Feb 112017
 

Okay, so now the searing heat of the day has dissipated a bit, I’ve dragged the cluster out and got everything running.

No homebrew charge controller, we have:

240v mains → 20A battery charger → battery → volt/current meter → cluster.

Here’s the volt meter, showing the battery voltage mid-charge:

… and this is what the IPMI sensor readings display…

Okay, the 3.3V and 5V rails are lower than I’d expect, but that’s the duty of the PSU/motherboard, not my direct problem.

The nodes also vary a bit… here’s another one. Same set-up, and this time I’ll show the thresholds (which are the same on all nodes):

Going forward… I need to get the cluster solar ready. Running it all from 12V is half the story, but I need to be able to manage switching between mains and solar.

The battery I am using at the moment is a second-hand 100Ah (so more realistically ~70Ah) AGM cell battery. I have made a simple charger for my LiFePO₄ packs that I use on the bicycle, there I just use a LM2576 switchmode regulator to put out a constant voltage at 3A and leave the battery connected to “trickle charge”. Crude, but it works. When at home, I use a former IBM laptop power supply to provide 16V 3A… when camping I use a 40W “12V” solar panel. I’m able to use either to charge my batteries.

The low output means I can just leave it running. 3A is well below the maximum inrush current capacity of the batteries I use (typically 10 or 20Ah) which can often handle more than 1C charging current.

Here, I’m using an off-the-shelf charger made by Xantrex, and it is significantly more sophisticated, using PWM control, multi-stage charging, temperature compensation, etc. It also puts out a good bit more power than my simple charger.

Consequently I see a faster rise in the voltage, and that is something my little controller will have to expect.

In short, I am going to need a more sophisticated state machine… one that leaves the cut-off voltage decision to the charger. One that notices the sudden drop from ~15V to ~14V and shuts off or switches only after it remains at that level for some time (or gets below some critical limit).

Feb 112017
 

So… in the last test, I tried setting up the nodes with the ATTiny24A power controller attempting to keep the battery between 11.8 and 13.8V.

This worked… moreover it worked without any smoke signals being emitted.

The trouble was that the voltage on the battery shot up far faster than I was anticipating. During a charge, as much as 15.5V is seen across the battery terminals, and the controller was doing exactly as programmed in this instance, it was shutting down power the moment it saw the high voltage set-point exceeded.

This took all of about 2 seconds. Adding a timeout helped, but it still cycled on-off-on-off over a period of 10 seconds or so. Waay too fast.

So I’m back to making the nodes more tolerant of high voltages.

The MIC29712s are able to tolerate up to 16V being applied with peaks at 20V, no problem there, and they can push 7.5A continuous, 15A peak. I also have them heatsinked, and the nodes so far chew a maximum of 3A.

I had set them up to regulate down to approximately 13.5V… using a series pair of 2.7kΩ and 560Ω resistors for R1, and a 330Ω for R2. Those values were chosen as I had them on hand… 5% tolerance ¼W carbon film resistors. Probably not the best choice… I wasn’t happy about having two in series, and in hindsight, I should have considered the possibility of value swing due to temperature.

Thinking over the problem over the last week or so… the problem seemed to lay in this set point: I was too close to the upper bound, and so the regulator was likely to overshoot it. I needed to knock it back a peg. Turns out, there were better options for my resistor selections without resorting to a trim pot.

Normally I stick to the E12 range, which I’m more likely to have laying around. The E12 series goes …2.7, 3.3, 3.9, 4.7, 5.6… so the closest I could get was by combining resistors. The E24 range includes values like 3.0 and 3.6.

Choosing R1=3.6kΩ and R2=390Ω gives Vout ~= 12.7V. Jaycar sell 1% tolerance packs of 8 resistors at 55c each. While I was there today, I also picked up some 10ohm 10W wire wound resistors… before unleashing this on an unsuspecting AU$1200 computer, I’d try it out with a dummy load made with four of these resistors in parallel… making a load that would consume about 5A for testing.

Using a variable voltage power supply, I found that the voltage could hit 12.7V but no higher… and was at worst .7V below the input. Good enough.

At 16V, the regulator would be dropping 3.3V, passing a worst case 3A current for a power dissipation of 9W out of the total 48W consumption. About 80% efficiency.

Not quite what I had hoped for… but this is a worst case scenario, with the nodes going flat chat and the battery charger pumping all the electrons it can. The lead acid battery has a nominal voltage of 13.8V… meaning we’re dropping 1.1V.

On a related note, I also overlooked this little paragraph in the motherboard handbook:

(*Do not use the 4-pin DC power @J1 when the 24-pin ATX Power @JPW1 is connected to the power supply. Do not plug in both J1 and JPW1 at the same time.)

Yep, guess what I’ve done. Being used to motherboards that provide both and needed both, I plugged them both in.

No damage done as all nodes work fine… (or they did last time I tried them… yet to fire them up since this last bit of surgery). It is possible there is no isolation between the on-motherboard PSU and the external ATX one and that if you did plug in power from two differing sources, you could get problems.

In a way if I had spotted this feature before, I could have done without those little PSUs after all, just needing a Molex-style power adaptor cable to plug into the motherboard.

Still… this works, so I’m not changing it. I have removed that extra connection though, and they’ve been disconnected from the PSUs so they won’t cause confusion in future.

I might give this a try when things cool down a bit … BoM still reports it being about 32°C outside (I have a feeling where I live is a few degrees hotter than that) and so I don’t feel energetic enough to drag my cluster out to the workbench just now. (Edit: okay, I know…those in NSW are facing far worse. Maybe one of the mob in New Holland should follow the advice of Crowded House and take the weather with them over here to the east coast! Not all of it of course, enough to cool us off and reduce their flood.)