The Sysco Foods Los Angeles fleet was having a problem: the liftgates they relied upon for deliveries were causing truck breakdowns.

The liftgate battery in the rear of the trailer is hardwired to the starting battery in the tractor, connecting via a two pole connector.
Drivers run the liftgate at each of their many deliveries during a working day. Frequent liftgate operation can deplete both the liftgate and starting batteries until the vehicle is unable to start. 

This proved to be a very expensive problem.
In addition to the wages of the road-call mechanic, other costs such as lost time in the shop, late penalties, lost driver time, led Sysco to estimate the cost of a jump start outside of their shop at over $1000. 

A Littelfuse CVP Sales Representative suggested the Cole Hersee brand 48530 Smart Isolator, and as a test, Sysco installed Smart Isolators on  their tractors, in the circuit  between the battery and the two pole connector. 
The test was so successful that Sysco installed the Smart Isolator on every one of the liftgate equipped trucks in the fleet.
Sysco now specifies the Cole Hersee 48530 to be installed in their new tractors before they take delivery from Freightliner.

How the Smart Isolator functions
to eliminate dead batteries for Sysco… and other tailgate users
The alternator and starter motor are connected to the starting battery, and via the Smart Isolator to the auxiliary battery.

When the truck is running,
the alternator charges both batteries.

When the truck makes a delivery,
the alternator is not charging the batteries, and there’s a draw on both batteries from the tailgate motor. If the draw reduces the battery voltage to 12.7, the Smart Isolator cuts the starting battery out of the circuit, thus preserving starting power. The auxiliary battery continues to power the liftgate.

When the truck is started,
the Smart Isolator enables the alternator  to charge the Starting Battery up to 13.2 V, then it allows charging of the auxiliary battery as well.

Pretty smart, really!