Where Cells Come From
Cells are the mod's core currency. They drop from almost every hostile mob in the Prisoners' Quarters and later areas — Buzzcutters, Grenadiers, Kamikazes, Leaping Zombies, Mutated Bats, Rancid Rats, Scorpions, Shockers, Sweepers, Protectors, and Inquisitors all contribute. ELITE variants and bosses like the Concierge and Conjunctivius drop significantly more.
Banking Without Losing Them
Cells are lost if you die before depositing them, so treat banking as a priority:
1. Clear a room, then immediately look for the 
Cell Holder.
2. Deposit everything you have before opening new doors or triggering Elite Obelisks.
3. Only carry Cells into a boss fight if you are confident — otherwise bank first.
4. Use Boss Stem Cells from boss kills to raise difficulty, which increases Cell drops but also mob strength.
Efficient Routes
Run the same early rooms repeatedly rather than pushing into unknown territory with a full Cell pouch. Kill spawner-driven waves, bank, and repeat. Once your permanent upgrades are strong enough, extend the route to include the 
Black Bridge Doorway and boss arenas for the biggest payouts.