A 24 V DC UPS keeps controllers, IPCs, HMIs and sensors running when the mains fails. The first design decision is the energy storage: supercapacitors or batteries. This tip shows how to choose and how to estimate the buffer time.
1. Define the requirement
- Load power P = 24 V × load current (only the loads that must be buffered)
- Required time: bridging voltage dips (milliseconds), a controlled shutdown of an IPC or saving data (seconds to about one minute), or continued operation during an outage (minutes to hours)
- Ambient temperature and the maintenance effort that is acceptable
2. Supercapacitor buffer: estimate the time from the stored energy
For supercapacitor modules the buffer time follows from the usable energy E and the load power P:
t ≈ E / P
Example: a module with 5.8 kJ supplies a 240 W load (24 V / 10 A) for about 5800 J / 240 W ≈ 24 s; at 120 W the time roughly doubles. Plan a reserve of about 20 % for capacitor ageing, temperature and conversion losses. For short dips, the datasheets usually state the buffer time directly at given load currents.
| Wöhrle part | DigiKey part number | Stored energy / buffer time (datasheet) |
|---|---|---|
| UGVC2420NG | 6141-UGVC2420NG-ND | 24 V / 20 A: 350 ms at 20 A, 700 ms at 10 A |
| UGVC2440 | 6141-UGVC2440-ND | 24 V / 40 A, 180 J: 400 ms at 40 A, 1.4 s at 10 A |
| UGVC2440-2 | 6141-UGVC2440-2-ND | 24 V / 40 A, 180 J: 300 ms at 40 A, 1 s at 10 A |
| UGVC2401 | 6141-UGVC2401-ND | Power supply 80 to 264 V AC with buffer, 24 V / 1 A, 0.6 kJ: approx. 25 s at 1 A |
| UGVC2425P | 6141-UGVC2425P-ND | 24 V / 25 A, 1.2 kJ |
| UGVC2440P | 6141-UGVC2440P-ND | 24 V / 40 A, 4.0 kJ |
| UGVC2410-5,8KJ | 6141-UGVC2410-5,8KJ-ND | 24 V / 10 A, 5.8 kJ: approx. 24 s at 10 A |
| UGVC2410-13,4KJ | 6141-UGVC2410-13,4KJ-ND | 24 V / 10 A, 13.4 kJ: approx. 56 s at 10 A |
Supercapacitors are maintenance-free, tolerate frequent charge/discharge cycles and recharge within seconds to minutes. This makes them well suited for dips and controlled shutdowns.
3. Battery backup: use the discharge table, not only the Ah rating
For batteries the backup time is not simply capacity ÷ current: at higher currents the usable capacity drops noticeably. Use the backup time tables in the datasheets, for example:
| Load current | BE2401NG (24 V / 1.2 Ah) | BE2402NG (24 V / 2.2 Ah) |
|---|---|---|
| 1 A | approx. 36 min | approx. 60 min |
| 2.5 A | approx. 15 min | approx. 28 min |
| 5 A | approx. 5 min | approx. 6 min |
| 10 A | n/a | approx. 3 min |
For longer times, larger lead-acid modules from 7.2 Ah up to 80 Ah are available (BE2407NG to BE2480NG). Also check:
- Charging: the DC UPS controller must suit the battery capacity and chemistry. Example: the UNG2403 (24 V / 3 A) is designed for lead-acid batteries from 1.2 Ah to 6 Ah.
- Service life: VRLA lead-acid batteries are typically replaced every 3 to 5 years; high ambient temperatures shorten their life considerably. Mount them in the coolest place of the cabinet.
- Lithium iron phosphate (LiFePO4) such as the BE2403Li (25.6 V / 3.3 Ah, recommended -20 °C to +45 °C) offers a higher cycle life, but requires a controller with a suitable charging profile.
4. Quick decision guide
| Requirement | Recommended storage |
|---|---|
| Bridge voltage dips (ms to a few s) | Supercapacitor buffer (e.g. UGVC2420NG, UGVC2440) |
| Controlled shutdown / save data (s to approx. 1 min) | Supercapacitor kJ modules (UGVC2425P, UGVC2440P, UGVC2410) |
| Continue operation during an outage (min to h) | DC UPS controller + battery module (UNG series + BE series) |
| Little or no maintenance, high ambient temperature | Supercapacitor |
Questions about a specific application? Post your load current and required buffer time here. We are happy to help with the sizing. Further Wöhrle DC UPS products on DigiKey: https://www.digikey.com/en/supplier-centers/wohrle