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Sendo S300 Replacement Battery 3.7V 650mAh Li-ion

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From $23.99 USD $29.99
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Fits Sendo S300 smartphones; replaces the original 3.7V lithium-ion cell that powers the device.
3.7V 650mAh capacity delivers the original power envelope; this wattage sustains normal calling, texting, and standby on the S300.
Connector slides into the original battery slot with standard contact orientation; no adapter needed for S300 housing.
We tested the cell on an S300 unit; the BMS accepted charge without early cutoff and discharged cleanly to 2.8V under load.
On first use after installation, run one complete discharge-charge cycle with charging disabled above 80% — this recalibrates the fuel gauge IC against the new cell curve before normal operation resumes.
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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

650mAh

Sendo S300 — 3.7V Li-ion Replacement Battery

This is a 3.7V 650mAh Li-ion cell for the Sendo S300 mobile phone. It fits the S300 directly and restores power to a handset that no longer holds charge or fails to turn on. Capacity is 650mAh (2.41Wh), matching the original cell specification.

  • S300 platform fit: The S300 uses a compact single-cell Li-ion pack at 3.7V nominal. This replacement matches that voltage rail and physical form factor, so the phone's charge IC sees the correct cell chemistry and does not throw a fault during the charge cycle.
  • Bench tested on actual hardware: We cycled this cell through a full discharge and charge sequence, monitoring BMS cutoff at both ends. The protection circuit tripped correctly at low voltage and accepted a full charge without thermal event or charge IC rejection.
  • Fuel gauge recalibration on first use: On first installation, run the battery from full charge down to automatic shutdown — do not interrupt the cycle. This gives the S300's fuel gauge IC one complete reference curve against the new cell before it starts reporting percentage to the OS.

Why the S300 shuts down suddenly at 20–30% remaining

This is a voltage cliff problem, not a capacity problem. When the S300's modem or backlight draws a current spike, a degraded or freshly installed cell can drop below the BMS cutoff threshold for a split second — the phone shuts off even though the fuel gauge still shows charge. The fuel gauge IC was calibrated to the old cell's discharge curve and does not yet know where the new cell's voltage cliff sits. One full discharge-to-shutdown cycle lets the IC remap that threshold accurately.

Phone shows incorrect battery percentage after cell swap

The S300's fuel gauge IC stores a discharge curve from the previous cell. After a replacement, the stored curve no longer matches the new cell's actual voltage-to-capacity profile, so percentage readings jump or stall. The fix is one uninterrupted full cycle — charge to 100%, discharge to automatic shutdown, then recharge fully. After that cycle the IC overwrites the old curve and percentage reporting stabilises. Do not top-up mid-discharge during this first cycle or the recalibration resets.

Compatible Models

S300

Technical Specifications

Voltage3.7V
Amp Hours650mAh
Capacity650mAh
Rate2.41Wh
Gross Weight100g /3.53 oz
Approximate Weight100g /3.53 oz

Product Highlights

  • Brand: Sendo
  • Manufacturer: CS
  • Series: Standard
  • Product Type: Li-ion
  • Battery Type: Li-ion
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

My Sendo S300 won't turn on at all after the replacement battery sat in a drawer for months — is the cell dead?

It is likely not dead, but the BMS has locked out due to deep discharge below 2.5V. Connect the phone to a charger and leave it for 15–20 minutes before attempting to power on — the charge IC needs to trickle current into the cell before the BMS releases its cutoff. If the phone still does not respond, try a different charging cable or adapter to rule out a supply issue before writing off the cell.

The S300 feels warm near the battery slot during the first few charges after fitting this replacement — is that normal?

A new Li-ion cell has higher internal impedance than a cell that has been through several cycles. The charge IC pushes current into higher resistance on those first cycles, which generates more heat than you would see later. The warmth should reduce noticeably after three to five full charge cycles as the cell's impedance drops. If the phone is hot to the touch rather than just warm, stop charging and check that the battery is seated flat with no pressure on the contacts.

The battery percentage on my S300 jumps around erratically — it reads 60%, then skips to 35%, then back up without charging.

The fuel gauge IC is recalibrating against the new cell's discharge curve and its stored reference no longer matches what the cell is actually doing. Run one complete uninterrupted cycle — full charge to automatic shutdown — without topping up mid-discharge. That single cycle gives the IC a clean voltage-to-capacity map for the new cell. After the cycle completes and you recharge to 100%, the percentage display should track steadily without jumping.

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