Following Q&A will teach you everything you wanted to know about batteries and how to optimize the usage.
The electrical capacity of different types or batches can differ. If such different batteries are used the difference in electrical capacity will grow during usage which could eventually cause one of the batteries to over discharge, leak and even explode.
Mixed use or leaving the battery in the appliance can cause battery leakage.
This should be avoided at any time. When the battery has been in a washing machine, it is no longer fit to use. Please dispose this battery.