State of Charge

What it is

State of Charge is the battery’s fuel gauge - expressed as a percentage from 0% to 100%. At 100% SoC the battery is fully charged. At 0% the car stops (though “0%” on the dashboard doesn’t mean the cells are literally empty - more on that below).

The percentage shown to the driver reflects usable capacity, not total cell capacity. Battery management systems deliberately hide a buffer at both ends of the pack.

The buffers you can’t see

EV manufacturers reserve capacity at the top and bottom of every pack. A battery that shows 100% on the dash might only be charged to 95% of the cells’ true maximum. A battery that shows 0% and won’t drive any further still has a few percent left - enough to avoid damaging the cells through deep discharge.

These buffers vary by manufacturer but the approach is consistent across the industry. They exist because lithium-ion chemistry degrades faster at very high and very low states of charge. Keeping cells between roughly 10% and 90% of their absolute capacity extends pack longevity significantly.

Why most owners charge to 80%

Daily charging to 100% is fine for most modern EVs occasionally - specifically the night before a long trip. But doing it every night accelerates degradation at the top of the charge curve.

The 80% recommendation you’ll see in most owner manuals and on most charging apps comes from the electrochemistry: the last 20% of a charge is where lithium plating risk increases and where cells spend more time at elevated voltage. Over hundreds of cycles, that adds up.

Practically speaking, 80% of a 75 kWh battery is 60 kWh - enough for 350–400 km of combined driving in most conditions. For the majority of daily use in Australia, most people never need more than 50% charged.

Charge limits in practice

Most EVs let you set a charge limit directly in the car’s interface or the companion app. Tesla calls it “Daily” vs “Trip” limit. Hyundai and Kia let you set it by percentage. The car simply stops charging when it hits the limit, whether you’re at a home wallbox or a public AC charger.

DC fast chargers at public stations automatically taper the charge rate as SoC rises above 80%. That’s why the jump from 10% to 80% is much faster than 80% to 100% - it’s not just the last 20% that’s slow, it’s an exponential slowdown that starts around 70–75% on most vehicles.

SoC and trip planning

For trip planning purposes, the practical window is roughly 20% to 80%. That’s the band where charging is fast, degradation risk is low, and you’re not caught out by the uncertainty that comes with estimating range at the bottom end of the battery.

If you’re departing on a long drive, charging to 100% the night before makes sense. Arriving at a destination with 20% remaining and plugging in overnight is a reasonable target - it means you haven’t over-stressed the pack but also haven’t pushed the range anxiety buttons by getting too low.