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I am trying to provide RTC power backup for a 3.3V system that will be running at the very least 8hrs during the work day. As such I only need up to a month of backup, enough to survive over long weekends and vacation periods just in case the system is powered-down.

  • Estimated RTC consumption is <3µA.
  • Minimum RTC voltage 2V.
  • Charge/discharge duty cycle better than 5:1 (40hrs running over a week).

The main driving design consideration is overall size. So the answers provided to related questions such as this one, or this one, are not satisfactory. I am looking for a minimum-size solution and I would like some input regarding the tradeoffs and filling-in important details.

The first solution that I approached was a super capacitor. I found a 220mF 3.5V supercap that in principle satisfies all of the requirements with only a diode and a resistor to charge it. However the vendor does not specify the self-discharge current, so the duration estimate might be very exaggerated. It clocks in at 11mmx11mmx5mm.

The second solution that I approached is an actual battery. I found this 3V coin cell that has a 1mAh stated capacity, a maximum 5µA discharge current and 30µA maximum charge current. This should last for two weeks. A simple two-resistor and diode charging circuit that accounts for that 30µA maximum current would only provide ~3µA for charging during normal conditions. So I have two choices:

  1. Ignore the maximum charging current and design the resistor divider to provide at least 15µA (to satisfy my charge/discharge duty cycle) during expected normal conditions.
  2. Add a transistor a resistor and a diode to limit the maximum charging current.

As size is the main driver, this solution is very attractive as the coin cell only takes 5mmx5mmx1.5mm. However this size advantage is somewhat offset by the extra transistors and 2 resistors.

schematic

simulate this circuit – Schematic created using CircuitLab

Do I have any other alternatives?

Would the super capacitor duration actually satisfy the requirements?

Are there better batteries with a reasonable form factor? (all of the ones I found have very bad mechanical placement for their SMD pads).

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  • \$\begingroup\$ Except for a low leakage diode to charge a super-cap, avoid using diodes. Even 2 Schottky diodes in series is a 1 volt drop. Typical backup time for a super-cap is about 4 days per 1/2 F (1uA per volt), so to last a month you need about 2F or a CR2032 sized coin cell. \$\endgroup\$ – Sparky256 Dec 6 '18 at 21:15
  • \$\begingroup\$ Basing on your area is paramount, I recall Maxim has some integrated trickle chargers (DS139x and others in uSOP packages). Also Abracon has an integrated RTC with trickle charge and crystal built into a 3.7 x 2.5 package (AB-RTCMC series). \$\endgroup\$ – isdi Dec 6 '18 at 21:28
  • \$\begingroup\$ @isdi unfortunately the consumption/RTC part of the problem is already designed into the miniature SBC we are using. So I have no choice on the matter. But the DS139x trickle chargers are minimalistic solutions that would destroy the rechargeable battery, although these would work with the super capacitor. \$\endgroup\$ – Edgar Brown Dec 6 '18 at 21:36
  • \$\begingroup\$ Just put a permanent lithium battery on it? Depending on the RTC you will get years of life. \$\endgroup\$ – MadHatter Dec 6 '18 at 23:00
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In all the times I have designed an embedded system with a need to have a power backup for an RTC it turned out that after all the analysis was completed a non-rechargable coin battery was the superior choice.

I've used CR2032 coin cell for this many times in a socket. Vertical types are available so that the PCB footprint is minimized.

Other products I have used a smaller battery like a CR1225 that is accommodated in a very simple socket like shown below. Although I use the through hole version of the socket bracket to save even more PC space.

enter image description here

Picture source: https://www.adafruit.com/product/1141</sup>

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