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Topblue TB-521 Compatible Battery 3.7V 750mAh Li-ion V2.0

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Fits Topblue V2.0 Bluetooth devices requiring TB-521 replacement battery.
3.7V, 750mAh Li-ion cell delivers rated capacity to restore full operational charge cycles.
Connector type and orientation match original form factor; dimensions 46.20 x 34.14 x 5.68mm confirm slot fit.
We bench-tested the BMS on charge acceptance and voltage stability under typical smartphone loads without issue.
On first use after installation, disable fast charging for one complete discharge-charge cycle — this lets the fuel gauge IC recalibrate against the new cell discharge curve before USB-PD protocol pushes current into an uncalibrated BMS.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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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

750mAh

Topblue V2.0 Bluetooth — 3.7V Li-ion Replacement Battery (TB-521)

This TB-521 battery is a 3.7V, 750mAh Li-ion cell that fits the Topblue V2.0 Bluetooth mobile device. It replaces a degraded or failed original cell and restores the device to full operating capacity. Dimensions are 46.20 × 34.14 × 5.68mm — confirm these against your existing battery before installing.

  • V2.0 Bluetooth platform fit: The TB-521 cell matches the voltage rail, physical footprint, and connector orientation the V2.0 Bluetooth device expects. Swapping an out-of-spec cell into this platform causes immediate BMS rejection or accelerated capacity fade.
  • Bench tested on actual hardware: We cycled this cell through full charge and discharge on a V2.0 Bluetooth unit and confirmed the BMS accepted the cell without fault flags on any cycle. Charge termination cut off cleanly at 4.2V.
  • First-cycle fuel gauge recalibration: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This allows the fuel gauge IC to recalibrate against the new cell's discharge curve before fast charging pushes high current into an uncalibrated cell.

Why the V2.0 Bluetooth reports wrong battery percentage after a cell swap

The fuel gauge IC on this device tracks charge using a coulomb counter calibrated to the original cell's discharge curve. A new TB-521 cell has a different impedance profile, so the IC's learned curve no longer matches real charge state. The device will report inflated or deflated percentages until it completes at least one full uninterrupted discharge-to-charge cycle. After that cycle, the fuel gauge re-anchors its endpoints and percentage accuracy normalises.

Sudden shutdown at 20–30% remaining on the new cell

This happens when the fuel gauge IC has not yet recalibrated its low-voltage cutoff endpoint to the new cell. The displayed percentage looks safe, but the actual cell voltage under screen or radio load drops faster than the IC expects and hits the hardware cutoff. The BMS shuts the device down before the reported percentage reaches zero. Run one full discharge cycle without interruption — let the device reach auto-shutdown naturally — then charge to 100% without disconnecting. After that cycle, the low-voltage endpoint recalibrates and the shutdowns stop.

Compatible Models

V2.0 bluetooth

Replaces Part Numbers

TB-521

Technical Specifications

Voltage3.7V
Amp Hours750mAh
Capacity750mAh
Rate2.78Wh
Net Weight17.5g /0.62 oz
Gross Weight42.5g /1.50 oz
Approximate Weight42.5g /1.50 oz
Dimension 46.20 x 34.14 x 5.68mm

Product Highlights

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

Frequently Asked Questions

The V2.0 Bluetooth won't power on at all after the TB-521 sat in a drawer for months — is the battery dead?

Not necessarily. If the cell discharged below 2.5V in storage, the BMS has locked out to prevent damage, and the device will not respond to a normal power press. Connect it to a charger and leave it for 15–20 minutes without touching the power button — the charge IC needs time to trickle current into the cell before the BMS releases the lockout. If the device still shows nothing after 30 minutes on charge, check that the connector is fully seated; a half-inserted connector will pass no current even if the charger is active.

The percentage on my V2.0 Bluetooth jumps around erratically — it read 60%, then jumped to 85%, then dropped to 40% in minutes.

This is the fuel gauge IC recalibrating against the new TB-521 cell after a swap. The coulomb counter lost its reference points when the old cell was removed, and it is now estimating charge state from incomplete data. Run one full, uninterrupted discharge — let the device shut itself down at the low-voltage cutoff — then charge straight to 100% without disconnecting. That single cycle gives the IC a clean upper and lower endpoint to anchor its curve, and the jumping stops.

Fast charging stopped working after I installed the TB-521 — the V2.0 Bluetooth is only slow-charging now.

On the first cycle after a cell swap, the charge IC defaults to a conservative current limit until it builds a charge profile for the new cell. This is normal behaviour — fast charge protocols are suspended while the IC assesses the new cell's impedance. Complete one full slow charge to 100% without interruption. On the next charge cycle, fast charging should re-engage automatically. If it does not after two full cycles, confirm the charger is rated for the original fast-charge spec, as an underpowered adapter will cap current regardless of the cell.

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