A lithium-ion battery is a black box you charge and discharge, and it never gives back exactly what you put in: its round-trip efficiency — energy out over energy in — falls short of 100% because current pushed through its internal resistance on both legs becomes heat, not stored or delivered energy. Charge faster and you raise the C-rate, and because resistive heating scales with current squared, a fast charge wastes far more energy as heat than a gentle one. Hold it fully charged and self-discharge quietly bleeds a little energy away even while idle. And every cycle nudges its usable capacity down — a fade that runs faster the deeper, faster, and hotter you cycle it — until it reaches the 80% end-of-life threshold. Set the capacity, charge rate, discharge load, hold time, resistance quality, and ambient temperature, then watch state of charge, power, temperature, and cycle-life fade play out live. It all runs on your device.
Energy in during charging, energy out during discharge — and where the rest goes.
State of charge—
Power—
Round-trip—
Temperature—
Phase—
Cycles to 80%—
Peak temp—
Power
Energy split — delivered vs lost as heat
Charge / discharge schematic
State of charge over the cycle
Cycle-life fade toward 80% capacity
A lithium-ion battery never returns 100% of the energy you charge it with. Charging and discharging both push current through the battery's internal resistance, and that resistive loss leaves as heat on both legs of the round trip, so round-trip efficiency — energy delivered over energy in — always falls short of 100%. Charge faster and the C-rate rises; because resistive heating scales with current squared, a fast charge wastes far more energy as heat and runs hotter than a gentle one. Even sitting idle, self-discharge quietly bleeds away a little charge during a long hold time. None of this is a currency cost — it's physical accounting in kWh, kW and °C — and the same stresses that raise losses in a single cycle (depth, rate, heat) are what fade the battery's usable capacity over hundreds of cycles, until it reaches the conventional 80% end-of-life threshold.
Reading the simulation
What the schematic, the energy-split bar and the two plots are telling you — then what repeated cycles do to the battery.
1
The schematic and the power gauge
Energy from the grid flows in during charging, some of it is lost as heat, the rest is stored, and on discharge it flows back out through another loss before reaching the load. The gauge beside the schematic tracks live power against the battery's rated power, positive while charging and negative while discharging.
2
The energy-split bar and round-trip efficiency
The energy-split bar breaks the round trip into three slices: the charge loss and discharge loss that both leave as resistive heat, and what's left as energy actually delivered to the load. Round-trip efficiency is delivered energy divided by energy in — the higher the C-rate or the weaker the resistance quality, the smaller that delivered slice gets.
3
State of charge and cycle-life fade
The state-of-charge plot tracks percent charge through the charge, hold and discharge phases of one cycle, including the slow bleed of self-discharge during the hold. Run more cycles and the fade plot tracks usable capacity declining toward the 80% end-of-life threshold — faster the deeper, faster, and hotter each cycle runs, reported live as cycles to 80% and peak temp.
How It Works
A resistive loss on every amp that flows, a self-discharge that never sleeps, and a cycle-life fade that plays out over hundreds of cycles, not minutes.
Energy from the grid passes through a charge loss on its way into storage, and back out through a discharge loss on its way to the load — both losses leaving as resistive heat, not stored or delivered energy. The battery itself is treated as a black box: only its capacity, resistance quality, state of charge and temperature matter here, not what's inside it. Every such cycle also nudges the battery's usable capacity down, a fade that runs faster the deeper, faster and hotter the cycle.
1
Round-trip efficiency: what current squared costs you
Any current flowing through the battery's internal resistance dissipates power as heat equal to current squared times resistance, on both the charge and discharge legs. Round-trip efficiency is energy delivered divided by energy in, and it drops whenever the C-rate (current relative to capacity) rises or the resistance quality is worse — a fast, high-current cycle wastes far more of the round trip as heat than a gentle one, even though it moves the same amount of charge.
2
Self-discharge while held
Even with no charger and no load connected, a lithium-ion battery slowly loses charge to internal leakage current — self-discharge. It's usually a small fraction per day, but over a long hold time between charging and discharging, it measurably lowers the state of charge the battery starts its discharge from, on top of any resistive loss already paid during charging.
3
Cycle-life fade over hundreds of cycles
Every charge-discharge cycle leaves the battery's usable capacity a little lower than before, a decline tracked here as cycles to 80% — the count until capacity fades to 80% of its original value, a common end-of-life threshold. That fade runs faster with a deeper swing in state of charge per cycle, a higher charge or discharge C-rate, and higher ambient temperature, all stacked together in the peak temp the battery reaches during the run.
Classroom activity
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Describe how a battery stores and releases energy.
Relate capacity, charge, and discharge.
Define round-trip efficiency.
Variables to change
Charge/discharge rate (C-rate)
Capacity
State of charge
Procedure
Charge the battery and watch the state of charge rise.
Discharge it under load and watch it fall.
Compare energy put in during charging with energy delivered on discharge.
Observations
Record energy in vs energy out, and how discharge rate affects delivered energy.
Questions
What form of energy does a battery store?
What is battery capacity?
What is round-trip efficiency?
Why does energy out fall short of energy in?
What does a high C-rate mean?
Explanation
A battery stores chemical energy and converts it to electricity on demand. Its capacity (Ah/Wh) sets how much it holds; round-trip efficiency (~85–95% for Li-ion) reflects losses to internal resistance, which grow at high charge/discharge rates.
Answer key (for instructors)
1. Chemical energy, converted to and from electrical energy.
2. The charge it can deliver, usually in amp-hours (Ah) or energy in watt-hours (Wh).
3. The fraction of energy recovered on discharge compared with energy used to charge (typically ~85–95% for lithium-ion).
4. Internal resistance and chemical inefficiencies dissipate some energy as heat.
5. Fast charging or discharging relative to capacity, which usually lowers efficiency and can heat the cell.
Educators: link or embed this simulation freely in your LMS or course guide.
What is round-trip efficiency in a lithium-ion battery?
Round-trip efficiency is the energy you get back out of a battery divided by the energy you put in to charge it. It's never 100% because charging and discharging both push current through the battery's internal resistance, and that resistive loss becomes heat instead of stored or delivered energy on both legs of the round trip. A battery with a lower-resistance path for the same current wastes less of the round trip as heat and reports a higher round-trip efficiency.
Why does fast charging waste more energy as heat?
Charging faster means pushing more current through the battery for a given capacity, described by its C-rate. Resistive heating scales with current squared, so doubling the charge current roughly quadruples the heat lost to internal resistance for the same charge delivered. That's why a high C-rate fast charge measurably lowers round-trip efficiency and raises battery temperature compared to a gentle charge, even though both eventually deliver the same state of charge.
Why does a battery lose charge just sitting fully charged?
A lithium-ion battery isn't a perfectly sealed store of energy: small internal leakage currents slowly discharge it even when nothing is drawing power, a behavior called self-discharge. It's usually only a few percent per month at room temperature, but the rate roughly doubles for every 10°C rise in storage temperature, so a battery left fully charged in a hot car loses charge noticeably faster than one stored cool. Held at that state for a long time before it's ever used, it will end up with measurably less energy in it than what was put in during charging, on top of any resistive loss.
What shortens a lithium-ion battery's cycle life?
Cycle life is usually measured as the number of charge-discharge cycles a battery can complete before its usable capacity fades to about 80% of its original value, a common end-of-life threshold. Three factors accelerate that fade: deeper cycles (a bigger swing in state of charge per cycle), a higher charge or discharge C-rate, and higher operating temperature — many chemistries lose a large share of their remaining cycle life for roughly every 10°C rise in operating temperature. Shallower, gentler, cooler cycling is what stretches a battery's usable lifetime.
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