Hacker News Reader

Comment by ilyt | Hacker News Reader

UAVs also have higher current requirement, and that means more weight "wasted" for chonkier electrodes Car batteries aren't pulling 50C worth of current

View on HN

Don't car batteries also have very high current requirements? Turning over an engine takes an enormous amount of power.

GP is probably talking about EV main batteries, not the lead-acid 12v accessory/starter batteries.

Most batteries that run starters are not energy dense, they're typically standard lead acid batteries.

FWIW, to provide the 225 amps (for a V8 starter motor) a Tesla car battery would only need a discharge capability of 3C (1C being around 80 amps), which is within its rated capabilities. This is also for batteries which provide higher voltages, so I'm vastly overestimating the C rate required.

C is the unit for charge/discharge rates, and is based off the capacity of the battery.

Sure but it's not about starter motors - these batteries power 400kW motors, that's a lot of power.

That's why how much power an EV has is closely related to the size of the battery. 400kW from a 75kWh battery is a little over 5C.

Honest question, though: does "Turning over an engine" (as an earlier commenter typed) have actual meaning for an EV?

No, it is referring to what the starter motor in an ICE vehicle does. It was most likely a misunderstanding, but it is more similar in that it pulls very high loads like a drone battery does.

Traditional ICE starter batteries are optimized for this cold-cranking power rating, but they only have to deliver this for a matter of seconds before being recharged. They are not designed to deliver this continuously nor to ever be operated at low states of charge.

Conversely, a BEV traction battery has to support a wider range of loads at any charge state between its minimum and maximum charge levels, in order to have decent driving range. Like a starter motor, the BEV is not going to sustain high power output for very long, since a car only takes seconds to accelerate to legal road speeds. After that, it requires continuous output at lower power levels to maintain a cruising speed.

Even with lead-acid batteries, there are regular starter batteries and then there are deep-cycle batteries which have far less cold cranking amps but more durability when depleted to low charge states before being recharged.

The low density of lead-acid batteries is what makes them unsuitable for mobile applications. They might have 30-50 Wh/kg while various lithium ions might be 100-300 Wh/kg. And now this announcement is talking about 500 Wh/kg so 10x the best lead-acid batteries...

This person seems to be getting downvoted but they aren't totally wrong, electric car batteries are massively paralleled so that the amperage draw per cell is kept to a reasonable figure. They are confused about cranking amps to turn over an ICE vs. batteries used in all-electric cars.

The 18650 Panasonic cells used in an older Tesla model S for instance are rated at only 10A draw per cell as their nominal 1S voltage (4.20V when full).

Starter current is ~150-200A at 12V.

At say 30C you'd only need 7Ah lithium batery

Reply on news.ycombinator.com