Mastervolt Batteries: Lithium (LiFePO4), AGM, and Gel for Marine, RV, and Mobile Power

Mastervolt batteries form a reliable house bank for boats, vans, and specialty vehicles. Pick LiFePO4 for maximum usable capacity, quick recovery from charging, and low weight with integrated battery management. Choose deep-cycle AGM or gel when you need a straightforward, sealed replacement that works with existing chargers and budgets.

Which chemistry should you choose?

• Choose LiFePO4 if you want high usable capacity (≈80–90% of rated Ah), frequent cycling, rapid charging, weight savings, and networked monitoring. Avoid if your alternator or charger cannot be configured/protected for lithium or you regularly charge below 32°F (0°C) without low-temp protection.
• Choose AGM for low-maintenance, sealed lead-acid that handles vibration and fits legacy compartments. Avoid if daily deep cycling and weight are critical—usable capacity is typically ≈50% of rated Ah.
• Choose Gel for steady, deep-cycle loads and good longevity at moderate charge rates. Avoid if you need high current charging or very large inverter surges.

Quick comparison

Feature	LiFePO4	AGM	Gel
Usable capacity (typ.)	80–90%	~50%	~50–60%
Cycle life (to 80% DoD)	~2,000–5,000	~400–800	~500–1,000
Recommended charge rate	0.2–0.5C	0.1–0.2C	0.1C
12 V absorption/float	14.2–14.6 V / 13.4–13.6 V (or no float)	14.4–14.7 V / 13.5–13.8 V	14.1–14.4 V / 13.5–13.8 V
Low-temp charging	Do not charge below 32°F (0°C) unless protected	Reduced acceptance when cold	Reduced acceptance when cold
Weight vs. capacity	Lightest	Heavier	Heavier

System design essentials

• Capacity planning: Size for at least one day of use plus 20–30% reserve. Lithium can be sized closer to daily need; AGM/gel typically need ~2× Ah to equal lithium runtime.
• Cabling & protection: Install a main fuse (Class-T for >2 kW inverters; ANL/MEGA for smaller) within ~7 in (18 cm) of the positive post. Use tinned marine-grade cable sized for <3% voltage drop on critical loads.
• Parallel/series rules: Use identical models and cable lengths. Parallel via busbars, not battery studs, for even sharing. Avoid mixing old/new or different chemistries.
• Mounting: Secure against vibration with access for inspection. Keep within specified temperature range and provide ventilation (AGM/gel can vent under fault).
• Inverter precharge: Precharge large inverters to avoid inrush tripping the BMS on lithium banks; many inverter/chargers have built-in soft start.

Charging and alternator guidance

Shore, solar, and DC-DC

• Program chargers and MPPT controllers to the correct absorption/float voltages for the chosen chemistry.
• Lithium: limit charge current to the battery’s recommended C-rate; float is optional—some systems hold at a reduced “storage” voltage (~13.4–13.6 V).
• AGM/gel: use temperature-compensated charging and allow a full absorption to 100% periodically to prevent sulfation.

Main engine alternator

• Lithium can overwork stock alternators. Use a DC-DC charger or an external regulator with temperature sensing and current limits.
• Provide a safe load path if the BMS disconnects while charging (e.g., alternator field disconnect or DC-DC isolation).

Temperature management

• Do not charge LiFePO4 below 32°F (0°C) unless the battery provides low-temp charge protection or heating.
• Keep all chemistries within manufacturer limits; typical storage sweet spot is 50–77°F (10–25°C).

Sizing and runtime examples

• Daily house loads: If your boat uses ~60 Ah/day at 12 V (~0.72 kWh), a 100 Ah LiFePO4 bank (≈80–90 Ah usable) covers a day with little reserve; a 200 Ah AGM bank (≈100 Ah usable) provides similar runtime.
• Inverter appliance: A 1,000 W load for 15 minutes uses ≈250 Wh. A 12 V 200 Ah LiFePO4 bank (≈2.0–2.3 kWh usable) can run this ~8–9 times before recharge; a 200 Ah AGM bank (≈1.2 kWh usable) supports ~4–5 cycles.

Monitoring and integration

Use a shunt-based monitor for accurate state of charge. LiFePO4 models with an integrated BMS can share status (voltage, current, temperature, alarms) with compatible chargers and displays over MasterBus for coordinated charging and protective shutdowns.

Care and storage

• Lithium: Store around 40–60% state of charge; check every 3–6 months. Update device firmware and keep terminals torqued.
• AGM/gel: Store fully charged and keep on a proper float or top up monthly. Inspect and clean connections annually.

Compatibility notes

• Available in 12/24/48 V configurations. Confirm inverter/charger and regulator settings match the battery chemistry.
• Most house lithium batteries are not designed for engine starting unless explicitly rated for cranking.
• Do not mix chemistries or capacities in the same bank. Replace batteries in a bank as a set.

FAQs

Can I replace my AGM bank with LiFePO4 using my existing charger?

Only if the charger supports lithium profiles or custom voltage settings and you address alternator protection. Without correct setpoints and current control, you risk poor performance or damage.

How close should the main fuse be to the battery?

Install the main positive fuse as close as practical—about 7 inches (18 cm) from the battery terminal—to protect downstream wiring.

Is ventilation required?

LiFePO4 does not normally vent in use, but follow manufacturer guidance and provide general airflow. AGM/gel are sealed but can vent under fault; avoid sealed compartments and protect against heat.

Can I run multiple batteries in parallel?

Yes—use identical models, matched cable lengths to busbars, and proper fusing per string. Keep parallel strings to the minimum needed for capacity and current.

What about cold weather?

Do not charge LiFePO4 below 32°F (0°C) unless the battery includes low-temp charge protection or heating. Discharge limits are wider, but capacity drops in the cold for all chemistries.