To choose a DC-DC converter, start from the load and work backwards. Note the voltage and continuous current the equipment needs, then pick a converter whose input range covers the whole voltage span of your real source and whose rated output current is at least 25% above that load. Efficiency, heat and fusing follow from those numbers.

Most wrong purchases come from the same slips: trusting the “12 V” or “24 V” label instead of the real voltage span, sizing for average instead of peak current, and mixing up input current with output current. New to the term? Start with what a DC-DC converter is, then follow the six steps below.

The rule in short

The input range must cover the whole span of the source, with margin at both ends. The continuous output rating should be at least 25% above the load: a 15 A load points to a 20 A class converter. Work out the input current from power, then fuse and wire the input for it.

Step 1: define the load

Write down three numbers for everything the converter will feed:

  • Voltage. What the equipment needs and how far it tolerates deviation. Some devices accept 9–15 V; others need a tightly regulated value.
  • Continuous current. From the nameplate or datasheet. If only watts are given, divide by the voltage: 180 W at 12 V is 15 A.
  • Start-up or peak current. Motors, pumps, compressors and equipment with large input capacitors briefly draw several times their running current.

Add the continuous currents of loads that run together. For routers, cameras and other electronics, the guide to running a router and devices on DC covers voltage and polarity checks.

Step 2: match the input range to the real source

“12 V” and “24 V” are labels. The real voltage moves with state of charge, load, cable drop and charging. For a lithium iron phosphate (LiFePO4) bank, the ends are set by its BMS and charge settings.

Typical real voltage spans of common DC sources
Source Typical span
“12 V” vehicle system roughly 11–14.8 V
“24 V” vehicle or boat system roughly 22–29 V
24 V LiFePO4 bank (8 cells) 20–29.2 V
48 V LiFePO4 bank (16 cells) roughly 40–58.4 V

The converter’s input range must contain the entire span, with margin. Its minimum operating voltage must sit below the lowest voltage at the converter’s terminals, after cable drop and under load; its maximum above the highest, including charger and alternator spikes. Some units need a higher voltage to start than to keep running, so check the start-up figure too.

Converters are often described by maximum input alone, as in “80 V to 12 V”. That is the ceiling, not the floor. On a 48 V bank, check the ceiling against 58.4 V; on a 24 V truck, check the floor, since the voltage can dip during engine start.

Steps 3 and 4: output voltage and current rating

Output voltage. A fixed output cannot be mis-set by accident. An adjustable output suits loads that want 13.8 V or another non-standard value, but measure it before connecting the load: an overvoltage can destroy equipment.

Current rating. Choose a continuous output rating at least 25% above the continuous load. For 15 A, 15 × 1.25 = 18.75 A, so the 20 A class fits. The margin matters because heat and ambient temperature reduce what a unit can deliver continuously: a rating quoted at 25 °C (77 °F) does not automatically hold at 50 °C (122 °F).

The 25% covers continuous running, not surges. If start-up current is a large multiple of running current, compare it with the converter’s documented overload behaviour or step up a class.

Worked example: a 12 V, 15 A load from a 24 V source

  1. Load power: 12 V × 15 A = 180 W.
  2. Converter class: 15 A × 1.25 = 18.75 A, so a 20 A class unit. At 12 V that is a 240 W rating, and the load uses 75% of it.
  3. Input power at 90% efficiency: 180 W ÷ 0.90 = 200 W. The 20 W difference is heat inside the converter.
  4. Input current at 24 V: 200 W ÷ 24 V ≈ 8.3 A. The input carries this, not the 15 A of the output.
  5. Input current at the low end: the same 200 W needs about 9.1 A at 22 V and 10 A at 20 V. Fuse and cable must suit that worst case, not 8.3 A.

From a 48 V bank the same job needs about 4.2 A (200 ÷ 48). From a 12 V source it is not possible with a step-down (buck) converter, because the input must stay above the output; a source at or below the output voltage needs a step-up or buck-boost type. The 90% is an assumption: use the efficiency your converter documents at that load.

Steps 5 and 6: heat, mounting, fuse and wiring

The 20 W lost in the example has to leave the case. Mount the converter on a surface or heatsink that carries heat away, leave airflow around it, and keep it out of sealed boxes and insulation. Efficiency also varies with load and input voltage, as the guide to DC-DC converter efficiency explains.

  • Fuse at the source. Put the input fuse at the battery or distribution point, close to the positive terminal, so the whole cable is protected. Choose it for the cable and the worst-case input current, and follow the converter’s manual.
  • Size the cable for current and length. Voltage drop on the input raises the current for the same power; on the output it lowers the voltage the load sees. Keep runs short, especially on the high-current 12 V side.
  • Mind polarity. Check the markings twice before connecting, and do not assume reverse-polarity protection unless the documentation states it.

For fixed installations, ask a qualified installer.

Common mistakes to avoid

  • Charging a lithium battery from a converter. A converter holds a voltage; it does not run a charge profile or limit current for a battery. Use a DC-DC charger.
  • An input range that is too tight. A unit that only just covers 24 V drops out when the real voltage dips or spikes.
  • A buck converter with the input barely above the output. Step-down types need headroom. A 12 V vehicle feeding a 12 V output needs a buck-boost type.
  • Ignoring start-up surge. The load runs fine alone, then the converter trips at switch-on.
  • Feeding sensitive equipment from an undersized unit. A converter at its limit sags, heats up and may cycle on thermal protection, resetting routers and instruments.

Typical scenarios and what to check

This is a checklist, not a product recommendation. If no single converter spans the gap, cascading converters is possible but multiplies the losses.

Typical conversion scenarios and the first things to check
Scenario Converter type What to check
24 V truck or boat to 12 V accessories Step-down Input range covers about 22–29 V with margin; output current 25% above the combined load; fuse at the 24 V source
48 V bank to 12 V network gear Step-down, wide input Maximum input above 58.4 V with margin; a stable, regulated output
12 V vehicle to a fixed 12 V supply Buck-boost Input swings above and below 12 V (roughly 11–14.8 V), which a buck converter cannot handle
12 V to 24 V step-up for a winch controller Step-up Input current is more than double the output current; heavy, short cable and fuse on the 12 V side; the controller’s real current draw

Step-down converters to compare

Use the cards as a shortlist. On each product page, compare the documented input range, output voltage and rated output current with your numbers from steps 1 to 4, and read the documented cooling and mounting information.

Frequently asked questions

How many amps do I need for a 24 V to 12 V converter?+

Count the amps on the 12 V output side. Add up the equipment that runs at the same time, then add at least 25%: a 15 A load calls for a 20 A class unit. The current drawn from the 24 V side is lower, about 8.3 A for 15 A out at 90% efficiency.

Can I run a DC-DC converter at its maximum current all day?+

Only if the datasheet gives that figure as a continuous rating for your temperature and cooling conditions. Heat reduces the usable current, so a margin keeps the unit cooler. A peak rating is higher than the continuous one.

Does a bigger converter waste power?+

Not in proportion to its rating, because the load decides how much power is drawn. A moderate margin costs nothing noticeable. A very oversized unit costs more and may be less efficient at very light load; check the documented no-load current if it stays on for long periods.

What is the difference between constant voltage and constant current output?+

A constant-voltage output holds a set voltage and lets the load draw what it needs, up to the current limit; most equipment expects this. A constant-current output holds a set current and lets the voltage float, which suits LED drivers and the early stage of charging.

Can I run a fridge or compressor from a DC-DC converter?+

A 12 V DC compressor fridge can, if the continuous rating covers its running current and the converter tolerates the compressor’s start-up surge, which can be several times higher. Check both figures in the fridge’s data. A fridge that runs on mains voltage needs an inverter, and a 24 V fridge on a 24 V bank usually needs no converter at all.

Figures above are arithmetic examples, including the assumed 90% efficiency, and the voltage spans are typical values. Ratings on product pages are supplier-stated and are not independently tested by PVShop. Follow the converter and battery manufacturers’ specifications, and consult a qualified installer for fixed installations.