NSLU2-Linux
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How to make a battery powered slug

Try this at your own risk and only if you have some electronic knowledge; No warranty! If you are not willing to take all responsibility from doing this, then don't!

As the slug is a very versatile device, it seemed an interesting challenge to see if the slug could be battery powered. Or as RonM stated: "Never leave home without it!".

First I did some measurements. My power supply (which is not the standard one) gives 5.3 V. When the slug boots without anything connected it takes 400 mA. With the Ethernet connected (but still nothing on USB) the current is about 480 mA.

This seems low enough to make battery power work. In order to get in the area of 5V it is best to use 3 or 4 batteries. When rechargeable batteries are used you'll definitely want to use 4 as these are typically 1.2V each.
For regular batteries I would recommend 3 (as that will give between 4.5 and 5 V). You could try 4. The slug itself can handle it, but it is unknown what the effect is on the voltage on the USB connector. Better safe than sorry, so stick to 3 batteries unless you really know what you are doing.

In order to battery power the slug you'll need:

  • a power connector (I fried my power supply in the past, so I just cut off the connector)
  • a container for batteries (and some fresh batteries)
  • a piece of cable
  • a voltage meter
  • a soldering iron and some solder

Just solder the cable to the battery container and to the connector. Make sure that the inside of the connector is connected to the + and the outside to the -. (This is where the voltage meter comes in handy).

Put some fresh batteries in the container. Again measure that the + is on the inside of the connector. Verify also that the voltage is around 5V.
If everything is sound, connect the power to your slug and enjoy the view of the LEDs? while it is booting.

Measurements with not-so-fresh batteries indicated that the voltage will drop when the batteries are connected (not really a surprise). I've seen a voltage drop from 6V to below 4V. The lowest I was able to get the slug running is around 3.5 V. In that case the current consumption is about 550 mA with the Ethernet connected.

If you are carrying around the slug it is probably not connected to the Ethernet. My guesstimate is that if you use only a single memory stick, the power consumption will be around 500 mA. This means that with a good set of batteries you should be able to power the slug for several hours. I did a test with 4 x 1700 mA/h AA batteries though and there the voltage dropped from 5.3 to 3.4 V within half an hour. After disconnecting the slug the voltage rose quickly back to 5.3V. Apparently my batteries cannot deliver sufficient current for a sustained period.

It has also been suggested to use a 6V lead acid gel battery and connect this through a 7805 (or similar) voltage regulator. This has not been tested yet though. And it almost certainly won't work well: the 7805 drops more than 1V between input and output and so is unlikely to provide a reliable 5V. But a low-drop regulator like the LP38690 could work.

More simply, you could try a diode such as a 1N4001 to drop around 0.6 Volt - providing 5.4V to the Slug. At this level, you're less likely to damage USB devices than by running the 6V directly to the slug, and as the battery voltage droops as it discharges, the slug and USB devices are likely to reliably keep on going for longer.

[Tibby says: Be careful using a 1N4001 to drop voltage. If you have lots of accessories attached to your Slug, you might exceed the 1A rating!]

Also it was suggested to use an ISun Portable Solar Charger. Again not tested.

Try this at your own risk and only if you have some electronic knowledge; No warranty! If you are not willing to take all responsibility from doing this, then don't!


As part of my Slug Carputer, I am using batteries in my Honda Hybrid

To provide consistent power, and meet the power needs of the slug, I am using a 2 Carnetix 15watt, 5v Point of load converters. These devices use 10-15v input and deliver up to 3A each of clean 5v. I have 2, one for a USB 80GB HDD on Disk2 and one for the Slug with a 512MB Flash Drive1. The whole unslung portion of my carputer uses less than 1A at 12v. To power this, I have a 34AH Gel Battery (12v) under the passenger seat. This, in turn, is recharged by the starter battery. (I don't mess with the 150volt battery used for locomotion)


The best way to run from a 12V gel or conventional lead-acid battery is to use a switching regulator; linear ones will waste more than half the energy supplied as heat. Instructions for building one can be found at [http://johnarthur.wordpress.com/2008/06/08/an-efficient-voltage-regulator this page]


I just did a test with an old (10 years old!) UPS battery - 12V 7AH 20H - I used a car USB converter to convert from 12V to 5V. I let it run overnight. Based on the cron entries in /var/log/messages - it lasted for 9 Hours.


I think that, given the right power cable, an iPad car charger would work to power a slug @ 12-15VDC. iPads need 5VDC @ 2A to charge properly. This is in theory, however. I'll order an iPad car charger and report back.

-freeflashstuff

The iPad charger does supply significant enough amperage to run my NSLU2 (Unslung) running a flash drive and my FTDI USB-to-Serial adapter running lighttpd and perl with FastCGI?. :)


Powering a NSLU2 from a 12 Volt Battery

Good for solar-powered applications, automotive applications, or battery-powered applications.

Power Requirements

The NSLU2 requires a regulated 5 volt power source up to 2 amps. To power a NSLU2 from a 12 volt battery, you need a power supply that can take the variable battery voltage (10.5 - 15 volts) and provide a constant 5 volt supply. I have tested three power supplies: a 7805 linear voltage regulator, a high efficiency LM2575T switching regulator, and Murata Power Solutions 78SR high efficiency, synchronous switching regulator. The 7805 and the LM2575T have the ability to provide up to 1 amp which should be adequate unless you are driving two high-powered USB devices. The Murata 78SR is rated at 2 Amps.

Power Tests

A basic NSLU2 connected to Ethernet and an externally-powered USB hard drive draws 0.53 amps at 5 volts (2.7 Watts) when idle. When transferring a large file to the NSLU2, it draws 0.59 amps at 5 volts (3 Watts).

I conducted tests of the three power supplies with an input power source set at 13 volts.

The 7805 linear voltage regulator provides a stable 5 volt power source. When powering the NSLU2, it draws 0.53 amps at 13 volts (6.9 Watts) when idle and 0.59 amps at 13 volts (7.7 Watts) when transferring a large file. This regulator has an efficiency of 39% and is hot to touch.

The high efficiency LM2575T switching regulator also provides a stable 5 volt power source. When powering the NSLU2, it draws 0.30 amps at 13 volts (3.9 Watts) when idle and 0.34 amps at 13 volts (4.4 Watts) when transferring a large file. This regulator has an efficiency of 68% and is slightly warm to touch.

The Murata 78SR provides a stable 5 volt source. When powering the NSLU2, it draws 0.22 amps at 13 volts (2.9 Watts) when idle and 0.25 amps at 13 volts (3.3 Watts) when transferring a large file. This regulator has an efficiency of 93% and has no noticeable temperature difference to touch.

Conclusion

The high efficiency switching regulators are far more efficient than the 7805 linear voltage regulator, and the Murata 78SR is very efficient (especially important if used in a solar-powered or battery-powered application). The kit for the 7805 and the LM2575T board cost <$10. The Murata 78SR cost around $14 from Mouser, and I had to build a little circuit board with some terminal blocks. It actually made a very nice, small power supply. Also the 78SR is pin compatible with the 7805 and could be soldered into the 7805 kit below (I'd recommend the 78SR meant to be mounted horizontally if you are going to do this as it will fit flush to the board in the area where the unneeded heat sink would go.

Sources of Power Supplies

7805 Linear Voltage Regulator kit(approve sites) produced by kitsrus.com (I bought my from electronics123.com). You also need to buy a separate 7805(approve sites) for this kit.

High Efficiency LM2575T Switching Regulator from futurlec.com.

Murata Power Solutions 78SR 2 Amp Series(approve sites) - I bought mine from mouser.com.


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Page last modified on March 14, 2011, at 05:14 AM