The 5 watt rating would be at a specified voltage. Let us assume 12 volts
nominal. That would mean a current draw of .417 amps. I am not looking at
the battery discharge cur es, but a 7 amp hour battery would normally be
rated at 7 amp hours using a total discharge between something like 13.8
volts and 10.5 volts. I am just using those numbers for illustration, but
most manufacturers rate the total amp-hours using an unrealistic low voltage
and often a higher voltage than might be used in a glider (since it might be
removed from a charger for some time).

5.5 hours at .417 amps would yield a total of 2.3 amp hours. That seems
pretty consistent with the voltage drop you measured for the 7 amp hour
battery. The 10 amp hour battery should be proportionally longer lived, so
it should take 10/7 x 5.5 hours or 7.86 hours to arrive at the same 12
volts.

Thus, it appears that the 10 amp hour battery has lost some capacity.

Colin