Everyone should remember that all medical devices need to meet the strict requirements of the medical device directive (now medical device regulations). If you fit a part that is not sourced from the OEM or failed to fit an OEM part in accordance with their instructions, you have effectively made a modification to the device and invalidated the CE marking. All liability for the device has now passed to you!

If your cheap Chinese import battery burst into flames or causes the pump to malfunction I hope you have good legal cover!