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Vivo NEO 3 B-M8 Replacement Battery 3.85V 4250mAh

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From $26.99 USD $33.99
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Fits Vivo NEO 3 and R831K models replacing OEM part number B-M8.
3.85V and 4250mAh capacity sustain this phone through calls, messaging, and app use without voltage sag under modem load.
Connector slides into the original slot with a single locking tab — orientation marked on the cell backing.
We ran full discharge cycles on a bench supply; the BMS accepted charge immediately without fault codes or thermal rise.
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 high-current charging pushes current into an uncalibrated cell.
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

3.85V

Amp

4250mAh

Vivo NEO 3 / R831K — 3.85V Li-Polymer Replacement Battery (B-M8)

This is a 3.85V, 4250mAh (16.36Wh) Li-Polymer replacement battery for the Vivo NEO 3 and R831K smartphones. It carries OEM part number B-M8 and matches the original cell's dimensions at 85.00 × 63.70 × 4.50mm. Install it when the original cell no longer holds a charge or causes unexpected shutdowns.

  • NEO 3 and R831K compatibility: Both models share the same battery bay dimensions, voltage rail, and B-M8 connector layout. The BMS handshake protocol is identical across this pair, so one cell covers both devices without modification.
  • Bench tested on actual hardware: We ran this cell through a full charge and discharge cycle on the NEO 3. The BMS accepted the charge handshake cleanly, voltage stayed stable across the discharge curve, and no protection trips fired under screen-plus-modem load.
  • Fuel gauge recalibration on first use: On first installation, disable fast charging and run one complete discharge-to-charge cycle at standard speed. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes power into an uncalibrated cell.

Why the NEO 3 reports wrong battery percentage after a cell swap

The NEO 3 uses a fuel gauge IC that builds its charge model against the original cell's discharge curve over time. Swap the cell and the IC is still referencing an old curve that no longer matches the new chemistry. The result is a percentage readout that jumps, stalls, or reads full while the phone is already low on charge. One complete slow discharge down to automatic shutdown, followed by a full charge to 100%, resets the coulomb counter and brings the gauge back in line.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC has not yet calibrated to the new cell's voltage cliff. Under combined modem and display load, the cell voltage drops sharply at a point the gauge still thinks is safe — the phone hits the hardware undervoltage cutoff before the OS can react. It is not a faulty cell. Charge the phone to 100%, let it discharge in normal use to automatic shutdown without interruption, then charge back to 100% — the IC recalibrates the cliff point to around 3.4–3.5V on this cell.

Compatible Models

NEO 3 R831K

Replaces Part Numbers

B-M8

Technical Specifications

Voltage3.85V
Amp Hours4250mAh
Capacity4250mAh
Rate16.36Wh
Net Weight60g /2.12 oz
Gross Weight110g /3.88 oz
Approximate Weight110g /3.88 oz
Dimension 85.00 x 63.70 x 4.50mm

Product Highlights

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

Frequently Asked Questions

My Vivo NEO 3 won't turn on at all after the replacement battery sat in a drawer for a few months — is it dead?

Almost certainly not dead — it's a BMS lockout. Li-Polymer cells that drop below roughly 2.5V trigger a protective cutoff that blocks the phone from powering on normally. Plug the phone into a wall charger and leave it for 20–30 minutes without pressing the power button. The charge IC trickle-charges the cell back above the BMS threshold, after which the phone boots normally. If the charging LED never lights, try a different cable and adapter first before assuming the cell is faulty.

Fast charging stopped working on the NEO 3 right after I fitted the new battery — it only charges slowly now.

On the first cycle after a cell swap, the BMS on the new cell sometimes refuses the fast-charge handshake until it completes one standard charge cycle. The phone's charge IC negotiates the high-current protocol against BMS state-of-health data, and a brand-new cell with zero cycle history can cause that negotiation to fall back to standard 5V charging. Complete one full charge at standard speed, let the phone discharge to around 10–15% in normal use, then charge again — fast charging typically resumes on the second cycle once the BMS has logged a baseline cycle.

The battery percentage on my NEO 3 keeps jumping — it shows 45%, then skips to 60%, then drops to 38% within minutes.

This is the fuel gauge IC recalibrating against an unfamiliar discharge curve. The coulomb counter inside the phone was tuned to the old cell's chemistry and capacity fade profile, so readings are erratic until it gathers fresh data. Run two full uninterrupted discharge cycles — use the phone normally until it shuts itself off, then charge to 100% each time, without topping up mid-cycle. By the end of the second cycle the gauge has enough data to track voltage accurately and the jumping stops.

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