Source
r/ECE•
akmark

Question about rechargeable battery life.

My background is some sort of vague mix of hobbyist and vocational training for digital electronics and a compsci person overall and my question is this:

I have this device (Sansa Mini-clip) and it is by far the most power efficient device I've ever had. Yes the actual work it does is not as intense as a laptop but in general the battery has negligible degradation over time when it comes to the charge kept. I've had it for at least two years now and at one point it kept a charge without being touched for 3 months that was still mostly full (literally one of those I found it behind the couch and switched it on and it was good to go). What makes this device so good? I really don't even know the particulars well enough to start looking for an answer, but I am interested more in the technical side of things. As a hobbyist I would just wonder what I can do to make my electronic projects this efficient (or if anything if I leave a battery hooked up how to minimize drain).

Anything would be great even if its just a list of words I can start researching from, I just don't have enough experience with ECE as a whole to figure it out.

Posted Sep 21, 2011, 7:41 PM UTC

119
eric_ja•

The main thing it's probably doing is just being a cool-running, low-power, low-clock speed device. What really wears batteries out is running a high level of charge and high temperatures for an extended period of time.

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

6
akmarkOP•

Way cool! This is a great place to start. Thanks.

1
goki•

It appears to have a "real" power switch, which means it can use zero power when off. The only loss in capacity being from the batteries internal self discharge (~5% per month).

Since the battery is not removable, I will speculate that it doesn't have always active protection circuitry (laptop example). To be fair some protection circuits draw can be in the 10's of uA, which is minimal (5% self discharge for a 1Ahr batt would be ~20uA).

You can easily use a hard power switch in your own projects :). If being "always on" is unavoidable, it gets more complex (sleep modes, quiescent currents, or dedicated power switch ICs).

3
akmarkOP•

Ah, I just re-examined the device and it's a little more clear that it's a real power switch. It just 'feels' like it has an active power switch IC because you have to hold the switch on for a few seconds before it actually turns off. I suppose they have figured out a good way to make it a full power off instead of just an IC like you mentioned.

2
goki•

Actually it looks like I'm wrong, the switch returns to its position and there is some logic to it (slide it for X seconds).

So, I can't really say without seeing the schematics. Its possible there is some logic that shuts off most circuitry and only manages that button. Or its also possible that all of the parts are just very low power.

Microcontrollers for example, can have a sleep state where most of the chip is unpowered. Only very basic sections are left on to be able to detect say, a button press. Once that happens the chip is powered up to be able to process the event.

http://embedded-lab.com/blog/?p=3237

2
elHuron•

It could be the same as with a mainboard, where you press the button and then (I guess) it closes a transistor using logic to stay on

1
jabies•

Is that 5% exponential or linear?

2
goki•

http://www.mpoweruk.com/images/self_discharge.gif
http://www.mpoweruk.com/performance.htm

Probably depends a lot on material, quality, etc. Supposedly some bare 18650's are <1%.

http://www.candlepowerforums.com/vb/showthread.php?115206-Li-Ion-Self-Discharge-Rate-Test

1
puguhwah•

yes Ni Cad actually more long life

make rechargeable battery

1