Long Answer
This touches on a number of issues all of these have had some sort of attention above mut very little hard fact.
The GR 6 (AN 88) requirment is for at least 30 min of battery life after the failure check list had been done, some people calculate this, however I wanted to be a little more practical when I upgraded the avionics on my aircraft and so decided to meet the requirment by demonstrating the the time to failure following the loss of the alternator.
We assumed that following the low bus warning light 5 min was required to do the check list so the battery ran the following equipment for 5 min:- all aircraft indicators, electric turn & slip, pannel lighting, KCS 55 compass system & RMI, 2 x KX 165 nav /com, KN 62 DME, KR87 ADF, KN89B GPS, AT 150 ATC transponder, Collins audio & intercom.
After 5 min we switched off all the equipment except the aircraft indicators, turn & slip, the KCS55 compass system, one KX165, KN89B GPS & AT150 ATC transponder.
We occationaly transmitted on the KX165 to simulate communicating with the ground.
After 2 hours we turned on all the eqipment for 10 min to simulate an IFR approach, the whole test lasted 2 hours 15 min and we had no equipment failure due to lack of electrical power.
This test was conducted with an "average" 12v battery.
The Cessna 152 has a 24V battery and is likely to have at least as much power if it is reasonably charged and has been serviced properly......... and here comes the problem!
The battery fitted to your average club aircraft is unlikely to get much in the way of TLC, there is no requirment for a capacity check as this is not reccomended by the battery makers. Quite quickly after I got both my C152's the batterys failed and following this I replaced both the batterys and got a spare, I rotate the batterys around the fleet at the 50 hour checks and do the servicing on the spare battery when it is not in use, over the last 4 years I have not had a battey fail. This would seem to indicate that the average flying club battery gets a hard life and is not to be trusted.
So the question on your lips now is how as a flying club member do I detect the imminent failure of a battery?
Shortly before both my batterys failed I had a number of reports of the pitot heat being very week, as this is one of the bigger loads that you can apply to the electrical system I think that it is probably the only "battery test" that a flying club member can use on an aircraft that he/she is renting.
I think that it is most important to not over react to an alternator failure, don't panic, do the GR 6 check list with care as with even a poor battery you are likely to have well over 30 min to get the aircraft on the ground with no loss of electrical services for the landing.
If it was a VFR day I would probably switch off all the electrics (turning them back on for a FREDA check at 20 min intervals) and continue to my destination, turning it all back on for landing, at night or IFR I would put the aircraft on the ground ASAP.
The last thing to note is that if the alternator fails don't forget to turn OFF the alternator part of the split master switch, powering the alternator field is of little use if the alternator has failed or is not turning due to a broken drive belt.