The Lenovo X220 represents the pinnacle of ThinkPad development, having in effect a 7 row keyboard, excellent display and battery life. Now that new old stock (NOS) 42T4967 slice batteries are showing up at reasonable prices, it’s worth thinking about adding one on. The Lenovo X220 can have a second “slice” battery that snaps onto the bottom, adding nearly 60 Watt-hour to the 86 Watt-hour 9-cell battery built-in to most Lenovo X220. By this time, you might not have your full 9-cell 42T4940 battery life anymore, so the slice battery will nearly double your battery working time.
The Lenovo firmware prioritizes charging first the built-in (non-slice battery) first, and the slice battery last.
When discharging, the slice battery discharges first all the way, then the built-in battery is used.
This is very smart and I applaud this battery charge/discharge order choice.