Twelve electric rigids against a 70 kVA charging headroom
A grocery distributor has put 12 battery-electric 18 t rigids into a depot served by a 400 A, 415 V three-phase supply shared with a cold store. Each truck has 280 kWh usable and averages 1.1 kWh/km. The site's maximum demand is already 180 kVA and the network operator will not authorise more than 250 kVA without a 14-month upgrade. There are eight 22 kW AC charge points, all OCPP 1.6J, and a nine-hour overnight window. Routes average 175 km; two trucks also do a 60 km twilight run after returning. Overnight, drivers report sessions tapering from 22 kW to 9–11 kW after about 90 minutes, and two trucks are not reaching 100% by 05:00. The depot manager wants every truck at 100% before departure and proposes adding four more 22 kW charge points to fix the problem.
1. Build the energy budget for the overnight window and test the manager's proposal against the authorised supply capacity.
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Traction: 175 km × 1.1 kWh/km = 192.5 kWh per truck; add roughly 5% for HVAC, lights and auxiliaries, about 202 kWh, and about 218 kWh at the grid side once 22 kW AC charger losses are counted. The two twilight trucks cover 235 km, about 258 kWh traction, roughly 280 kWh from the grid. Twelve trucks therefore need on the order of 2,900 kWh in nine hours - an average of about 320 kW.
The site has 250 kVA authorised and 180 kVA already in use, leaving about 70 kVA; over nine hours that is roughly 630 kWh, enough for about three trucks. The manager's four extra charge points add load, not capacity, and cannot change that arithmetic. Also check the cold store's night-time profile: if base load falls, real headroom may be larger than 70 kVA, but it must be measured over 24 hours rather than assumed from the daytime peak.
2. Diagnose the taper from 22 kW to 9–11 kW. What would you measure before ordering any hardware?
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A 22 kW AC feed into a 280 kWh pack is about 0.08C, far below the point where a healthy pack tapers, so the taper is almost certainly upstream of the vehicle. Roughly 11 kW is 16 A per phase at 400 V and 9 kW is near 13 A, which is the shape of an imposed current limit.
Measure: the charge point's control-pilot duty log and the OCPP SetChargingProfile currently active; total site current at the main board against the load-management setpoint; phase voltages and currents at each charge point (a lost phase produces exactly this pattern); connector and cable temperature, since thermal derating in the plug also caps current; ambient temperature at the charge point; and the vehicle's own reported maximum AC acceptance from the scan tool.
If the profile is the cause, the fix is scheduling and load management, not new charge points. If one unit alone is limited, swap that unit with a known-good one to separate hardware from configuration.
3. Propose a workable charging plan within the constraints, including what you would tell the board about the two twilight trucks.
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Sequence by energy need: the ten 175 km trucks each need about 202 kWh, which is roughly 9.2 hours at 22 kW, so they must arrive at or above 30% state of charge and be plugged in first. With 70 kVA available, run three at 22 kW overnight, and use the daytime idle window for the remainder once the cold store load is measured and known to be lower. Cap total site import with an OCPP charging profile and a fallback contactor, not with driver discipline.
The two twilight trucks need about 280 kWh grid-side but have only about four hours after the twilight run - 88 kWh at 22 kW. They cannot be recovered by AC overnight. Options are a 50 kW DC opportunity charge at the store during the twilight run, or removing them from the electric roster until capacity arrives. Present the 14-month connection timeline to the board with an interim option such as a battery-buffered charger that shaves peaks without a supply upgrade.