Autopapyrus · Toyota Hiace (H200)
https://autopapyrus.com/guides/toyota/hi-ace/installing-a-second-battery-with-an-isolator-relay/

Compiled automatically from service literature and owner experience. Specs are for the typical configuration — check them against your exact car before you start.

Toyota Hiace (H200): Installing a Second Battery with an Isolator Relay

The isolator relay connects the second battery to the main circuit while the engine is running and the alternator is charging. After the engine stops, the relay opens the circuit, so auxiliary equipment (refrigerator, inverter, lighting, winch) runs from the second battery without draining the starter battery. The example below is described for a Hiace H200 with the 1KD-FTV diesel. The exact routing and mounting locations depend on the body style (van or passenger), so adapt them to your vehicle.

Difficulty: Medium Time: 3-5 hours Steps: 12 Tools: Socket and wrench set (10/12/13 mm), torque wrench, cable lug crimping tool (or hammer and chisel for soldering), multimeter, screwdrivers, zip ties, split loom conduit, fuse holder, drill and rivet tool (or bolts) for mounting the battery

Part

🔩 100-200 A isolator relay (battery separator) controlled by the ignition (or a smart battery isolator with voltage sensing), second battery (AGM or gel, deep cycle, 80-120 Ah), 16-25 mm² power cable (25 mm² is better for runs longer than 4-5 m), fuses for the power circuits next to each battery, cable lugs, heat-shrink tubing, terminals, battery box or tray

Step by step

12 steps

Choose the location for the second battery. It is best inside the cabin or cargo area in a sealed, ventilated box (acceptable for AGM), or under the body in a sealed box. Do not place it near the exhaust, and do not leave it without rigid mounting. The battery weighs 25-35 kg, so you need a rigid steel platform and a hold-down that will keep it from shifting in an impact.
Disconnect the negative terminal of the main battery. Wait 5-10 minutes so the control modules can settle. On a 1KD-FTV diesel with an immobilizer and a radio, settings may be lost. This is normal. Find the main battery in the engine bay or under the seat (the location depends on the body style and steering wheel side).
Choose a location for the relay. Ideally it should be near the main battery, so the power cable on the starter side is short. Mount the relay on a flat surface, away from the heat of the exhaust manifold and the turbocharger. If you chose a unit with electronic control, connect it according to the manufacturer's diagram.
From the positive terminal of the main battery (or from the power terminal at the starter, if that is more convenient), run a power cable to one power terminal of the relay. Be sure to install a fuse (about 100-150 A, depending on the cable cross-section and the relay) within 20-30 cm of the battery positive terminal. Without a fuse, a short circuit in the wiring creates a fire hazard.
From the second power terminal of the relay, run a cable to the positive terminal of the second battery, also through a fuse next to that battery. Route the cable away from hot parts, moving components and sharp edges. Pass it through metal only via rubber grommets, secure it with zip ties every 30-40 cm, and use split loom conduit on exposed sections.
Connect the relay control wire (coil) to a positive feed that is live only when the engine is running. The alternator "D+" output or an ignition line through a fuse works best. Do not use a constant positive feed, or the relay will always be on and will drain the main battery. On the 1KD-FTV, find the connection point with a multimeter: voltage should appear on the correct wire with the ignition on, and preferably only after the engine starts (smart relay units will switch on at a voltage threshold of 13.2-13.5 V).
Connect the relay negative and the second battery negative to the body at a point cleaned down to bare metal, close to the battery itself (use a cable of the same cross-section as the positive one). For extra reliability, also connect the second battery negative to the main battery negative or to the engine block with a separate cable. Coat the ground points with conductive grease and protect them from moisture.
Connect the auxiliary equipment (inverter, refrigerator, lighting) only to the positive of the second battery through its own fuse block. The negatives of the loads go to the common ground of the second battery. Do not connect high-power consumers to the wiring of the Hiace's factory circuits.
Check the circuit before applying power: the resistance between positive and negative must not be close to zero, and all power connections must be clean and tight. Do not overtighten the terminals: as a guide, use 5-8 N·m (4-6 lb-ft) for battery terminals, and for the bolted cable lug connections on the relay, follow the relay manufacturer's instructions.
Reconnect the main battery negative and start the engine. Check with a multimeter: with the engine running, the voltage on both batteries should be 13.8-14.5 V and the relay should be on. Shut off the engine: the relay should open, and the battery voltages become independent. Turn on a load and make sure the starter battery is not draining.
Check the cables, relay and terminals for heat after 15-20 minutes of operation under load. Noticeable heating indicates a poor connection, a cable that is too thin, or an overloaded relay, and the cause must be fixed.
If the second battery is often deeply discharged or the vehicle has heavy loads (an inverter over 1000 W), consider the capacity of the factory alternator and think about using a DC-DC charger instead of a simple relay: it charges AGM and gel batteries better and protects the main alternator.

Common mistakes and warnings

Work with the negative terminal disconnected, and do not let the positive cable short to the body: a 16-25 mm² cable can carry hundreds of amps, which can cause burns and fire. Fuses on the positive cables are mandatory, the battery must be securely fastened, and the cables must be protected from chafing. If you have no experience with high-current electrical work, it is better to have an auto electrician do the installation, and do not overload the factory alternator and wiring beyond their rated capacity. Check the exact output of the factory alternator and the permissible load on the label or markings on your alternator.