Quick answer: Which DC fuse should I use with a lithium (LiFePO4) campervan battery?
LiFePO4 batteries can deliver huge short-circuit currents, so the fuse at the battery needs a high interrupting capacity (AIC). Our rule of thumb is Ah × 50, or 5,000A per 100Ah. On 12V we allow an MRBF up to 200Ah per battery. Above that, or on 24V, it's Class-T. A MEGA's 2kA is too low.
LiFePO4 fuse selection is one of the most important safety decisions in a mobile, marine or off-grid electrical system. Fuses provide critical overcurrent protection, safeguarding equipment and preventing fire hazards.
This guide focuses specifically on LiFePO4 (Lithium Iron Phosphate) battery systems and details the characteristics of Class-T, MRBF, and MEGA fuses, assisting in proper selection for these high-performance battery chemistries.
Quick answer
In short
LiFePO4 batteries have very low internal resistance, so a short circuit can deliver huge fault currents. Choose a fuse whose interrupting capacity (AIC) covers that current, not just its amp rating. Our rule: the interrupting capacity should be at least each battery’s capacity in Ah × 50. That is 5,000A per 100Ah, with a separate fuse on each battery in a parallel bank.
- AIC = each battery’s Ah × 50
- MEGA: around 2kA AIC
- Fuse each parallel battery
- LiFePO4 systems only
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On this page 8 sections
Important Note: This guide is exclusively directed at LiFePO4 battery systems. LiFePO4 batteries possess significantly lower internal resistance compared to lead-acid or AGM batteries. This characteristic allows them to deliver exceptionally high short-circuit currents, making proper fuse selection absolutely critical. While fuse selection remains important for lead-acid and AGM systems, the potential for extremely high fault currents is considerably lower due to their inherently higher internal resistance. The recommendations in this guide are not necessarily applicable to lead-acid or AGM battery systems.
Why LiFePO4 fuse selection matters
A fuse is the deliberate weak link in a circuit, interrupting current flow during overloads. When looking at LiFePO4 batteries, the potential for rapid and substantial short-circuit currents necessitates careful fuse selection. Incorrect fuse selection can lead to premature failures, or more dangerously, inadequate protection, potentially causing catastrophic damage or fire. A properly rated fuse is crucial for safety and system reliability in LiFePO4 systems.
LiFePO4 fuse types: Class-T, MRBF and MEGA
Class-T Fuses
These fuses handle high-current applications common in inverters, battery banks, and critical power circuits. Their high interrupting capacity (AIC) allows them to safely interrupt substantial fault currents, crucial in LiFePO4 systems with large battery banks. Class-T fuses are typically bolt-on and require compatible holders.
Up to 200Ah per battery, 12V only
MRBF
10,000A AIC at 12V, in a terminal fuse block on a 3/8 inch stud or busbar next to the battery, never bolted to an M8 battery post. Our exception: up to 200Ah per battery, one per battery in a parallel bank. 200Ah is a hard ceiling.
Above 200Ah, and every 24V bank
Class-T
20,000A AIC in a bolt-on holder. Always our choice above 200Ah per battery, and on every 24V or 48V bank.
MRBF Fuses (Marine Rated Battery Fuse)
Designed for marine environments, MRBF fuses offer corrosion and vibration resistance. Their compact size suits tight spaces. The fuse sits in a terminal fuse block, never directly on the battery terminal, as the maker specifies. Blue Sea’s MRBF terminal fuse block mounts on a 3/8 inch stud, so we never bolt it to an M8 battery post, such as a Fogstar Drift’s. It goes on a 3/8 inch stud or busbar next to the battery, fed by a short cable from the post. We hold that cable to BS 7671 Regulation 434.2.1: no longer than 3 m, inherently short-circuit and earth-fault proof, and installed to keep the risk of fire to a minimum. While typically used in marine applications, MRBF fuses also suit campervans and motorhomes where space is tight and a secure connection close to the battery matters. They also provide relatively fast-acting protection.
The maker’s MRBF datasheet rates an MRBF to break 10,000A at 14V, 5,000A at 32V and 2,000A at 58V. So on a 24V system it breaks only half what it does on 12V. That’s why we only use MRBF on 12V banks.
MEGA Fuses
Bridging the gap between smaller automotive fuses and larger Class-T fuses, MEGA fuses are common in automotive and recreational vehicle (RV) applications. They protect high-current circuits like charging systems, alternators, and starter motors. It’s important to note that MEGA fuses typically have a lower AIC rating (around 2000A or 2kA) compared to MRBF or Class-T fuses.
Our LiFePO4 fuse AIC sizing rule
At Tiny Build Electrics, we prioritise safety and recommend a specific protocol for determining the appropriate Ampere Interrupting Capacity (AIC) for your fuses in LiFePO4 systems. Due to the inherent characteristics of LiFePO4 batteries, AIC selection is paramount.
Tiny Build Electrics rule (LiFePO4 only): The fuse’s interrupting capacity should be at least each battery’s capacity in Ah × 50. So 100Ah needs 5,000A, 200Ah needs 10,000A and 300Ah needs 15,000A. Each battery in a parallel bank still gets its own fuse.
Calculating the AIC
1. Determine Each Battery’s Capacity: Identify the Ah capacity of each battery you’ll fuse.
2. Calculate Required AIC: Multiply each battery’s Ah capacity by 50. This gives you the recommended AIC in Amperes.
3. Select Fuse Type: Choose a fuse whose AIC at your system voltage is equal to or greater than the calculated value. AIC falls as voltage rises, so check the maker’s figure for 12V or 24V.
LiFePO4 fuse selection examples
100Ah LiFePO4 Battery
100Ah * 50 = 5,000A AIC. A MEGA fuse, with its limited 2,000A AIC, is not sufficient for this application. An MRBF or a Class-T fuse would be required.
200Ah LiFePO4 Battery
200Ah * 50 = 10,000A AIC. That is exactly an MRBF’s 10,000A rating at 12V, with no spare margin. We allow an MRBF here as a clearly stated exception, and we treat 200Ah as a hard ceiling. We only do it because we control the rest: a fuse on every battery, quality cells, and a short, protected cable run to the fuse. Above 200Ah, or on 24V, it’s always a Class-T.
Fogstar explains how its BMS handles a short circuit but publishes no fault-current figure (Fogstar Knowledge Hub). That’s why we work from Ah x 50 rather than guess.
2X 200Ah LiFePO4 Batteries in Parallel (400Ah Total)
400Ah * 50 = 20,000A AIC. It is crucial to use a separate fuse for each battery in a parallel configuration. In this example, you would install two MRBF fuses, one for each 200Ah battery, even though the total AIC requirement is 20kA. Each fuse only has to break its own battery’s 10,000A, which is right at the MRBF’s limit. This is as far as we take an MRBF.
300Ah LiFePO4 Battery
300Ah * 50 = 15,000A AIC. A Class-T fuse, offering a 20,000A AIC rating, is the recommended choice due to its higher AIC capacity.
Important considerations for LiFePO4 fuse selection
- 01
Always consult the fuse manufacturers specifications: Ensure the selected fuse’s voltage rating is appropriate for your system.
- 02
Consider other circuit characteristics
The AIC calculation is a guideline. Factors like cable length and other components can influence the actual short-circuit current.
- 03
This guide is ONLY applicable to LiFePO4 systems
The very high short circuit currents possible with LiFePO4 necessitate this specialised guidance.
- 04
MEGA Fuse Limitations
Due to their lower AIC rating, MEGA fuses are generally not recommended for direct connection to larger LiFePO4 battery banks. They might be suitable for protecting specific high-current circuits branching off the main battery bank, but only if the calculated AIC for that specific circuit is within the MEGA fuse’s rating.
- 05
When in doubt, consult a professional
Electrical system design and safety are critical, especially with LiFePO4 batteries. If you’re unsure about fuse selection, seek advice from a qualified electrician or marine electrical specialist.
Getting LiFePO4 fuse selection right
Correct LiFePO4 fuse selection and installation are critical for electrical system safety, especially with the high-current potential of LiFePO4 batteries. We highly recommend consulting a qualified electrician or marine electrical specialist. Proper fuse selection is a crucial investment in safety and system reliability.
LiFePO4 fuse selection FAQs
Which DC fuse should I use with a lithium (LiFePO4) campervan battery?
LiFePO4 batteries can deliver huge short-circuit currents, so the fuse at the battery needs a high interrupting capacity (AIC). Our rule of thumb is Ah × 50, or 5,000A per 100Ah. On 12V we allow an MRBF up to 200Ah per battery. Above that, or on 24V, it’s Class-T. A MEGA’s 2kA is too low.
What AIC rating do I need for a LiFePO4 fuse?
Multiply each battery’s capacity in Ah by 50. A 200Ah battery needs 10,000A, exactly an MRBF’s rating at 12V, so 200Ah is our ceiling for an MRBF. A 300Ah battery needs 15,000A, so we fit a Class-T. On 24V an MRBF breaks only 5,000A, so we fit Class-T.
Can I use a MEGA fuse on a lithium battery?
Not as the main battery fuse on most banks. A MEGA fuse interrupts around 2,000A, well short of the 5,000A a single 100Ah battery calls for. It can protect a branch circuit, but only where that circuit’s fault current is within its rating.
Do I need a fuse on each battery in a parallel bank?
Yes. Each battery can deliver its own short-circuit current, so each one gets its own fuse, rated for that battery’s AIC.
About the reviewer: Tom Alderdice
Tom holds the 18th Edition (BS 7671) and inspection and testing qualifications, plus NCC training in caravan and motorhome electrics.
Tiny Build Electrics is an Authorised Victron Energy Installer. See Tom’s full qualifications
