If I buy an older EV that has lost some of its original battery capacity—let’s say it’s down to 50%—would charging it use only about 50% of the original energy and cost roughly half as much as when the battery was new?
Yep, that’s correct.
The capacity is reduced, so you’d charge roughly the amount that was discharged. With a 50% state of health (SoH), the cells are pretty worn, and there could be some small losses due to heat from higher internal resistance, but it shouldn’t be a huge factor.
I sold my Nissan Leaf when it was down to 4 battery bars. I usually got about 30 miles of range, and it took much less time to charge compared to when the battery was newer.
Jason said:
I sold my Nissan Leaf when it was down to 4 battery bars. I usually got about 30 miles of range, and it took much less time to charge compared to when the battery was newer.
Does the DC fast charging speed slow down as much as the battery capacity drops?
@Gabriel
No, DC charging speed doesn’t necessarily drop with battery health. For example, on the Leaf, the CHAdeMO port is limited to 50kW regardless of battery health. But a degraded battery with higher internal resistance might generate more heat, which can slightly affect efficiency.
If a battery’s capacity is down to 50%, it’s almost at the end of its life. As batteries degrade, their efficiency drops too, so they don’t hold energy as well, and more energy can be lost as heat, especially during fast charging.
@Imani
Actually, faster charging can sometimes lead to fewer losses because higher voltage reduces current for the same amount of energy. The most losses tend to happen with slow, low-voltage charging, like Level 1 (120V).
@Peggy
Also, with DC fast charging, you avoid the AC to DC conversion loss. When charging at home (AC), you lose around 10-15% of energy during that conversion.
A battery that’s down to 50% is basically on its way out. Most older EVs still have around 70-80% capacity left. Even though degraded batteries have slightly less efficiency due to higher internal resistance, it’s not a massive difference in most cases.
@zendaya
I used 50% as an easy example. I was mostly wondering how efficiency changes when a battery degrades.
jeff said:
@zendaya
I used 50% as an easy example. I was mostly wondering how efficiency changes when a battery degrades.
If the battery is still functional, you’ll likely see around 80% capacity or more. Charging costs mostly depend on how efficient the car is (miles per kWh), so with a degraded battery, you’ll just need to charge more often.
If a battery originally has 70 kWh capacity, it would take about 70 kWh to fully charge it (plus any losses). If it’s degraded to 50%, it would take around 35 kWh to fully charge it. But a battery degraded to 50% capacity is probably nearing the end of its life.
Think of it like a crumpled plastic bottle—it holds less energy, so it takes less energy to fill it up, and you get less out of it.
Sure, it might cost less to charge a degraded battery, but you won’t be able to drive as far, so you’ll end up charging more often. It probably won’t save you money in the long run.