There are two main types of NVRAM+RTC solutions for Philips CD-i players to store user data and settings: 8 KB and 32 KB. They have a built-in battery that powers the non-volatile memory and the real-time clock. These two types are not compatible and thus cannot be replaced with each other (at least, not without hardware and/or ROM modifications).
There are other types, also 8 KB and 32 KB, but they concern only very few and rare devices. I point them out when necessary.

In this article, you’ll find information about the different NVRAM types and their alternatives. You’ll also find tips for repair and upgrades. If this looks familiar to you then you have probably read parts of it in my Modifications for Philips CD-i Players article before. Since the NVRAM section grew so large, I’ve completely rewritten it and will continue to update it in its own article.