Charging HP Prime

[matalog]

I had my HP Prime plugged in during a 2 hour train journey, plugged in a USB power outlet on the train, which charged my modern smart phone in 20 mins. The screen on the HP Prime stayed lit throughout the journey with the lightning symbol over the battery.

I got home a day later and turned on the calculator, and it was giving me warnings about needing to charge as the battery was at a critically low level.

I assume something caused it not to charge correctly on the train.

I then had to charge my calculator at bed time last night, but I have found that it does not charge unless the device is turned on, and therefore the screen is on. Obviously charging overnight, I would rather that I could charge with the screen off.

Should I have to charge with the screen on, or should this HP Prime G2 charge from a 2.1A mains adapter when plugged in, and not turned on?

Thanks for any help.