From AGM to Lithium: How to Upgrade Correctly – Step-by-Step to a Modern Onboard Power System
Changing from AGM to lithium (LiFePO4) batteries is one of the most effective upgrades you can make to your onboard power system. More usable capacity, less weight, faster charging times, and significantly longer lifespan – the benefits are compelling. However, a simple "remove lead, install lithium" rarely works without problems. Those who understand the peculiarities of LiFePO4 batteries and plan the system correctly from the start will subsequently experience a significantly more reliable and powerful onboard power system.
This guide explains what you need to consider when upgrading from AGM to lithium, what variations are available, and which components you will need.
Why Switch from AGM to Lithium at All?
Before we delve into the technical details, a brief 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, on the other hand, can be almost completely discharged. 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 easily 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 Just a Drop-in Replacement
Remove the lead-acid battery, install lithium, and you're done – many manufacturers of energy storage systems with integrated BMS promote this approach. If you pay attention to a few things, you can indeed get a functional system. At least the battery provides power and is recharged. However, it's not optimal. Because when charging, LiFePO4 batteries behave quite differently from lead cells. For example, there should be no float charge, and the power supply must definitely be terminated when full capacity is reached.
This is the crucial difference that makes many upgrades 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, triggering of the BMS protection.
What LiFePO4 Batteries Do Differently When Charging
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 maritime applications is DIN EN IEC 62619 (VDE 0510-39). It specifies tests for the safe operation of lithium secondary cells and is a good indicator for classifying a battery manufacturer's safety documentation.
The Three Conversion Options – From Simple to Complete
Option 1: Simple Replacement with Lithium-Compatible Charging Sources
The simplest option: 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 curve (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 curve is always better
Suitable for: Simple onboard power systems with little engine runtime and already lithium-compatible chargers.
Caution with the alternator: A standard alternator with an internal regulator will generally charge the LiFePO4 battery – but not optimally. The BMS of the lithium battery can disconnect 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.
Option 2: Charge Booster (B2B Charger) as Protection for Alternator and Starter Battery
The safer and more recommendable option for anyone who wants to use lithium as a house battery while retaining an AGM or lead-acid starter battery.
To charge the house battery with a standard alternator, a battery-to-battery charger (B2B charger, charge booster) is used. This unit loads the alternator, along with the starter battery, in a controlled manner and transfers the alternator power to the house battery 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).
Option 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 most out of their onboard power system.
In this option, 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 charging according to the LiFePO4 battery's requirements, continuously monitoring battery and alternator temperature.
Advantages of the complete conversion:
- High charging currents even at low engine speeds
- Precise LiFePO4 charging curve via 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 charger or B2B charger sufficient for the expected loads, and is there protection directly at the battery? The current limit of the BMS only protects the storage unit, not the cables.
LiFePO4 batteries can supply and accept 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 present directly at the battery and adequately sized?
- Check fuses of all consumer lines
2. Check All Charging Sources for LiFePO4 Compatibility
Go through each charging source:
Shore Power Charger: Does it support a LiFePO4 characteristic 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? Modern MPPT controllers usually support this.
Alternator: Does it have an external regulator (e.g., Balmar) or an internal standard regulator? For standard regulators: B2B charger recommended. For external regulators: Switch to LiFePO4 characteristic curve.
Wind Generator: As with the solar controller – check the controller for LiFePO4 characteristic curve.
3. Battery Main Switch and BMS Shutdown Behavior
Depending on the BMS, the battery may shut down completely in certain irregular operating conditions. This is not possible with AGM batteries – but with LiFePO4, it is a reality 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 Components Required for Conversion
Depending on the chosen option, you will need different components:
Mandatory for all options:
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 suitable for 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 its LiFePO4 characteristic curve. Recommended charging voltage 14.2–14.6 V, no continuous floating.
Battery Monitor (e.g., Philippi BLS or BTM2) LiFePO4 batteries do not reliably display their state of charge via voltage – the discharge curve is almost flat. A precise shunt-based battery monitor is therefore a necessity, not a luxury, with lithium.
Protection Directly at the Battery Bolt-on 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 onboard power 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 curve or dedicated LiFePO4 profile.
Additionally for the complete conversion (Option 3):
High-Performance Alternator with External Regulator (Balmar) Replaces the standard alternator – for optimal LiFePO4 charging at high currents and low RPMs. 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 Which Option?
| Component | Option 1 (Simple) | Option 2 (B2B) | Option 3 (Complete) |
|---|---|---|---|
| LiFePO4 Battery | ✓ | ✓ | ✓ |
| Lithium-Compatible Charger | ✓ | ✓ | ✓ |
| Battery Monitor | ✓ | ✓ | ✓ |
| Battery Protection | ✓ | ✓ | ✓ |
| Charge Booster / B2B | – | ✓ | – |
| Lithium-Compatible Solar Controller | ✓ | ✓ | ✓ |
| High-Performance Alternator | – | – | ✓ |
| External Regulator (Balmar) | – | – | ✓ |
| Belt Conversion Kit | – | – | Possibly ✓ |
Recommendation: Which Option for Whom?
Option 1 is suitable if you primarily charge from shore power, have little engine runtime, and already own or are replacing a lithium-compatible charger. Inexpensive, quickly implemented – but only with a fully checked charging infrastructure.
Option 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.
Option 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, and the charging process is repeatedly interrupted.
Alternator without protection: No B2B charger, no external regulator – the BMS of the lithium battery shuts down when fully charged, the alternator produces voltage without load, and 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 read reliably.
Frequently Asked Questions About AGM-to-Lithium Conversion
Can I keep my AGM starter battery? Yes, in most cases, it is even recommended. Starter batteries are usually left as AGM or lead-acid batteries in lithium conversions – LiFePO4 as the house battery, AGM as the starter battery. A charge booster separates both batteries and charges them optimally.
Do I have to re-wire everything? Not necessarily – but you must check all cable cross-sections for suitability for the higher possible currents. Especially the feeder lines to the charger and alternator must be able to continuously handle the expected charging currents.
What happens if the BMS shuts down? All connected consumers lose power – this is like a main switch. With a running alternator without B2B protection, this can destroy the alternator's 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 as a replacement for my previous AGM battery 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 standby times.
