on June 08, 2026
From AGM to Lithium: How to convert correctly – Step-by-step to a modern electrical system
The switch from AGM to lithium batteries (LiFePO4) is one of the most effective modernizations you can make to your electrical system. More usable capacity, less weight, faster charging times, and significantly longer lifespan – the advantages are compelling. But: A simple "lead out, lithium in" rarely works without problems. Those who understand the specifics of LiFePO4 batteries and plan the system correctly from the start will subsequently experience a significantly more reliable and powerful 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 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 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 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
Lead-acid battery out, lithium in and done – many manufacturers of energy storage systems with integrated BMS advertise this approach. If a few things are considered, you can actually get a functioning system. At least the battery provides power and is recharged. But it is not optimal. This is because LiFePO4 batteries behave very differently from lead-acid cells when charging. For example, there should be no float charge, and the power supply must definitely be stopped when the full state is reached.
This is the decisive difference that makes many conversions a compromise: All charging sources – shore power charger, alternator, solar charger – must be adapted to the specific charging parameters of LiFePO4. Those who ignore this risk batteries that are not fully charged permanently, premature wear, or in the worst case, the BMS protection being triggered.
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 sounding an alarm or being damaged.
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 to 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 electrical systems with little engine running time 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 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 recommendable variant for anyone who wants to operate lithium as a consumer battery while maintaining an AGM or lead 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 along with the starter battery and transfers the alternator power to the service accumulator along 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 consumer 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 consumer).
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 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 a full conversion:
- High charging currents even at low engine speeds
- Precise LiFePO4 charging characteristic due to 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 suitable for the expected loads, and is there protection directly at the battery? The current limitation of the BMS only protects the storage, 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 sufficiently sized?
- Check fuses of all consumer lines
2. Check all charging sources for LiFePO4 compatibility
Go through each charging source individually:
Shore power charger: Does it support a LiFePO4 characteristic? Can the float charging 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 a standard internal regulator? For standard regulators: Charge booster recommended. For external regulators: Switch to LiFePO4 characteristic.
Wind generator: As with the solar controller – check the controller for LiFePO4 characteristic.
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 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 necessary components for the 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 appropriate 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 it for LiFePO4 characteristics. 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 not a luxury with lithium, but a necessity.
Protection 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 conversions on boats:
Charge booster / B2B charger For electrical systems with a standard alternator and a mixed battery bank (AGM starter + LiFePO4 consumer). 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.
Additionally for the full conversion (Variant 3):
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 over 100 A and V-belt systems: Switch to serpentine or dual V-belt for reliable power transmission.
The complete overview: What do I need for which variant?
| Component | Variant 1 (Simple) | Variant 2 (B2B) | Variant 3 (Complete) |
|---|---|---|---|
| LiFePO4 battery | ✓ | ✓ | ✓ |
| Lithium-compatible charger | ✓ | ✓ | ✓ |
| Battery monitor | ✓ | ✓ | ✓ |
| Protection 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 from shore power, have little engine running time, and already own or replace a lithium-compatible charger. Inexpensive, quickly implemented – but only with 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 travelers, 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 interrupts.
Alternator unprotected: 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 for 50 A are overloaded with 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 the consumer battery, AGM as the starter battery. A charge booster separates both batteries and charges them optimally.
Do I have to re-lay 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 off? 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 to replace my existing 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 bit more capacity – for more comfort and longer standby times.
AGM to lithium conversion boat · LiFePO4 conversion yacht · Lithium battery electrical system · B2B charger charge booster lithium · LiFePO4 alternator protection · BMS boat · Bulltron LiFePO4 · Balmar alternator lithium · Charger LiFePO4 boat · Lithium electrical system planning · Cable cross-section lithium boat · Philippi lithium battery
