From AGM to Lithium: How to Get the Conversion Right – Step-by-Step to a Modern On-board Electrical System
Switching from AGM to lithium batteries (LiFePO4) is one of the most effective modernizations you can make to your on-board electrical system. More usable capacity, less weight, faster charging times, and significantly longer lifespan – the advantages are compelling. However, a simple "lead out, lithium in" rarely works without problems. Those who understand the peculiarities of LiFePO4 batteries and plan the system correctly from the outset will subsequently experience a much more reliable and powerful on-board electrical system.
This guide explains what you need to consider when converting from AGM to lithium, which variants are available, and which components you need.
Why Switch from AGM to Lithium at all?
Before we dive into the technical details, a quick look at the essentials: What do you gain by making the switch?
Usable Capacity: AGM batteries should not be discharged below 50% to avoid significantly shortening their lifespan. LiFePO4 batteries, however, can be discharged almost completely. A 100 Ah lithium battery effectively provides twice as much usable energy as a 100 Ah AGM battery.
Weight: A 210 Ah LiFePO4 battery weighs approx. 23 kg – a comparable AGM battery weighs around 70 kg. Especially on sailing yachts, this means less weight and better weight distribution.
Charging Speed: LiFePO4 batteries readily accept high charging currents (up to 1C) and are fully charged in a fraction of the time.
Lifespan: While AGM batteries are at the end of their life after 300–500 cycles, LiFePO4 batteries achieve several thousand charging cycles.
The Biggest Misconception: Lithium is Not Simply a Drop-in Replacement
Remove the lead-acid battery, insert lithium, and you're done – many manufacturers of energy storage systems with integrated BMS advertise this approach. If you pay attention to a few things, you can indeed get a functional system. At least the battery provides power and recharges. However, it is not optimal. This is because LiFePO4 batteries behave significantly differently from lead-acid cells during charging. For example, there should be no float charging, and the power supply must definitely be stopped when full charge is reached.
This is the crucial difference that turns many conversions into a compromise: All charging sources – shore power charger, alternator, solar charger – must be adapted to the specific charging parameters of LiFePO4. Ignoring this risks permanently undercharged batteries, premature wear, or, in the worst case, the activation of the BMS protection.
How LiFePO4 Batteries Charge Differently
LiFePO4 batteries have specific charging parameters that differ significantly from lead/AGM:
| Parameter | AGM / Lead | LiFePO4 |
|---|---|---|
| Charging Voltage (12V) | 14.4–14.8 V | 14.2–14.6 V |
| Float Charge | 13.6–13.8 V | Not required / 13.2 V |
| Charge Termination | Voltage-based + current drop | Voltage-based, strict |
| Charging Temperature | –10 to +50 °C | 0 to +45 °C (no charging below 0 °C) |
| Recommended Charging Current | 0.1–0.2C | 0.2–1.0C possible |
Important: LiFePO4 batteries must not be charged at temperatures below 0 °C – the BMS prevents this, but the charger must be able to handle it without alarming or damaging itself.
The internationally recognized safety standard for lithium cells and batteries in industrial and marine applications is the standard DIN EN IEC 62619 (VDE 0510-39). It defines tests for the safe operation of lithium secondary cells and is a good benchmark for evaluating the safety documentation of a battery manufacturer.
The Three Conversion Variants – From Simple to Complete
Variant 1: Simple Replacement with Lithium-Compatible Charging Sources
The simplest variant: The AGM battery is replaced by a LiFePO4 battery, and all existing charging sources are set to LiFePO4 characteristics or replaced.
Prerequisites:
- Shore power charger must support a LiFePO4 characteristic (charging voltage 14.2–14.6 V, no floating)
- Solar controller must be switchable to LiFePO4
- As a rule of thumb: If the charger can be set for gel or AGM batteries with a charge termination voltage of 14.4 volts, it might work – but a dedicated LiFePO4 characteristic is always better
Suitable for: Simple on-board electrical systems with little engine running time and already lithium-compatible chargers.
Caution with the Alternator: A standard alternator with an internal regulator will charge the LiFePO4 battery, but not optimally. The BMS of the lithium battery can shut off the circuit when fully charged, which can lead to dangerous voltage spikes with a running alternator. Without protective measures, you risk damaging the alternator regulator.
Variant 2: Charge Booster (B2B Charger) as Protection for Alternator and Starter Battery
The safer and more recommended variant for anyone who wants to operate lithium as a house battery while maintaining an AGM or lead-acid starter battery.
To charge the service battery with a standard alternator, a battery-to-battery charger (B2B charger, charge booster) is used. This unit controls the load on the alternator together with the starter battery and transfers the alternator power to the service accumulator according to an adjustable characteristic curve.
Advantages of the B2B charger:
- Protects the alternator from uncontrolled load shedding by the LiFePO4 BMS
- Converts the alternator voltage into a clean LiFePO4 charging curve
- Electrically separates starter and house batteries
- Also works with standard alternators without an external regulator
Suitable for: Most conversions on boats and yachts with a standard alternator and a mixed battery bank (AGM starter + LiFePO4 house).
Variant 3: Complete Conversion with High-Performance Alternator and External Regulator
The most professional and powerful solution for long-distance cruisers, blue-water sailors, and anyone who wants to get the maximum out of their on-board electrical system.
In this variant, the standard alternator is replaced by a high-performance alternator (e.g., Balmar 6-series or XT-series) with an external regulator. The external regulator – for example, the Balmar MC-618 – precisely controls the charging according to the requirements of the LiFePO4 battery and continuously monitors battery and alternator temperature.
Advantages of the complete conversion:
- High charging currents even at low engine speeds
- Precise LiFePO4 charging curve through external regulator
- Temperature sensor protects alternator from overheating
- Short charging times, maximum battery utilization
- No uncontrolled load shedding due to intelligent regulator behavior
What You Need to Check Before Conversion
1. Cable Cross-Sections and Connections
When converting to lithium batteries, the entire electrical installation should always be inspected. Are the cable cross-sections between the battery and the charger or B2B charger sufficient for the expected loads, and is there a fuse directly at the battery? The current limitation of the BMS only protects the battery, not the cables.
LiFePO4 batteries can supply and absorb very high currents – significantly higher than AGM batteries. What was tolerated with AGM can lead to overloading of cables and connections with lithium.
Checklist:
- Check cable cross-sections for plausibility
- Check all cable connections for oxidation and contact resistance
- Is a main fuse directly at the battery present and adequately dimensioned?
- Check fuses of all consumer circuits
2. Check All Charging Sources for LiFePO4 Compatibility
Go through each individual charging source:
Shore power charger: Does it support a LiFePO4 charging curve? Can the float charge voltage be set to ≤ 13.2 V or deactivated? If not: Replace the charger.
Solar controller: Can it be set to LiFePO4 or a charge termination voltage of 14.2–14.4 V? Most modern MPPT controllers support this.
Alternator: Does it have an external regulator (e.g., Balmar) or an internal standard regulator? For standard regulators: Charge booster recommended. For external regulators: Switch to LiFePO4 charging curve.
Wind generator: As with the solar controller – check the controller for LiFePO4 charging curve.
3. Battery Main Switch and BMS Shutdown Behavior
Depending on the BMS, the battery can completely shut down in certain irregular operating conditions. This is not possible with AGM batteries – but it is a reality with LiFePO4 that all downstream consumers and chargers must cope with. A sudden load shedding with a running alternator without protection can destroy the alternator's voltage regulator.
The Required Components for Conversion
Depending on the chosen variant, you will need different components:
Mandatory for all variants:
LiFePO4 battery (e.g., Bulltron) The battery itself – with integrated BMS, active 5A balancer, and P-Bus interface for real-time data transmission to compatible monitors. Choose capacity according to the battery bank (at least current usable AGM capacity × 2).
Lithium-compatible shore power charger (e.g., Philippi ACE series) In most cases, replace the existing AGM charger or at least check it for LiFePO4 compatibility. Recommended charging voltage 14.2–14.6 V, no continuous floating.
Battery monitor (e.g., Philippi BLS or BTM2) LiFePO4 batteries do not reliably indicate their state of charge via voltage – the discharge curve is almost flat. A precise shunt-based battery monitor is therefore not a luxury with lithium, but a necessity.
Fusing directly at the battery Bolt fuse or high-current circuit breaker directly at the positive terminal of the battery – as short as possible.
Recommended for most boat conversions:
Charge booster / B2B charger For on-board electrical systems with a standard alternator and mixed battery bank (AGM starter + LiFePO4 house). Protects the alternator, provides a clean LiFePO4 charging curve, and decouples the battery banks.
Lithium-compatible solar controller If a solar system is present – MPPT controller with adjustable LiFePO4 characteristic or dedicated LiFePO4 profile.
For the complete conversion (Variant 3) additionally:
High-performance alternator with external regulator (Balmar) Replaces the standard alternator – for optimal LiFePO4 charging at high currents and low speeds. Includes temperature sensors for battery and alternator.
Belt conversion kit (if necessary) For alternator outputs above 100 A and V-belt systems: Switch to serpentine or double V-belt for reliable power transmission.
The Complete Overview: What Do I Need for Each Variant?
| Component | Variant 1 (Simple) | Variant 2 (B2B) | Variant 3 (Complete) |
|---|---|---|---|
| LiFePO4 Battery | ✓ | ✓ | ✓ |
| Lithium-compatible Charger | ✓ | ✓ | ✓ |
| Battery Monitor | ✓ | ✓ | ✓ |
| Fusing at Battery | ✓ | ✓ | ✓ |
| Charge Booster / B2B | – | ✓ | – |
| Lithium-compatible Solar Controller | ✓ | ✓ | ✓ |
| High-Performance Alternator | – | – | ✓ |
| External Charge Regulator (Balmar) | – | – | ✓ |
| Belt Conversion Kit | – | – | Possibly ✓ |
Recommendation: Which Variant for Whom?
Variant 1 is suitable if you primarily charge with shore power, have little engine running time, and already own a lithium-compatible charger or are replacing it. Inexpensive, quickly implemented – but only with a fully checked charging infrastructure.
Variant 2 is the right choice for most conversions on boats with a standard alternator. The charge booster protects the alternator, ensures clean LiFePO4 charging, and can be easily integrated into existing installations.
Variant 3 is recommended for long-distance cruisers, blue-water sailors, and anyone who needs maximum independence from shore power. The complete high-performance system with Balmar alternator and external regulator gets the most out of the lithium battery bank.
The Most Common Mistakes in Lithium Conversion
Charger not adapted: The old AGM charger continues to charge with float voltage – the LiFePO4 battery is continuously exposed to voltage, the BMS reacts, the charging process repeatedly stops.
Alternator without protection: No B2B charger, no external regulator – the BMS of the lithium battery switches off when fully charged, the alternator produces voltage without load, the internal regulator is destroyed.
Cable cross-sections underestimated: AGM cables rated for 50 A are overloaded by a lithium battery that can supply 200 A.
No main fuse at the battery: The BMS protects the cells – but not the cables between the battery and the first fuse.
Battery monitor forgotten: Without a shunt monitor, the state of charge of a LiFePO4 battery cannot be reliably read.
Frequently Asked Questions About AGM to Lithium Conversion
Can I keep my AGM starter battery? Yes, in most cases, this is even recommended. Starter batteries are usually left as AGM or lead-acid batteries in lithium conversions – LiFePO4 as a house battery, AGM as a starter battery. A charge booster separates both batteries and charges each optimally.
Do I have to rewire all cables? Not necessarily – but you must check all cable cross-sections for suitability for the higher possible currents. Especially the supply lines to the charger and alternator must permanently withstand the expected charging currents.
What happens if the BMS shuts down? All connected consumers lose power – it's like a main switch. With a running alternator without B2B protection, this can destroy the alternator regulator. That's why a charge booster or external alternator regulator is so important for the conversion.
Can I charge LiFePO4 batteries in cold weather? No – LiFePO4 batteries must not be charged at temperatures below 0 °C. The integrated BMS automatically prevents this, but your charging sources must be able to handle it without producing errors. Some high-quality LiFePO4 batteries have a heating function for winter operation.
How much capacity do I need to replace my previous AGM bank? Since AGM batteries should effectively only be discharged to 50%, you only need half as much nominal lithium capacity for the same usable energy. If you previously had 200 Ah AGM and used 100 Ah of it, 100 Ah LiFePO4 will suffice. However, it is usually worthwhile to choose a little more capacity – for more comfort and longer standing times.
