Orange light

Yesterday on the way back from school an orange light came on on the dashboard. This was a bit disconcerting. Not having a manual I went on Google and found out it was the engine warning light which was really encouraging. I found out how you could do a diagnostic test by pressing the odometer button then switching on the ignition then letting go of the button then pressing it repeatedly until the screen says DTC then reading the code and looking it up on Google. Right. Feeling quite clever I managed to do this and came up with code 9318 which means low battery voltage. So is it a duff battery, is the alternator going or is it a faulty engine warning light or is it something else? Have suggested to P that he takes car back to garage where he got it on Thursday when he's off. Meanwhile he charged the battery this morning and I'm hoping I don't have a break-down between now and then.