The IBM ThinkPad main battery connector is a six-pin proprietary interface used on ThinkPad T20, T21, T22, and T23 laptops (machine types 2647/2648) with Li-Ion battery packs rated at 10.8 V (nominal), up to 12.6 V fully charged. It conforms to the Smart Battery System (SBS) era of portable power, governed by the Smart Battery Data Specification Revision 1.1 (SBS Implementers Forum, December 1998) and the System Management Bus (SMBus) Specification. The IBM Hardware Maintenance Manual (P/N 62P9631) specifies that the main battery voltage should be measured between terminal 1 (+) and terminal 5 (−), with a healthy pack reading 0–12.6 V DC and a discharged pack reading below 11.0 V DC; the backup (CMOS) battery is a separate unit measuring 2.5–3.2 V DC.
| Pin | Signal |
|---|---|
| 1 | VCC |
| 2 | KEY |
| 3 | CLK |
| 4 | DATA |
| 5 | THERMAL |
| 6 | GND |
Signals
- VCC — positive supply rail from the battery pack; Li-Ion pack voltage ranges from 0 V (discharged) to 12.6 V DC (fully charged).
- KEY — mechanical key slot; not an electrical contact. Present to prevent incorrect battery insertion.
- CLK — SMBus Serial Clock (SCL); one of the two-wire SMBus lines used by the internal fuel-gauge IC to communicate state-of-charge, voltage, current, temperature, and health data to the host. No internal pull-up resistor is present on the battery side; the host system provides pull-ups.
- DATA — SMBus Serial Data (SDA); the bidirectional data line of the SMBus pair. Together with CLK, it operates at up to 100 kHz (standard-mode SMBus). Logic LOW is below 0.8 V; logic HIGH is above 2.1 V.
- THERMAL — Safety Signal / thermistor pin. Implements a 10 kΩ NTC thermistor (other end connected to GND) that presents a variable resistance to the charger, signalling whether charging is permitted. Per SBData 1.1, the charger must not charge when the resistance between this pin and GND falls in the range 425–3150 Ω.
- GND — battery negative terminal; circuit reference ground.
Notes
- The SBS standard does not specify a physical connector pinout; the six-pin layout is IBM-proprietary.
- The HMM terminal-numbering for voltage measurement (terminal 1 to terminal 5) reflects the five electrically active contacts — pin 2 (KEY) is a mechanical slot and is excluded from the count.
- SMBus CLK and DATA lines have no pull-up resistors inside the battery pack; the host motherboard supplies the required pull-ups.
- Compatible battery part numbers for T20–T23 include 02K6620, 02K6626, and 02K6859 (all 10.8 V, 6-cell Li-Ion).