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Constant Power Battery Discharge

Recently I’ve been looking at battery datasheets, in preparation for an off-grid solar project. I’ve noticed something strange about the “constant power discharge” numbers in the datasheets of several 12V lead acid batteries. Here’s an example from a 20 Ah Euroglobe sealed lead acid battery.

In the Constant Current Discharge table, if you discharge to a final voltage of 1.80V/cell (10.8V total voltage), the entry circled in yellow shows that you can get a current of 1.00 amps over 20 hours. The voltage will drop from around 13V down to 10.8V during that time. Let’s call it an average of 12V times 1A – that means you can average about 12 watts for 20 hours.

But wait. In the Constant Power Discharge table, if you discharge to a final voltage of 1.80V/cell over 20 hours, the entry circled in yellow shows a power of just 1.98 watts. That’s far less than 12 watts. Why?

Other table entries show something similar. It’s 11.3 amps constant current for 1 hour – that should be an average rate of about 136 watts, but the Constant Power Discharge table shows a measly 21.6 watts. It’s not just this particular battery either. Here’s a 35 Ah lead acid Mighty Max battery that shows the same curious pattern in the Constant Power Discharge table.

So what’s going on here? Am I misunderstanding what these tables mean? Or is there some other factor that limits the power to a much lower number than is suggested by the constant current data? I’ll keep digging for answers.

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Shelter In Place

My family and I have been ordered to shelter at home until at least April 7, to help stop transmission of COVID-19 in the San Francisco area. Many of you may already be living under similar orders, or will be shortly. Travel is restricted to only the “most essential needs” – basically food and health care.

As you can imagine, this will severely impact business shipments. BMOW will still be accepting new orders during this time, but it will likely be impossible to ship anything until April 7 or later. Please be patient, and if you’re not prepared to wait at least 3-4 weeks for delivery, then please hold off until mid-April to place your order.

Stay safe everyone. Remember to wash your hands.

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Backyard Metal Foundry Dreams

Sometimes my brain works in unexpected ways. I haven’t started any new electronics projects lately, but my thoughts have been spinning in other directions.

Last weekend I jokingly told my kids that I was starting a home-based ore smelting business. Because today’s busy families just don’t have time to process their bauxite, taconite, and other ores at home, the way Grandma used to. Keeping up with the household’s demand for antimony and zinc can be such a chore – but now there’s a better way! My friendly staff will pick up your ore, lovingly smelt it, dispose of the slag responsibly, and return the processed metal in 100g nuggets stamped with your choice of fun logo designs. Naturally, this ore smelting business will be named He Who Smelt It Dealt It.

Three things I learned from this groan-worthy joke:

  1. It’s taconite, not taco night
  2. I’ve been pronouncing the word antimony (an·tuh·mow·nee) wrong for my entire life
  3. Melting converts a solid into a liquid. Smelting converts ore to its purest form.

Yet somehow this smelting comedy gradually transformed from a bad joke into a semi-serious idea for a fun backyard project. Actual smelting probably isn’t a great plan, because where the heck would I find ore? And do I really want to process large piles of messy rocks to extract a bit of tin? Instead of smelting, I soon found myself researching designs for a backyard metal foundry.

I was fascinated. This Mini Metal Foundry design looks simple to build and operate, but can easily reach temperatures of 660 C (1221F) – hot enough to melt aluminum, zinc, lead, tin, and pewter. The molten metal can then be poured into steel molds or sand cast to make tools, toys, and trinkets. Sure the quality won’t be great, but if you perked up at hearing the words “molten metal”, then I like your thinking and we should hang out sometime. Check out this video:

Of course I immediately began planning for my backyard metal foundry. My wife, however, was considerably less enthusiastic about my prospects for doing this without making the neighbors call the fire department or outright killing myself. She has an advanced degree in materials science, and actually has real lab experience working with large pools of molten lead, germanium, and other metals, so she probably knows what she’s talking about. I began to pay more attention once I learned about what happens if there’s a metal spill onto outdoor concrete. Moisture held in the concrete can instantly flash into steam, shooting globs of molten metal in all directions at high speed. See the example at time index 7:35 in the video. It looks horrific. So I’ll hold my backyard foundry plans in the “maybe” category for now.

Enter plan B, an inexpensive 500 Watt electric ladle. Designed for small metal casting projects, this little gem can’t melt aluminum, but it’s still hot enough to melt lead and maybe zinc (though I’m not sure exactly what I’d do with molten zinc). For about $50, it could be the perfect tool for DIY-enthusiasts who want to melt some metals without burning down the house.

A few metals that might pair nicely with this tool, ordered by melting point:

zinc (maybe) – 419C, 787F – The tool says it’ll melt lead, but the melting point of zinc is not too much higher. What can you do with zinc? I’ve heard of zinc plating, but don’t think I’ve ever seen a solid zinc object.

lead – 327C, 621F – Lead has a bad reputation these days, but how great is the risk assuming you’re not eating the stuff? Maybe it’s best to avoid it anyway.

pewter – 295C, 563F – In my mind, pewter is what 18th century candlesticks are made from. It’s an alloy of tin, antimony, and copper. It’s also sometimes used for jewelry and can be polished to a shiny finish.

bismuth – 271C, 521F – I have no mental concept of bismuth except as an ingredient of Pepto-Bismol. What does metallic bismuth look like? Is it safe to handle? Is it ever used for metal casting?

babbitt – 249C, 480F – I’m including babbitt on this list because I’d never even heard of it until yesterday. I learned that babbitt is an alloy of tin, lead, copper, and antimony, and is commonly used for making low-friction bearings.

tin – 232C, 449F – In years past, tin was popular for making cups and dishes. It should be cheap and safe, but maybe not very exciting. At this melting point, I wouldn’t even need any special heating tools: I could just melt tin ingots in steel molds with my kitchen oven.

solder – 183C, 361F – Solder wouldn’t normally be used for casting, but why not? Probably because it’s too easily bendable, and there are better alternatives. Lead-free solder has a higher melting point of 217C/422F, but it’s still lower than any other metal on this list.

Why do all the metals with low melting points have a silver/gray color? It would be nice to have more variety. To find a metal that’s a difficult color, I believe you have to climb the temperature scale to about 890C/1630F to melt brass and bronze. Copper and gold have melting points that are even higher. If I were a super chemist, I’d probably have some explanation why metal colors are related to their melting points.

Have you ever experimented with metal casting for making jewelry or tools? Ever built a backyard foundry and melted some aluminum soda cans? Leave a note in the comments and tell us your story.


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BMOW Store Trouble

Yesterday’s upgrade to PHP 7.1 has broken the BMOW store checkout process. You can add items to your cart and authorize your payment, but the final “confirm payment” step leads to a blank screen. The payment never gets processed, and the items remain in the shopping cart. Please bear with me while I sort this out… UPDATE: It’s working now.

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Floppy Emu SoftSP Warning

I’m calling attention to a hardware issue with the third-party “softSP” card that can damage your Floppy Emu when the two are used with a Disk II controller card in an Apple II+ or Apple IIe. This issue creates a power-to-ground short circuit that will cause accumulating damage to the transistor structures on Floppy Emu’s interface chip. The symptoms don’t appear immediately, and it may seem that everything’s OK for days or weeks, until the Floppy Emu begins to fail irreversibly. The good news is that a simple cable modification is all that’s needed to use softSP and Floppy Emu together safely.

softSP Pseudo-Smartport

BMOW isn’t affiliated with the softSP card – it’s sold by a third party and is designed for use with another type of disk emulation product. It does appear to work with the Floppy Emu initially, and in recent months the Total Replay game collection has inspired a few people into using softSP with the Floppy Emu. Some popular YouTube videos even specifically recommend this combination, even though softSP isn’t designed for use with the Floppy Emu.

The problem is that softSP provides a software patch for Apple II disk controller functions, but does nothing to address the resulting low-level electrical problems on the disk interface. The softSP card contains a small ROM that overrides the built-in ROM on a standard Disk II controller card. It essentially reprograms the Disk II card, so instead of functioning as a 5.25 inch floppy disk controller, it now functions like a Smartport disk controller, which supports block-based disk I/O for disk sizes up to 32 MB. Neat! But there’s a catch.

You can’t safely connect a Smartport device to a Disk II controller card, no matter how the card’s internal logic might be modified. That includes Floppy Emu when it’s configured in Smartport emulation mode. The reason is that Smartport devices connect pin 12 internally to ground. This is how other connected equipment and daisy-chained drives know that they’re Smartport drives, and it’s essential for correct daisy-chain operation of Smartport drives with the BMOW Daisy Chainer or the Apple Unidisk 3.5 drive. For other types of Apple II disks as well as the Macintosh and Lisa, pin 12 is used for the SELECT signal. But on the Disk II controller card, pin 12 is connected to the +5 volt power supply. So when you connect a Smartport device to a Disk II controller, you create a direct power-to-ground short circuit. Ouch!

To be clear, there’s no specific hardware problem with the softSP card itself – it’s just a ROM. The problem arises when using the softSP card to reprogram a Disk II controller card, which is then connected to a Floppy Emu that’s configured in Smartport emulation mode.

Accumulating Chip Damage

The Floppy Emu board has a small inline protection resistor that will prevent immediate damage and failure due to this short circuit, but it’s only meant to protect against brief transients during power-up and power-down, or brief accidental mis-configuration. The Floppy Emu’s CPLD interface chip will likely not survive sustained operation in this mode, because it will cause a continuous current on pin 12 due to the short circuit, with a current level that’s more than twice the absolute maximum rating of the chip. This can eventually cause damage to the chip that will appear as intermittent disk errors or total failure of the device. Unfortunately this type of damage is cumulative, so even if you stop using Smartport mode with softSP and a Disk II card, the damage is already done.

With the continuous over-current, the insulating silicon layers between parts of a transistor can wear away, or develop small holes. At first the effect is minor – maybe the leakage current is more than it should be, or the noise margins are reduced below the spec. The chip may still work OK under normal conditions, but problems may appear under extraordinary conditions at high/low temperatures, or when the supply or signal voltages are close to the rated margins, or when substantial EM noise or voltage transients are present. A problem might cause a 0 to become a 1 somewhere, resulting in a visible I/O error, or it might cause the whole chip to stop functioning until power is turned off. As chip wear grows worse, you may start to see these kinds of problems during ordinary usage. Eventually the problems will grow so frequent that the chip is no longer really usable, or the wear will progress all the way to an internal short-circuit or open circuit within the chip itself, effectively destroying it.

This kind of chip damage can be viewed as a type of gradual wear, like wearing down the engine in your car, rather than a simple yes/no question of is it damaged or not-damaged. Even normal use causes chip wear, and chips do have finite lifetimes, but normally the lifetime is measured in decades or longer. In this case the power-to-ground short circuit is like driving your car without enough oil in the engine. It’ll work for a while, but you’ll start to notice it’s running increasingly rough, and maybe it’ll develop occasional trouble with stalls or failure to start. Then one day the engine will completely seize up and the car will no longer run at all.

Cable Modification Fix

A simple work-around is to sever the 12th wire of the 20-conductor ribbon cable. The red wire is number 1, so simply count wires from there and cut number 12 using a small nail or a razor blade. The resulting cable will work for softSP Smartport emulation with a Disk II controller card, without creating a power-to-ground short circuit. It will also work for standard Apple II 5.25 inch floppy disk emulation. But the modified cable won’t work for true Smartport emulation with other Smartport hardware, nor for 3.5 inch floppy disk emulation, nor for Macintosh or Lisa disk emulation. If you don’t want to modify your original ribbon cable, you can get a spare cable from DigiKey for a few dollars.

Unfortunately modifying the cable won’t undo any damage that’s already been done, so if you plan to use softSP with your Floppy Emu, you’ll need to make this cable modification right from the start. Be safe!

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Windows 10 External Video Part 5 – Failure

I’m either very persistent or very stupid. After 4+ months, I’m still chasing unexplained problems with external video on the HP EliteBook x360 1030 G2 laptop that I bought in May. Recently I thought I’d finally solved it, but I was wrong, and now the problem is worse than ever. I am slowly going insane.

For the previous chapters of this story, see part 1, part 2, part 3, and part 4. Here’s a short refresher:

  • Windows 10 laptop with an external monitor, ASUS PB258Q 2560 x 1440
  • works OK during normal use
  • problems appear every couple of days, after a few hours of idle time or overnight
  • random crashes in Intel integrated graphics driver igdkmd64.sys (stopped after upgrading the driver)
  • computer periodically locks up with a blank screen and fan running 100%
  • Start menu periodically won’t open, must kill WindowsShellExperienceHost.exe
  • Windows toolbar sometimes disappears
  • Chrome window sometimes gets resized to a tiny size

This might sound like a random collection of symptoms, but I’m 99% sure they’re all somehow related to the external video. I suspect the external video is periodically disconnecting or crashing or entering a bad state, which causes errors for the Start menu, toolbar, and applications, and sometimes causes the computer to freeze.


Here’s a recap of the past month. On August 15, I switched from using an HDMI cable to a USB-C to DisplayPort cable. This did not fix the problems. On August 22, in the Windows advanced power options, I changed the Intel Graphics Settings power plan when plugged in to “maximum performance”, and also changed the minimum processor state when plugged in to 100%. The computer then went seven days without problems. On August 29, I also installed two “critical” HP updates: a BIOS update and an Intel Management Engine Driver update. Neither one mentioned anything about video issues in its release notes. The computer went a further 11 days without problems – 18 total days problem free!

Believing that the issue was resolved, on September 9 I switched back to an HDMI cable, which I prefer over DisplayPort for reasons of convenience. Within a few hours, the problems returned. I swapped the DisplayPort cable back in, and everything seemed OK.

But this morning, I once again experienced a frozen computer with a blank screen and fans running 100%. After forcing a shutdown and restart, the external monitor via DisplayPort no longer works at all. When I connect the cable, the Device Manager shows “Cable Matters USB-C Video Cable” but no external display is detected. I’ve tried rebooting and unplugging/replugging the cable at both ends, and cycling through options in the monitor’s menus, but nothing helps. Maybe the cable is broken? It was working last night, and I didn’t touch it after that.

Where Do We Go From Here?

I’m at wits’ end. Is this is a Window 10 driver problem? Laptop hardware problem? External monitor hardware problem? Cable problem? Multiple such problems at once? The sometimes long delay between problem episodes is frustrating any attempt to find a solution. Typically the problems appear every couple of days, but I recently went 18 days between problem events. That makes it almost impossible to make changes one at a time and draw conclusions about whether they helped.

A few people have suggested I try running Linux, to help determine whether this is a Windows problem or a hardware problem. I have briefly run Linux a few times, and it works OK. But I can’t devote 18+ days to running Linux simply as an experiment. Much of the software that I use is Windows-only, and I’m not interested in switching to Linux as my desktop OS right now.

I could look into repairing or returning the computer, but I’m not interested in that. It was a used/refurb machine, and not terribly expensive. While the money is not insignificant, my time and the disruption to my work are more important. I can’t be without this computer for three weeks while a reseller or PC tech experiments with it. If I need to move everything to another PC anyway, then I’ll simply go with that as the final solution.

I could try another DisplayPort cable. Maybe it simply broke somehow. That would be cheap and easy, and everything did seem OK via DisplayPort up until today.

I could also try switching back to my old 1920 x 1080 external monitor. In earlier testing from June-July, that seemed to work OK, but perhaps I just didn’t test it long enough. Reverting to the smaller and lower-resolution monitor would be a disappointment, but at this point I would be happy with any solution that works.

Finally, I could just replace the whole computer. That’s what I keep threatening to do, but I haven’t yet. Reinstalling and reconfiguring all the software on this PC would be a major pain, and the memory of that experience is still fresh in my mind from May. Several multi-gigabyte packages would need to be downloaded again, and node-locked licenses re-requested and regenerated for several software tools. When I went through the process in May, it took close to a week to get through it all.

Perhaps I could clone the existing hard drive and use it in the new PC, avoiding having to repeat all the software setup. But then I’d also be cloning all the HP-specific drivers and plugins and utilities, while missing out on all the vendor-specific stuff for the new computer. And I’d still need to regenerate the node-locked licenses for some software, even if I didn’t need to download and install them again. Maybe it’s better to start with a clean slate, even if it takes longer.

I know any computer can have hardware problems, and any OS can have bugs. But I can’t escape the feeling that this type of driver/hardware mystery is precisely why so many people dislike Windows. I feel like I’m living in one of those “I’m a Mac, I’m a PC” television commercials. If you have any suggestions for other solutions I could try (short of a sledgehammer), please share them in the comments.

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