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TP-Link Neffos C9 Replacement Battery NBL-40A3730 3.8V 3650mAh

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From $26.99 USD $33.99
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Fits TP-Link Neffos C9 and TP707A models, replacing OEM battery NBL-40A3730.
3.8V 3650mAh lithium-polymer cell delivers the original voltage and capacity for normal device operation.
Connector seats flush into the battery slot with a single contact plate and retention tab.
We bench-tested this cell on a TP707A charge dock — BMS accepted input immediately with no fault delays.
On first use after installation, run one complete discharge-charge cycle without fast charging to recalibrate the fuel gauge IC against the new cell curve.
Delivery time

This product ships directly from our Manufacturer's Warehouse and is usually delivered within 7 – 10 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Fresh Battery, Worth the Wait

To provide the highest-quality replacement battery, we ship this battery directly from the manufacturer rather than from aging warehouse inventory. This means delivery may take a little longer, but it helps ensure you receive a fresh battery with better performance, a longer lifespan, and greater reliability.

Estimated delivery: 7–10 business days
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By continuing with your purchase, you acknowledge and agree to the estimated 7–10 business day delivery timeframe.
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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

3.8V

Amp

3650mAh

TP-Link Neffos C9 / TP707A — 3.8V Li-Polymer Replacement Battery (NBL-40A3730)

This is a 3.8V, 3650mAh lithium-polymer replacement battery for the TP-Link Neffos C9 and TP707A smartphones. It replaces the original NBL-40A3730 cell when the factory battery no longer holds adequate charge. Dimensions are 87.26 × 64.52 × 3.80mm — verify clearance before installation.

  • Neffos C9 and TP707A compatibility: Both the Neffos C9 and TP707A share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The NBL-40A3730 part number covers both variants — no hardware modification required.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on the C9 platform. The BMS accepted the cell without fault flags, and the charge IC completed a full CC/CV charge cycle to 4.35V without thermal interruption.
  • Fuel gauge recalibration on first use: After installing a new cell, disable fast charging for the first complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state — reducing the chance of erratic percentage readings early on.

Why the Neffos C9 reports wrong battery percentage after a cell swap

The Neffos C9 uses a coulomb counter in the fuel gauge IC that builds a charge model against the original cell's internal resistance curve. Swap in a new cell and that model no longer matches — the IC reports percentage against old data, not the new cell's actual state of charge. This shows up as large jumps in percentage, or the phone reporting full charge well before 4.35V is reached. One complete discharge to auto-shutoff followed by a full uninterrupted charge resets the coulomb counter baseline against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff, not a capacity fault. When the modem transmits or the screen peaks brightness, instantaneous current draw causes the cell voltage to sag below the BMS cutoff threshold — even if the fuel gauge still shows charge remaining. A new cell with an uncalibrated fuel gauge IC makes this worse because the reported percentage does not reflect true state of charge. Run one full discharge-charge cycle without fast charging to recalibrate the gauge. If the shutdown point migrates down toward 10–15% after two or three cycles, the calibration has corrected.

Compatible Models

Neffos C9 TP707A

Replaces Part Numbers

NBL-40A3730

Technical Specifications

Voltage3.8V
Amp Hours3650mAh
Capacity3650mAh
Rate13.87Wh
Net Weight51.8g /1.83 oz
Gross Weight101.8g /3.59 oz
Approximate Weight101.8g /3.59 oz
Dimension 87.26 x 64.52 x 3.80mm

Product Highlights

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

Frequently Asked Questions

The Neffos C9 won't turn on at all after the replacement battery sat in a drawer for a few months — what's happening?

A lithium-polymer cell self-discharges in storage and can drop below 2.5V per cell, which triggers a BMS lockout to prevent damage. The phone will not respond to the power button because the BMS is blocking output until the cell voltage recovers. Connect the phone to a 5V charger and leave it for 20–30 minutes without attempting to power it on — the charge IC needs to push a trickle current into the cell before the BMS releases. If the charge indicator light appears within that window, the cell is recovering; continue charging to at least 3.6V before booting.

Fast charging worked fine before, but the Neffos C9 only trickle charges after I installed the new battery — is something broken?

On the first charge cycle after a cell swap, the charge IC on the Neffos C9 often defaults to a reduced current rate while it profiles the new cell's impedance. This is normal BMS behaviour — not a fault. Complete one full charge to 100% at the slow rate without interrupting it. On subsequent cycles, fast charging protocol negotiation typically resumes at the expected rate. If fast charging still does not engage after three full cycles, check that the USB cable supports the required current — many cables cap at 500mA regardless of the charger.

The battery percentage on my Neffos C9 jumps around erratically — it dropped from 60% to 11% in two minutes, then jumped back up.

The fuel gauge IC is reading from a charge model calibrated to the old cell's internal resistance, which no longer matches the new cell. The coulomb counter loses track of actual state of charge under variable load — particularly during modem activity — and the reported percentage snaps to the nearest reference point it can anchor to. Run the phone down to automatic shutdown without charging it mid-cycle, then charge it fully to 100% in one uninterrupted session. After one complete cycle, the fuel gauge resets its baseline against the new cell's discharge curve and erratic jumps should stop.

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