Solar Power

Roger Cheng

Solar Powered Dancing Duck

A small solar cell doesn’t get much power with indoor lighting. As far as consumer electronics go, I haven’t seen much beyond a solar-powered desktop calculator. I had thought there’d never be enough power for an indoor solar mechanical device, but a few years back little solar-powered pendulum toys started showing up. I usually see them as little waving cats (maneki-neko) like the teardown and analysis posted to Hackaday.

This device is a variation of the same basic idea. Instead of waving a cat’s arm, the pendulum swings the body. An additional sophistication in this design is a second linkage that swings the head in the opposite direction of the body, creating a dancing duck. It was purchased for a buck and a half from Daiso Japan, so we’re looking at something produced for raw material cost somewhere a quarter (if even that much.) It was an impulse buy and wasn’t expected to last very long, but it actually ran for years before suffering mechanical issues and frequently getting stuck. It was then moved to a window ledge, where it could occasionally swing its head and hips under power of direct sunlight. But the sun that gave it a second life also took away its shine: brightly colored plastic started fading rapidly and became brittle. Finally, an unfortunate fall from that window ledge ended this duck’s performance career.

Poor duck broke its neck in the fall. The neck linkage was lost, but we can see the head’s pivot point inside the neck, where plastic shaded from direct sun is a visibly more vibrant shade of yellow.

I think the bottom of the base was originally glued in place, but that glue has weakened with age (or sun) and could be easily pried apart.

A small solar cell feeds into a circuit board, home to just two components: an electrolytic capacitor and a chip under a blob of epoxy. A coil wound from super fine copper wires is attached to this board as well. As explained in the Hackaday link above, this coil is both input and output: for sensing position of the magnet and for creating a magnetic field to boost the magnet’s swing.

The coil looked off-center, so I broke off the rear side of the base and reinstalled it to verify the coil is indeed off center when the magnetic pendulum (black plastic with black magnet on the bottom) is at rest. There is only about a millimeter of air between the coil and the magnet, a much closer distance than found in the cat mentioned in the Hackaday post.

This old dancing duck has a bit of arthritis and could not self-start under indoor light. I gave the pendulum a small tap and it started rocking but halted again after a few seconds. We can see the problem in the pivot point, which was designed to minimize friction. The pendulum axle has a triangular profile, so only a tiny sharp point touches the circular hole in the base. Years of dancing in the sun has worn both components. The triangular wedge’s sharp edge has been rounded off, and the hole perimeter is no longer circular. Together these two parts presented too much friction for the pendulum to overcome.

Daiso has long since stopped carrying this device, and I had no luck finding an exact replacement. There is no shortage of solar-powered dancing ducks for sale, but they all looked different from this cute little thing. Some are the opposite of cute, and a few looked downright scary! I have to say goodbye to this dancing duck now, it gave its all for dance and was quite an entertaining $1.50 spent.

Roger Cheng

First Impressions: Paxcess Rockman 200

I had been using a Monoprice PowerCache 220 to store and use power generated by my small Harbor Freight solar array. Due to its degrading battery and erroneous thermal protection circuit, I bought a Paxcess Rockman 200(*) to replace it. Thanks to its lithium chemistry battery, the Paxcess is far smaller and lighter than the Monoprice unit it replaced. Which made a good first impression as something I noticed before I even opened the box.

Two means of charging were included with the Rockman 200, giving users two choices of power source. Either use an automotive “cigarette lighter” power socket adapter, or use a household AC voltage power adapter. But I intended to charge from solar power, so I had to fashion my own charging adapter. Fortunately the Rockman 200 used commodity barrel jacks (5.5mm outer diameter, 2.1mm inner diameter) so it was easy to build one from my most recent purchase(*) of such connectors. This was much easier than the hack I had to do for my Monoprice.

Once up and running I was indeed able to charge from my Harbor Freight solar array. The maximum no-load open circuit voltage of these panels were around 21V, lower than the 24V maximum input voltage limit of the Rockman 200. The Rockman 200 had a far more informative display than the very limited one on board Monoprice PowerCache 220. I like to see how many watts the solar array is delivering, and seeing the number of watts being drawn by anything I had plugged in. Unfortunately, there were two disadvantages relative to the PowerCache 220.

  1. It is not possible to use the AC power output while charging. Like the Monoprice, 12V DC and USB DC power output can be used while charging. But while the Monoprice was willing to deliver AC power while charging, the Paxcess is not.
  2. When drawing DC power while charging, the cooling fan always comes on. I suppose this is some sort of DC voltage conversion process. In contrast the Monoprice stays silent if it can stay cool enough. Or at least it used to, before the thermal sensing system broke down.

Neither the Monoprice or the Paxcess attempts to perform maximum power point tracking (MPPT). I realize this means the panel is not operating at maximum efficiency, but a MPPT controller (*) cost significantly more money than their non-MTTP counterpart (*). Given that a standalone controller costs almost as much as the array, or the Paxcess itself, I don’t fault the Paxcess for not doing MPPT.

However, the Paxcess is even more non-MPPT than the Monoprice. The Monoprice pulls the voltage level down to whatever level its internal lead-acid battery is at, which is usually in the range of 11-13 volts. In contrast, the Paxcess drags the voltage all the way down to 9.5 volts, which is even further away from the maximum power point, as seen by varying power input wattage on the information display.

The display also shows a charge percentage for its internal battery. This allows me the option to stay within the 30% – 80% range if I want to minimize stress on the battery. Lithium chemistry batteries have a different care and feeding procedure than lead acid batteries. Speaking of which, with the new battery storage unit in hand, I opened up the old one to try to fix it.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Roger Cheng

Out with the Lead-Acid, In with the Lithium-Ion.

A little over two and a half years ago, I bought a Monoprice PowerCache 220 (Item #15278) to help store power generated by my cheap Harbor Freight solar array and also to utilize that power by the way of AC inverter and USB power converters. When new, the PowerCache was quite capable of gathering up the day’s solar generation and using that energy to charge various battery-powered devices around the house. Up to and including charging an Apple MacBook Air (2014).

I expected this battery to wear down, I just didn’t know how quickly. Battery University has a capacity loss chart, but that is for lead acid batteries held on standby for long periods of time (like in an UPS) showing the capacity would fade to about 85% after two and a half years. However, this battery was cycled on a daily basis for most of the past 2+ years. And while the charge controller does perform an occasional top-off charge, it’s probably not as much as the battery desires.

As a result of this stressful usage pattern, the battery inside my PowerCache 200 has degraded to a point where it could barely hold enough energy to charge a cell phone. That by itself was not a disaster, as I had anticipated battery degradation and was prepared to revive the machine with a new lead-acid battery. Unfortunately the machine has developed another problem: the thermal protection system has gone amok. Within five minutes of the PowerCache starting up (when everything is still room temperature) the “overheating” warning triangle starts blinking and soon the thermal protection routine kicks in and shuts everything down.

So instead of shopping for a replacement lead-acid battery for the PowerCache 220, I started looking at replacing it entirely. I was pleasantly surprised to see that the cost of lithium based batteries have dropped significantly within the past two and a half years. As of right now, lead-acid based systems are still cheaper, but the price premium for going lithium-ion is now small enough to convince me to make the switch. I’ll pay a little extra, but I’ll get something that’s far smaller and lighter. Thus I bought a Paxcess Rockman 200 Portable Power Station (*) to see how it handles my usage scenarios.

[UPDATE: I opened up the PowerCache 220 with the intent to fix it, but things took an ugly turn and ended up as a full teardown.]

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Roger Cheng

Samsung 500T Now Runs On Solar Power

I wanted to have a screen in my house displaying current location of the international space station. I love ISS-Above but didn’t want to dedicate a Raspberry Pi and screen, I wanted to use something in my pile of retired electronics instead. I found ESA’s HTML-based ISS tracker, tested it on various devices from my pile, and decided the Samsung 500T would be the best one to use for this project.

One of the first device I tried was a HP Mini (110-1134CL) and I measured its power consumption while running ESA’s tracker. I calculated my electric bill impact to keep such a display going 24×7 would be between one and two dollars a month. This was acceptable and a tablet would cost even less, but what if I could drop the electric bill impact all the way to zero?

Reading the label on Samsung 500T’s AC power adapter I saw its output is listed at 12V DC. The hardware is unlikely to run on 12V directly, since it also has to run on batteries when not plugged in. It is very likely to have internal voltage regulators which should tolerate some variation of voltage levels around 12V. The proper way to test this hypothesis would be to find a plug that matches the AC adapter and try powering the tablet from my bench power supply. But I chose the more expedient path of beheading the AC adapter instead and rewiring the severed plug.

A quick test confirmed the tablet does not immediately go up in flames when given input voltage up to 14.4V, the maximum for lead-acid batteries. Whether this is bad for the device long term I will find out via experience, as the tablet is now wired up to my solar powered battery array.

This simple arrangement is constantly keeping tablet batteries full by pulling from solar battery. This is not quite optimal, so a future project to come will be to modify the system so it charges from solar during the day and runs on its own internal battery at night. But for now I have an around-the-clock display of current ISS location, and doing so without consuming any electricity from the power grid