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Redmi K40 5G Compatible Battery 3.87V 4400mAh Li-Polymer

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From $28.99 USD $35.99
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Fits Redmi K40 5G, K40 Pro 5G, and K40 Pro+ 5G smartphone models.
3.87V nominal voltage, 4400mAh capacity delivers full charge cycles to the Snapdragon processor and display.
Connector seats flush into the battery slot with no locking tabs — orientation marked on the cell.
We ran discharge curves at 2A load; the BMS held voltage stable through mid-drain before expected taper.
On first use after installation, disable fast charging for one complete discharge-charge cycle so the fuel gauge IC recalibrates against this cell's discharge curve.
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.

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⚠️ 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

3.87V

Amp

4400mAh

Redmi K40 5G / K40 Pro 5G / K40 Pro+ 5G — 3.87V Li-Polymer Replacement Battery

This 3.87V, 4400mAh Li-Polymer cell replaces the original battery in the Redmi K40 5G, K40 Pro 5G, K40 Pro+ 5G, and M2012K11Q variants. It restores full power to the processor, display, modem, and all onboard systems. Dimensions are 87.00 x 64.20 x 4.30mm — physically identical to the factory cell.

  • K40 series platform fit: The K40, K40 Pro, and K40 Pro+ share the same physical battery bay, connector pinout, and BMS handshake protocol. One cell fits all listed variants without modification.
  • Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on K40 hardware. The BMS accepted the cell on first contact, and charge IC communication registered correctly at 3.87V nominal without faults.
  • Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current charging begins on an uncalibrated reference.

Why the K40 reports wrong battery percentage after a cell swap

The K40's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches actual voltage behaviour. The gauge reads a voltage, compares it to the old model, and outputs an incorrect percentage. One full slow-charge cycle — charge from flat to 100% without fast charge enabled — forces the coulomb counter to rebuild its reference map against the new cell's real chemistry.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC is still running the old cell's voltage-to-capacity curve. The new cell hits a voltage cliff — typically around 3.50–3.55V — that the OS doesn't recognise as low, so no warning fires before the phone shuts down. It is not a defective cell. Run one full slow discharge-charge cycle, and the gauge recalibrates its low-voltage cutoff threshold to match the new cell. After that cycle, shutdowns below 20% should stop.

Compatible Models

K40 5G K40 Pro 5G K40 Pro+ 5G M2012K11Q K40 Ulta M2012K11C M2012K11AC

Technical Specifications

Voltage3.87V
Amp Hours4400mAh
Capacity4400mAh
Rate17.03Wh
Net Weight58.8g /2.07 oz
Gross Weight108.8g /3.84 oz
Approximate Weight108.8g /3.84 oz
Dimension 87.00 x 64.20 x 4.30mm

Product Highlights

  • Brand: Redmi
  • 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 Redmi K40 won't turn on at all after sitting in a drawer for months — is the new battery dead on arrival?

Almost certainly not. A cell stored at low charge drops below 2.5V, and the BMS locks out to prevent damage — the phone won't power on even with a new battery installed if that lockout trips during storage. Plug into a wall charger (not a PC USB port) and leave it for 20–30 minutes without pressing anything. The charge IC trickle-charges the cell back above the BMS recovery threshold, usually around 3.0V, and the phone will power on normally from there.

Fast charging stopped working after I swapped the battery — the phone only charges slowly now.

The K40's USB-PD and proprietary fast-charge handshake runs through the BMS on the new cell. On the first cycle, some BMS units don't complete the authentication exchange until they've seen at least one full slow charge. Charge the phone to 100% using the standard charger without triggering turbo mode — hold the fast-charge toggle off in settings if your ROM exposes it. After that first full cycle, reconnect with the original fast charger and the handshake should complete correctly.

The phone feels warm near the battery during the first few charges after replacement — is that normal?

Yes, and it's specific to new high-impedance cells. A fresh Li-Polymer cell has slightly higher internal resistance than a broken-in one, so the charge IC pushes more voltage across that resistance during the initial cycles, generating mild heat. It's measurably warmer than a seasoned cell but not a fault condition. After two or three full cycles, internal resistance drops and the warmth reduces. If the back stays hot throughout a full charge after five cycles, check that the charge IC isn't stuck in a continuous trickle state by confirming charge current in a battery diagnostics app — it should reach at least 1.5A during bulk charge.

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