How does the potential difference across the light when batteries are added in series compare to parallel?

When the batteries are connected in parralell, the voltage remains the same as a single battery, but the current capability is doubled. When connected in series, the voltage doubles at the light, but the current remains the same as if a single battery was connected.

Ohm's Law

E=IxR

R=E divided by I

I=E divided by R

E=Voltage

R=Resistance

I=Current