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https://autopapyrus.com/guides/toyota/hi-ace/installing-a-second-battery-in-parallel-with-the-factory-battery/
Compiled automatically from service literature and owner experience. Specs are for the typical configuration — check them against your exact car before you start.

Toyota Hi-Ace (H200): Installing a Second Battery in Parallel with the Factory Battery

Connecting the batteries in parallel (positive to positive, negative to negative) keeps the voltage at 12 V and adds up the capacity. On the 1KD-FTV this is usually done for more reliable cold starts of the diesel or to power additional equipment. The factory alternator charges both batteries, and it normally does not need to be replaced.

Difficulty: Medium Time: 2-3 hours Steps: 11 Tools: 10/12/14 mm wrenches, crimping tool or soldering iron for cable lugs, cable cutters, multimeter, terminal cleaner, heat-shrink tubing

Parts

6
  • 🔩 a second battery with the same voltage (12 V), capacity and type (AGM or conventional) as the factory battery, preferably of the same age
  • 🔩 heavy-duty copper cable, 25-35 mm² (positive and negative)
  • 🔩 two 100-150 A fuses (one in the positive lead at each battery)
  • 🔩 cable lugs
  • 🔩 a mounting bracket or tray for the second battery. Toyota has no specific OE part numbers for this kind of add-on kit
  • 🔩 these are universal parts

Step by step

11 steps
  1. Choose the second battery: same voltage, similar capacity and cold cranking current, and the same type (do not mix AGM and conventional). Batteries of different capacity or age are not recommended in parallel: the weaker one will be drained by the stronger one. Make sure the dimensions fit the chosen location.
  2. Choose the installation location. Ideally it should be next to the factory battery so the cables are short (up to 1-1.5 m). If you install it in the cabin or cargo area, you need a sealed AGM battery or a vented box with a gas outlet, rigid fixing to the body with a bracket, and protection of the terminals against shorting to metal.
  3. Turn off the engine, remove the key, and disconnect the negative terminal of the factory battery, then the positive one. Wait a few minutes.
  4. Secure the second battery to a rigid base with bolts through reinforced washers or plates. The battery must not shift during braking.
  5. Make the cables: crimp the lugs with a crimping tool, then seal them with adhesive-lined heat-shrink tubing or heat-shrink tubing. On the positive cable, install a fuse at each battery end, as close to the terminal as possible (within about 20-30 cm (8-12 in)).
  6. Connect the positive of the second battery to the positive of the factory battery through the fuses. Connect the negative of the second battery to the body at a point cleaned down to bare metal (chassis ground) and, preferably, also with an additional cable to the negative of the factory battery or to the engine ground. The contacts must be clean and tight; clean any painted surfaces.
  7. Route the cables away from sharp edges, hot exhaust parts and moving components. Secure them with cable ties, and fit rubber grommets or corrugated conduit where they pass through metal.
  8. Before connecting, make sure both batteries have similar voltage (a difference of no more than about 0.2 V; check with a multimeter). If one is noticeably discharged, charge it separately before connecting.
  9. Connect the terminals: positive first, then negative. Tighten moderately without excessive force (over-tightening breaks the posts on lead terminals). Use the exact value from the battery manufacturer's specifications.
  10. Start the engine and measure the voltage at the terminals of both batteries: at idle it should be about 13.8-14.5 V. Check that the cables and fuses do not heat up after several minutes of operation and after a start.
  11. After a day or two of the vehicle sitting, measure the voltage of both batteries again (at rest, 12.4-12.8 V). Make sure there is no leakage: the overnight parasitic draw of the factory electronics should remain the same as before.

Common mistakes and warnings

  • Without a fuse at each battery, a short circuit in the cable will cause a fire. Do not install batteries in the cabin without ventilation (except sealed AGM). If you need to power additional equipment (a refrigerator, an inverter) with the engine off, a direct parallel connection without isolation is dangerous: you can drain the starter battery as well. In that case, use an isolation relay or a DC-DC charger instead of a direct parallel connection. If you have no experience with high-current wiring, have an auto electrician do the work.