Battery-Fed DC-DC Converters: Calculate the Real Input Range

Build the battery-to-converter voltage envelope

A “24 V battery” describes a system class. Its actual voltage depends on chemistry, series configuration, charging settings, load, temperature, and the battery-management strategy. Obtain the limits from the battery and charger documents rather than selecting a generic cutoff from an online chart.

A battery specification should separately identify nominal voltage, charge voltage, and end-of-discharge voltage, as illustrated by the external battery technical-data example. Use the values for the battery actually specified. A converter’s undervoltage shutdown is not automatically the correct battery-protection threshold.

Record these at the actual converter boundary. An upstream battery-monitor reading cannot include a downstream loose contact or the voltage lost in the converter’s own input branch.

Calculate low-voltage current with the cable included

The basic loaded-input estimate is:

Input current ≈ output power ÷ (efficiency × converter-terminal input voltage)

The referenced discussion of DC-DC input current explains why efficiency must correspond to the operating conditions. Do not use no-load quiescent current as the loaded input demand, or assume peak efficiency at the lowest input and highest temperature.

Consider a hypothetical auxiliary branch delivering 12 V at 8 A, or 96 W. Assume 90% conversion efficiency for this calculation, a loaded battery-source voltage of 20.0 V, and total input-loop resistance of 0.10 Ω. The battery-source value is already a loaded value; the resistance here represents only the downstream input branch.

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Because input current causes cable loss, solve both relationships together:

• Terminal voltage: V_in = 20.0 V − (I_in × 0.10 Ω).
• Loaded input power: I_in × V_in = 96 W ÷ 0.90 = 106.67 W.

The coupled steady-state calculation gives V_in ≈ 19.45 V and I_in ≈ 5.48 A. The input branch loses approximately 0.55 V and 3.01 W. This is a steady-state estimate; it does not predict the switching converter’s stability or its startup pulse.

All three rows use the same assumed 96 W output, 90% efficiency, and 0.10 Ω loop. Real resistance changes with conductor temperature and contact condition, while efficiency varies with input and load. Replace these assumptions with worst-case supported values before selection. Coordinate conductor and protective-device ratings using the calculated demand and the applicable installation requirements, rather than choosing a fuse from this example.

Compare operating range and startup threshold separately

Suppose a candidate’s published continuous input range is 19–36 V. The 19.45 V calculation lies inside that range, but leaves only about 0.45 V above its lower boundary. Whether that is acceptable depends on the uncertainty in source voltage, resistance, efficiency, and dynamic load. It is not a general approval of a 19 V minimum converter.

At startup, the same candidate may require a different threshold or draw a different current. A converter that can keep running at a low voltage may fail to start there. After a low-voltage shutdown, battery voltage can rebound when the load disappears; inadequate hysteresis or a poorly coordinated enable signal can produce repeated start-stop behavior.

Use the voltage-budget method for both ends of the system. The converter input must stay valid, and the distant 12 V load must receive its permitted voltage after output-side losses. If placement reduces one cable loss while increasing the other, compare both paths rather than optimizing only the output run.

Read the complete WEHO input suffix

Read the complete input suffix before assessing a DC-DC product. The SD-50 specification provides a concise example: it lists A, B, and C input windows of 9.2–18 VDC, 19–36 VDC, and 36–72 VDC respectively. “SD-50” by itself therefore does not identify the input variant. Select the full model code and check its output rating and environmental limits together. The SD-50 and WH-C241210 product-detail links in the isolation section below identify separate products and must be read with their corresponding specifications.

For the 96 W branch, shortlist the required power class first, then check its own input suffix and output derating. The SD-50 document explains nomenclature and is not a candidate recommendation for that branch. Use temperature and mounting derating when reading that continuous rating.

Ask for the current revision if a specification has no visible revision identifier or if its public page differs from the supplied document. For battery-powered products, also resolve input-output isolation and grounding: an isolated converter and a common-negative converter solve different interface problems.

Distinguish isolated and non-isolated converters

Resolve the required isolation and grounding arrangement before selecting the enclosure. The WEHO SD-50 specification explicitly identifies input-to-output isolation for the SD-50 family, including SD-50B-12. The WH-C241210 specification explicitly identifies a non-isolated design with a common input/output negative connection.

These are different interface architectures, not interchangeable versions of one converter. Select the required isolation arrangement, then check the complete model, input window, output power and installed conditions. Neither enclosure shape nor the number of wires proves isolation. The two product images below illustrate the documented types; neither is being selected for the hypothetical 96 W branch.

Resolve differences between the public product listing and the exact-model specification before using their parameter values in a project design. Both models above are documentation examples of isolation and common-reference arrangements, not capacity selections for the hypothetical 96 W branch.

Separate a steady voltage window from transients

The highest normal charging voltage belongs inside the continuous-input envelope. A brief spike from a contactor, charger connection, or nearby inductive load is a different requirement. Specify its waveform or bounded test profile, duration, repetition, and source impedance. A 36 V upper operating limit does not state what happens during a 50 V pulse.

Where an external filter or surge-limiting stage is needed, include its series drop, heating, and effect on converter startup. Review DC-DC input-filter resonance and source impedance as well: an added LC network and the converter’s approximately constant-power input behavior can interact. Extra input capacitance can increase connection inrush. Do not assume that an arbitrary capacitor or suppressor makes the converter compatible with a named transient standard.

Battery protection is another separate function. Coordinate the battery-management system, low-voltage disconnect, converter enable, standby current, and recovery sequence. If the battery protection disconnects, establish which controller can restart the system and whether unattended recovery is intended. These decisions come from the battery-system design, not merely the converter’s output-voltage setting.

Validate the corners that drive the choice

For a representative assembly, record input voltage directly at the converter, input current, output voltage at the load, temperatures, and the operating state. Cover the lowest loaded source, highest sustained charging voltage, largest credible load step, and permitted restart conditions using rated test equipment and the approved battery-system procedure.

A useful record distinguishes three outcomes: continuous operation stays inside the documented window; a shutdown occurs and is the intended battery-protection action; or an unintended reset or cycling event remains to resolve. Capture recovery as well as the initial interruption. A successful nominal-voltage bench run covers only one point.

Contact WEHO about the battery-input converter requirement. Include the battery’s loaded voltage range, required output voltage and current, and enclosure temperature. Add a known startup or transient requirement if one exists; those facts are enough to begin checking the correct converter family and input variant.