I also have a 172M (N5132H) but I get about 8 years minimum on the last
three ordinary lead acid battery - with no between flight recharges.
The clock still works too. I suspect if it ever quits, there is the
possibility that it could draw current continuously rather than as a
simple pulse. Otherwise the golden rule on batteries is to keep the
plates covered at all times - even if the water has to come freom the
tap or a mud puddle.

The battery can be recharged thru my ground service plug, but it
requires a fairly high voltage from the charger to charge the 12 V
battery. From the circuit it seems to bring the charge current thru a
series resistor and an isolating diode. It also requires the basic bus
to be energized running the rate-of-turn, in order to charge.

If you let the A/C battery become fully discharged (such as leaving the
master on) there is nothing to self energize the alternator even if you
get it started thru the ground service receptacle (sp?). Leaving the
ground service plug in place for a few minutes before attempting a
start, allows a little bit of charge to get in the A/C battery,
allowing the alternator to start. These few minutes are not going to
hurt the rate of turn. If you wanted to charge more extensively, you
could reach behind the panel and unscrew the connector on the back of
the R/T. Don't let it dangle though as it might get in the control
column travel.

Why Cessna did it this way, I don't know unless to protect the
electronics?. Does anyone else know? In general I think they had a
very good electrical system.

For that matter, do I have it right? Where is that darn charge-thru
diode and resistor? What is the value of the resistor (maybe 5 ohms?)?
The schematics I have looked at in the pilots manual and the Cessna
service manual don't seem correct, consistent, or even look complete in
that area.

nrp