Battery Question_2 - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: October 19, 2021, 05:43:19 pm
content: Hello guys the meaning of underlined lines is not clear to me. What I understand is that a battery car with a high energy density will cover long distances at low speed. Conversely, a battery car with a high relative capacity will cover short distances at high speed. Can anyone tell me whether I am right or not? What is the reason for batteries with high specific power usually have low energy density?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 19, 2021, 05:57:00 pm
content: What it is saying that high energy density batteries (batteries that are physically small for the amount of energy stored) tend to not be able to deliver their power quickly. A car with a small battery might not be able to accelerate quickly (high power delivery) in spite of being able to go a long ways.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 19, 2021, 06:47:38 pm
content: The last highlighted sentence is a bit confusing.It's all about optimizing the tiny details in cell manufacturing.A manufacturer can make a so-calledenergy cell, which has energy density like 300Wh/kg and cost of $150/kWh. But it can be discharged at some 1C max, meaning it must be discharged during one hour or longer. Short peaks of higher current are of course acceptable.Or, they can make apower cell, which has energy density of just 250Wh/kg and cost of $200/kWh, so it's inferior in energy storage. But it can be discharged at 4C max, meaning you can run it flat in 15 minutes; peaks even higher.Those power cell are used in power tools like drills, and maybe in hybrid EVs because the capacity is so small yet you need quite some power in acceleration so therelativecurrent to capacity is high.Today's full electric vehicles are not very "high power" in this regard. If you have 250km of driving range, maybe you discharge it in two hours, i.e., 0.5C discharge rate. This is well within the capabilities of classical "energy cells". This doesn't matter though because EVs are such large business today that the battery manufacturer will micro-optimize the cells for exactly what the car manufacturer wants, so it makes no sense to group the cells in the two strict categories.But it helps understanding the literature to know about those two classic categories, and also understand there is no large fundamental difference, just manufacturing details. At cell chemistry level, high power cells prefer to have more active surface area so that reactions can happen faster, at the expense of having less "bulk" material. The materials and chemicals might be exactly the same, or there can be some small "secret sauce" differences I have no idea about.

author: Daixiwen
author_rank: Frequent Contributor
timestamp: October 20, 2021, 07:15:17 am
content: The high power Li-Ion cells I used to work with used the same chemistry as the regular ones, but had thicker electrodes and more connections between the electrodes and the casing in order to reduce the internal resistance. This is in addition to a higher surface area as Siwastaja says.