I agree with Loius - one or more cells went dry and that's that - it's gone.
I haven't seen a self-maintainable auto battery in years, but that's how it is in the U. S.
12 volt batteries contain 6 cells and the ideal charge in each is 2.1 volts. 6 x 2.1 = 12.6 volts - the ideal reading one would get with a VOM set on the 20 scale and after the auto had been at rest for 6-8 hours.
If you cancel out one cell(2.1 volts), and the other 5 are healthy(2.1 volts each), you're down to 10.5 volts. That won't work. In all likelyhood if one cell is dry the others are probably below optimum.
Time for a new battery.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.