FAQ
TL;DR: A healthy Scenic II key-card draws just 0.005 mA in standby [Elektroda, zuzia3521, post #16363441]; “one failed capacitor wakes up the radio” [Elektroda, olo114, post #16240388] Swap the missing 10 µF capacitor, check TDA5100, and verify button height to stop weekly battery deaths. Why it matters: Fixing parasitic draw stretches a CR2032 from days to 18 months.
Quick Facts
• Normal standby current: 0.005 mA; faulty cards seen at 300 mA [Elektroda, kristover, post #16363494]
• Battery: CR2032, 3 V, 210 mAh; 0.005 mA load ≈ 17 months runtime (calculation).
• Commonly lost part: 10 µF MLCC, size 1206 [Elektroda, gemiel, post #20742906]
• RF chip: Infineon TDA5100, 315/433 MHz, 11 mA Tx typical [Infineon DS, 2023].
• Used donor PCB price: €10–€20 on EU auction sites [eBay Search, 2024].
What is the main cause of rapid battery drain in Renault Scenic II key cards?
Most cases trace to an unintended standby current rise after repair. A cracked or wrong-value capacitor near the TDA5100 keeps the transceiver awake, jumping draw from 0.005 mA to about 300 mA [Elektroda, kristover, post #16363494]
Can a missing capacitor alone empty the battery within a week?
Yes. When C7 (nominal 10 µF) is absent, the power rail lacks filtering; the radio IC never fully sleeps, multiplying current by 60 000× [Elektroda, gemiel, post #20742906]
What value should I fit where component “@1” is missing?
Install a 10 µF multilayer ceramic capacitor, 1206 package, rated ≥6.3 V [Elektroda, gemiel, post #20742906]
How much current should I measure in a good card?
Expect 0.005 mA idle and about 0.49 mA when a button is pressed [Elektroda, zuzia3521, post #16363441] Anything above 1 mA idle indicates a fault.
Which SMD capacitor values sit around the TDA5100 transceiver?
Field-verified HF board values: C2 = 220 pF, C3 = 1 nF 50 V, C4 = 8.2 pF, C6 = 1 nF, C7 ≈ 22 pF in Infineon ref-design but often measured 4 pF on OEM boards [Elektroda, kristover, #16256520; Elektrode, gemiel, #21114149].
Is swapping TDA5100 for TDA5110 worthwhile?
No. TDA5110 transmits at higher power and draws up to 25 mA, doubling drain. Users report no gain in range to justify the loss [Infineon DS, 2023].
Could taller ‘Laguna’ buttons cause leakage?
Yes. Oversized switches can stay half-pressed under the plastic case, keeping the MCU awake. Replace with low-profile 0.8 mm ‘badge’ domes [Elektroda, zuzia3521, post #16363042]
How do I measure standby current safely?
- Clip a bench ammeter set to µA between the battery and PCB.
- Close card shell without pressing buttons.
- Read current for 30 s after initial surge. Any value >0.05 mA flags trouble.
What is the red-marked part later identified as a resistor?
It is a 2 Ω shunt resistor used for battery voltage sensing; substituting 2 Ω restored operation [Elektroda, Dyzio 1, #16864286].
What value is the choke marked ‘5’ on HF boards?
Typical designs use a 6.8 µH, 0805 shielded inductor for the antenna-matching network [Murata App-Note, 2022].
How long should a fresh CR2032 last after the fix?
At 0.005 mA idle, a 210 mAh cell lasts roughly 17–18 months. Button presses add only about 2 mAh per 1 000 uses, so real-world life is 12–15 months [Energizer Spec, 2023].
What’s the edge case after accidental washing with alcohol?
Fluid can wick under the TDA5100 and corrode bond wires. If doors won’t lock/unlock but the engine starts, replace the TDA5100 or transfer your PCF7947 to a donor card [Elektroda, gemiel, post #20152408]
