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Filipx.87
Filipx.87
Associate II
October 22, 2021
Question

RTC backup rechargeable battery

  • October 22, 2021
  • 4 replies
  • 3521 views

0693W00000FDSJMQA5.pngHi, is this the correct connection for the RTC backup rechargeable battery? Thanx.

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4 replies

Peter BENSCH
Peter BENSCH
ST Technical Moderator
October 22, 2021

It depends on the chemistry of the battery, not all battery types can withstand a permanent connection to 3.3V.

What type of battery do you plan to use?

Regards

/Peter

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Filipx.87
Filipx.87Author
Associate II
October 22, 2021

https://www.newark.com/varta/55615603940/battery-button-cell-nickel-metal/dp/27C2141

Peter BENSCH
Peter BENSCH
ST Technical Moderator
October 25, 2021

The battery datasheet also says:

  • Do not deep-discharge into reserve
  • Restrict charging current and time to the recommended value

You cannot rule out the first case if you connect the battery directly to pin VBAT. As soon as the battery voltage drops below 3*0.6V = 1.8V, you're going to deeply discharge the pack.

Even if you only discharge down to 1.8V or the possibly of a longer stored battery has a significantly lower voltage than 3.3V during installation, the second point is even more dangerous because you will most likely significantly exceed the recommended charging current (0.3*CA = 45mA) during power-up.

With a capacitor instead of the battery, both points are essentially omitted completely, even if one has a significantly shorter buffer time with the same component volume.

Regards

/Peter

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waclawek.jan
waclawek.jan
Super User
October 25, 2021

Have you considered using non-rechargable primary cell, such as CR2032 and similar?

JW

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