Apple A3121 iPhone Replacement Battery 3.85V 4750mAh

Apple iPhone 15 Plus — 3.85V Li-Polymer Replacement Battery (A3121)

This is a 3.85V, 4750mAh Li-Polymer cell that fits the iPhone 15 Plus (model numbers A2849, A3105, A3106, A3108, and one additional regional variant). It replaces the original Apple A3121 cell when the installed battery no longer holds adequate charge or causes unexpected shutdowns. Capacity is rated at 18.29Wh.

• iPhone 15 Plus model variants: Apple releases the same phone under multiple model numbers for different regional markets — A2849, A3105, A3106, A3108 all share the same chassis dimensions, connector pinout, and BMS communication protocol, so they use the same cell.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on an iPhone 15 Plus unit, confirmed the BMS handshake completed without error flags, and verified the charge IC accepted current at both standard and fast-charge rates after one full calibration cycle.
• Fuel gauge recalibration on first use: After installation, disable fast charging and complete one full discharge-to-charge cycle before resuming normal use. The fuel gauge IC is still calibrated to the old cell's discharge curve — running one full cycle lets it remap against the new cell before high-current charging begins.

Sudden shutdown at 20–30% on the replacement cell

A fresh cell in a new state of charge should not shut down at 30%, but the fuel gauge IC doesn't know that yet. It still has the old cell's internal resistance map loaded, so under peak load — modem switching, display at full brightness, GPS active simultaneously — the IC predicts a voltage cliff that may not actually exist on the new cell. iOS interprets that prediction as imminent shutdown and cuts the display. One full discharge-charge cycle rewrites the coulomb counter baseline, and the false shutdowns typically stop after that.

USB-PD fast charge not working after cell replacement

On first boot after a cell swap, the charge IC may refuse to negotiate the fast-charge protocol with the power adapter. This happens because the BMS reports a high internal impedance on a brand-new, uncycled cell — the IC interprets this as a fault condition and falls back to standard 5W charging as a precaution. This is normal behaviour, not a defective battery. Plug into a USB-PD adapter after the first full calibration cycle and fast charge will reinitiate automatically.