Reversed battery · diode · MOSFET
Reverse polarity protection: what happens if you connect a battery backwards?
Swap the jump leads for a second and current flows the wrong way through everything that conducts in reverse: the alternator’s diodes, relay coil diodes, ECU input clamps. Protection either blocks that current or makes it blow a fuse before anything else fails.
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What gets damaged
- The alternator’s rectifier diodes conduct straight across the reversed battery — often the first part to fail.
- Relays with a built-in coil diode short the supply through it.
- Modules without protection: their regulators and inputs can be destroyed.
- Fusible links and main fuses may blow — that is the protection working.
Three ways to protect a circuit
| Method | How it works | Trade-off |
|---|---|---|
| Fuse + shunt diode | Diode across the supply after the fuse; reversed, it conducts and the fuse blows | No loss in normal use; the fuse must be replaced, and the diode must survive until it blows |
| Series diode | Diode in the + line blocks reverse current | Simple; loses about 0.3–0.7 V and heats up at high current |
| MOSFET or ideal-diode controller | A transistor conducts only with correct polarity | Almost no loss, works for high currents; costs more and needs a proper circuit |
Which one where
A series diode suits small electronics where 0.5 V does not matter. For power feeds of several amps the diode’s heat is a problem, so a MOSFET circuit or the fuse-and-shunt method is used. Whole-car protection is rarely fitted: the practical defence is marking the leads clearly and using jump leads with polarity protection.
Model the fuse-and-shunt method in WireForge
- The simulator has no diodes, but you can model what a shunt diode does when the battery is reversed: it shorts the supply after the fuse.
- Draw Battery → Fuse → a load → Ground, and add a Switch (hold) named “Shunt diode (reversed)” from the fuse output to ground.
- Simulate and hold the button: the fuse blows instantly and the load is cut off.
- Remove the fuse and try again: the simulator warns “short circuit without a fuse — fire risk”, which is why the fuse comes first.
Questions
What should I check after connecting a battery backwards?
Main fuses and fusible links, the alternator (charging voltage with the engine running) and every module that no longer wakes up.
Does a car ECU have polarity protection?
Many do on their supply input, but not on every pin. Do not count on it.
Why not put a big diode in the main battery line?
The starter current would burn it, and its voltage drop would take power from everything. Protection is fitted per circuit instead.