Unless it charges @ 13VDC + , it's never going to have a fully charged battery .

Do the basics before you go throwing $ at it ~ I had low charge from all brands other than BOSH, several were German made, a surprise to me .

The jumper charge pack can have 1,000 amperes but if it has 4Ga. cables it isn;t going to work .

The Mercedes OM661 engine requires 800 CCA to crank quickly ~ yes i have jumpered with 400 CCA batteries but that's marginal at best .

This whole thing in the winter cold, maybe just a case of low battery charge ~ I hope you understand to never quick charge a battery ~ it needs slow and gentle charging to be right as there's a chemical reaction going on inside it as the current is replaced .

As mentioned : a voltage test under load (whilst the starter is cranking or just clicking) .

I can go on about BOSCH starter failures but no point until you know what's what .

remember ; never use 1st./rev. to bump start any vehicle ! .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better