Sizing a DC UPS: calculate the battery in 4 steps
A DC UPS keeps the 24 V control voltage up when the mains fails. For this to work reliably, charge controller and battery must match the application. Follow these four steps to size your DC UPS.
1. Determine the load current
Add up the current of all loads that must keep running during a power failure. If only the power is known: current = power ÷ 24 V. Example: controller 1.5 A, operator panel 1 A, sensors 0.5 A gives 3 A.
2. Define the bridging time
If the system only needs to shut down in a controlled way, a few minutes are often enough. If controller and communication must keep running, 30 minutes to several hours are common. In our example we use 30 minutes.
3. Calculate the battery capacity
The basic formula is: capacity in Ah = load current in A × time in h. In the example: 3 A × 0.5 h = 1.5 Ah. But this is only the starting point:
- When discharged in less than one hour, a lead battery delivers only about 50 to 65 percent of its rated capacity.
- Batteries lose capacity as they age. Plan for about 25 percent reserve.
- Cold reduces the available capacity, heat shortens the service life. Around 20 °C is ideal.
For the example this means: 1.5 Ah ÷ 0.55 × 1.25 ≈ 3.4 Ah. An energy storage with 3.5 Ah fits. For more reserve, choose 7 Ah.
4. Select the DC UPS
The DC UPS charge controller must be able to supply the full load current continuously. Choose it with some headroom, for example 10 A for a 3 A load with later extensions. The upstream power supply must cover load and charging current together.
Short outages: capacitor instead of battery
If only seconds need to be bridged, a capacitor buffer is often the better choice. It is maintenance free and needs no battery replacement. Here the calculation is based on energy: a buffer with 1.2 kJ supplies 72 W (3 A at 24 V) for about 16 seconds in theory. The usable time is somewhat lower depending on the unit.
We help with sizing
Not sure which combination fits? Write to dc-vertrieb@woehrle-svs.de or call +49 7157 7374 506. We are happy to calculate your application.