An emergency vehicle frequently has to sit on scene, engine running, with the crew away from the cab. Run-lock makes that safe and legal; load-shedding makes it survivable for the battery. The two work together, and getting the interaction right is central to a well-engineered ambulance, police or fire build.
What run-lock does
Run-lock lets the engine keep running after the ignition key is removed. The crew arms it with a button, removes the key, and the engine stays running to maintain alternator output. That output powers what the vehicle needs on scene: radios and MDT, scene and warning lighting, the saloon environment, medical equipment and refrigerated drug storage.
The safety logic is what makes it acceptable. The system is designed so the vehicle cannot be driven away in the run-lock state. Typically, operating the footbrake, releasing the handbrake, or pressing the clutch will cut the engine, so an opportunist who jumps in cannot simply drive off. They have to restart with the key. Exact triggers vary by installation and transmission type, but the principle is constant: the engine runs for the equipment, not for driving.
Why load-shedding is the other half
Run-lock keeps the alternator turning, so on paper the battery is not the immediate concern. In practice, two situations put the starter battery at risk:
- The engine stops on scene (stalled, run out of fuel, or deliberately shut down) while heavy auxiliary loads keep drawing.
- The vehicle is parked and dormant for long periods with tracking, telematics and standby loads slowly pulling the battery down.
In both cases the danger is the same: a front-line vehicle that will not start when it is called. Load-shedding manages that by prioritising loads and disconnecting the non-essential ones before the battery falls to the point where it can no longer crank the engine.
How load management is engineered
The building block is a battery protection device. A Battery Guard continuously monitors the battery while the charging system is off and isolates non-essential loads at a set threshold, protecting the engine-start capability with very low quiescent current of its own. It is the backstop that guarantees the vehicle starts.
On a full ambulance or response build, this sits inside a wider task and power management system:
- Prioritised loads. Loads are grouped by importance. Medical and communications equipment sits above saloon comfort loads, which sit above convenience loads.
- Staged shedding. As battery voltage falls, the controller sheds the lowest-priority groups first, giving the crew warning before anything critical is affected.
- Defined switching. Driving these loads through a programmable controller and CAN switching such as iConnex, rather than discrete relays, means the shedding logic is deterministic and diagnosable rather than dependent on a loom of timers.
- Operator feedback. Switch panels and displays show the crew what is on, what has been shed, and the state of the battery.
Designing the two together
Run-lock and load-shedding should be specified as one system, not bolted together afterwards:
- Decide which loads must survive an engine-stop and which can be shed, before wiring anything.
- Set the shedding thresholds above the Battery Guard’s hard disconnect, so the vehicle degrades gracefully rather than dropping loads all at once.
- Make sure the run-lock safety triggers suit the transmission and the crew’s working pattern.
- Verify behaviour with the engine off, not just idling, because that is the failure case that strands a vehicle.
Done properly, the crew gets full power on scene, the battery is never taken below start capability, and the vehicle is ready the moment it is needed again. Intellitec’s task and power management controllers and Battery Guards, all UK manufactured, are used on front-line NHS ambulances for exactly this job.