Learning more about rechargeables; what works for what, when to re-charge...etc 
Here are the battery chemistries I pay the most attention to:
NiCd sucks.
LiCoO2 AKA ICR (you probably know them as Lithium Ion batteries) are used in most cellphones, laptops, and so on.  Great capacity and cycle life, with low self-discharge rates.  Prone to "vent with flame" when damaged or discharged too quickly.  Don't use these unless they have built-in protective circuitry.  I use these in most non-AA/AAA devices.
LiMn2O4 AKA IMR are used in vapes and other high-discharge uses like high-performance flashlights.  Significantly safer than ICR chemistry but has less capacity and cycle life.  Most think they don't need protective circuitry.  Great in high-discharge devices.  I use these in one specific device.
LiNiMnCoO2 AKA INR supposedly combine the best features of ICR and IMR.  I don't have any of these.
NiMH is the chemistry used by Eneloops.  I use these extensively in place of primary (non-rechargeable) batteries in AA and AAA devices.
None of these (other than NiCd) have a "memory" effect and handle top-up charges well.
Long-term storage of rechargeables is best, supposedly, at around 40% of charge to prevent degradation over time.  I keep most of my rechargeables in "ready to use" mode, fully charged or nearly so.