"dead" battery info
changing a dead battery is so hard on a c4, so when mine kept going flat I did some investigation before leaping to the conlusion the battery was in fact dead or dying.
I have not been using my c4 lately, and found that every week it needed a batter recharge. There are only two reasons for this - either the battery cannot hold a charge, or something is draining it.
Turn off the car and put an amp-meter in line on the positive connection. Mine was drawing 300ma.
Go round the fuse panel and see if pulling a fuse changes the current reading. If so, investigate wiring, connectors and switches.
On one c4, I found a current of 1amp, this turned out to be the interior hood light. The switch was rusted and always on. Fixed that, and no more problems with the battery.
On another c4 with 300ma with the doors closed, lights off and start key not plugged in, none of the fuses had any effect. That leaves the security system... try turning it off, or flipping the valet switch.
Otherwise... if not security system, how bad is a 300ma leak? enough to discharge a new battery within 7 days! I think, experiment shows this is correct with a brand new fully charged battery.
final check - charge the battery and then disconnect it, leave for a week & reconnect. The engine will start, this proves the battery is ok and you need to find what is drawing the 300ma.
That is as far as I have got. Anyone have an idea?