Description
The 1756-ESMCAP is the capacitor-based Energy Storage Module (ESM) that ships fitted to every Allen-Bradley ControlLogix 5570 controller — the 1756-L71 through L75 and their GuardLogix L7 siblings. It is not a battery and it is not an I/O module. It is a small, controller-mounted supercapacitor pack that catches the falling edge of a power loss and buys the processor the few seconds it needs to finish writing its program, data tables and WallClockTime to non-volatile memory.What makes the 1756-ESMCAP worth specifying is what it quietly removes from the maintenance plan: no lithium cell to age, no two-year replacement cycle, no electrolyte leak down the side of a £15,000 CPU, and no “red BAT LED at 2 a.m.” because a battery tray was keyed for the wrong series. Rockwell replaced the 1756-BA2 era with exactly this part, and the L7x controller is designed around it .
Application Scenarios
A control engineer on a North Sea platform once described the old failure mode with the kind of precision only a pager-duty veteran has: “The battery didn’t fail. The tray did.” A 1756-BA2 in a humid, vibrating rack had sweated its terminals, the cell voltage collapsed during a routine switchover, and the next unscheduled trip wiped the L6x program. The restart took nine hours, six of them spent proving the logic was still on the laptop and the other three re-sequencing the line.The 1756-ESMCAP is the answer to that specific class of failure. On a modern L7x rack, a capacitor does not care about humidity, orientation or a ten-year sit on the shelf — its chemistry does not dry out. When the 24 V supply dips, the controller sees the event, the 1756-ESMCAP discharges through the internal bus, and the save cycle runs to completion before the last of the charge is gone. The operators see a bump on the SCADA trend and a controller that comes back with its recipe, its fault history and its clock intact. Nobody opens a battery log, because there is no battery.
Parameter
| Main Parameter | Value / Description |
|---|---|
| Product Model | 1756-ESMCAP (conformal-coat variant: 1756-ESMCAPK; extreme-temperature version: 1756-ESMCAPXT) |
| Manufacturer | Allen-Bradley / Rockwell Automation |
| Product Category | ControlLogix energy storage module (ESM), supercapacitor-based, field-replaceable |
| Host Controllers | 1756-L71, L72, L73, L74, L75 and the GuardLogix L7 variants; ships installed on every 1756-L7x. Also the L7xXT extreme-temperature build, which takes the 1756-ESMCAPXT instead |
| Storage Technology | Industrial supercapacitor, automatic charge cycle, no primary lithium cell, no rechargeable battery chemistry. RoHS-compliant, no hazardous-material transport or disposal paperwork |
| Bus Supply | 5.1 V DC from the controller’s internal power bus — the module is powered and charged by the host, with no external PSU, cable or chassis slot required |
| Operating Current | 330 mA during charge/operation; peak dissipation around 13.3 W while the capacitor bank is topping up. The inrush is brief and internal to the CPU’s supply budget |
| Charge Time | Approximately 2 minutes from fully depleted to operational, 2–3 minutes to settle. Contrast that with a fresh lithium cell, which is simply “there” — the ESM is ready again before the shift supervisor finishes the outage report |
| Hold-Up Role | Supplies the controller long enough to complete the NVS (non-volatile storage) write at power-down and to retain the WallClockTime through a short off-period. It is not a UPS and does not keep the processor running, scanning or controlling the line |
| RTC Retention | Wall-clock hold measured in days at room temperature, shorter as the enclosure warms; Rockwell documents the depletion curve against temperature. If your audit trail needs the clock after a long weekend, verify the curve against your ambient before trusting it |
| Physical Fit | Direct-mounts to the side of the L7x CPU, no chassis slot consumed, no backplane position, no DIN rail. Approx. 100 × 25 × 80 mm, around 0.1–0.15 kg — the weight varies by whether the seller quotes the bare module or the box |
| Status Indication | Charge/health shown on the controller face and surfaced in Controller Properties → Health / minor-fault status in Studio 5000. A “needs maintenance” or ESM fault condition is a minor fault, not a major, and the OK LED behaviour during the save cycle is documented in the manual |
| Environmental | 0…+60 °C operating, -40…+85 °C storage; IP20 by virtue of being installed inside the controller enclosure. Pollution Degree 2 / IP54-minimum cabinet requirement applies in Zone 2 installations per the system-level warning |
| Agency / Compliance | CE, UL 508 / cUL, RCM (as declared on the ControlLogix system declaration records); the module inherits the controller’s system-level ratings rather than carrying its own panel-nameplate rating |
| Software Footprint | None. No ladder logic, no AOI, no tag, no EDS. Studio 5000 sees it as a controller health attribute. That is the point: a part that protects the project without becoming part of the project |
| Lifecycle | Active, supported by Rockwell; field guidance is to proactively replace on a long-interval schedule (common practice: ~10 years) even though the capacitor has no hard expiry date |
