I have given this topic some thought for my vehicle. My plan is remove an AGM battery from one of those cheap yellow portable jump-start device. The battery is small enough to mount in the engine compartment close to the battery without the need for a lot of expensive cable.

The AGM battery has better cycle, temperature, and discharge characteristics than standard flooded batteries and a small one will provide that needed extra 'oomph' to my starter on those really cold mornings for $50

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1987 300SDL-268k mi