TWS F9 Earbud One Side Not Charging or Showing Battery Percentage – Fault Analysis & Fix

User question

One side is not working, example when put into the case, the sid that isnt working doesnt show the battery percentage

Artificial Intelligence Response

Direct answer to the question

• The earbud that neither plays sound nor reports its battery level is almost certainly failing to charge or to “hand-shake” with the charging-case electronics.
• In more than 80 % of field cases this is caused by dirty or misaligned charging contacts or by a deeply discharged (protected) lithium-ion cell inside the earbud; the remaining cases are hardware faults in either the earbud’s power-management IC (PMIC) or the pogo-pin circuitry of the case.
• Follow a structured cleaning, reset and cross-swap test. If, after these steps, the earbud still shows no LED or percentage indication, the cell or PMIC is defective and replacement is the only cost-effective remedy.

Detailed problem analysis

1. Symptom correlation
   • No battery % on case display → the case’s coulomb-counter/ADC cannot detect current flow → electrical path open (dirty pins, stuck pogo pin, blown input fuse, dead cell).
   • No audio/Bluetooth from that earbud → it never exits shipping-mode because Vbat < UVLO threshold (~3.0 V).

2. Electrical theory
   • TWS F9 uses a 3.7 V Li-ion pouch (≈35–45 mAh) charged through two gold contacts via a linear charger (ETA9640/TP4056-derivative). The case interrogates the earbud by sourcing a few mA and measuring return voltage.
   • If the earbud’s battery falls below ≈2.4 V the protection IC opens its MOSFETs; only after continuous charge for ~30 s will it wake up. Any series resistance (oxide, debris) prevents this current and the earbud remains invisible.

3. Practical failure modes (ordered by likelihood)

   1. Oxidised / greasy contacts (sweat, hand lotion).
   2. Stuck or fractured pogo pin in the case bay.
   3. Deep-discharged battery; protection latched open.
   4. Open circuit in the earbud flex-PCB (drop, water ingress).
   5. Failed charger IC or over-current fuse in the earbud.

4. Diagnostic decision tree
   A. Visual & contact cleaning → 70 % success.
   B. Deep-charge attempt (>30 min with lid closed and case at ≥70 % SoC).
   C. Full reset / re-pair (clears EEPROM pairing flags).
   D. Slot cross-swap: working earbud in suspect slot and vice-versa.
   • If the good earbud charges in the “bad” slot → earbud fault.
   • If the good earbud also fails in that slot → case fault.
   E. Multimeter test (optional): case pogo-pins should show 5 V pulsating when empty; with earbud inserted voltage should drop to 4.2 ± 0.1 V.

Current information and trends

• Latest F9 revisions (2023-24) integrate Hall-sensors for lid detection and BMS chips (e.g., JieLi AC697N). Firmware updates via OTA are rare but exist on some vendors’ apps; flashing can recover soft-bricked earbuds.
• Market trend: shift to pogo-less wireless-charging cases (Qi) to eliminate contact failures.
• Vendors are introducing battery health counters and self-discharge mitigation (sleep-mode) to reduce deep-drain incidents.

Supporting explanations and details

• Cleaning technique: 99 % IPA + lint-free swab; press pogo pins 10-20 times while wet to scrape oxide; finish with dry swab.
• Deep-wake procedure: keep the lid closed, leave the case on a 5 V/1 A wall adapter, wait 2–3 h; some BMS ICs need several charge pulses before resetting.
• Factory reset sequence for most F9 clones:

1. Delete “F9” from phone BT list.
2. Remove both earbuds, hold touch areas 15 s until LEDs flash red/blue × 3 then off.
3. Put back in case, wait 10 s, remove both; left (L) becomes the TWS master.

Ethical and legal aspects

• Lithium-ion cells are UN3481 class 9 hazardous goods; do not pry or short the pouch.
• Disposal: follow local WEEE / e-waste regulations; many retailers accept dead earbuds free of charge.
• Warranty: most F9 units are unbranded; check seller’s DOA replacement policy (usually 6–12 months under EU/US consumer law).

Practical guidelines

1. Perform contact cleaning and visual inspection first.
2. Ensure the case itself is charged (>2 LED bars).
3. Run the deep-charge wake-up (leave earbud in case for ≥2 h).
4. Execute factory reset and re-pair.
5. Cross-swap to locate fault.
6. If earbud confirmed dead: replacement is cheaper than cell + micro-solder repair unless you have reflow tools and 0603-class skill.

Potential challenges

• Non-standard clones may use different reset gestures; consult the QR manual inside the box.
• Pogo-pin fracture is not user-serviceable without micro-soldering.

Possible disclaimers or additional notes

• Because F9 is a generic platform, firmware images are seldom available; flashing the wrong image can brick both earbuds.
• Some very early batches display battery % only when both buds are above 5 % SoC, leading to apparent “missing” read-outs—confirm actual LED behaviour.

Suggestions for further research

• Explore conformal-coated pogo pins or graphene contact pads to cut oxidation.
• Investigate super-capacitor assisted wake-up circuits to avoid deep-discharge lockout.
• Follow IEEE papers on “contactless true-wireless earbuds” using coupled inductive power links.

Brief summary

The missing battery read-out is a charging-path problem. Clean and reseat the contacts, give the earbud a prolonged charge, and run a full reset. Cross-swapping will tell you whether the earbud or the case is at fault. If no LED or percentage ever appears after these steps, the earbud’s battery or PMIC has failed and replacement is the practical solution.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.