PSU Thoughts

Posted on by crackingcontraptions_3voi0v

SuperCap Feasibility

I’d like to start by seeing how much I can get from a supercap.  This seems to be one with a reasonable capacity, and a decent shape for a wearable – although sadly it isn’t curved.
The SuperCap is (14x21x3.6mm) ~ 1058 mm3.   The LiPo below is Size: 11.5mm x 31 x 3.8mm ~ 1355 mm3
LiPo:     11.5*31*3.8/105 =  13 cu mm / mAh
SupC:   14*21*3.6/24 = 44 cu mm / mAh
So the SuperCap is about 30% of the energy density.
At the maximum 10C charging rate which is 240mA it takes about 6 minutes to charge up.
Discharge is fairly flat for about 55/65 = 85% of the time.  After that it dips away quickly.  So let’s assume that there are 24 * 85% mAh in the SuperCap.
I assume I need a boost regulator to get to 3v3, or a buck regulator to get down to 1v8.  Running at 1v8 means logic-level converters to talk to some sensors.  A booster will waster 5% of the energy at best.  I’m not sure what is a reasonable estimate.  Let’s say 10%.  
So now we are talking about 24 * 0.85 * 0.9 which is 18mAh.  So I wonder how many hours I can listen on BLE with 18mAh to spend.

The Amazon BiP sports a 190 mAh battery. Our data shows that it can listen for BLE packets for at least 10 days.  It’s also running sensors, a display, and a GPS periodically.

So its sucking 190/10 mAh per day = 19mAh / day

So a supercap is in the ballpark.  Maybe with an e-ink display it can do better.

Charging the SuperCap

The SuperCap can only be charged at 2v7 max (?).  And the maximum current it can handle is 10C which is 240mA.  (Does that mean fat traces?)
Given this will be charged initially from a regular 5V USB PSU (I assume) then there needs to be something to limit the current, and voltage.
I know I can limit the voltage with an ordinary regulator, but this needs to limit the current too.  
MAX17525 , or MAX14575 seem like they might do the trick.  I suppose there are lots of contenders.  These are the first I found.  

0 thoughts on “PSU Thoughts”

  1. mgl says:

    Is the straight "supercap quick charge" energy harvesting? I can see that having a single architecture for both energy harvesting and quick-charge is good, but do they each need a specialized solution? What matters?

    Quick charge needs 240mA, whereas TEG, or photovoltaic can charge much moer slowly, presumably at lower currents?

  2. mgl says:

    Some energy harvesting regulators limit their input current, and their output voltage, but is that significantly different from output current and voltage? Could some current get lost in translation?

  3. mgl says:

    My plan was to get the biggest (conceivably wearable) supercap and run some trials to see how long, or how often it could perform some representative benchmark tasks, such as:
    – continuously listen out on BLE (how many mins/hours)
    – connect and transmit one packet (how many)
    – rewrite an e-ink display (full and partial rewrites)
    – wakeup and proces IMU interrupt (e.g. pace counting)

    The 10C charge current for my example supercap was 240mA!

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