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I want to identify a rechargeable battery pack from inside an old portable hard-drive.

enter image description here

It is unlabelled. (I ripped open the blue covering - it isn't labelled on the internal metal cover either.)

Battery is ~55m x ~33m x ~5mm - so relatively flat. The plug is 3-4 mm wide.

Device takes +5V DC, so I imagine that is the charging voltage. One part of the circuit takes 3.3V DC, so I imagine that's the output voltage.

How is this battery specified?

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It looks like one of a standard Li-Ion or Li-Po pouch cells, size 5.0 x 33 x 55 mm, more like 50 mm, or 503350 for short notations, could be 503250. An example from Adafruit of a 503035 battery:

enter image description here

The example has h=5mm, w=30mm, and L=35mm

To see the actual label and determine the size, you need to carefully remove the blue wrap and exclude battery protection part from measuring.

CORRECTION: the battery protection part is included into length measurement.

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  • 3
    \$\begingroup\$ After the whole mess on meta, TIL that these flat battery packs have short notations like individual round cells do. \$\endgroup\$ – Passerby Aug 31 '18 at 16:31
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Aside from physical dimensions, such rechargeable cells are specified by:

  • Chemistry (LCO, LTO, LFP, etc.) LCO is the most common for older cells.
  • Min and max operating voltage (those largely depend on the chemistry)
  • Capacity (in A*h)
  • Max charging and discharging currents. Those are usually proportional to the capacity for a given battery type, so they are often specified in relative units called C rate. A battery capable of 1C charge/discharge can be fully charged/discharged in 1 hour. 2C means 30 minutes, etc.
  • Other parameters like temperature range which are less relevant in this case.

AFAIK there is no reasonable way to determine the parameters above for an unlabelled battery.

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Given a blue chunk of plastic, there is no way to tell what the underlying battery might be.

  • It could be Li/Ion, Li/Poly or Li/FePO4.
  • It could have one of several different charge voltages and charge currents.

If you replace it with "some other battery that is also square-shaped", you might end up damaging the cell or in worst case blow it up. Notably there are only 2 wires, not a 3rd one for a NTC thermistor, so you can't just use any charger either.

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