Q:
I have 2 x 12 v 100 ah LIFEPO4 batteries, each with 100 amp BMS. If I connect in series, I have 24 v 100 amp. If I connect parellel, I have 12 v 200 amp. What value does a fuse on the positive post have?
A:
The specific amperage value (rating) of the fuse depends on whether you configure the batteries in Series or Parallel, because the current (Amps) flows differently in each setup.
However, the value (purpose) of the fuse remains the same: It is a fail-safe to prevent fire. While your Battery Management System (BMS) protects the battery cells, the fuse protects the wire connecting your battery to the rest of the system.
Here is the breakdown of the fuse ratings you need for each configuration.
1. Series Connection (24V system)
- Total Voltage: 24V
- Total Capacity: 100Ah
- Max Continuous Discharge: 100 Amps (Current must pass through both BMS units in a line, so you are limited to the rating of a single BMS).
Recommended Fuse Size: 125A – 150A In a series connection, the current stays the same (100A). You want a fuse that is slightly higher than your maximum potential current so it doesn’t blow during normal usage spikes, but low enough to blow instantly if a short circuit occurs.
Why: A 125A fuse allows your full 100A current to flow without “nuisance blowing,” but cuts the circuit if a short occurs before the wire melts.
Wire Requirement: You must use wire capable of handling roughly 1.25x the fuse rating (e.g., 2 AWG or 1/0 AWG recommended).
2. Parallel Connection (12V system)
- Total Voltage: 12V
- Total Capacity: 200Ah
- Max Continuous Discharge: 200 Amps (Current is additive; 100A from Battery A + 100A from Battery B).
Recommended Fuse Size: 250A – 300A In parallel, the two batteries combine their power output. Your main wire carrying the combined load must handle 200A, and the fuse must protect that wire.
- Why: Since you can theoretically pull 200A continuous, a 250A fuse (Class T or ANL) is standard to handle the load plus surge headroom.
- Wire Requirement: This requires much thicker wire (e.g., 2/0 AWG or 4/0 AWG) to handle the 200A load safely.
Pro-Tip for Parallel: In a parallel bank, it is safest to put a smaller fuse (e.g., 125A MRBF terminal fuse) on each battery positive terminal, rather than just one big fuse on the main line. This ensures that if one battery develops an internal short, the other battery cannot dump all its energy into the bad battery and cause a fire.
| Configuration | System Voltage | Max Amps | Recommended Main Fuse | Wire Gauge Needed (Approx) |
| Series | 24V | 100A | 125A – 150A | 2 AWG or 1/0 AWG |
| Parallel | 12V | 200A | 250A – 300A | 2/0 AWG or 4/0 AWG |
The specific amperage value (rating) of the fuse depends on whether you configure the batteries in Series or Parallel, because the current (Amps) flows differently in each setup.
However, the value (purpose) of the fuse remains the same: It is a fail-safe to prevent fire.1 While your Battery Management System (BMS) protects the battery cells, the fuse protects the wire connecting your battery to the rest of the system.
Here is the breakdown of the fuse ratings you need for each configuration.
1. Series Connection (24V system)
- Total Voltage: 24V
- Total Capacity: 100Ah
- Max Continuous Discharge: 100 Amps (Current must pass through both BMS units in a line, so you are limited to the rating of a single BMS).
Recommended Fuse Size: 125A – 150A
In a series connection, the current stays the same (100A). You want a fuse that is slightly higher than your maximum potential current so it doesn’t blow during normal usage spikes, but low enough to blow instantly if a short circuit occurs.
- Why: A 125A fuse allows your full 100A current to flow without “nuisance blowing,” but cuts the circuit if a short occurs before the wire melts.
- Wire Requirement: You must use wire capable of handling roughly 1.25x the fuse rating (e.g., 2 AWG or 1/0 AWG recommended).
2. Parallel Connection (12V system)
- Total Voltage: 12V
- Total Capacity: 200Ah
- Max Continuous Discharge: 200 Amps (Current is additive; 100A from Battery A + 100A from Battery B).
Recommended Fuse Size: 250A – 300A
In parallel, the two batteries combine their power output. Your main wire carrying the combined load must handle 200A, and the fuse must protect that wire.
- Why: Since you can theoretically pull 200A continuous, a 250A fuse (Class T or ANL) is standard to handle the load plus surge headroom.
- Wire Requirement: This requires much thicker wire (e.g., 2/0 AWG or 4/0 AWG) to handle the 200A load safely.
Pro-Tip for Parallel: In a parallel bank, it is safest to put a smaller fuse (e.g., 125A MRBF terminal fuse) on each battery positive terminal, rather than just one big fuse on the main line. This ensures that if one battery develops an internal short, the other battery cannot dump all its energy into the bad battery and cause a fire.
Summary Comparison
| Configuration | System Voltage | Max Amps | Recommended Main Fuse | Wire Gauge Needed (Approx) |
| Series | 24V | 100A | 125A – 150A | 2 AWG or 1/0 AWG |
| Parallel | 12V | 200A | 250A – 300A | 2/0 AWG or 4/0 AWG |
Important Safety Note
The fuse must always be the “weakest link” in the chain. The hierarchy of amperage capacity should be:
$$Device\ Load < BMS\ Limit < Fuse\ Rating < Wire\ Limit$$
If your wire can only handle 100A, but you put a 150A fuse on it, the wire will become the fuse (i.e., it will melt and catch fire) before the actual fuse blows.