Voltage Under Load
A battery's voltage is not constant — it drops as the battery discharges, following a characteristic curve that depends on chemistry, load current, and temperature. Lithium-ion cells start at ~4.2V and drop to a 3.0V cutoff. Lead-acid starts at ~12.7V (6 cells) and drops to ~10.5V. Understanding these curves is essential for designing battery-powered systems that perform reliably across the full charge range.
The C-Rate Trade-Off
Discharging a battery faster (higher C-rate) means less of its nominal capacity is actually usable. A battery rated at 50Ah at the 20-hour rate (2.5A) might deliver only 40Ah at the 1-hour rate (50A). Internal resistance causes voltage to sag under load, reaching the cutoff voltage sooner. This effect is captured by Peukert's law, first described in 1897 for lead-acid batteries.
Chemistry Matters
Different battery chemistries produce dramatically different discharge curves. Lithium-ion (NMC) has a sloping curve that makes voltage a useful state-of-charge indicator. LiFePO4 is extremely flat — great for stable output but challenging for charge estimation. Lead-acid has a gradual slope with a steep drop-off near depletion. Each chemistry trades off energy density, cycle life, safety, and cost.
Temperature and Real-World Performance
Battery performance in the real world often disappoints compared to datasheet specifications measured at 25°C. At -10°C, a lithium-ion battery may lose 30% of its capacity. At 45°C, degradation accelerates. This simulation lets you explore how C-rate, chemistry, and temperature interact to determine actual delivered energy — the number that matters for system design.