THE CURVE

Voltage as the cell empties

Plot voltage against charge delivered and every chemistry has its own shape. Push a cell harder and the whole curve sinks, so it reaches its cutoff voltage sooner.

A discharge curve is a battery's fingerprint.

WHERE VOLTAGE GOES

Three kinds of loss

🔻

Ohmic (I × R) Resistance of the electrolyte, foils and contacts: an instant drop when current flows

Activation The energy needed to push the reaction at each electrode surface

Concentration Ions can't reach the surface fast enough, worst near empty and at high rates

Every loss turns useful voltage into heat.

LIMITS

Cutoff voltages

🛑

⬆️

Upper cutoff

About 4.2 V for NMC and 3.65 V for LFP: charging past it damages the cathode

⬇️

Lower cutoff

About 2.5–3.0 V: going lower can dissolve the copper foil

🧠

Who enforces them

The battery management system and the charger

Cells are only used between their cutoffs.

HOW FULL?

State of charge and depth of discharge

🔋

State of charge (SOC) How full the cell is now, from 0 to 100%

Depth of discharge (DOD) How much was used in a cycle: 0–100% SOC used is 100% DOD

State of health (SOH) Capacity now ÷ capacity when new

A flat curve, like LFP's, makes SOC hard to read from voltage alone.

EFFICIENCY

What comes back out

🔁

Coulombic efficiency

Charge out ÷ charge in, over 99.9% for a good lithium-ion cell

🔥

Energy efficiency

Energy out ÷ energy in, about 90–95%: the rest is heat

📉

Why it matters

99.9% sounds perfect, but over 1,000 cycles small losses add up

Every cycle loses a tiny bit; long life means making that bit tinier.

UNIT 4 STUDY COMPLETE

📉

Ready for the Fab Challenge?

You've read a discharge curve, seen what pulls the voltage down, and learned the words engineers use for how full a cell is.