EV batteries, Ah rating - Page 1
Forum section: electronics / general

author: woodchips
author_rank: Frequent Contributor
timestamp: February 22, 2022, 10:15:48 am
content: Talking to a neighbour and electric Freelander (little Land Rover) came up. Did some searching and there seems to be lots of conversion places.My question is, what batteries do EVs use? Lithium obviously, but what cell size? One thread mentioned 160Ah 3.6V lithium cells added together to give 197V. Are single lithium cells now that size? Last time I looked, some time ago, the battery packs had thousands of 16500 or similar cells, so each was only a few Ah.At 160Ah and charge currents of 1000's amps, how is the charge balancing and over/under voltage correcting done?Thanks.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 28, 2022, 01:15:30 pm
content: A typical EV pack will be around 400 V fully charged, with a typical capacity in the 50 - 250 Ah range. Most packs are constructed with the capacity divided between a number of parallel cells, but not all.First generation Tesla Model S, for example, uses 96 series banks of 74 parallel 3400 mAh 18650 cells, while the Volkswagen e-Golf uses 88 series banks of 3 parallel 36 Ah prismatic cells.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 28, 2022, 01:28:52 pm
content: It's possible to build a "single" cell of any capacity, and it's possible to parallel any number of cells, which act together like it was a single larger cell to begin with.Paralleling many smaller cells have some benefits in thermal management (larger surface area to volume ratio) and possibly manufacturing yield (smaller cells are easier to manufacture). Too small, and manufacturing costs from cell packaging go up again, though.Different EV manufacturers have settled with different sizes of cells, the variation is large, from about 3Ah to around maybe 50Ah.Any of this has no role in current rating, voltage monitoring, or balancing, whatsoever. All of this works the same regardless of using a single 160Ah cell, or 160pcs of 1Ah cells in parallel. Look at Kirchoff laws and you see how paralleled cells act: exactly like one bigger cell. Dynamic connection/disconnection of cells is never performed, the parallel connections are permanent.

author: Cerebus
author_rank: Super Contributor
timestamp: February 28, 2022, 03:19:50 pm
content: It makes little sense to talk about EV battery packs in terms of Ah. The usable voltage range is going to vary a lot between vehicles and without that information as well the Ah figures are meaningless. Much better to talk in terms of Wh for the pack, or just Joules if you want to be properly SI about it.As it is, the commonest types of EV battery packs are made up of lots and lots of little 18650 cylindrical cells wired up in both series and parallel. Some vehicles do use much larger individual prismatic cells, but the industry seems to be settling on 1860 as the commonest base cell. This makes a sort of sense, as one can see how limiting the size of individual cells can, in theory, with adequate other precautions, make for much better behaviour of the whole pack under catastrophic failure conditions. If you can limit a fire to one cell, better it be an 18650 rather than a large, multi-kilogram prismatic cell.

author: woodchips
author_rank: Frequent Contributor
timestamp: March 04, 2022, 06:50:59 pm
content: Thank you for your replies.So the packs are still made from lots and lots of small cells. Certainly easier to arrange for packaging rather than cardboard box sized cells. But, do they then still have the isolating switches? At what current?Oh well, don't think I will change my Range Rover.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 05, 2022, 09:40:09 am
content: Paralleled cells never ever have any kind of power switches.Tesla's fusible wires (I don't know if they still use them) is the furthest anyone has gone in that direction. And cells internally tend to have some kind of passive protection, like pressure vent which opens the circuit. But the whole idea is that the cell will never be reconnected.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 05, 2022, 09:52:20 am
content: Quote from: Cerebus on February 28, 2022, 03:19:50 pmPardon my ignorant intrusion, but why this specific cell of all the things?

author: wraper
author_rank: Unknown
timestamp: March 05, 2022, 10:12:40 am
content: Quote from: Cerebus on February 28, 2022, 03:19:50 pmNot really. Tesla model 3 and Y (models with the largest numbers produced) use 2170 cylindrical cells. Model S and X use 18650 but their production is way smaller. Other western EV manufacturers use prismatic cells. EV made in China including Tesla mostly use prismatic LiFePO4 cells.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 05, 2022, 10:27:15 am
content: Yes, 18650 is not "the commonest" by any means.But Tesla did build all their success around this cell type, and the reason is not completely (directly) technical. It was just that 15-10 years ago, li-ion manufacturers made their flagship products (best energy density) but also had the best price (per kWh) in this form factor. Cylindrical cells are cheap to manufacture, but that is partially because of the existing infrastructure.Technically, small cells offer superior surface area to volume ratio, enabling better cooling and possibility to stop thermal runaway.But finally when Tesla grew to being significant enough so they can design whatever they want, they concluded d=18*l=65mm istoosmall; optimum sweet spot with small enough manufacturing costs, but still the benefits regarding surface area and production yield, is something bigger such as d=21*l=70 mm.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: March 05, 2022, 11:52:57 am
content: Chevy Bolt EV uses 288 pouch-style cells in a 96x3 arrangement, 350 V pack voltage, 58 kWh. They run aluminum plates between the cells, which transfers heat to two larger plates with coolant channels. The WeberAuto Youtube channel has a teardown video showing some details. Compared to the Tesla packs, they seem more complex internally, so maybe that's why Tesla uses the cylindrical cells.

author: wraper
author_rank: Unknown
timestamp: March 05, 2022, 12:01:33 pm
content: Quote from: radar_macgyver on March 05, 2022, 11:52:57 amAlso they are prone to catch fire, therefore 100% of the Bolt ever made were recalled.