Industrial automation cabinets rarely fail in dramatic, cinematic ways. They fail in the most annoying way possible: a brief mains dip, a breaker trip upstream, a motor starting next door, and suddenly your PLC reboots, your Ethernet switch resets, your HMI goes dark, or your I/O goes into an “I have no idea who I am” state. In many plants, the downtime cost of a 200-millisecond interruption is wildly disproportionate to the event itself.
That’s the niche Siemens built the SITOP DC UPS ecosystem for: keeping the 24 V DC control power alive long enough to ride through short interruptions, perform controlled shutdowns, preserve data, and prevent nuisance faults. The Siemens 6EP1935-6ME21 is a dedicated SITOP battery module rated at 24 V with 7 Ah capacity, designed to be used with SITOP DC UPS modules (6 A, 15 A, and 40 A classes). It is a maintenance-free battery pack based on sealed lead-acid batteries, intended as the energy storage element in a DC UPS architecture.
Instead of treating “backup power” as a separate, bulky AC UPS, this approach keeps everything on the DC side: a 24 V power supply feeds your loads and the DC UPS module, and the battery module is connected on the UPS battery interface. When the AC input fails, the UPS module seamlessly switches to battery buffering, keeping the 24 V rail available for the loads that matter most. The result is a cleaner cabinet layout, predictable behavior, and a backup system sized specifically for controls rather than for the entire panel’s AC input.
What 6EP1935-6ME21 is ?
6EP1935-6ME21 is an energy storage module, not a power supply and not a complete UPS on its own. It stores energy (7 Ah at 24 V nominal), and the SITOP DC UPS module manages charging, monitoring, and changeover behavior. The battery module is engineered for the electrical and mechanical expectations of the SITOP DC UPS platform, including defined charging parameters and protection elements.
Key technical specifications (official data)
| Item | Siemens 6EP1935-6ME21 Specification |
|---|---|
| Product type | SITOP battery module, maintenance-free sealed lead-acid |
| System compatibility | For SITOP DC UPS modules 6 A, 15 A, and 40 A |
| Battery capacity | 7 Ah |
| Nominal output voltage | 24 V DC |
| Max output current (buffering mode) | 30 A |
| Peak current | 30 A |
| Max charging current | 1.75 A |
| End-of-charge voltage recommendation (temperature dependent) | -10 °C: 29 V; 0 °C: 28.4 V; 10 °C: 27.8 V; 20 °C: 27.3 V; 30 °C: 26.8 V; 40 °C: 26.6 V; 50 °C: 26.3 V |
| Short-circuit protection design | Battery fuse 20 A / 32 V (blade-type fuse + holder) |
| Operating temperature | -15 to +50 °C |
| Transport / storage temperature | -20 to +50 °C |
| Self-discharge / temporary capacity loss | Typical 3% per month at 20 °C |
| Typical service life (to 80% capacity, EUROBAT reference) | 20 °C: ~4 years; 30 °C: ~2 years; 40 °C: ~1 year; 50 °C: ~0.5 year |
| Connection method | Spring-loaded terminals; UPS interface via +BAT / -BAT terminals |
| Dimensions (enclosure) | 186 × 168 × 121 mm (W×H×D) |
| Mounting | Wall mounting (keyhole mounting); DIN-rail mounting: No |
| Net weight | 5.2 kg |
| Certifications (examples) | CE, UL/cURus recognized (UL 1778 / CSA), EAC; Marine approvals listed (ABS, DNV); ATEX: No |
Why this battery module matters in real DC UPS designs
1) Predictable buffering for critical 24 V loads
A 7 Ah, 24 V battery module represents a meaningful amount of stored energy for control circuits (nominally 24 V × 7 Ah = 168 Wh before losses and real discharge behavior). That energy can keep PLC CPUs, safety controllers, industrial switches, remote I/O, and essential sensors alive during outages long enough to avoid uncontrolled stops or to complete a safe shutdown sequence.
Because runtime depends heavily on load current, battery age, temperature, and discharge rate, the most honest way to discuss buffering is with engineering-style estimates, not marketing guesses.
2) Charging behavior defined by temperature (battery longevity depends on it)
Lead-acid chemistry is sensitive to charging voltage vs temperature. Siemens provides recommended end-of-charge voltages across a range of temperatures (for example, 27.3 V at 20 °C, higher at low temperatures, lower at high temperatures). That is not trivia: charging too aggressively at high temperature accelerates aging; charging too gently at low temperature reduces usable capacity. This module is designed to work within those defined parameters under the control of the SITOP DC UPS system.
3) Designed for cabinet safety practices
The datasheet explicitly references standards/regulations for storage, mounting, and operation of lead-acid batteries (e.g., EN 50272-2 / VDE guidance), including ventilation requirements and keeping ignition sources at least 50 cm away. That’s a practical reminder that even sealed, maintenance-free batteries still deserve respect in enclosure design.
Practical runtime estimation (engineering examples)
The table below gives ballpark buffering estimates using nominal energy, then applying a conservative “usable energy factor” to reflect conversion losses, discharge curve effects, and aging. These are not guarantees; they are planning numbers to help you size a DC UPS design.
| Example load on 24 V rail | Load power (W) | Simple theoretical runtime (168 Wh / W) | Conservative planning runtime (assume ~60% usable) |
|---|---|---|---|
| 2 A (small PLC + switch + I/O) | 48 W | ~3.5 h | ~2.1 h |
| 5 A (PLC + HMI + distributed I/O) | 120 W | ~1.4 h | ~0.8 h |
| 10 A (larger control cluster) | 240 W | ~0.7 h | ~0.4 h |
| 20 A (heavy 24 V distribution) | 480 W | ~0.35 h | ~0.2 h |
These estimates should be refined using the specific SITOP DC UPS module behavior, the real duty cycle of your loads, and the fact that high discharge currents reduce effective capacity (a classic battery reality often described by Peukert effects). The key point is that 6EP1935-6ME21 is well-suited to “keep controls alive” scenarios, and can be paralleled in systems that allow multiple battery modules to extend buffer time. (Parallel capability is commonly offered within SITOP DC UPS battery module ecosystems; confirm for your exact UPS module configuration and wiring approach.)
Installation and maintenance considerations that affect total cost of ownership
Temperature is the silent lifetime killer
Siemens provides a clear service-life expectation versus temperature: roughly 4 years at 20 °C, dropping to about 2 years at 30 °C, 1 year at 40 °C, and 0.5 year at 50 °C (to reach 80% of original capacity). This is why cabinet thermal management is not “nice to have” when batteries are involved. Placing the battery module away from hot components (drives, braking resistors, transformers) can directly translate into fewer replacements and more reliable buffering.
Storage and logistics planning
For storage, Siemens notes typical temporary capacity loss of 3% per month at 20 °C and recommends storing batteries fully charged and preferably within 0 to +20 °C for best longevity. For global supply chains, this matters: a battery that sits discharged or stored hot for too long will arrive already “older” in a chemical sense.
Mounting style and space planning
This model is specified as wall-mountable with keyhole mounting and is not intended for DIN-rail mounting. Dimensioning and mounting height/installation width are defined so you can plan door clearance and service access.
Product lifecycle note (procurement reality)
Siemens Industry Mall currently lists the product lifecycle as active, but also indicates a “product phase-out since 01.05.2026” and notes that successor information is planned to follow (with a referenced timeframe). If you are standardizing a BOM for long-running projects, it is smart to confirm availability and the recommended successor path early rather than discovering it mid-project.
Summary
Siemens 6EP1935-6ME21 is a purpose-built SITOP 24 V / 7 Ah battery module designed to pair with SITOP DC UPS modules (6 A, 15 A, 40 A) to buffer critical 24 V control power during outages. Its value is not “big battery energy” in the abstract, but predictable integration: defined charging limits (including temperature-compensated charge voltage recommendations), specified buffering current capability up to 30 A, maintenance-free sealed lead-acid construction, and documentation that addresses real industrial constraints like ventilation and thermal impact on service life.
