Why is my volvo v40 trying to overcharge its battery, and what can I do about it?

Gas, diesel, 4cyl, or 6cyl.? turbo? (options) ??????
as always, fuses tested first .
if the Alt sense fuse is blown, all bets off.

overcharged, please define that. ? Back battery dry, ruptures?
voltages outside the norm of 13.3 to 15v??????

1. Bad batteries can fool the regulator
2. Bad wiring to reg.
3. bad REG (in alternator in the PCM)
4. a burned out charge lamp in dash , can cause issues.
5. This lamp must glow key on, if not fix that first.
6. fuse 11a/22 15amp blown. (sense fuse)

the reg is found in the alternator or in the PCM on most cars.
but Volvo is the odd duck..
depends on years and models of cars.
2000 V40
there are 3 systems MELCO , 1 and 2, (3 sys)
The ALT has a field driver in the ALT, its not a regulator as drawings say.its a slave to the MELCO box. 5 or 6 pins, 6 is diesel.

the swedes, call this a 4/32 module, how it works is some secret.
this is a seperate module.
for sure it controls the Field pin on the ALT. so... is a key player.
if all connections are good, must be, MELCO DOA.
4 ground pins, and 1 field pin, what could it do with just those pins.
ambient air temp ? sensor?
a real FSM would answer that.

Did you check the fluid in the battery. Level ok?
But the more worrying thing is the voltage regulator is not working properly anymore. Check the manual, to see at what voltage it should stop and measure if it does.