When a conversion comes back with an electrical complaint, the DC-DC converter is often where it started. Too small and it runs hot or sags as soon as the load comes on. Set to the wrong charge profile and the auxiliary battery never gets full. Non-isolated where the job needed isolation and you end up chasing earth faults and noise on the radio. None of this is difficult to get right. You just have to look at the figures in a sensible order.
Supply or charger?
Two quite different jobs both get called a DC-DC converter, and they are sized in different ways.
One is supply conversion. The vehicle runs at one voltage and a piece of payload equipment wants another, so you step it up or down. A typical case is 24V down to 12V for a radio, a fridge or a camera recorder on a truck. All that unit has to do is hold a steady output.
The other is battery-to-battery charging, where the converter takes power from the vehicle’s charging system and charges a separate auxiliary battery. That needs a charge curve matched to the battery chemistry, which a plain supply converter does not have. We keep the two in separate ranges, DC-DC converters and battery chargers.
Voltage, and the range around it
Match the input to the vehicle system (12V or 24V) and the output to the load or the battery being charged. Then look at the input range on the data sheet, not just the nominal figure. A 24V truck can sit well below 24V while cranking and well above it while charging hard, and the converter has to stay in regulation across that whole swing. If it drops out every time the engine turns over, the equipment on its output reboots every morning.
Size on continuous current
Headline peak ratings flatter. What matters is the current the unit will carry for hours. Add up the loads that genuinely run at the same time, then pick a rating with real headroom above that total. A converter worked at its limit all day runs hot, and heat is what ages the electrolytic capacitors inside it.
As an example, the SMC20 steps 24V to 12V at 20A continuous (276W). If your 12V loads add up to 17 or 18A running together, that is the point to move up to the 35A unit rather than hope.
Isolated or not
Galvanic isolation separates the input and output grounds completely. You want it when you are combining supplies from different sources, breaking an earth loop or protecting sensitive equipment from stray currents. The SMG2412-360 is an isolated 24V to 12V unit at 360W (30A), and the SMG range also covers 48V combinations.
Non-isolated units such as the SMC range are simpler, cheaper and more efficient, and they are fine wherever a shared ground is acceptable. That is most conversions. Pick isolation when you can say why you need it.
Charge profile
A battery-to-battery charger has to suit what it is charging. A LiFePO4 bank needs a lithium profile. Lead-acid and AGM need their own multi-stage profiles. Charge lithium on a lead-acid curve and it will never reach full charge, however long the vehicle drives.
Lithium also must not be charged below freezing, which matters on a vehicle parked outside in January. The ADC-1250 handles this with a low-temperature cut-off and heater control when its optional battery sensor is fitted (more in our post on lithium low-temperature charging).
Heat and where you mount it
Efficiency decides how much energy is wasted and how much heat the unit has to get rid of. The SMC20 is around 90% efficient, so at full output roughly 30W ends up as heat in the housing. On a bench that is nothing. Bolted into a sealed locker under a seat base in August, it can be enough to make a unit throttle back or shut down. Give it airflow, keep it off other hot kit and check the ambient temperature where it will actually live.
How hard it works
An intermittent load can run from a smaller converter than the same peak load held for hours. Emergency and utility vehicles parked under load all shift are the ones that cook undersized units.
The rest of the circuit
Fuse it on both sides and feed it from a supply rated for the current it will pull. Pair any charger with a Battery Guard so the auxiliary bank cannot be run flat and, on a split system, the engine-start battery stays protected. On a complex build, let the task and power management controller decide when auxiliary charging runs relative to everything else.
Checklist
- Decide whether it is supply conversion or battery charging.
- Confirm the input range and the output voltage.
- Total the continuous load and pick a current rating with margin.
- Choose isolated or non-isolated, with a reason.
- For chargers, set the profile to the battery chemistry.
- Check heat and mounting in the real installed position.
- Fuse both sides and protect the auxiliary battery.
Get those right and nobody thinks about the converter again, which is the best outcome you can have. Intellitec builds its DC-DC converters, chargers and Battery Guards in the UK.