Originally Posted by moparado

The problem with permanently connecting two batteries in parallel is the stronger one will discharge (try to charge) the other battery when the engine is off then vice versa depending on the two batteries amp hour ratings.
With the the long wires going to the (gulp!) lawn mower battery there could be a large enough voltage drop across them to screw up charging from the alternator.
The safe charging rates between the larger and smaller batteries are different meaning the smaller battery could be over charged resulting in degradation and finally failure of the battery.
Further, if the smaller battery becomes dead shorted there is no appreciable impedance to restrict its charging current, a potential fire or explosion hazard.
And even further yet, putting a lead acid battery in the cabin is a potential safety hazard due to hydrogen gas emission another potential safety hazard.