Hacking Duracell's Powermat Battery

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The box, and the $3 sticker.

Since this is a work in progress, where I don't know exactly where I'm going, and with several directions to explore, this is going to ramble. Once I've got somewhere, I'll probably branch off a conclusions article for the people in a hurry.

This isn't so much a hack article yet, as exploring something cool for a good price that can be used in lots of future hack projects.

Introduction

While camping summer of 2014, I discovered that my cell phone will quickly suck its battery dry in marginal coverage areas. So, when the local discount warehouse had Duracell Powermat battery packs for $5, I picked one up. It works great for keeping my phone charged, especially when I'm using it as a wifi hotspot for my tablet or netbook.

I have no idea why this unit is suddenly being dumped on the market at low prices. Huge overproduction, replaced by a new model, some hidden defect, or perhaps the latest iPhones are finicky about what chargers they accept? Dunno. (If anyone has any hard info, email me. It's on the About page.)

When the local Dollar Store had bunches of them for $3, it was the time and price to hack them for other uses! I should have bought a bunch because, as great stocking-stuffers, they were gone when I went back next week. Hopefully there'll be more later.

  • 12/30/2014: Success! They had more in stock at $3, so I picked up another three. I might get more after I have purposes for them.
  • 1/8/2014: The discount warehouse lowered its price to $2.75. It's a race for the bottom.
  • 2/8/2015: And now My Charge 2000Mah Trek Power Bank W/Apple Dock battery prices are headed for the floor. What is it about this class of product where the lack of demand is forcing the price down? All the more for me, I guess.

The tiny manual says "Do not disassemble the charging device. There are no user-serviceable parts inside." Bwahahahaha! (Read Vernor Vinge's Rainbows End for what I think of that!)

Battery unit

  • Charge inputs:
  1. Micro-USB connector and 5" USB cable.
  2. Duracell Powermat inductive pickup, PMA standard. (Base charger not included.)
  • Output: Built-in Apple and micro-USB connectors, 5V 1A.
  • 1850 mAh, 3.7V lithium battery.
  • Power button and 4 charge level LEDs.
  • Case: 3"x3"x3/4", rounded corners, faux-aluminum plastic body, black or white colored top and bottom inserts.

Gallery

Thoughts

  • Because batteries don't keep a steady voltage during their discharge cycle, and smart phones are picky eaters, it has some kind of boost/buck converter to maintain a constant output just under 5 volts from the 3.7V battery. Likely it also has one on the charging input pickup to turn the induced AC into DC at the correct battery charging voltage. That has the possibility of being hacked as an input from a number low power sources: small solar cells, wind belts, lemons, etc. If not, then rolling your own boost converter isn't hard. I also have similar 1.5V AA to USB chargers that could be re-purposed too.
  • Reading the manual, the Powermat base unit can detect when the battery has been fully charged through the inductive pickup. Interesting. I looked to see if anyone had pulled one apart to see how the system works, but didn't find anything, and the specs are club-members only. No doubt this unit has a tiny microcontroller embedded in there somewhere to manage everything. It'd be nice if it had overcapacity, spare I/O and could be hacked and harnessed, but I wouldn't bet on it. (It's a battery pack, not a Qeng Ho localizer.)
  • If you get a bunch of these, put a different label or sticker on each one, otherwise you'll never remember which ones are charged or drained.

Sample applications

  • Emergency light with a three LED head and resistor left over from a broken flashlight.
  • Rechargeable battery pack for portable radio. Replaces 3 AA batteries.
  • Remote power. Re-purpose a cheap solar patio light as a case and low power charger. Use it to power wireless sensors, alarms, data collection, etc.
    • It already has a magnet with a surrounding coil. Perhaps another magnet moved by a wind-belt could generate power?
  • A cheap UPS for projects like a Raspberry Pi or Arduino. A data line to signal that the battery pack is running low, save everything and shutdown now would be good. (Line from the 2nd last power level LED?)
  • A USB charger pack. Okay, not much of a hack there-just cut off the Apple connector and add a mini-USB plug, and it'll charge my GPS on long-distance scooter trips when it sits in my pocket with an earplug in my ear for directions. Those AA to USB chargers fall behind, and I've spent some time sitting on back roads in the pitch-black cold, burning AA batteries and waiting for it to charge up. (Not a total waste: Pitch-black with fireflies!)
  • Power pack for portable projects that can be wireless recharged at any Starbucks.

Problems

It's not automatic, you have to press a button!

  • When the battery is drained or there is no load on the output, the battery will switch off after several seconds. Switching it back on requires a button press.
    • Low power applications might not be enough load to keep the battery on. (e.g. a nightlight that's off during the day.)
    • If the battery drains, it won't switch back on when it has recharged.
    • Work-around: hard-wire the button on?

Sleepy at the switchover

  • Power cycling as a UPS. The battery will charge a phone while being charged/maintained itself. However an initial test showed that, on input power loss, it doesn't smoothly switch from its charge input to its battery power, and caused a reset of my Raspberry Pi.
    • Work-around: A big cap on the output to smooth-out the fluctuation?

Dissection

  • The unit is sealed by the top and bottom insert pieces snap-fitted together through the main body.
  • There doesn't seem to be a non-destructive way of getting the ends off to look inside this.
  • I slipped a screwdriver under one of the ends and pried up until I snapped the four posts that hold the top and bottom inserts together.
  • At it happens, I removed the bottom insert which covers the centering magnet and inductive pickup for the Powermat input.
  • This is the boring side of the PC board, with traces but no components.
  • I should be able to trim the remaining posts and remove the top insert non-destructively.
  • That PCB coil is probably for communication with the base. Likely there's a real copper coil under it for power transfer.
  • The battery is held down by something under it, so there might be other components there. I suppose following the leads from the power level LEDs would lead to the smarts on the board.

Progress

More later, with pictures, as I disassemble my sacrificial unit.

Experiments to try:

  • What happens when the USB charge input gets less than 5V, perhaps a lot less, like 1.5V from a solar cell? Most USB devices would turn up their nose, but if there's a voltage converter on that input, it might just up the voltage and drink what current it can take. If that worked, it wouldn't be necessary to dissect units to get at the inductive charge circuit or use my own converter in front of that input.
    • From a 1.5V button cell, no result. From a 3.27V 2032 cell, charge lights came on.
    • Using a solar garden light cell @ 3.5V, the charge lights came on. Now the question would be: Is the solar cell gaining on the charge LED drain? I should drain one of the batteries, and leave it in the sun for a day.
  • Get another strong magnet and vibrate it close to the induction pickup. If it can use the induced current as charge input, it would make a dynamite core for a wind-belt power station without even having to take it apart.
    • Does it need to handshake with the base unit first? Who knows. The specs for PMA appear to be members-only.

External links

Remember: Don't throw away those old printer parts. They could be the core of your next giant killer robot!