Partner HDI 90 PS battery ????

Hello, fucking cold!!! Yesterday already happened – put in the car and turned the ignition lock – “klag”, no more!! Two second attempts the diesel started and we moved the thing. Even several times turned off and at some point turned on again – NoProblem!! This morning again an attempt – “klag”, no more! Next attempt “klag” ……. So this morning it was never possible to start the box. The neighbour says, if it only makes “kack” then the starter or. the magnetic switch on the starter and therefore I need a new starter, because the thing is firmly attached to it. An acquaintance says that a battery monitoring is installed on the newer cars and even no attempt to turn the starter is started with undervoltage – you can test if you close another car with a bridge cable to it. ALSO: Tested and ……. goes times and can’t do it!!! Now I’m not so sure if it’s done with a new battery although it is clear to me that our partner is already 6 years old and still owns the first battery!!! What do you think about this??? Thank you JBR