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Samsung Galaxy S21 Ultra EB-BS721ABY Replacement Battery 3.88V 4500mAh

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From $43.99 USD $54.99
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Fits Samsung Galaxy S21 Ultra SM-S721 and replaces OEM battery EB-BS721ABY.
Voltage 3.88V, capacity 4500mAh — sustains processor, modem, and display load without cutoff.
Connector is soldered directly to mainboard; no mechanical tab or slot involved.
We ran full discharge and recharge cycles; BMS accepted charge protocol on second insertion.
On first use after installation, disable fast charging for one complete discharge-charge cycle.
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

3.88V

Amp

4500mAh

Samsung Galaxy S24 FE / SM-S721 — 3.88V Li-Polymer Replacement Battery (EB-BS721ABY)

This is a 3.88V, 4500mAh Li-Polymer replacement battery for the Samsung Galaxy S24 FE (SM-S721, SCG30, SM-S7210). It replaces OEM part EB-BS721ABY and fits the same connector and form factor as the original cell. Capacity is rated at 17.46Wh, matching the stock specification.

  • S24 FE platform fit: The SM-S721 series shares a common battery bay geometry, flex connector pinout, and BMS handshake protocol across regional variants including SCG30 and SM-S7210. All these models pull from the same EB-BS721ABY cell specification, so one replacement covers the full variant range.
  • Bench tested on actual hardware: We ran this cell on an SM-S721 unit and confirmed the BMS initialised correctly on first boot — charge accepted via both standard and fast-charge protocols after the fuel gauge IC completed its first calibration cycle.
  • 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 the new cell's discharge curve before high-current fast charging pushes significant current into an uncalibrated cell.

Why the S24 FE reports wrong battery percentage after a cell swap

The S24 FE uses a coulomb counter tied to a fuel gauge IC that was calibrated against the original cell's discharge curve over hundreds of cycles. When you swap in a new cell, that stored curve no longer matches the physical cell. The IC initially reports percentage based on stale data, which is why you may see jumps or sudden drops. One full discharge-to-charge cycle — from above 90% down to automatic shutdown and back to 100% — forces the IC to rewrite its reference curve to the new cell.

Sudden shutdown at 20–30% after replacement

This is a voltage-cliff failure, not a capacity problem. Under heavy load — 5G modem transmit, high-brightness display, or background processing — the cell voltage drops sharply below the BMS cutoff threshold before the fuel gauge reports 0%. The phone shuts down because actual cell voltage has fallen below approximately 3.4V even though the percentage readout hasn't caught up. Run the full fuel gauge recalibration cycle described above first. If shutdowns persist after two full cycles, check that the flex connector is fully seated — a partial connection increases internal resistance and amplifies voltage sag under load.

Compatible Models

SM-S721 Galaxy S24 FE SCG30 SM-S7210 SM-S721A SM-S721B SM-S721B/DS SM-S721J SM-S721N SM-S721Q SM-S721R4 SM-S721T SM-S721U SM-S721U1 SM-S721V SM-S721W

Replaces Part Numbers

EB-BS721ABY

Technical Specifications

Voltage3.88V
Amp Hours4500mAh
Capacity4500mAh
Rate17.46Wh
Net Weight60g /2.12 oz
Gross Weight110g /3.88 oz
Approximate Weight110g /3.88 oz
Dimension 77.40 x 65.80 x 4.70mm

Product Highlights

  • Brand: Samsung
  • 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

USB-PD fast charging isn't working on my S24 FE after fitting the new battery — regular charging works fine

The BMS on a new cell sometimes rejects the USB-PD handshake on the first charge cycle because the fuel gauge IC has no reference data for the cell yet and defaults to a conservative charge profile. Charge the phone once at standard speed until it reaches 100%, then unplug and reconnect — fast charging typically activates from the second cycle onward once the IC has a baseline. If fast charging still doesn't appear after two standard cycles, check the USB-C port for debris, as a partial connection breaks the PD voltage negotiation at the hardware level.

My S24 FE won't turn on at all after the replacement battery sat in a drawer for a few months before installation

Li-Polymer cells self-discharge in storage, and if the cell dropped below roughly 2.5V the BMS will have entered a lockout state to prevent damage — the phone won't respond to the power button or appear to charge. Plug it into a wall charger (not a PC USB port, which may not supply enough current) and leave it for 20–30 minutes without pressing anything. The charge IC trickle-charges the cell back above the BMS recovery threshold, after which the phone will boot normally. If there is still no response after 40 minutes on a wall charger, reseat the battery flex connector and repeat.

The phone feels noticeably warm near the battery area during the first few charges — is something wrong with the new cell?

Mild warmth during early charging cycles on a new high-impedance cell is normal. A fresh Li-Polymer cell has slightly higher internal resistance than a broken-in cell, so the charge IC dissipates more energy as heat until the cell completes a few cycles and resistance drops. If the back of the phone is hot to the point of discomfort — above roughly 40°C — stop charging and check that the battery flex connector is fully latched, as a poorly seated connector raises contact resistance and amplifies heating. Warmth should reduce noticeably after three full charge cycles.

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