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BoostMobile Legacy CPLD-428 Compatible Battery 3.85V 3900mAh

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Fits BoostMobile Legacy and CP3705AS models, replaces OEM part number CPLD-428.
3.85V nominal, 3900mAh capacity delivers full-day runtime on the Legacy platform without voltage sag.
Connector seats flush into the battery slot with a single locking tab — no adapter required.
We bench-tested this cell in a Legacy unit; the BMS accepted the pack on first insertion with normal charge acceptance and steady voltage ramp through the fuel gauge IC.
On first charge after installation, run one complete discharge-charge cycle with fast charging disabled — the coulomb counter needs to map the new cell's discharge curve before USB-PD 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

3.85V

Amp

3900mAh

BoostMobile Legacy / CP3705AS — 3.85V Li-Polymer Replacement Battery (CPLD-428)

This is a 3900mAh Li-Polymer cell at 3.85V nominal, built to the CPLD-428 specification. It fits the BoostMobile Legacy and CP3705AS smartphones directly. If the original cell is swelling, no longer holding a charge, or causing unexpected shutdowns, this is the correct replacement.

  • Legacy and CP3705AS compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — the CPLD-428 spec covers both. The cell footprint is 88.24 × 66.30 × 4.25mm, so fit is exact with no case modification.
  • Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Legacy platform. The BMS accepted the new cell without triggering a fault code, and the charge IC stepped through trickle, CC, and CV phases correctly at each stage.
  • Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging pushes current into an uncalibrated register.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff problem, not a capacity problem. Under modem transmission load or peak screen brightness, the new cell briefly drops below the BMS cutoff threshold — even though the OS reports 20–30% remaining. The fuel gauge IC is still calibrated to the old, degraded cell's curve, so reported percentage and actual cell voltage are mismatched. Run one complete discharge cycle down to automatic shutdown, then charge uninterrupted to 100%. After that cycle, the coulomb counter recalibrates and shutdowns at low percentage typically stop.

OS reporting wrong battery percentage after cell swap

The Legacy's fuel gauge IC stores a learned discharge curve from the old cell. Swapping the physical cell does not reset this stored curve — the IC continues to calculate percentage against data that no longer matches the new cell's chemistry. You will see the percentage jump erratically or read high while the phone shuts down under load. The fix is one complete uninterrupted discharge to zero, followed by a full charge to 100% with fast charging disabled — this forces the IC to rewrite its learned curve against the new cell.

Compatible Models

Legacy CP3705AS

Replaces Part Numbers

CPLD-428

Technical Specifications

Voltage3.85V
Amp Hours3900mAh
Capacity3900mAh
Rate15.02Wh
Net Weight57g /2.01 oz
Gross Weight107g /3.77 oz
Approximate Weight107g /3.77 oz
Dimension 88.24 x 66.30 x 4.25mm

Product Highlights

  • Brand: BoostMobile
  • 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 phone powers on but shuts off the moment I make a call or open the camera — why?

This is a voltage sag failure under high-draw load. The modem radio and camera sensor together pull enough current to briefly drop cell voltage below the BMS cutoff, causing an immediate shutdown even at reported charge levels above 20%. The fuel gauge IC is reading percentage from a stored curve that does not match the new cell yet. Run one full discharge cycle to automatic shutdown, recharge to 100% without fast charging, and the IC will recalibrate — shutdowns under load typically stop after that single cycle.

Fast charging stopped working after I installed the replacement battery — is something wrong with the charger?

The charger is almost certainly fine. On the first cycle after a cell swap, the BMS on the new cell may not complete the handshake with the phone's charge IC, so the device falls back to standard 5V charging instead of accepting the fast-charge protocol. This is normal behaviour on the first cycle. Run one full standard-speed charge to 100%, then reconnect your fast charger — the handshake completes correctly on subsequent cycles in most cases.

The phone won't turn on at all after sitting in a drawer for several months with a new battery installed — what happened?

Li-Polymer cells self-discharge in storage, and if the cell dropped below approximately 2.5V, the BMS entered lockout mode to prevent damage — the phone will not respond to the power button in this state. Connect the phone to a wall charger (not a computer USB port) and leave it for at least 30 minutes without touching it. The charge IC runs a trickle current into the cell to bring it back above the BMS recovery threshold, typically around 3.0V, at which point the phone will resume normal charging and boot.

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