DC-DC charger or VSR isolator: which do I need?

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Quick answer

A VSR (voltage sensitive relay, or isolator) simply connects your auxiliary battery to the vehicle’s charging system when the engine is running. A DC-DC charger converts the vehicle’s power into a proper multi-stage charge for the auxiliary battery. Choose a DC-DC charger if your vehicle has a smart (variable-voltage) alternator, your auxiliary battery is lithium, the cable run is long (canopy, tray or caravan), or you want solar charging built in. A VSR can still suit an older vehicle with matching lead-acid batteries close together under the bonnet.

Is this a DIY job?

  • Difficulty: Medium for a VSR under the bonnet; Pro for a DC-DC charger with long cable runs, lithium or a smart alternator
  • Time: 2 to 6 hours
  • Tools: heavy cable crimper, cable cutter, multimeter, drill and grommets
  • Skills: high-current wiring and fusing

Step by step: choosing

Dual battery system overview The alternator charges the starting battery. A charging device, either a VSR or a DC-DC charger, links the starting battery to the auxiliary battery, with a fuse or circuit breaker near each battery. Solar can feed the auxiliary battery through a regulator or the DC-DC charger. Accessories such as the fridge and lights run from the auxiliary battery through a fuse block. How a dual battery system works Alternator engine running Starting battery starts the engine, runs the vehicle Charging device VSR (isolator) or DC-DC charger Auxiliary battery deep-cycle AGM or lithium securely mounted Fuse block one fuse per accessory Solar panel via regulator or the DC-DC solar input Fridge Lights, USB fuse fuse Key points Fuse or circuit breaker close to each battery. Engine off: batteries are separated, so accessories can’t flatten the starting battery. Smart alternator, lithium or long cable run: a DC-DC charger is usually the better choice.
Dual battery system overview showing where the VSR or DC-DC charger sits
  1. Find out what alternator you have. Many newer vehicles (roughly 2015 on, and some older ones) have smart or variable-voltage alternators, often alongside stop-start or a battery sensor on the negative terminal. Check the owner’s manual or ask the vehicle maker. A smart alternator may drop its output once the starting battery is full, so a VSR may not see enough voltage to charge the auxiliary battery properly.
  2. Check your auxiliary battery type. Lithium (LiFePO4) batteries need a charge profile to suit them. A DC-DC charger with a lithium setting is the usual choice. A VSR generally suits matching lead-acid batteries.
  3. Measure the cable run. Long cables lose voltage. A DC-DC charger mounted near the auxiliary battery makes up for some of that loss. A VSR can’t, so the auxiliary battery may only partly charge. (see What size cable do I need for my 12V accessory?)
  4. Decide on solar. Many DC-DC chargers have a built-in solar input (often MPPT). With a VSR, you need a separate solar regulator.
  5. Consider cost and simplicity. A VSR is simpler and cheaper. A DC-DC charger costs more but usually charges more completely.
  6. Check the charger’s input rules. Some DC-DC chargers need an ignition or “D+” signal to work with smart alternators. Follow the maker’s instructions.
  7. Fuse both ends. Fit a fuse or circuit breaker near the starting battery and near the auxiliary battery (or at the charger output), sized for the cable.

Specs table

Situation Usually suits
Older vehicle, conventional alternator, two lead-acid batteries side by side VSR
Smart or variable-voltage alternator DC-DC charger
Lithium auxiliary battery DC-DC charger with lithium profile
Auxiliary battery in a canopy, tray or caravan (long run) DC-DC charger near the auxiliary battery
Want solar charging through the same unit DC-DC charger with solar input
Lowest cost, simplest install VSR (if the situation above suits)

What you’ll need

  • DC-DC charger or voltage sensitive relay
  • heavy duty single core auto cable or twin core auto cable
  • circuit breaker / MIDI fuse holder (one each end)
  • cable lugs and adhesive heat shrink
  • Anderson plug 50A (for caravan or trailer connections)

When to call an auto electrician

  • Your vehicle has a smart alternator, a battery sensor, stop-start or a battery management system.
  • You’re installing a lithium battery or running cable to a canopy, tray or caravan.
  • The auxiliary battery isn’t fully charging after a day’s driving.
  • You want a combined system with solar, an inverter and a battery monitor.

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Related questions

Q: Can I use a VSR with a lithium auxiliary battery? A: Generally not recommended. Lithium batteries need a suitable charge profile, and they can draw very high current from the alternator through a VSR. A DC-DC charger with a lithium setting is the usual approach.

Q: How do I know if I have a smart alternator? A: Signs include stop-start, a sensor on the battery negative terminal, or charging voltage that varies a lot while driving. Check the owner’s manual or ask the vehicle maker. (see What is a smart alternator and how does it affect accessories?)

Q: Does a DC-DC charger need solar? A: No. Solar input is an option on many units. It’s useful if you camp for several days without driving.

Q: Where should the DC-DC charger go? A: Usually close to the auxiliary battery, with heavy cable from the vehicle side. Follow the maker’s limits on heat and mounting.

Sources

  1. REDARC, “Choosing the right charger for a dual battery system” – isolators connect on sensed voltage and rely on alternator output; DC-DC chargers set the charge to suit the battery and handle voltage drop to a canopy better; isolators generally suit older fixed-voltage vehicles, DC-DC chargers newer variable-voltage ones; solar input. https://www.redarcelectronics.com/au/discover/choosing-the-right-charger-for-a-dual-battery-system/ (checked October 2026)
  2. REDARC, “BCDC v DC-DC dual battery chargers” – variable-voltage (smart) alternators need a DC-DC charger for the auxiliary battery; chemistry-specific charge profiles including lithium; solar charging. https://www.redarcelectronics.com/au/discover/bcdc-v-dcdc-dual-battery-chargers/ (checked October 2026)

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