These items were purchased new for ~$130 from defender.com in 2017 and I went a different route on our boat. They were never installed and all components are new and in the box with instructions. The pair of items will allow you to top off your trolling motor batteries when your house battery is fully charged.

Yandina Trollbridge 24 Battery Combiner:
https://www.defender.com/product.jsp?id=672772

Features:

• Allows charging or running 24 Volt trolling motor battery from 12 Volt alternator on main engine, single output charger or trailer hook-up
• Automatically puts (2) 12 Volt batteries in parallel when charging, or in series when running your trolling motor
• Both batteries operate in parallel when not trolling
• Fully automatic, no switches or connectors to change (optional manual 12 / 24 Volt switching available)
• The (2) trolling batteries do not need to be matched or balanced
• Single-output 12 Volt charger will charge all batteries
• Works with minimal 2-battery systems or in conjunction with house battery
• LED status indicator
• 99% efficient when charging
• Waterproof – will operate when submerged in salt water
• Simple 5-wire connection to (2) batteries and charging input line
• Supplied with all cables required for installation
• Withstands ambient temperature to over 175°F for engine compartment mounting
• No diodes to burn out if accidentally shorted
• Works automatically with 97% of trolling motors and manually with 100%

Specifications:

• Yandina Model No. Trollbridge24
• Voltage: 12 Volt DC
• Rated for 12 Volt alternators up to 150 Amps
• Rated for 24 Volt trolling motors up to 85 Amps

Yandina Combiner 100 Battery Combiner:
https://www.defender.com/product.jsp?id=605576

Features:

• Compact, voltage-sensing relay (13.3 Volts) which connects (2) batteries in parallel when either is receiving a charge
• When charging ceases, relay opens so that each battery operates independently
• Suitable for alternators up to 100 Amps, up to 18 Volts
• Can be used on alternators with internal or external regulators
• Waterproof – ideal for mounting in open boats
• Electronic thermal monitoring with shutdown and automatic restart
• Easy 3-wire installation – includes all required cables and terminals
• Draws no current when batteries are not being charged
• No diodes to burn out if accidentally shorted
• LED status indicator

Specifications:

• Yandina Model No. C100
• Voltage: 12 Volt DC
• Amperage: 75 Amps (continuous)
• Dimensions: 1-1/2" H x 1-3/4" W x 1-1/8" D

Asking $80 for the pair