Renault Logan II Battery Drain: Tracing the Overnight Parasitic Draw

Why your Logan II battery goes flat overnight

It is frustrating to walk out in the morning and find a Renault Logan II that will not crank, especially when nothing was left switched on. In most cases this is not a bad battery — it is a parasitic draw, meaning a circuit stays live after the car should have gone to sleep. A healthy Logan II typically settles to around 30–50 mA after 20–60 minutes; anything above roughly 80 mA is worth investigating. Before chasing wiring, load-test the battery and confirm the alternator is charging properly. A weak battery can mimic a draw, and a failed alternator diode can drain the battery through the charging circuit even with the key off.

Most likely culprits to check first

• Alternator diode fault — a common cause of overnight drain; check for AC ripple and current flowing back through the alternator with the engine off.
• Boot or glovebox light staying on — easy to miss, especially if a boot latch or switch is misaligned.
• Aftermarket radio, alarm, tracker or dash camera — wired to permanent live instead of ignition live.
• Body control unit (UCH) not sleeping — a known area on the Logan II; door lock actuators, central locking or a faulty door switch can keep it awake.
• Towbar wiring or trailer module — corrosion or a stuck relay can create a constant drain.
• Heater blower resistor, relay or interior light — a stuck relay or faulty resistor can keep a circuit live.
• LPG or accessory wiring — if fitted, check for poor splices and permanently powered modules.

How to test it without guessing

Fully charge the battery, then let the car sit locked for at least 30–60 minutes so all modules can go to sleep. Use a clamp meter around the negative cable, or a multimeter set to 10 A in series with the negative terminal — do not crank the engine with the meter connected. Once you see an excessive draw, pull fuses one at a time and watch the reading; a significant drop points you to the circuit. Voltage-drop testing across fuses is a safer, quicker method on modern cars. Also check that the boot light actually goes out, that the radio shuts down, and that no aftermarket accessory has been tapped into a permanent live.

Is it safe to keep driving?

For a few short trips, yes — but do not ignore it. Repeated deep discharges damage the battery and can leave you stranded, and if the draw is caused by a chafed wire or failing relay there is a small fire risk. If the battery is new and still drains overnight, book a parasitic draw test soon rather than replacing parts blindly. A proper diagnosis on your Logan II is usually faster and cheaper than guessing.

This material is AI-generated and for informational purposes only — it does not replace an in-person diagnosis by a qualified mechanic.