Parking AC on Electric Trucks: Sizing the 24V Auxiliary Load and Avoiding Range Surprises

The question that gets asked in the wrong order

A fleet electrification team usually starts with the traction side: pack size, charging curve, route energy. Cabin climate is treated as a rounding error until someone points out that a driver who cannot sleep at 26 °C in July will not accept the vehicle, and that idling the traction system to run the HVAC is both wasteful and, at depots with strict rules, not allowed.

By then the auxiliary architecture is frozen. The DC-DC converter has been specified for hotel loads and a microwave, not for a compressor that pulls 33 A at 24V for hours. The result is a vehicle that technically has a parking air conditioner and practically cannot run it overnight without flattening the 24V battery.

This article is about getting that order right: understand the auxiliary bus first, then choose the cooling unit, then decide the operating policy.

What "24V auxiliary" actually means on an electric truck

On a diesel truck, the 24V system is fed by an alternator bolted to the engine. On a battery-electric truck, it is fed by a DC-DC converter that steps the high-voltage pack (typically 600–800 V) down to 24–28 V.

Three numbers describe that converter, and they are not the same number:

• Continuous output rating — what it can supply indefinitely at nominal temperature. This is the figure that matters for a parking air conditioner.
• Peak output rating — a short-term capability, often 1.3–1.5× continuous, used for steering and brake actuation.
• Derating curve — how output falls as the converter's coolant or ambient temperature rises. A converter rated 3 kW at 25 °C may deliver 2.2 kW at 65 °C, and the converter sits in a hot part of the chassis.

A parking air conditioner is unusual among auxiliary loads because it is continuous and large. A microwave runs for three minutes. A coffee maker for five. A parking air conditioner runs for eight hours at 700–900 W, which at 24V is 29–38 A. Nothing else on the vehicle behaves like that except the heater, and the heater mostly runs when the AC does not.

The energy budget, with numbers

Start with the unit. Take a 24V rooftop model in our range — VS02-PRO at 24V draws 10–33 A, CU2700 draws 9–33 A, EcoMax 3S 10–33 A in cooling. At full compressor speed and 24V nominal, that is roughly 790 W at the top of the range, plus indoor fan and controls, so call it 830–850 W at the converter output.

In an eight-hour night, if the compressor ran continuously, that would be 6.6–6.8 kWh. It does not run continuously, because the thermostat cycles it. At 45 °C ambient with a reasonable cab, expect 50–65 % duty over the night:

For a long-haul electric tractor with a 500–600 kWh pack, the answer is comfortable: overnight cooling costs 1 % of range, and no sensible fleet would refuse it. For a medium-duty electric truck with a 120–160 kWh pack, the same night costs 3–5 % — which on a 250 km route with 15 % reserve is a real constraint, and needs either a smaller cooling unit, a shorter cooling window, or a charging plan that includes a top-up.

The arithmetic also explains a pattern we see in enquiries: European and North American medium-duty fleets asking for the smallest 24V unit that will hold a 22–24 °C cab in a 1.8 m³ day-cab bunk, rather than the largest unit available. In an electric vehicle, the efficiency of the unit matters more than its headline capacity, because every watt has a range consequence.

Efficiency is the specification that matters here

On a diesel truck, a 10 % efficiency difference is a fuel-number argument. On an electric truck, it is a range argument that a route planner can see.

Two figures in our data illustrate the spread:

• The CU2700 / CUDU2700 platform publishes a maximum EER of 3.4, measured at 35 °C ambient. That is 3.4 W of cooling per watt of electrical input at that condition, and it is the most efficient platform in the range.
• A larger-capacity rooftop unit at full speed in the same conditions delivers more cooling but at a lower ratio, because condenser and compressor losses grow faster than capacity.

Efficiency also degrades in heat, in both directions: capacity falls and input power rises. The derating behaviour described in our high-ambient guide applies identically here, and on an electric truck it shows up as a range number rather than a comfort complaint.

Integration: five connections, one of them critical

Mechanically, mounting a parking air conditioner on an electric truck is no different from a diesel one: roof aperture, sealed base, drainage. Electrically, there are five connections, and the ordering matters.

1. Power — a fused feed from the 24V bus, sized by voltage drop at peak current (see our cable-sizing guide; the method is identical, the current is what changes).
2. Ignition / wake signal — how the unit knows the vehicle is parked and authorised to run. On an EV this is usually a body-control output rather than a key position.
3. Ground — a dedicated return to the 24V battery negative, not a chassis path through bonded aluminium structure.
4. Communications (optional) — CAN or LIN if the fleet wants telematics control, or dry contacts for a simple run-permission signal.
5. HV interlock awareness — the unit itself does not touch high voltage, but the installation must not route 24V cabling near HV orange looms without the OEM's specified separation, and any work near HV components requires the OEM's de-energisation procedure.

The critical one is the wake signal, because an EV's auxiliary system typically sleeps. If the AC is wired to permanent 24V and left to its own remote control, it will run while the vehicle believes it is shut down, and the converter may not enable to support it. The unit then drains the small 24V buffer battery, and the truck will not start its systems in the morning — an electric equivalent of a flat cranking battery, with the same diagnosis time and none of the same jump-start options.

The 24V buffer battery is not optional

Electric trucks keep a small 24V battery — typically 2–4 kWh of lead-acid or a small lithium pack — to buffer loads the converter cannot follow instantaneously and to keep safety systems alive during HV disconnection.

That buffer does the same job for the air conditioner: it absorbs the compressor's start surge, which the converter may not supply because of its current limit. A 24V compressor starting against a hot condenser can pull two to three times running current for tens of milliseconds. If the buffer is undersized or aged, the bus voltage sags, the unit's controller sees a low-voltage event, and — depending on its logic — it may refuse to start or restart repeatedly.

Two practical consequences:

• Do not delete the buffer battery in a weight-reduction exercise without checking the converter's transient response. The converter is a steady-state device; the battery is the transient device.
• Set the low-voltage cutoff for the buffer chemistry, not for a large traction-adjacent pack. A 24V lead-acid buffer wants about 23.6–24.0 V cutoff; a lithium buffer can go lower, but the BMS will disconnect without warning if pushed.

Operating policy: the part engineering cannot decide for you

Once the vehicle can physically run the AC all night, someone has to decide under what conditions it should. That is a fleet policy question, and the options are not equivalent.

The SOC-gated approach is the one most fleets converge on, because it is enforceable in software and auditable afterwards. It also creates a support question: when a driver overrides it at 02:00 and the truck is at 18 % in the morning, whose problem is it? Answer that in writing before the first vehicle is delivered, not after.

Pre-cooling: the cheapest cooling is the cooling you do on shore power

Every kWh of cooling done while plugged in is a kWh the driver does not need to carry. Two habits make a measurable difference on an electric truck:

• Pre-cool on the charger for 20–30 minutes before departure, pulling the cab from 35 °C to 23 °C while the vehicle is on shore power. The AC's own consumption is then external to the route energy, and the driver starts with a cool, dry cab.
• Park in shade and use covers. Solar gain is the largest single variable load on a parked cab. A reflective windscreen cover and closed sleeper curtains can cut the cooling load by 300–500 W, which on an eight-hour night is 2.4–4 kWh — the difference between 1 % and 2 % of a mid-size pack.

Neither of these requires a product change, which is why they are usually skipped in favour of a bigger battery.

What to specify in an RFQ to the AC supplier

If you are the vehicle integrator or the fleet specifying the cooling unit, ask for:

• Current at 24V, maximum compressor speed, at a stated ambient. Not nameplate watts.
• Maximum inrush current and duration, so the converter and buffer can be checked.
• Efficiency (EER or COP) at 35 °C and at 45 °C, with the test condition stated.
• Control interface options: remote-only, ignition-signal, CAN, dry contact.
• Sleep current — what the unit draws when parked but off. A unit that pulls 40 mA continuously will flatten a small buffer battery over a long weekend.
• Behaviour on undervoltage: does it restart automatically, latch a fault, or require a manual reset?

That last item matters more on an EV than on a diesel, because the recovery procedure for a latched fault may require walking to a vehicle that is deliberately immobilised.

A worked converter check

Take a medium-duty electric truck with a 3 kW continuous DC-DC converter, rated at 25 °C and derated to 2.2 kW at 65 °C. Assume the vehicle's other continuous auxiliary loads while parked are modest: body control, telematics, a fridge at 60 W, interior lighting at 30 W, and periodic brake-system wake-ups averaging 40 W. Call the non-AC baseline 250 W.

Add the air conditioner at 850 W worst case and the total is 1,100 W — half of the derated converter capacity, which sounds comfortable. Then add the loads that are easy to forget:

• cab heater on a cold morning, 400–800 W;
• seat heating, 100 W;
• a driver's inverter for a laptop and phone chargers, 150 W;
• the 24V battery recharge itself, which can pull 300–500 W after a deep night.

A cold morning with heating, a laptop and a depleted buffer battery lands at roughly 2,050 W — inside the 2.2 kW derated rating, but with almost no margin, at the exact moment the converter is hottest after eight hours of work. This is why the OEM's derating curve and priority logic matter more than the headline converter rating, and why a heat-pump unit that delivers 2,000 W of heating for 620 W of input is a materially different proposition from a resistive heater.

What changes on a 12V van platform

Light commercial electric vans usually run a 12V auxiliary bus, and the arithmetic is less forgiving: the same 850 W of cooling draws about 70 A at 12V instead of 35 A at 24V. The converter is smaller, the cable has to be much larger, and the vehicle's 12V buffer battery is often a 70–95 Ah AGM unit that was never intended to absorb a compressor start.

Two practical consequences. First, choose the smallest unit that meets the cab load — in a 1.8 m³ van cab, a 7,000 BTU/h rooftop at 12V is usually sufficient, and the current band is more important than the capacity headline. Second, treat the 12V buffer battery as a consumable with a state-of-health check at every service, because an aged AGM will produce exactly the intermittent shutdown pattern that gets misdiagnosed as a controller fault.

Where electric-truck cooling is heading

Two trends are visible from the supplier side.

First, cooling is being absorbed into the thermal management system. Instead of a standalone rooftop unit, some OEMs want the sleeper evaporator plumbed into a shared refrigerant loop with a chiller or heat pump. That improves efficiency and reduces roof clutter, but it makes the AC a vehicle-level system with vehicle-level validation, and it removes the aftermarket simplicity that fleets value.

Second, parked cooling is becoming a telematics feature rather than a hardware feature. The unit is one component; the value is in the policy engine that decides when it runs, at what set point, and against which state of charge. Fleets that instrument the first fifty vehicles learn more about their real duty cycles than any specification sheet will tell them.

For now, the pragmatic answer for most fleets is a standalone 24V rooftop unit of modest capacity, an efficient platform, a properly sized converter feed, a healthy buffer battery, and a written operating policy. That combination costs less than a bespoke thermal integration and covers the requirement that actually matters: a driver who sleeps, and a truck that starts. It also leaves room to change your mind about the thermal architecture later, because a standalone unit can be removed from one vehicle and fitted to the next one in the fleet — an option that disappears the moment cooling is plumbed into the vehicle's own refrigerant loop.

Maintenance differences on an electric truck

Two service habits change. First, there is no engine bay to warm the condenser, but there is a battery thermal system that can dump heat near the unit on a hot day, so roof-mounted condensers on electric trucks should be checked for recirculation in the same way as diesel installations — parked over a hot HV pack, a rooftop unit can see inlet air several degrees above ambient.

Second, diagnostics split in two. The 24V side behaves like any other truck: measure voltage under load, check current against the specification, verify the cutoff setting. The high-voltage side is the OEM's domain, and technicians should not be asked to work around HV components to reach a 24V fault. Write the boundary into the service procedure so nobody improvises near an orange cable.

Frequently asked questions

The air conditioner is a 24V load, so the converter and buffer battery decide feasibility before the cooling capacity does. Energy cost is about 1 % of a long-haul pack and 3–5 % of a medium-duty pack per night. And the operating policy — when cooling is allowed, against which state of charge — is the thing that turns a working installation into a deployable one.