Upgrading Your Auxiliary Car Battery Installation Kit from AGM to Lithium (LiFePO4)

If you’re considering upgrading your auxiliary car battery installation kit from an AGM battery to a Lithium Iron Phosphate (LiFePO4) battery, you’re making a smart investment in performance, weight savings, and longevity. As more 4WD owners, overlanders, RV enthusiasts, and off-road adventurers switch to lithium power, it’s important to understand the changes required to ensure your dual battery system operates safely and efficiently.

Why Upgrade from AGM to LiFePO4?

AGM batteries have been the standard for auxiliary power systems for years, but LiFePO4 batteries offer several significant advantages.

Why LiFePO4 Outperforms AGM

Weight: LiFePO4 batteries are typically 40–60% lighter than comparable AGM units, reducing overall vehicle load.
Charge Speed: Lithium chemistry accepts charge current far more readily than AGM, shortening recharge time during normal driving.
Usable Capacity: While AGM batteries are generally limited to ~50% depth of discharge, LiFePO4 batteries can be discharged to 80–100% without meaningfully impacting longevity.
Cycle Life: LiFePO4 batteries commonly exceed 3,000 charge cycles, compared to 300–800 for AGM — a substantial difference in long-term value.

Compatibility Considerations

Not all AGM-rated installation kits are suitable for lithium batteries. LiFePO4 charging profiles differ from AGM, and mismatched lithium battery isolators or charging systems can lead to incomplete charging or premature battery degradation. Before upgrading, verify your kit includes:

A lithium-compatible smart battery isolator like True brand
Correct charging voltage support for LiFePO4 chemistry
Wiring rated for the charging current
Quality terminals and fusing
Waterproof, vibration-resistant components for off-road conditions

Feature	AGM Auxiliary	LiFePO4 Auxiliary
Usable Capacity	50% (50Ah out of a 100Ah battery)	80% to 100% (80–100Ah usable)
Weight (per 100Ah)	~60–70 lbs	~22–28 lbs
Lifespan	300–500 cycles (2–4 years)	3,000–5,000+ cycles (8–10+ years)
Alternator Connection	VSR / Solenoid or DC-DC	DC-to-DC Charger Required
Resting Voltage	~12.6V – 12.8V	~13.3V – 13.4V

Components Likely Requiring an Upgrade

Smart Battery Isolator: A standard voltage-sensitive relay is typically not optimized for lithium charging; a True lithium battery isolator is essential.
Battery Cables: Confirm gauge and condition; undersized or degraded cabling increases voltage drop and reduces charging efficiency. No worries as all lithium dual battery kits are complete with installation kit.
Battery Tray/Mounting: LiFePO4 units are often smaller and lighter — ensure the tray still secures the battery firmly, especially off-road.
Battery Monitoring: A dual battery voltmeter or monitor lets you track charging status and health across both batteries. The True dual voltmeter is functional with either lithium and AGM batteries.

Benefits of a Lithium Auxiliary Battery System

After upgrading, you’ll notice several improvements:

Faster recharge while driving
Longer runtime for camping accessories
Reduced vehicle weight
More usable battery capacity
Longer battery lifespan
Better performance for refrigerators, inverters, lighting, and air compressors
Lower long-term maintenance costs

Is the Upgrade Worth It?

For users running fridges, inverters, lighting, communications gear, or recovery equipment off an auxiliary battery, LiFePO4 delivers a clear performance and longevity advantage despite the higher upfront cost. Occasional users with minimal auxiliary power needs may find AGM still sufficient.

What to Look for in a Kit

LiFePO4 compatibility
Original True lithium battery isolator kit
Plug-and-play installation
Heavy-duty cabling and connectors
Waterproof housing
Automatic battery protection
Compatibility with your vehicle’s charging system

The 3 Non-Negotiable Rules for the Upgrade

The Big Takeaway: The single most important change is swapping your old isolator/VSR for a DC-to-DC charger. Without a DC-DC charger, your lithium battery will overwork your alternator and never fully charge.

1. Replace Your Isolator/VSR with a DC-to-DC Charger

Why AGM used VSRs: Standard Voltage Sensitive Relays (VSRs) or smart solenoids simply link your starter battery and AGM battery together when the engine is running.
Why Lithium needs a DC-to-DC Charger: LiFePO4 batteries have extremely low internal resistance. If tied directly to the alternator via a VSR, the lithium battery will pull as much current as possible, which can overheat or burn out your vehicle’s alternator. Additionally, standard alternators do not supply the precise 14.2V–14.6V constant voltage required to charge lithium to 100%.

2. Check Temperature Limits & Battery Placement

Keep out of extreme heat: AGM batteries can live under the hood, but engine bay temperatures often exceed 140°F (60°C), which degrades LiFePO4 cells and trips their internal BMS (Battery Management System).
Watch out for freezing: LiFePO4 batteries cannot be charged below 32°F (0°C) without permanent chemical damage unless the battery has a built-in low-temp heating pad. Mount your new lithium battery inside the cabin, bed, or a insulated compartment.

3. Reconfigure Solar & Shore Power Chargers

If your kit includes an MPPT solar controller or an AC-to-DC shore power charger, change their dip switches or app settings from “AGM/Sealed” to “Lithium / LiFePO4”. AGM charging profiles include an “equalization” mode (high voltage spikes) that will cause a lithium battery’s BMS to trigger a safety shutdown.

Step-by-Step Upgrade Procedure

1.Disconnect All Power Sources: Safety First.

Disconnect the negative (-) terminal from your main vehicle starter battery first. Then disconnect and safely remove the old AGM auxiliary battery. Ensure all solar panel inputs are covered or switched off.

2.Remove the Old Isolator & Install DC-DC Charger: Wiring Setup.

Remove your old solenoid or VSR. Mount a smart DC-to-DC charger (typically 20A to 50A, matched to your alternator’s spare capacity). Connect the starter battery positive wire to the DC-DC charger input, and route an ignition trigger wire (D+ wire) if your charger requires it to detect when the engine is running.

3.Verify Fuses and Wire Gauges: Protection Check.

Ensure high-current fuses (MRBF or ANL type) are installed within 7 inches of both the starter battery positive post and the auxiliary battery positive post. Verify wire thickness: a 40A DC-DC charger over a 15-foot run generally requires at least 4 AWG copper wire.

4.Mount and Connect the LiFePO4 Battery: Final Hookup.

Securely mount the new LiFePO4 battery in a temperature-controlled area (cabin or bed box). Connect the DC-DC charger output positive (+) and ground (-) cables to the battery terminals. Reattach ground points to the vehicle chassis or dedicated busbar.

5.Reconfigure Charger Settings & Test: System Check.

Set solar controllers and shore power units to Lithium Mode. Reconnect the main starter battery negative terminal. Start the engine and use a multimeter to verify that the DC-DC charger is outputting ~14.2V–14.6V to the new lithium battery.