A better explanation can be had by reading the mfg web site description, a portion of which is quoted below.
I highlighted the relevant part:
"Xantrex’ digital echo~charge (part#82-0123-01) is designed to charge auxiliary or starting batteries from an inverter/charger or other charging source with limited voltage drop. The Xantrex echo~charge detects when the house battery bank is being charged and directs a portion of the charge current to auxiliary or starting batteries
. The maximum charger current offered by echo~charge is limited to 15 amps.
- 15 A maximum charge current
- Automatically switches on/off without affecting the in-house battery bank or over-charging
- Utilizes excess current from the primary charging source"
The start battery should never be discharged significantly and could cook while the house bank gets charged slowly.