When you need a fail-safe 12V power supply for a router, Arduino cluster, or telemetry station, wiring a circuit two batteries can power reliably requires more than just twisting red wires together. The direct answer for a redundant 12V setup is an OR-ing diode topology. By placing a Schottky diode or an ideal-diode MOSFET on the positive terminal of each battery before joining them at a common output node, you double your amp-hour capacity while preventing a failing battery from dragging down the healthy one.
This guide walks through the exact node mapping, component selection, and breadboard testing protocol for a dual 12V LiFePO4 redundant circuit, ensuring your system survives extreme failure modes without a total blackout.
Topology Description & Node Mapping
The OR-ing topology isolates each power source so current can only flow toward the load, never backward into a parallel battery. This is critical when dealing with lithium chemistry, where a 0.2V mismatch between parallel cells can cause dangerous cross-charging currents.
- NODE_BAT1_POS: Positive terminal of Battery 1 (Post-BMS, Pre-Diode).
- NODE_BAT2_POS: Positive terminal of Battery 2 (Post-BMS, Pre-Diode).
- NODE_ANODE_1 / 2: The input side of the isolation component (Diode anode or MOSFET source).
- NODE_CATHODE_COMMON: The joined output side of the isolation components. This is your V_OUT.
- NODE_GND_COMMON: The shared negative bus. (Grounds are tied directly together; isolation happens only on the high side).
In this configuration, if Battery 1 sits at 13.4V and Battery 2 sits at 13.2V, Battery 1 will supply the load through its diode. The diode on Battery 2 will be reverse-biased (blocking) because the voltage at NODE_CATHODE_COMMON (13.4V minus the diode drop) is higher than NODE_BAT2_POS. As Battery 1 depletes and its voltage drops below Battery 2, the system seamlessly hands over the load without any switching transients.
Why This Topology Over Series or Direct Parallel?
When evaluating how to configure a circuit two batteries can support, you generally have three choices. Here is the failure-mode contrast that makes OR-ing the superior choice for 12V redundancy.
| Topology | Output | Failure Mode: One Battery Opens (BMS Trip) | Failure Mode: One Battery Shorts Internally |
|---|---|---|---|
| Series (2S) | 24V / 50Ah | Total system failure. 0V output. Inverter shuts down. | Output drops to 12V. BMS on the good battery trips on over-current or undervoltage. |
| Direct Parallel | 12V / 100Ah | Seamless handover. Good battery powers load. | Catastrophic. Good battery dumps massive current into the shorted battery, risking fire or melted wires. |
| OR-ing (Isolated Parallel) | 12V / 100Ah | Seamless handover. Good battery powers load. | Isolation diode blocks reverse current. Good battery powers load safely. |
Series wiring is excellent for high-power 24V inverters, but it lacks fault tolerance. Direct parallel is heavily discouraged by Battery University and most BMS manufacturers unless the cells are perfectly matched and permanently bonded. The OR-ing circuit gives you the capacity of parallel with the safety of isolated sources.
Design Walkthrough: Picking Real Component Values
Let’s design a practical system for a 12V 5A continuous load (like a Starlink router and a network switch). We will use two 12V 50Ah LiFePO4 batteries.
The Low-Current Schottky Approach (Under 5A)
If your load is strictly under 5A, you can use Schottky diodes. Standard silicon diodes (like the 1N4007) drop ~0.7V, which wastes power and pushes your 12V system dangerously close to the 11V low-voltage cutoff of many appliances. Schottky diodes drop ~0.3V to 0.4V.
- Batteries: 2x Ampere Time 12V 50Ah LiFePO4 (Built-in 50A BMS).
- Isolation Diodes: 2x SB560 (5A, 60V Schottky). The 60V rating provides a massive safety margin against 12V system voltage spikes.
- Fuses: 15A ANL fuses on each
NODE_BAT_POSbefore the diode.
The High-Current Ideal Diode Approach (Up to 30A)
For loads exceeding 5A, the SB560 will overheat. A 0.4V drop at 20A is 8W of heat per diode, requiring massive heatsinks. Instead, we use an Ideal Diode Controller IC to drive a P-Channel MOSFET, reducing the voltage drop to under 0.05V (20mV). Refer to the Analog Devices LTC4359 datasheet for the exact switching characteristics.
- Controller IC: LTC4359 (Ideal Diode Controller).
- MOSFET: IRF9540N (P-Channel, -100V, -23A, Rds(on) = 0.117Ω). *Note: At 20A, 0.117Ω still yields heat; for high continuous loads, use a lower Rds(on) MOSFET like the IRF4905 (0.02Ω).*
- Wiring: 10 AWG silicone wire from battery to MOSFET source; MOSFET drain to
NODE_CATHODE_COMMON.
Behavior Matrix: What Happens When Elements Change
Understanding the exact node behavior during faults is what separates a reliable UPS from a fire hazard. Here is the behavior matrix for the OR-ing topology using the LTC4359 + MOSFET setup.
| Event / Fault | Node Voltage Changes | System Result & Component Action |
|---|---|---|
| Normal Operation (Matched) | BAT1 = 13.4V, BAT2 = 13.4V | Both MOSFETs conduct. Load current splits roughly 50/50 based on wiring resistance. |
| BAT1 Sags (Heavy Load) | BAT1 drops to 12.8V, BAT2 holds 13.2V | LTC4359 senses V_source < V_drain on BAT1. Turns off BAT1 MOSFET. BAT2 takes 100% of load. |
| BAT1 Opens (BMS Trip) | NODE_BAT1_POS drops to 0V |
BAT1 MOSFET body diode blocks reverse current. BAT2 powers load seamlessly. Zero interruption. |
| BAT1 Internal Short | NODE_BAT1_POS drops to near 0V |
BAT1 MOSFET turns off instantly. BAT1 fuse blows if internal short draws from BAT2, but MOSFET prevents this. Load survives on BAT2. |
| Load Transient (0A to 20A) | NODE_CATHODE_COMMON sags momentarily |
Both MOSFETs are already fully enhanced (Vgs driven by IC). Output sags only by I × Rds(on) + wire resistance. |
Step-by-Step Breadboard Testing Protocol
Never test a new battery topology directly with high-capacity LiFePO4 cells. A wiring error can result in hundreds of amps of short-circuit current. Use this bench protocol to validate your circuit two batteries logic safely.
- Configure Power Supplies: Set two bench power supplies (PSU1 and PSU2) to 13.5V. Set the current limit on both to 1.0A.
- Wire the Isolation: Insert two SB560 Schottky diodes into the breadboard. Connect PSU1 (+) to Diode 1 Anode. Connect PSU2 (+) to Diode 2 Anode.
- Join the Output: Tie Diode 1 Cathode and Diode 2 Cathode together on a common breadboard rail. This is
NODE_CATHODE_COMMON. - Connect Grounds: Tie PSU1 (-), PSU2 (-), and the load ground to the common negative rail.
- Attach Dummy Load: Connect a 12V automotive bulb or a power resistor (e.g., 10Ω, 20W) across
NODE_CATHODE_COMMONand Ground. - Test Handover (Voltage Sag): With both PSUs at 13.5V, measure the voltage at the common rail (expect ~13.1V due to diode drop). Slowly turn down PSU1 to 12.0V. Watch the multimeter on the common rail; it should hold at ~13.1V (powered by PSU2). PSU1's ammeter should drop to exactly 0.00A, proving the diode is blocking reverse current.
- Test Open Circuit: Turn off PSU1 completely (simulate BMS trip). Verify the load remains powered by PSU2 without the light flickering.
- Test Short Circuit Simulation: With PSU1 off, use a jumper wire to short the PSU1 side of Diode 1 to ground. Verify PSU2 voltage does not sag and Diode 1 does not overheat. The diode successfully blocks PSU2 from dumping into the short.
FAQ: Circuit Two Batteries Long-Tail Questions
Can I wire a circuit two batteries in parallel without diodes or MOSFETs?
You can, but it is highly discouraged for standalone batteries with separate BMS units. If you wire two LiFePO4 batteries directly in parallel without isolation, the battery with the slightly higher voltage will force current into the lower-voltage battery to equalize them. If the voltage difference is large (e.g., connecting a 100% charged battery to a 20% discharged one), the equalization current can exceed the BMS charge-current limit, tripping the BMS or damaging the cells. Direct parallel is only safe if the batteries are permanently bonded, share a single common BMS, and are manually balanced to the exact same voltage before connection.
What breaks at the extremes if one battery shorts completely?
In a direct parallel setup, a complete internal short on Battery 1 turns it into a dead short across Battery 2. Battery 2 will dump its entire short-circuit current (often 300A to 1000A for LiFePO4) into Battery 1, melting wires and potentially causing a thermal event before the fuse clears. In the OR-ing circuit two batteries topology described above, the isolation diode or MOSFET on Battery 1 becomes reverse-biased the millisecond Battery 1's voltage drops below Battery 2. The component blocks the reverse current, Battery 2's fuse never sees a spike, and your load continues running on Battery 2 alone.
Do I need a BMS for each battery in this two-battery circuit?
Yes. Every individual lithium battery pack in an isolated parallel setup must have its own dedicated BMS. The BMS protects against cell-level over-voltage, under-voltage, and over-temperature. Because the OR-ing diodes prevent the batteries from sharing charge current dynamically, if Battery 1 hits its low-voltage cutoff under load, its BMS will open the internal disconnect. The OR-ing circuit handles this gracefully by shifting the load to Battery 2, but without a BMS, Battery 1 would be driven into deep discharge, permanently damaging the lithium cells.
