Weighing a mobile charging trailer

logistics5 min read2026-09-22

A mobile charging station is a battery, a charger, and a surprising amount of steel, sitting on something with a certification label. The weight question decides the trailer class, which decides the tow vehicle, which decides whether the person driving it needs a commercial licence.

The battery dominates, and chemistry is the lever

At system level, meaning cells plus modules plus racking plus BMS plus enclosure rather than bare cells, the numbers are roughly:

LFP: 10 kg/kWh
NMC: 7 kg/kWh

A 250 kWh LFP pack is 2,500 kg before anything else goes on the deck. The same energy in NMC is 1,750 kg. For a 1 MWh unit that difference is 3 tonnes, which is the difference between trailer classes.

LFP is cheaper and far more cycle-tolerant, which matters enormously for a unit that cycles daily. NMC is the choice when weight is the binding constraint, which on a trailer it very often is. This is one of the few places where the chemistry decision is made by the department of transportation rather than by the accountants.

Everything else, in order

For a 250 kWh LFP unit with a 350 kW charger, one liquid-cooled dispenser and an onboard grid inlet:

Component	Weight	Basis
Battery system	5,512 lb	250 kWh x 10 kg/kWh
Charger cabinet	2,646 lb	350 kW rating
DC-DC converter	1,543 lb	2 kg/kW of charger
Balance of system	1,516 lb	8% of equipment + 300 kg
Dispenser and cable	551 lb	liquid-cooled
Enclosure	534 lb	5% of equipment
Add-ons	463 lb	fire suppression, jacks
Grid front end	431 lb	3 kg/kW of service
Thermal system	220 lb	3% of battery, min 100 kg
Payload	13,416 lb
Trailer itself	7,500 lb	gooseneck tri-axle 30K
Total towed	20,916 lb

Which lands on a gooseneck tri-axle at 60% of its payload capacity, needing a medium-duty Class 5 to 6 truck to tow it, and crossing the threshold where a CDL Class A is likely.

The coupling surprise

Conventional wisdom says DC coupling is lighter: the battery feeds a DC-DC converter and goes straight to the dispenser, with no inverter in the path. AC coupling puts a PCS inverter in between, which is heavier.

Except the two are sized to different things:

DC-DC converter: 2 kg per kW of charger rating
PCS inverter:    4 kg per kW of battery discharge limit

If the battery’s output limit is less than half the charger rating, the PCS is the smaller box. A 250 kWh battery at 0.5C behind a 350 kW charger gives 1,102 lb of PCS against 1,543 lb of DC-DC, so AC coupling is 441 lb lighter. Raise the output limit past 175 kW and it flips back.

The calculator computes both and names the winner for your configuration rather than repeating the rule of thumb, because the rule of thumb is wrong for exactly the configuration that a mobile unit tends to have: a modest inverter behind an oversized dispenser.

Clearance sets the deck, not the equipment

The equipment footprint for that 250 kWh unit is about 5 m². On an 8.5 ft wide deck, that is 2.2 m of length. The trailer needs 6.2 m.

The difference is service clearance: 0.8 m around each block so a technician can open a door and stand in front of a live cabinet. Four blocks means five gaps, and the gaps are three times the equipment. Anyone who has sized a trailer from an equipment list and then had to add three metres knows this one already.

Two thresholds worth knowing

26,000 lb gross combination, with a trailer rated over 10,000 lb, is where a CDL Class A likely enters the picture. The 250 kWh unit above crosses it once the tow vehicle is counted.
80,000 lb gross combination is the federal interstate limit. A 3.9 MWh unit is well past it and moves as a permitted load with route review.

Both are informational, both vary by state, and both are the sort of thing that turns a procurement decision on its head late in the process. Which is the argument for putting the weight model in front of the purchase order rather than after it.

Why fast charging slows down

Three reasons the curve falls away: lithium plating risk at high state of charge, heat, and the cabinet's own voltage and current ceilings.