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Battery-Powered Apple IIc?

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I’ve got the crazy idea in my head to run an Apple IIc system completely from battery power, so I could take it on the road. I know it’s possible – there’s already one person selling a pre-made kit. That’s a nice solution, but the pictured setup involves an automotive jump-start battery that costs around $150. Would something simpler be possible?

The Belkin Residential Gateway Battery Backup pictured above is designed to keep your VOIP phone or similar device working during a power outage, but it’s found a second life as a popular portable 12V supply for electronics projects. Inside the Belkin unit is a 12V/7.2Ah lead-acid battery. It charges from a standard AC wall outlet, and has a DC barrel plug for connection to the powered device. Hack that off and attach a 12V car cigarette lighter receptacle, or other connector of your choice, and you’ve got a nice little portable power source.

Is 7.2Ah enough to do anything interesting? To answer that question, we need to know how much power an Apple IIc requires. My Apple original AC adapter says it outputs 15V @ 1.2 amps. The IIc can accept any DC voltage between 9V and 20V, so the Belkin’s 12V should be fine here. 15V @ 1.2 amps is 18 watts, and to get the same power at 12V we would need 1.5 amps. That tells us the maximum power the AC adapter can deliver, based on what’s printed on its case.

What about the average power? That’s much more relevant, if we’re trying to estimate the possible run time while connected to a battery. I used a Kill-a-Watt to measure the AC adapter’s power draw from a wall outlet, and it was 12 watts while the IIc was idle doing nothing. So the adapter draws 12 watts from the wall, and delivers something less than 12 watts to the IIc, depending on its efficiency. I don’t know if it’s a switching regulator, or how efficient it is, but I’ll be generous and assume it’s 75% efficient and delivers 9 watts to the IIc. If my reasoning is correct, that means a IIc at idle would draw 0.75 amps from a 12V battery (9 watts).

Back to the Belkin unit: its 7.2Ah battery should theoretically be good for 9.6 hours of runtime, assuming the average power draw of the IIc is close to the idle draw of 0.75 amps. 9.6 hours seems a bit hard to believe, but I’ll go with it for the moment.

Now what about a monitor to use with the computer? Fortunately I’ve got an old portable DVD player with a composite video input that I know works with the IIc. And the DVD player runs off 12V! I haven’t measured its power draw, but I don’t think it can be more than the IIc itself. I’ll assume it’s equal to the IIc’s draw, so the total battery runtime of a IIc with DVD player / monitor would theoretically be 4.8 hours.

If the Apple IIc used any type of standard DC power connector, I’d give this a try right now. Instead, I need to go digging for a female DIN-7 plug. Why must every vintage Apple system include some kind of obscure connector? 🙂

Read 13 comments and join the conversation 

13 Comments so far

  1. Steve June 6th, 2016 6:15 am

    For giggles I also investigated powering the whole setup from standard D cell alkaline batteries. These have quite a large capacity – about 13Ah. Eight D cells in series would provide 12 volts. However, the maximum current a D cell can deliver efficiently might be too low, at around 1 amp or less. And of course it’s not rechargeable like the lead acid battery in the Belkin.

  2. Tux2000 June 6th, 2016 9:19 am
  3. John Honniball June 6th, 2016 10:24 am

    There’s an Amstrad PC-compatible laptop that runs on ten C cells: https://en.wikipedia.org/wiki/PPC_512

  4. Steve June 6th, 2016 2:00 pm

    @Tux2000 – yes, that looks like it. Now I just need to find a local supplier that sells in single quantities. 🙂

  5. Tux2000 June 7th, 2016 11:03 am

    @Steve:

    Shouldn’t be that hard, it’s a standard DIN connector (see https://en.wikipedia.org/wiki/DIN_connector), in North America also known as “small Tuchel”. Farnell part numbers 1193167, 1321490, 1193883, 1620536, 1814810 and Mouser part number 164-60170 look good.

    Note that there are two variants of the 6/7/8 pin DIN connectors, see the Wikipeda article. The DC connector at http://computarium.lcd.lu/photos/albums/APPLE_IIc/album/slides/DSC03623.html looks like a 7 pin 360°. If you can’t find a 7 pin 360° female plug, the 8 pin 360° female plug will also fit.

    According to ftp://ftp.apple.asimov.net/pub/apple_II/documentation/hardware/misc/Apple%20II%20pinout.txt, the connector uses only five pins including the shield. The two extra pins not present in a 5 pin 180° plug (as used for MIDI and ancient german HiFi devices) are unused. So if you absolutely can’t get a 7 pin plug, try a standard 5 pin 180° plug and drill two extra holes into the plug for the unused pins. All holes are on a circle around the center of the plug, with a 45° angle between every two holes. Often, you can see traces of the mold on the plastic, used to produce plugs with more contacts, showing you quite exactly where to drill.

    Try to get a non-metal plug, so you can cut away parts of the outer shell if it conflicts with the IIc.

  6. Tux2000 June 7th, 2016 11:21 am

    @Steve (again):

    Searching for “DIN plug female” on amazon.com returned several usable 7/8 pin female plugs, typically sold in female+male pairs, for “just” 10 times the price of a single plug: http://www.amazon.com/dp/B00K855I54, http://www.amazon.com/dp/B00REFGS78, http://www.amazon.com/dp/B00PZSGGZ0

  7. Steve June 7th, 2016 11:34 am

    Thanks! I actually ordered the first of those Amazon options yesterday, as well as one of the Belkin battery units and a female DC barrel connector with screw terminals. Once everything arrives, we’ll see what kind of silliness I can get up to.

  8. Tux2000 June 7th, 2016 12:39 pm

    I found a scan of the IIc reference manual. Page 207 confirms the DC pinout, Pins 1 and 7 are unused. Page 208 spec\’s the internal switch-mode converter to work from 9 V to 20 V, that fits perfectly to what the lead acid battery can deliver. Please add a fuse (3 A, maybe 4 A) next to the battery.

    Make sure that both IIc and display connect DC GND and signal GND, or that at least one of them uses insulated signal GND. If in doubt, measure resistance to DC GND and voltage between signal GND of both devices.

    Changing a lead-acid battery is quite easy:

    A regulated supply with current limiter is perfect, use 13.8 V, 0.1 A per Ah. It charges using constant current, switching to constant voltage when the battery is full. A simple regulated supply of about 14 V in series with a 10 W car light bulb works fine, too.

    A low current car or motor cycle battery charger will also do the job, but disconnect the IIc and the display first. Cheap chargers use pulsed DC (transformer plus diodes, no caps), and the peak voltages might kill the IIc and/or the display.

  9. Steve June 7th, 2016 3:15 pm

    Could you explain what you mean about connecting the different grounds? As you pointed out, the IIc power connector only has two ground pins, and they’re the same signal.

    I’ll be using this with the Belkin UPS unit mentioned in the post, not with a naked 12V battery, so the Belkin already provides charging circuitry and a fuse. But if it comes to it, I do have a fancy battery charger used for RC vehicles with a huge assortment of different charging modes. Hopefully I won’t need it.

  10. Hales June 7th, 2016 8:20 pm

    Steve: any reason that you are not considering a lipo pack? As I understand they cost more per joule but give much better weight to power and weight to volume ratios.

  11. Tux2000 June 8th, 2016 12:31 pm

    @Steve – hard to do with the limits of the comment function.

    The IIc has a DC in GND at the DC socket, using pins 2 and 3. There is also a Chassis GND at pin 4. Video output has another GND pin, signal GND for the video signal. Chassis GND is connected to most metal parts you can touch, and it is very likely connected to PE through the power supply. Video signal GND is very likely connected to chassis GND. I guess that DC in GND is also connected to Chassis GND, i.e. all GNDs at the IIc are the same. BUT: You could also implement a switch-mode converter that isolates DC in GND from any of its outputs, including from what is called GND at the output side. It could even choose an arbitary voltage to connect to DC in GND, or some internal point (like a heatsink). So you could have substantial voltage differences between the various GND points.

    The same applies to the display. It has signal input, signal GND, positive supply, negative supply (very likely called GND), and most likely, it also has a switch-mode converter. It could connect signal GND and negative supply, allowing to use a simple, non-isolating converter. But on those Murphy days, the display connects its signal GND to positive supply or some internally generated voltage. No problem as long as no input device connects the signal GND of its output via its internal GND plane to negative supply.

    That\’s why I would measure the various GND connections for resistance and voltage differences.

  12. Tux2000 June 8th, 2016 12:57 pm

    @Steve: I thougth that the Belkin box would output low power mains voltage for a wall wart, sorry for the confusion.

    A smart battery charger for lithium- or nickel-based cells will very likely have problems with a lead-acid battery. LiXXX, NiCd, NiMH, and Pb use very different chemistry and thus have very different charging requirements and charging characteristics. Unless your charger has a mode for lead-acid, it will probably mishandle or refuse the lead-acid battery.

    As I wrote, lead-acid cells and batteries have the very convenient property that their voltage is a very good charge indicator (unlike, e.g. lithium cells). And if you connect a lead acid battery via a current limiter (resistor, light bulb, current limiter of lab supply) to a power supply set up to 2.3 volts per cell, the lead-acid cells will automatically charge to 100% and stop there, just compensating self-discharge. No electronics needed.

    Reduce the voltage and the cells will charge or discharge to less than 100%, depending on the voltage. I still have my improvised car charger, using a cheap transformer specified for 11.5 V AC, 5 A, and a 10 A bridge rectifier connected to a cigarette lighter plug. It charged the car battery to a little bit more than 12 V, good enough to start my first car after a cold winter night. The internal resistance of the transformer and the leads act as current limiting resistors, the bridge rectifier prevents discharge through the transformer.

  13. Hales June 14th, 2016 5:24 pm

    @Tux2000: Lithium

    > lead-acid cells and batteries have the very convenient property that their voltage is a very good charge indicator (unlike, e.g. lithium cells)

    Lithium cobalt (the common \’lithium\’ chemistry) does this too. Fully charged: 4.2V, fully depleted: (chosen) 2.7-3V. They slowly reduce over their discharge cycle.

    Indeed lead-acid are much easier to charge, provided you don\’t need to charge them quickly.

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