How to use UltraBay batteries
ThinkPad laptops only charges/discharge one battery at a time. If you have two batteries present (a system battery and an Ultrabay battery), the laptop will completely deplete the UltraBay battery before using the main battery.
Switching between the batteries is instant, so if you pull the Ultrabay battery from the bay when it is being discharged, the system will instantly switch to the main battery. You can therefore use the UltraBay battery to hot-swap the system battery (i.e., replace it without the need to reboot, hibernate or use an external power adapter).
When charging, the system will completely charge the main battery before it starts on the UltraBay battery. When discharging, the system will completely charge the main battery before it starts on the UltraBay battery. This greatly reduces the lifetime of the Ultrabay battery, and also reduces its usefulness for enabling hot-swapping of the system battery. There are two ways to prevent this:
- Keep an eye on the charge in the UltraBay battery and physically remove it from the bay when it gets too low.
- Use the tp_smapi module to control which battery is charged (inhibit_charge on the other battery) or discharged (force_discharge). This only works on some ThinkPad models - see the model-specific status.