For most auxiliary LED light installs, an automatic charging relay (ACR) is the right pick, because it keeps charging voltage intact and suits AGM and lithium batteries. A simple relay or diode isolator is a fallback for older setups. Whichever you choose, disconnect the negative battery terminal before you touch any wiring.
TL;DR:
- An ACR combines batteries at 13.0V for 2 minutes and separates them at 12.75V for 30 seconds; check your model’s thresholds before troubleshooting.
- A diode isolator can lose about 0.7 volts, so reserve it for a legacy setup with a flooded lead acid battery and modest lighting loads.
- Match continuous rating to combined accessory draw: a standard unit handles up to 120A, while high output models reach 500A for heavy systems.
- Disconnect the negative terminal before wiring, install the isolator in the positive circuit, and place a correctly sized fuse close to the battery.
- Power lights from the auxiliary battery and connect their fused harness on the load side; direct starting battery wiring can still strand you after engine shutdown.
Table of Contents
- Which battery isolator types will you run into?
- How do these devices behave while charging?
- Why does an ACR usually beat a diode isolator for LED lights?
- What should you check before you buy?
- How do you install a battery isolator for your lights?
- What wire size and fuse should you use?
- Which isolator works with wheel lights, rock lights and portal lights?
- How do you match an isolator to your vehicle and lighting load?
- What safety steps should you follow during installation?
- How do you troubleshoot isolator and lighting problems?
- Where do wiring errors usually creep in?
- Prefer a pre-built kit?
- FAQ
- Sources
Which battery isolator types will you run into?
Shopping for a battery isolator for lights, you will meet three device families. Each does the same basic job, keeping accessory loads from pulling down the battery that starts your engine, but they get there differently.
- Diode isolators: low-cost, one-way devices that let current flow from the alternator to each battery but never back. The trade-off is a voltage drop across the diode.
- Solenoid or relay isolators: mechanical switching that loses very little voltage. They tend to be bulkier, and some versions are switched manually or by an ignition-fed signal.
- ACR or voltage-sensitive relay (VSR): a relay that watches system voltage, joins the batteries while charging, and separates them when the engine is off. Many include extras such as start isolation.
Littelfuse notes that solenoid-based isolators use relays for current control, while newer electronic isolators use Schottky rectifiers, power MOSFETs or relays to cut losses and size compared with older diode designs. Once you know which family a part belongs to, spec sheets get much easier to read.
How do these devices behave while charging?
An ACR is a voltage detective. It watches the charge voltage and decides when the two batteries should share the alternator. Blue Sea Systems gives a working example: the relay combines the batteries at 13.0V held for 2 minutes and separates them again at 12.75V held for 30 seconds. Treat those numbers as an operating model, since each model publishes its own thresholds.
Example thresholds: combine at 13.0V for 2 minutes, disconnect at 12.75V for 30 seconds, per Blue Sea Systems. That delay is why your light battery only joins the party once the alternator is truly charging.
A diode isolator works differently. It simply allows current in one direction only, so no sensing is needed, but every amp pays a small voltage toll on the way through.
Start isolation is the other feature worth knowing. According to Defender’s listing for the Blue Sea SI-ACR, advanced Blue Sea units offer Start Isolation and Engine Isolation, which protect electronics during cranking and allow paralleling the batteries for an emergency start.
Why does an ACR usually beat a diode isolator for LED lights?
The deciding factor is voltage. Samlex America says traditional diode isolators introduce a drop of about 0.7 volts. A diode isolator’s voltage drop can prevent the auxiliary battery from reaching a full state of charge. That matters most for AGM and lithium chemistries, which are less forgiving of chronic undercharging.
ACRs avoid that penalty. Samlex adds that they run cool, waste very little charging energy, and suit modern dual-battery systems better.
Here is where each type still makes sense:
- ACR or VSR: AGM or lithium auxiliary batteries, solar or inverter chargers on the system, and any build where a full charge matters.
- Diode isolator: an existing legacy install that already works, with a flooded lead-acid battery and modest lighting loads.
- Basic relay: a very low-budget retrofit where you accept a manual or ignition-switched setup.
Pro Tip: If your light kit draws only a few amps and rides on a battery you rarely discharge deeply, a simple relay can work. The moment you add a second charging source or a lithium battery, move to an ACR.
What should you check before you buy?
Spec sheets can be dense. Run any candidate through this checklist, or hand it to your installer.
- Continuous amp rating: match it to your alternator output and total accessory draw. Look at the intermittent or cranking rating too if the device sees high surges.
- Battery chemistry: pick an ACR or VSR for AGM and lithium auxiliary batteries, and for any system with solar or inverter chargers.
- Start isolation: confirm whether the device isolates or assists during cranking so sensitive electronics ride through engine starts.
- Wire and fuse capacity: plan the cable gauge and the fuse at the battery for the charging amps you expect. The reference table below helps with that.
Ratings are where buyers slip most. A standard unit like the SI-ACR is rated up to 120A, while Blue Sea Systems notes that high-output units like the ML-ACR are available for 500A systems. A weekend light bar does not need a 500A relay, but a heavily loaded work truck with winch, compressor and lighting might.
How do you install a battery isolator for your lights?
Battery-isolator installation involves electrical hazards. Have a qualified installer do the work, or follow the device manufacturer’s installation instructions and applicable safety requirements. The device is typically installed in the positive charging circuit between the two battery positive terminals, as Blue Sea Systems explains.
Pro Tip: Photograph your finished wiring and label both ends of every cable. Troubleshooting a year later is much faster when you can see what you did.
For the load side, our guide on powering multiple high-lumen lights on your truck covers alternator capacity and power management for bigger lighting builds.
What wire size and fuse should you use?
Undersized wire is the most common cause of hot cables and failed installs. This quick reference maps charging amps to minimum wire and fuse, based on examples from Blue Sea Systems.
| Charging amps | Minimum wire size | Recommended fuse |
|---|---|---|
| 120A | 2 AWG | 120A |
| 120A | 1 AWG | 120A |
Blue Sea’s own guidance is the authority here, and the page presents a full chart with more rows for other current levels. Final selection depends on the length of your cable run and the specs of your alternator and device, so always follow the manufacturer’s recommendation if it differs. For fusing habits that pair well with this, see our write-up on the 125% fuse rule and cable management for truck builds.
Which isolator works with wheel lights, rock lights and portal lights?
LED lighting is a gentle load compared with winches or compressors, and that shapes the choice. A set of wheel lights, rock lights or portal lights typically draws steady, low current, so the isolator’s continuous rating is rarely the limit. The bigger question is where the lights get their power.
If the lights run from the auxiliary battery, any isolator type will keep them from draining the starting battery, but an ACR keeps that auxiliary battery properly charged. If you wire the lights straight to the starting battery for simplicity, an isolator is not involved, and a long session of rock lights with the engine off can still leave you stranded.
Two practical points for accessory kits:
- Many lighting kits ship with their own fused harness. Treat that harness as the load side, and put the ACR between the batteries, not between the battery and the light harness.
- Wheel and rock lights are often switched from the cab. Wire the switch to the auxiliary side, so the lights stay on only when you want them.
Because an ACR combines the batteries during charging, your lights see charging voltage while the engine runs and battery voltage when it is off. LED controllers generally tolerate that range, but confirm the operating voltage printed on your kit.
How do you match an isolator to your vehicle and lighting load?
Start with your vehicle’s charging capacity, then add up what hangs off it. A half-ton pickup with a stock alternator and a pair of light kits sits at the small end. A work truck or overland rig with a winch, fridge and inverter sits at the large end.
Use this approach:
- Light-duty (a few light kits, stock alternator): a standard ACR in the 120A class is plenty, and a basic relay may do for flooded batteries.
- Mid-range (lighting plus a fridge or compressor): choose an ACR with a continuous rating comfortably above your combined draw, with start isolation if you run sensitive electronics.
- Heavy-duty (high-output alternator, winch, inverter): look at the high-output class. Defender’s listing for Blue Sea’s ML-ACR shows models rated up to 500A with dual sensing.
Add up the amps your accessories pull at once, then leave headroom. Choosing a device rated close to your maximum load leaves no margin and raises heat. Match wire and fuse from the table above, and revisit your plan if you add more accessories later.
What safety steps should you follow during installation?
Battery work is routine, but the energy behind it is not forgiving. Follow these steps in order.
- Park on level ground with the engine off and the key out. Remove metal jewelry.
- Disconnect the negative battery terminal first. Blue Sea Systems lists this as the standard before starting work.
- Fuse at the source. Put the fuse as close to the battery positive as practical, sized to the wire, never to the load.
- Cover exposed terminals while you work, so a dropped wrench cannot bridge them.
- Route cable away from heat and edges, and use grommets wherever it passes through sheet metal.
- Provide the dedicated ground the ACR needs, on clean bare metal.
- Reconnect last, then test. Reconnect the negative terminal only when the wiring is complete, then verify operation with the engine running.
If any step feels beyond your comfort, hand the job to a professional installer. Our guide on connecting a lighting kit to a truck fuse box shows fuse placement habits that complement these steps.
How do you troubleshoot isolator and lighting problems?
Most issues trace back to a short list of causes. Work through them from the simplest.
- Auxiliary battery never charges: check that the ACR has a good ground and that system voltage actually reaches the combine threshold. If your alternator rarely climbs above about 13V, the relay may never close.
- Batteries combine and separate repeatedly: voltage is hovering near the threshold. Check for a weak alternator, a loose connection or a voltage drop in the charging cable.
- Lights dim or flicker: inspect crimps and terminals for corrosion, and confirm the wire gauge matches the load. Undersized wire drops voltage.
- Device runs hot or a fuse blows: compare the fuse and wire to the table above, then check the continuous rating against your real load.
- Diode isolator and a weak auxiliary battery: the roughly 0.7V drop is the likely culprit. An ACR upgrade fixes it.
Test with a multimeter at the battery, at the device and at the lights. A reading that sags along the path points straight to the weak link.
Where do wiring errors usually creep in?
After enough installs, the same mistakes keep showing up: undersized cable, no fuse at the source, an electronic isolator left ungrounded, and no thought given to start isolation for sensitive electronics. The cures are boring and reliable. Use the table, label every cable, and check combine and disconnect behavior once the job is done. Treat the first test as part of the install, not an extra.
— Christopher
Prefer a pre-built kit?
If you would rather skip the wiring puzzle, we design our lighting kits in Louisiana and offer wheel lights, rock lights, portal lights and solar powered cab lights, along with harnesses made for them.
Browse our collections at Wheellightexpress and pick the kit that fits your build.
FAQ
What is the difference between a battery switch and a battery isolator?
A battery switch is manual: you turn it on or off yourself. An isolator or ACR manages current automatically, as Samlex America explains. Isolators use diodes for one-way flow, while ACRs use relays with voltage sensing.
Do you put a battery isolator on the positive or negative side?
The positive side. The device normally sits in the positive charging circuit between the two battery positive terminals, according to Blue Sea Systems. Disconnect the negative terminal before you start, and give electronic units a proper ground.
What is the best battery isolator for auxiliary lights?
For most modern setups, an ACR or voltage-sensitive relay. It avoids the voltage drop of diode isolators and suits AGM and lithium batteries. Match the continuous rating to your alternator and accessory load.
How do you hook up a battery isolator?
Mount it near the battery positive, run correctly sized cable fused at the source, ground the device if it is electronic, then reconnect the negative terminal and test. The installation steps above walk through the full order.