Toyota Hilux dual battery setup: a checklist
Short answer: the right setup depends on which alternator your Hilux has, so check that first. Then size a DC-DC charger to your auxiliary battery, wire its trigger to suit the alternator, and earth everything to the chassis.[1][2][3] Alternator and wiring details differ across Hilux model years and grades, so this page doesn't claim one answer for every Hilux.
1. Work out your alternator type
REDARC says smart (variable-voltage) alternators rely on a battery sensor, usually on the start battery negative terminal. If you can see a sensor module clipped to the negative terminal, assume you have a smart alternator.[1] If there isn't one, it is probably a fixed or temperature-compensating type. Confirm with Toyota or an auto electrician for your exact build.
2. Choose isolator or DC-DC charger
- Smart alternator: use a DC-DC charger. REDARC says an isolator is unsuitable because the voltage can be too low, too high or not charging at all.[1]
- Fixed alternator: an isolator can work, but a DC-DC charger still gives a proper multi-stage charge, which matters for lithium and for a battery in the tray where cable runs are long.[2]
3. Size the charger
REDARC's guidance is that its 25 A unit suits roughly 75–200 Ah battery banks, and 40 A or 50 A units suit larger single batteries or multi-battery banks.[2] Run your own numbers in the DC-DC charger size calculator, then read 25A vs 40A.
4. Trigger wiring
On REDARC's BCDC Classic range, connect the blue wire to an ignition-switched source on smart-alternator vehicles and leave it unconnected (or grounded) on fixed or temperature-compensating alternators.[2] REDARC lists a vehicle fuse, the 12 V accessory socket circuit, or the supply to wipers, mirrors or a dashcam as typical pick-up points. Confirm the point is live with ignition on and dead with ignition off.[2] Other brands differ, so follow your manual. See smart alternators explained.
5. Under bonnet or in the tray?
If the battery is in the tub or canopy, the cable run from the start battery is long, so voltage drop matters. Use the cable size calculator with your real run length. Install fuses close to each battery positive. REDARC's install guides, for example, call for the auxiliary fuse within 150 mm of the battery terminal.[4]
6. Earthing
Earth the charger and auxiliary battery to the chassis so everything shares a common ground. On smart-alternator vehicles don't connect accessories directly to the start battery negative, because that bypasses the vehicle's battery sensor.[3]
7. Check runtime
Use the fridge runtime calculator to check the battery size gives you the days off-grid you want.
FAQ
Does my Hilux have a smart alternator?
Check rather than guess. REDARC says smart (variable-voltage) alternators use a battery sensor, usually fitted on the start battery negative terminal. If your Hilux has one, plan for a DC-DC charger wired for a smart alternator. If you are unsure, ask Toyota or an auto electrician.
Can I use a plain isolator in a Hilux?
Only if the alternator is a fixed-voltage type. REDARC says a smart battery isolator is unsuitable with a variable-voltage alternator because the voltage can be too low, too high or not charging at all.
Where should the earth go?
To the chassis, not straight to the start battery negative. On smart-alternator vehicles a direct connection bypasses the battery sensor, which REDARC warns can leave the start battery undercharged.
Sources
- REDARC Help Centre: How do I know if I have a variable voltage alternator?
- REDARC Help Centre: How to use a BCDC charger to charge a secondary battery in any vehicle
- REDARC Help Centre: How to ground my BCDC, BMS or TVMS to my vehicle
- REDARC BCDC Alpha 100R install guide (PDF)
This is general information, not a substitute for the manuals for your specific charger, battery and vehicle. Fixed wiring and anything near the starter battery should be installed by a qualified auto electrician.
Last updated: 15 October 2026