Homemade Tablet – Auxiliary Battery (5)


Posted: 07/30/2015:

I prefer my batteries to be cool all the time. Since the stress on a battery is dependent on its load, all of the descriptions I have given assume a load of either the Odroid by itself, or the Odroid in combination with the LCD, and I select the power configurations accordingly.  I don’t know about powering any other devices, and make no suggestion to do such a thing.  The Odroid board itself, and its associated VU LCD display have known power consumptions. In addition,  the Odroid board can draw considerably more if many high-current USB devices are attached to it.  In my situation, I have only low current USB devices attached. The total current draw from the Odroid C1 board (by itself) is only about a half amp (500 mA) – and a little more running Linux).  But, this is another criterion to take into account.

Some comparative figures are in order. An absolute number, without comparison, is pretty worthless. Using the high-to-low DC-DC converter, with both the Odroid C1 board and the VU LCD running, we have ~ 750 mA of pull from the battery (there’s more on the converter output, since it’s at 5V).  How does our 750 mA of battery current stack up against other things we know about? Well, a Kodak digital camera, with four AA batteries, can draw 1200 mA when its LCD screen is being viewed. A typical old-timey style flashlight uses 400 mA of current from old-style dry cell batteries. Those batteries are not capable of great things, which is why digital cameras burn up regular batteries in no time at all. Camera manufacturers recommend something like non rechargeable lithium ion batteries to stand up to the 1200 mA current draw. NiMH are not old-timey batteries, are are similar to lithium in terms of the energy they can provide. So – our 700 mA is substantially below the load of a digital camera, especially considering that we are using C cells rather than AA cells.

So, considering the power capacity of both the internal and external NiMH packs added together, we have batteries capable of delivering 100 watts of power, while the cell phone people hold their computers in the palms of their hands, meaning they have much smaller batteries.  Are we just plain silly? The cell phone people don’t have 1.5 Ghz, four core computers with damned nice Mali graphics GPUs and nice 9 inch, high res displays, do they? They can’t reasonably compile Firefox with the GNU C compiler either, run Gimp to make pretty pictures like the ones in this article, or make connections to a GPIO interface, ADC converter, or I2C interfaces.  Nuff said.

Just a curious note, for those who wonder where the Odroid is popular. I don’t get very many hits from the United States, other than my own (of course). I get a bunch of German and Spanish visitors, quite a few British, Australians, Canadians, and New Zealanders, and a spackling of other European (French, Polish, Romanian, Italian) visitors. In asia, I get India, Thailand, Vietnam, and Maylasia – but oddly, no Chinese. The most unusual hits came from Columbian visitors and also a single one from Panama. There’s an occasional hit from Argentina and Chile. So, maybe the website should have a german and spanish translation, since that seems to be a lot of the hits. These numbers are pretty casual, taken from memory. Maybe I’ll collect some info in a more structured way, and list the results later.

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Note: the author does not have a recent, applicable background in circuit building, or battery related issues, so this is presented as the work of a hobbyist, and is not meant for duplication by others. Readers should look elsewhere for design advice and info.


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