HEMO1987

Hello all, I'm new here!!
Have been having a read through the forum and finding some very helpful bits.
I've just brought a mondeo today after my last car give up on me. I thought a mondeo, can't go far wrong there…
I've got a 2001 2.0 litre (petrol) zetec. Driving home I was dead chuffed drove lovely especially for the price. I get home and see the engine management light lit up. After doing some searching I found out I could check for errors with the trip comp. did that fine and found this error.

DTC 9318

I've found out this means battery voltage low. When I brought the car he did say it had been standing for a while, and he had to use a battery charger to get it going.
Obviously as whenever you buy a car there was no petrol. So I drive to a petrol station about 10 min away and after filling up it won't start. I manage to get someone to give me a jump and drive home about 20 miles away in slow traffic.
This is when I seen the light. The battery has enough power to start car fine now.
But my question is should The battery not be charged by now?
Also should I charge it or just leave it a while?
Thanks