Real-time-clock backup is one of the few places in industrial electronics where a primary CR2032 has held the design line for two decades. ML rechargeable coin cells have been quietly displacing it since 2020. Here is when ML wins on engineering merit, and the four design rules that get it right the first time.

The CR2032 status quo

A typical industrial controller (PLC, motor drive, network gateway) uses a CR2032 lithium primary in a tray holder to keep the RTC alive between power cycles. Self-discharge is roughly 1 % / year, capacity 220 mAh, expected life 8–12 years on a 1 µA RTC load. Replacement is a service call.

The ML approach replaces that with a soldered-in 65 mAh ML2032 that recharges from main power whenever the controller is on. Self-discharge is higher (~2 %/month under voltage hold), but because the cell continuously trickle-charges, the RTC sees full capacity indefinitely — until the cell hits ~10 years of total cycle exposure, at which point cycle ageing limits life rather than capacity loss.

When ML wins

  • The product runs from main power most of the time and the RTC backup is for short outages (< 30 days continuous).
  • Service replacement is expensive (industrial, telecom, solar inverter) — eliminating a tray holder and a service call typically pays back at unit volumes above 5,000.
  • Operating temperature exceeds CR2032''s rating of +60 °C. ML is rated to +85 °C with a different electrolyte.
  • The PCB sees lead-free reflow. ML can be reflowed; CR2032 cannot.

When ML loses

  • The product is fully off-grid for months at a time. Self-discharge eats the cell long before cycle life matters. Stay with CR2032.
  • RTC current draw exceeds 5 µA average. The 65 mAh ML reservoir runs out in months without recharging.

Four design rules for an ML RTC

Rule 1 — current-limit the charge path. ML wants ≤ 0.05 C constant-current trickle until the cell hits 3.0 V, then constant-voltage hold. A 1.5 kΩ resistor on a 3.3 V rail is the simplest implementation, sized for ≤ 1 mA charge current. Or use a one-cell charger IC like the MCP73831.

Rule 2 — diode-isolate the discharge path. When main power is off, the ML cell becomes the only source. A Schottky diode on the cell positive prevents reverse-current sneaking back into other rails. Use a low-leakage Schottky (BAT54-type, ~30 nA leakage at 25 °C).

Rule 3 — undervoltage-protect the cell. ML below 1.8 V is irreversibly damaged. Add a comparator that disables the discharge path at 2.2 V cell voltage. The 0.4 V margin protects against the diode forward drop and ageing.

Rule 4 — temperature-protect the charger. ML below 0 °C should not charge. A simple PTC thermistor in series with the trickle resistor takes care of this without adding firmware.

Compliance trail

Because ML is rechargeable lithium, it triggers IEC 62133-2 instead of IEC 60086-4 (which CR2032 uses). UN 38.3 also requires a 50-cycle pre-test before shipment. Both are routine; we ship every ML lot with both reports.