Laser Pointer Forums - Discuss Laser Pointers | High Power Lasers

Hi,
Can someone explain why it is not recommended to hook up 2 or more Lithium Polymer batteries in series (or parallel) when each have protection circuitry, i.e. PCB/PCM, already built in? The explanations I read are that one battery could charge another but why would that be undesirable if protection is present?

Or in other words, why is protection for the battery as a pack a must?

Thanks.