# Energy: Comparison of internal combustion engine efficiency versus EV battery packs

How many pounds of Lithium batteries do we need to replace 10 gallons of gasoline? We can calculate this out and find that we need about 1,700 pounds of Lithium-based batteries to replace about 10 gallons of fuel because of the much higher energy density of gasoline.

Modern gasoline engines have a maximum thermal efficiency of about 20% to 35% when used to power a car. In other words, even when the engine is operating at its point of maximum thermal efficiency, of the total heat energy released by the gasoline consumed, about 65-80% of total power is emitted as heat without being turned into useful work, i.e. turning the crankshaft

The battery is highly efficient. Li-ion has 99 percent charge efficiency, and the discharge loss is small. In comparison, the energy efficiency of the fuel cell is 20 to 60 percent, and the ICE is 25 to 30 percent. At optimal air intake speed and temperature, the GE90-115 on the Boeing 777 jetliner achieves an efficiency of 37 percent. The charge efficiency of a battery is connected with the ability to accept charge

Specific energy (capacity)

Compared to fossil fuel, the energy storage capability of the battery is less impressive. The energy by mass of gasoline is over 12,000Wh/kg. In contrast, a modern Li-ion battery only carries about 200Wh/kg; however the battery has the advantage of delivering energy more effectively than a thermal engine.

A kg of gasoline has about 12,000 watt hours of energy but, per the above, the internal combustion engine translates perhaps 30% or 4,000 watt hours into useful work (moving the vehicle).

A kg of a Li-Ion battery is about 200 watt hours. To achieve similar power output to 1 kg of gasoline, we need about 20 kg of Li-Ion cells. To replace the roughly ten gallon fuel tank in my compact car …

10 gallons is about 38 kg of fuel. Mulitply that by 20 and we need about 760 kg of Li-Ion battery pack. Sort of. In some scenarios, the Li-Ion pack will be recharged by regenerative braking. Otherwise, we need about 1,700 pounds of Li-Ion cells to replicate the 10 gallons of gas. This is right in line with the Tesla model 3 having a battery pack weight of 1,054 pounds (and less range than my compact car) and a specific energy of about 168 watt hours/kg.

This does not take in to account the efficiency of the electric motor converting the battery’s electricity into useful work (motion). Electric motors are typically around 90% efficient so the above “ball park” calculations are reasonable. Some new designs are up to 98% efficient.