How to know when DEVO 10 batteries are charged?

Hi guys,

I have a DEVO 10 with the last FW 4.0.1.

I use a wall CHG to charge my batteries in the TX. How I could know when are they full charged? When I connect the CHG the TX Voltage shows 13.¿¿ V (it's changing around 13V) but I don't know how to know the real voltage of the batteries along the charge.

Thanks!

CONGRATZ BY THE FW!