You need to know how the mod is wired for calculating how the batterieswill be used ...most mods work on a series basis as a general rule of thumb but always look it up...on regulated mods you will always be limited by the chip and onboard regulator anyway but it helps to use batteries that won't put the mod under strain
So
Connecting in Series
When connecting your batteries in Series you are doubling the voltage while maintaining the same capacity rating (amp hours) and CDR value of one battery. Example below
Connecting in Parallel
When connecting in Parallel you are doubling the capacity (amp hours) and CDR while maintaining the voltage of one of the individual batteries.
Important notes: Don't use two different chemistries when connecting batteries.
Always use married batteries I.e bought together always charged and used together, otherwise voltages will be different, but more importantly the charge rates will be different and the capacities may be different, thus resulting in a shortened life span.
Match capacities. When connecting batteries in a pack you should try to match the capacities as much as possible to avoid discharging one battery quicker than another. A pack operates at a combined voltage so your one cell that discharges quicker will likely discharge deeper than it may be able to recover from.
The RX300 you mentioned at the beginning of this thread is a little different ....I believe it uses both of these I.E uses four batteries but are wired in a way that uses series and parallel (2 on each side)
So taking a 20CDR battery with a 3000mah capacity fully charged at 4.2v as an example
You'll have 40CDR, 6000mah, 8.4v best of both worlds I could be wrong on this though so hoping
@SirRichardRear can verify what I'm saying here