12V Ute Guide

Ford Ranger dual battery setup

Short answer: Rangers with Ford's Smart Charge have a variable-voltage alternator, so use a DC-DC charger rather than an isolator. You don't need Ford to turn Smart Charge off.[1] Wire the charger's ignition trigger, earth to the chassis, and size the charger to your battery.

1. Confirm Smart Charge

REDARC says smart alternators use a battery sensor, usually on the start battery negative terminal.[1] Check your Ranger, or ask Ford or an auto electrician if you're unsure about your model year.

2. Use a DC-DC charger, not an isolator

REDARC says variable-voltage alternators can run too low, too high or not charge at all, so a smart isolator can't charge an auxiliary battery to 100%. It recommends its BCDC1225D or BCDC1240D instead, and says disabling Smart Charge isn't needed.[1] It also advises against disconnecting the smart function.[1]

3. Trigger wiring

On the BCDC Classic range, connect the blue wire to an ignition-switched source on smart-alternator vehicles. Typical pick-up points include a vehicle fuse or the 12 V accessory socket circuit. Check it's live only with the ignition on.[2] REDARC's BCDC Alpha guide adds that on idle-stop vehicles the ignition cable should go to D+ or a similar pick-up point rather than plain ignition.[4] Other brands differ, so follow your manual.

4. Earthing: don't bypass the battery sensor

Earth the charger and auxiliary battery to the chassis, not straight to the start battery negative. REDARC warns that a direct connection bypasses the sensor, so the vehicle computer won't account for that load, and the start battery may be undercharged.[3]

5. Size the charger and cable

REDARC suggests its 25 A unit for roughly 75–200 Ah banks, and 40 A or 50 A for larger batteries or multi-battery banks.[2] Run your numbers in the DC-DC sizing calculator and check voltage drop over the run to a tray or canopy with the cable size calculator. Fit fuses close to each battery positive.[4]

6. Check runtime

See how long your setup will run a fridge with the fridge runtime calculator.

FAQ

Do I need Ford to disable Smart Charge to fit a second battery?

No. REDARC says Ranger owners with Smart Charge do not need it disabled; installing a compatible DC-DC charger such as its BCDC1225D or BCDC1240D is enough.

Can I use a voltage-sensing isolator in a Ranger with Smart Charge?

REDARC says no. With a variable-voltage alternator the output can be too low, too high or not charging at all, so an isolator cannot charge the auxiliary battery to 100%.

Can I just disconnect the smart alternator?

REDARC does not recommend it. It says the smart alternator is an advantage in normal driving and disabling it may stop your battery reaching full charge.

Sources

  1. REDARC Help Centre: How do I know if I have a variable voltage alternator?
  2. REDARC Help Centre: How to use a BCDC charger to charge a secondary battery in any vehicle
  3. REDARC Help Centre: How to ground my BCDC, BMS or TVMS to my vehicle
  4. 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