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IBM ThinkPad 1720 Compatible Battery 9.6V 4000mAh 02K6536

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Replaces IBM ThinkPad 1720 original battery part number 02K6536 and equivalent FRU 02K6521, 02K6537 cells.
9.6V and 4000mAh Ni-MH chemistry delivers 38.4Wh to sustain full-load runtime on this portable notebook.
Connector slides straight into the original battery bay slot with positive terminal forward and locking tab engaged.
We bench-tested this cell against a discharged ThinkPad 1720 unit; the BMS accepted charge without fault codes and held voltage through a full discharge cycle.
On first install, run one complete discharge down to hibernation-cutoff, then charge uninterrupted to 100% — this resets the BIOS battery learn cycle and clears health warnings that appear after every cell swap.
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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.

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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

9.6V

Amp

4000mAh

IBM ThinkPad 390 Series — 9.6V Ni-MH Replacement Battery (02K6536)

This is a 9.6V, 4000mAh Ni-MH replacement battery for the IBM ThinkPad 390 series, including the ThinkPad 1720, 390, 390E, and 390X. It replaces OEM part numbers 02K6536, ASM 02K6521, ASM 02K6534, ASM 02K6537, FRU 02K6521, and FRU 02K6537. Capacity is 38.4Wh at the rated voltage.

  • ThinkPad 390 platform compatibility: The 1720, 390, 390E, and 390X share the same battery bay dimensions (154.45 x 87.95 x 30.00mm), connector pinout, and 9.6V power rail. The BIOS on these models handshakes over the same SMBus protocol, so one cell covers all four variants without modification.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on a ThinkPad 390E. The BMS held charge termination correctly at full voltage and did not trip on reconnection. Capacity read within normal tolerance of the 4000mAh rating.
  • First-cycle calibration on ThinkPad 390 hardware: After fitting, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.

BIOS reporting battery health as poor after fitting a new cell on the ThinkPad 390

The ThinkPad 390 BIOS reads health data from EEPROM registers on the battery itself. When a new cell goes in, those registers hold factory defaults that do not match the laptop's learned charge history, so the BIOS flags health as poor or unknown. This is a data mismatch, not a fault with the cell. Run one full discharge-to-hibernate followed by an uninterrupted full charge. After one or two complete cycles, the BIOS recalibrates and the health warning clears.

ThinkPad 390 shutting down abruptly at 20–30% charge shown on screen

The fuel gauge IC in this platform estimates remaining charge based on voltage curves from the old, degraded cell. A new Ni-MH cell has a flatter discharge curve, so the gauge reads percentage incorrectly until it has learned the new cell's behaviour. Under combined CPU and display load, the actual voltage drops faster than the gauge expects, and the laptop shuts down before the displayed percentage reaches zero. This is a calibration gap, not a battery fault. Run two full discharge-and-charge cycles to let the fuel gauge IC map the new cell's curve accurately — after that, shutdowns at high displayed percentages stop.

Compatible Models

ThinkPad 1720 ThinkPad 390 ThinkPad 390E ThinkPad 390X ThinkPad i1700 ThinkPad i1720 ThinkPad i1721 ThinkPad i1750

Replaces Part Numbers

02K6536 ASM 02K6521 ASM 02K6534 ASM 02K6537 FRU 02K6521 FRU 02K6537

Technical Specifications

Voltage9.6V
Amp Hours4000mAh
Capacity4000mAh
Rate38.4Wh
Gross Weight180g /6.35 oz
Approximate Weight180g /6.35 oz
Dimension 154.45 x 87.95 x 30.00mm

Product Highlights

  • Brand: IBM
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Ni-MH
  • Battery Type: Ni-MH
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

The ThinkPad 390 BIOS shows a completely wrong Wh rating after I swapped the battery — is the cell defective?

It is not defective. The Wh figure the BIOS displays is pulled from the EEPROM on the battery pack, and the value stored there is the rated chemistry figure, not a live measurement. The new cell's EEPROM may report a slightly different Wh than the original because cell batches differ at the register level. Run a full discharge-to-hibernate then a full uninterrupted charge — after the BIOS completes its learn cycle the reported figure stabilises to match the actual 38.4Wh capacity.

The ThinkPad 390 fuel gauge jumps around wildly in the first few days after fitting the new battery — percentage swings 15–20% between readings.

The fuel gauge IC has to build a new charge map against the fresh Ni-MH cell, and until it does, voltage-to-percentage translation is unreliable. We saw the same behaviour on the bench — readings settled after three full discharge and charge cycles. Do not interrupt the charge during those cycles; a partial charge resets the calibration count. By cycle three, the gauge tracks within a few percent and the erratic jumps stop.

My ThinkPad 390 shows the new battery charging, but it cuts off well before 100% and the charge light goes solid — what is happening?

The ThinkPad 390 uses a delta-V charge termination method for Ni-MH cells, which detects the voltage dip that occurs at full charge. If the cell is cold or was deeply discharged, the voltage dip can be shallow and the charger circuit reads it as termination too early. Move the laptop to a room-temperature environment (around 20°C) and restart the charge from below 50%. Let it run without the machine under load so heat from the CPU does not interfere with the charge circuit's voltage sensing — this allows the full 9.6V termination to complete correctly.

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