Curious no start behaviour

[ukkid35]

I often find that when attempting to start the car from cold there’s enough juice in the battery to crank the engine for 20 or 30 seconds, but without a hint of it firing. If I put the battery charger on it for 20 minutes it will start instantly. I presume this is because there’s insufficient voltage at the injectors to open them. Does that make sense?

[gruffalo]

Injectors or coil packs, battery not able to sustain a decent voltage when delivering cranking current.

What is trhe bettery voltage when turning the starter?

[ridds]

Yep, ECU voltage most likely.

Plug in laptop and watch for dropped comms or low Vbat when cranking.

If so, check the ECU earth in the passenger footwell.