring parts.  The LED can stay as an indicator to show when solar is “good enough”.

In purple, occupying most of the board, is the controller itself.  It stays for obvious reasons.

In red, is the fan control circuitry.  No reason why this can’t stay.

In blue is the circuitry for monitoring the battery voltage.  Again, this stays.

The main advantage of doing this is I already have the boards, and a number of microcontrollers already present.  There’s a board with all except the big MOSFETs populated: with the MOSFETs replaced by 3-pin KK sockets.

How would the logic work?  Much the same as the analogue version I was pondering.

  • If battery voltage is low, OR, the sun has set, enable the mains charger.

What concerned me about an analogue solution was what would happen once the charger got to the constant-voltage stage.  We want to give it a bit of time to keep the battery topped up.  Thus it makes sense to shut down the charger after a fixed delay.

This is easy to do in a microcontroller.  Not hard with analogue electronics either, it’s fundamentally just a one-shot, but doing it with an MCU is a single-chip solution.  I can make the delay as long as I like.  So likely once the battery is “up to voltage”, I can let it float there for an hour, or until sunrise if it’s at night.

2018/09/01 by Computing Projects Public Syndication Solar-powered Cloud Computing 0

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