At 06:40 on a Tuesday in March 2025 we closed the main breaker at Ostrand. Four hundred megawatt hours of battery were connected to the grid for the first time. Nothing dramatic happened, which was the plan. I stayed on site for the next three days anyway. I always do.

The first 72 hours of a battery site are when the design meets the weather, the grid and the people who will run it. Models and factory tests tell you what should happen. Those three days tell you what does.

The first night is about heat

Batteries are fussy about temperature. Each container has its own cooling, and on paper every container behaves the same. On the first night at Ostrand, container 31 ran four degrees warmer than its neighbours. Our technician Jarek found the cause with a torch at two in the morning. A shipping cover had been left on an air intake.

No alarm would have caught that for weeks. The container was still inside its limits. But a battery that runs warm ages faster, and over fifteen years that one cover would have cost real money. We now walk past every container by hand on the first night, whatever the screens say.

The screens show you what you thought to measure. Walking the site shows you the rest.

The second day is about the grid. Every connection point has its own character. At Ostrand the voltage rose each afternoon when a nearby wind farm picked up, and our control system corrected for it a little too eagerly. We softened one setting and the wobble stopped.

What I watch for in those three days

• Any container that is warmer or cooler than the ones beside it.

• Alarms that appear and clear by themselves. They are never nothing.

• How the site reacts when the grid voltage moves, morning and evening.

• Which screens the operators really look at, and which they ignore.

• Every question the night shift asks. Each one is a gap in the handover pack.

By the third day the site usually settles and the questions slow down. That is when I hand over, and not before. Leaving early saves three days. Staying has shown me at least two problems on every site that would otherwise have surfaced months later, with nobody around who remembered why.