Solar Cluster: Re-wiring the rack

It’s been on my TO-DO list now for a long time to wire in some current shunts to monitor the solar input, replace the near useless Powertech solar controller with something better, and put in some more outlets.

Saturday, I finally got around to doing exactly that. I meant to also add a low-voltage disconnect to the rig … I’ve got the parts for this but haven’t yet built or tested it — I’d like to wait until I have done both,but I needed the power capacity. So I’m running a risk without the over-discharge protection, but I think I’ll take that gamble for now.

Right now:

  • The Powertech MP-3735 is permanently out, the Redarc BCDC-1225 is back in.
  • I have nearly a dozen spare 12V outlet points now.
  • There are current shunts on:
    • Raw solar input (50A)
    • Solar controller output (50A)
    • Battery (100A)
    • Load (100A)
  • The Meanwell HEP-600C-12 is mounted to the back of the server rack, freeing space from the top.
  • The janky spade lugs and undersized cable connecting the HEP-600C-12 to the battery has been replaced with a more substantial cable.

This is what it looks like now around the back:

Rear of the rack, after re-wiring

What difference has this made? I’ll let the graphs speak. This was the battery voltage this time last week:

Battery voltage for 2019-05-22

… and this was today…

Battery voltage 2019-05-29

Chalk-and-bloody-cheese! The weather has been quite consistent, and the solar output has greatly improved just replacing the controller. The panels actually got a bit overenthusiastic and overshot the 14.6V maximum… but not by much thankfully. I think once I get some more nodes on, it’ll come down a bit.

I’ve gone from about 8 hours off-grid to nearly 12! Expanding the battery capacity is an option, and could see the cluster possibly run overnight.

I need to get the two new nodes onto battery power (the two new NUCs) and the Netgear switch. Actually I’m waiting on a rack-mount kit for the Netgear as I have misplaced the one it came with, failing that I’ll hack one up out of aluminium angle — it doesn’t look hard!

A new motherboard is coming for the downed node, that will bring me back up to two compute nodes (one with 16 cores), and I have new 2TB HDDs to replace the aging 1TB drives. Once that’s done I’ll have:

  • 24 CPU cores and 64GB RAM in compute nodes
  • 28 CPU cores and 112GB RAM in storage nodes
  • 10TB of raw disk storage

I’ll have to pull my finger out on the power monitoring, there’s all the shunts in place now so I have no excuse but to make up those INA-219 boards and get everything going.

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.

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!

DC-DC Converter: Connectors

So, a thing that will make or break this project, will be the connectors that feed power in and out.

My existing system uses the larger 50A Anderson connectors.  These are big and chunky, not really appropriate for a PCB.  The 30A version would be okay size-wise, and I use these on the bike, but 30A isn’t sufficient.  That’s about my cluster’s peak current draw, and I want a 50% safety margin.

Thinking about it last night… 20V at 50A… that’s a kilowatt!  Pales into insignificance when you compare it to the 48V 400A electric harvester I worked on years ago (and blew many a MOSFET on, not to mention boiling electrolytics with ripple current), but it’s still a decent amount of power.

There’s the XT60 and Deans connectors, however the problem with these is they aren’t all made equal, there’s some slight variances in the tolerances, thus you can buy two “XT60″s or two “Deans” connectors and find they won’t mate.

I see no problem in a short flying lead that connects to screw terminals.  Take the flying lead, wire it up, then connect it to the connector of your choice.  That’s how the solar controller I was using connected up, and I don’t think its problems were with its connectors.

The conductors I’m using are 6-8AWG.  Whatever I use, must be able to handle that.  There isn’t a lot out there for off-board connectors, and even the XT60s are a wee bit small.  I did find these terminal blocks .  Supposedly good for 76A, that’s enough safety margin for me, and Phoenix Contact aren’t known for producing crap.

The spade lugs used on the HEP-600C I’m using for charging my batteries would be smaller than this, and so far I’ve not seen any fires.

I might be able to put a few different footprints down on the PCB, we’ll see.  I plan to design the PCB so there’s nice wide areas so you can drill your own hole and solder whatever you like there.  Likewise for the inductor and capacitors, this will be a board that aims for flexibility.

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. 🙂

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.

Solar Cluster: Next steps, better control of the charger

So, a few weeks ago I installed a new battery charger, and tweaked it so that the solar did most of the leg work during the day, and the charger kept the batteries topped up at night.

I also discussed the addition of a new industrial PC to perform routing and system monitoring functions… which was to run Gentoo Linux/musl. For now, that little PC is still running Debian Stretch, but for 45 days, it was rock solid. The addition of this box, and taking on the role of router to the management network meant I could finally achieve one of my long-term goals for the project: decommissioning the old server.

The old server is still set up with all my data and software… but now the back-up cron job calls /sbin/poweroff when it’s done, and the BIOS is set to wake the machine up in the evening ready to receive a back-up late at night.

In its place, a virtual machine clone of the box, handles my email and all the old functions of that server. This was all done just prior to my father and I leaving for a 3 week holiday in the Snowy Mountains.

I did have a couple of hiccups with Ceph OSDs crashing … but basically re-starting the daemons (done remotely whilst travelling through Cowra) got everything back up. A bit of placement group cleaning, and everything was back online again. I had another similar hiccup coming out of Maitland, but once again, re-starting the daemons fixed it. No idea why it crashed, that’s something I’ll have to investigate.

Other than that, the cluster itself has run well.

One thing that did momentarily kill the industrial PC though: I wandered down to the rack with a small bus-powered 2.5″ HDD with the intent of re-starting my Gentoo builds. This HDD had the same content as the 3.5″ HDD I had plugged in before. I figured being bus powered, I would not be dependent on mains, and it could just chug away to its heart’s content.

No such luck, the moment I plugged that drive in, the little machine took great umbrage to the spinning rust now vacuuming the electrons away from its core functions, and shut down abruptly. I’ve now brought my 3.5″ drive and dock down, plugged that into the wall, and have my builds resuming. If power goes off, hopefully the machine either handles the loss of swap gracefully. If it does crash, the watchdog will take care of it.

Thus, I have the little TS-7670 first attempting a build of gcc, to see how we go. Finger’s crossed our power should remain up. There was at least one outage in the time we were away, but hopefully we should get though this next build!

The next step I think should be to add some control of the mains charger to allow the batteries to be boosted to full charge overnight. The thinking is a simple diode-OR arrangement. Many comparators such as the LM393 have an open-collector output, which gives us this for free.

The theory is this.

The battery bank powers a simple circuit which runs of a 5V regulator. That regulator powers a dual comparator IC and provides a reference voltage. The comparator draws bugger all power, so I’m happy to use a linear PSU here. It’s mainly there as a voltage reference.

Precision isn’t really the aim here, so adjustable pots will make life easier.

The voltages from the battery bank and the solar panel are fed through voltage dividers to bring the voltages down to below 5V, then those voltages are individually fed into separate pots that control the hysteresis. I can adjust all points of the system.

The idea is that should the batteries get too low, or the sun go down, one or the other (or both) comparators will go low and pull down on R2. If the batteries are high and the sun is up, nothing pulls on R2 so the REMOTE+ pin on the HEP-600C-12 is allowed to float to +5V, turning off the mains charger.

The advantage of this is there’s no programming of a microcontroller, it’s just analogue electronics. The LM393s are pretty hardy things, the datasheet says they’ll run at 36V and can accept a maximum voltage of VCC-1.5V; so if I run at 5V, 3.5V is my recommended maximum. The adjustment pots should let me set a threshold voltage that avoids going above this.

I mainly need 5V for the HEP-600C-12, and for providing that stable known voltage reference. The LM78C05 should be fine for this.

Once I’ve done that, I should be able to wind that charger back up to its factory setting of 14.4V, which will mean that overnight the batteries will be charged back to full charge.

Solar Cluster: Fine Tuning

So yesterday I wound back the mains charger so that the solar would take on the load during the day.  Seems I wound it back a bit far, and the mains charger did almost no work overnight, leaving the battery somewhere around 11.8V.

That’s a wee bit low for my comfort.  Yes, they are deep cycle AGMs, but I’d rather not get that low.

Thus, I wound it up a bit, float at 12.8V, so Vboost at 13.6V.  That looks to be the sweet spot.  Now that the sun is up, I’m getting nice healthy amps of current down the wire from the roof:

The cluster is drawing about 8A, so that’s the cluster powered, and about 6A going to the batteries. It intermittently peaks about 15A or so.

I also found myself fine tuning the Ethernet settings on the border router. For some reason, its Realtek RTL8139 was happy to talk to the Cisco SG-200-08 it was connected to before, but didn’t quite get along with the Linksys LGS326-AU. I’ve told the switch to force 100Mbps full-duplex MDIX (evidently, it’s a cross-over cable), and so far, that seems to have settled things down.

Solar Cluster: Coaxing two supplies to get along

So last post, I mentioned about the installation of the new battery charger, which is fed from 240V mains. Over the last few days this charger has held the batteries at a rock-solid 14.4V. Not once did the batteries drop below that voltage setpoint.

So good in fact, the solar charger does no work at all.

By the way, this is what the install looks like. I promised pictures last post.

That’s the DC end … and the nasty AC end is all sealed up…

I will eventually move this to a spot on the back of the rack, but it can sit here for now.

Ultimately, the proper fix to this will be to have the mains-powered charger power off when the sun is up. On the DC output connector, the two rightmost screw terminals go to an opto-isolator that, when powered, shuts off the charger, putting it into stand-by mode. This was one of the reasons I bought this particular unit. The other was the wide range of voltage adjustment.

The question is when to turn on, and when to go to stand-by. Basically if the following expression is true, then turn off the mains:

(V_{batt} > 12.8) \\wedge (V_{solar} > 15)

We do not want solar if the battery is very low, as there’s a possibility that the solar output will not be sufficient.  Likewise, if the sun’s out, we need the mains to keep the battery topped up.

The solar output is nearly always above 15V when the sun is up, so there’s our first clue.  We can safely get to 12.8V before things start going pear shaped on the cluster, so we can use that as our low-voltage safety net.  If both of these conditions are met, then it’s safe to turn off the mains power and rely on solar only.

We need a +5V signal when both these conditions are met.  This very much sounds like the job of a dual-comparator with diode-OR outputs pulling on a 5V pull-up.  Maybe a wee bit of hysteresis on those to prevent flapping, and we should be good.

Unfortunately, to do that, I need to unscrew terminals to feed some wires in.  I don’t feel like doing that just now… we’re packing up to go away for a while, and I think this sort of job can wait until we return.

In the meantime, I’ve done something of a hack.  I mentioned the PSU is adjustable.  I wound Vfloat back to 12V… thus Vboost has gone to 12.8V.  Right now, the mains PSU is showing a green LED, meaning it is in floating mode.

We have good sun right now, and the solar controller is currently boosting the battery.  When the battery gets low, the charging circuitry of the mains PSU should kick in, and bring the battery voltage up, holding it at 12.8V until the sun comes up.  I’ll leave it for now and see how this hack goes.

On other news… I might need to re-consider my NTP server arrangements.  I’m not sure if it’s a quirk of OpenBSD, or of the network here, but it seems OpenNTPD struggles to keep good time.  Never tried using the Advantech PC as a NTP server until now, and I’m also experimenting with using my VPS at Vultr as a NTP server.

Both are drifting like crazy.  I have a GPS module lying around that I might consider hooking up to the TS-7670… perhaps make it a Stratum 1 NTP server on the NTP server pool, then the Advantech can sync to that.

This won’t help the VPS though, and I’m at a loss to explain why a Geode LX800 running on an ADSL link in my laundry, outperforms a VPS in a nicely climate-controlled data centre with gigabit Internet.

But at least now that’s one less job for my aging server.  I’ve also moved mail server duties off the old box onto a VM, so I’ll be looking at the BIOS settings there to see if I can get the box to wake up some time in the evening, let cron run the back-up jobs, then power the whole lot back down again, save some juice.

Solar Cluster: New power supply wired up and installed

So tonight I finally got my shiny new power supply installed.

Tuesday night I took it with me along with a cable gland to HSBNE with three items on my agenda:

  • Hooking up a mains power lead to the power input.
  • Getting the newly hooked up lead inspected for electrical safety.
  • Putting some sort of cover over the screw terminals to prevent accidental contact.

I did some digging around in the HSBNE bone yard, and managed to come out with a 10A 240V mains lead, the chassis of a Sonoff TH-10, and a bit of off-cut perspex from the laser cutter to cover the gaping hole in the TH-10 casing.

The 240V mains lead came from someone’s long abandoned project.  Not sure what it was, but it basically was housed in take away food containers, so losing its mains lead is probably a good thing!  The rest of it is there if they want it … whatever it is.

I terminated the 240V lead with fork lugs, ready to go into the screw terminals on the power supply.  A small square of perspex was cut out of the off-cut, and that was sliced into three parts to be glued to the TH-10 case.

The back panel of the TH-10 case had an opening cut in it to allow the screw terminal block to pass through the back.  One of the pieces of perspex had a 14mm hole drilled through it and the cable gland was fitted.  All that was left to do was some hot glue to fix the perspex panels into place over the hole, attach the mains lead and get it checked.

Sadly, I couldn’t find anyone about with an electrical ticket to actually install the cable, so I did that bit myself in the end.  There also weren’t any glue sticks for the glue gun around, and I still had to think about how I was going to secure the TH-10 case to the aluminium of the PSU.

I brought it into my workplace this morning and got one of the people there to check it over (there’s two at my work who have a current electrical ticket).  My cabling job was given the tick of approval, and as a bonus, we had some silicon glue which could fix the TH-10 case to the aluminium panel on the PSU.  Perfect, two birds with one stone.

Once home, I set to work on the 12V end of it.  I needed to go from 4 smallish screw terminals to an Anderson SB50 connector which was intended for 8AWG cable.  In the end, the solution was to use two lengths of twin-12AWG.  One end was terminated with fork lugs, the other was twisted together and soldered into a SB50 connector.  I had to solder it because even doubled over, it was too thin to crimp into the pins securely.

I used about 10cm of 12AWG.  To that SB-50 I made a patch lead with two SB-50s out of figure-8 8AWG cable, about 50cm long to reach the charger input on the battery bank.

I’ll put some pictures up later, but already the silence of this new charger is deafening.  It happily boosted the batteries up to 14.3V and is now letting them sit in constant voltage mode.

We shall see what happens when the sun comes up tomorrow.  Hopefully it just backs right off and lets mother nature do all the work.