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TCL Q3 OT-BY40 Replacement Battery 3.7V 800mAh

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From $22.99 USD $28.99
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Fits TCL Q3, I802, E59 smartphones; replaces OEM part numbers OT-BY40 and TB-4X.
3.7V, 800mAh lithium-ion cell delivers 2.96Wh to restore full charge cycles on aging devices.
Connector type is proprietary; physical dimensions 53.30 x 33.80 x 4.50mm match original slot exactly.
We ran full discharge cycles on bench; BMS accepted charge current without fault codes or thermal delay.
On first use after installation, disable fast charging for one complete discharge-charge cycle to recalibrate fuel gauge.
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

Delivery and Shipping

🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

Disclaimer

⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.7V

Amp

800mAh

TCL Q3 / I802 / E59 — 3.7V Li-ion Replacement Battery (OT-BY40)

This is a 3.7V, 800mAh Li-ion replacement cell for the TCL Q3, I802, and E59 smartphones. It replaces OEM part OT-BY40 and TB-4X. Swap it in when the original cell no longer holds a usable charge or fails to power the device through a normal day.

  • Q3, I802, and E59 compatibility: These three models share the same battery bay dimensions, connector pinout, and charge IC protocol. The OT-BY40 footprint — 53.30 × 33.80 × 4.50mm — fits all three without modification.
  • Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Q3 platform. The BMS accepted the full charge cycle, voltage held steady under screen-on and modem loads, and no spurious shutdowns occurred above 10% state of charge.
  • Fuel gauge recalibration on first use: Disable fast charging for the first complete discharge-charge cycle after installation. This allows the fuel gauge IC to map its coulomb counter against the new cell's actual discharge curve before high-current charging begins on an uncalibrated cell.

Why the TCL Q3 reports wrong battery percentage after a cell swap

The Q3 uses a fuel gauge IC that builds its charge model from accumulated coulomb-counting data on the original cell. When you install a new cell, that stored model no longer matches the new cell's actual voltage-to-capacity curve. The phone pulls percentage readings from stale calibration data until it runs a full cycle. One complete discharge to auto-shutdown followed by an uninterrupted charge to 100% resets the calibration. After that cycle, the percentage readout tracks the new cell accurately.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC hasn't finished recalibrating and the cell voltage drops below the modem or display's minimum operating threshold before the reported percentage reaches zero. The phone cuts out not because the cell is defective, but because the voltage sags under peak load at a state of charge the gauge hasn't correctly mapped yet. Let the phone discharge fully to auto-shutdown — do not force-restart mid-cycle. After two full discharge-charge cycles, the coulomb counter aligns to the new cell curve and the early shutdowns stop.

Compatible Models

Q3 I802 E59

Replaces Part Numbers

OT-BY40 TB-4X

Technical Specifications

Voltage3.7V
Amp Hours800mAh
Capacity800mAh
Rate2.96Wh
Net Weight16.2g /0.57 oz
Gross Weight41g /1.45 oz
Approximate Weight41g /1.45 oz
Dimension 53.30 x 33.80 x 4.50mm

Product Highlights

  • Brand: TCL
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Li-ion
  • Battery Type: Li-ion
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

My TCL Q3 won't turn on at all after the replacement battery sat in a drawer for a few months — is the cell dead?

Probably not dead — the BMS has locked the cell out after voltage dropped below 2.5V during storage. Connect the phone to a wall charger and leave it untouched for 20–30 minutes before attempting to power on. The charge IC trickle-charges the cell back above the BMS recovery threshold before allowing a normal boot. If the phone still won't respond after that window, check that the battery connector is fully seated.

Fast charging stopped working the first time I plugged in after fitting the new battery — what's happening?

The USB-PD negotiation or proprietary fast-charge handshake on the Q3 will sometimes default to standard 5V charging on the first cycle with a new cell. This is the BMS being conservative with an uncalibrated cell. Complete one full charge cycle at standard speed, then reconnect to the fast charger — the protocol handshake re-establishes correctly once the BMS has a baseline reading from the new cell.

The phone feels warm near the battery area while charging the new cell — is that normal?

A new high-impedance cell generates slightly more heat during the first few charge cycles than a broken-in cell does. This is normal and tapers off after two or three full cycles as internal resistance drops. If the warmth is uncomfortable to hold or the phone triggers a thermal warning on-screen, remove it from the charger and let it cool to room temperature before resuming. Persistent overheating beyond the first three cycles points to a connector seating issue — reseat the battery connector first.

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