How do you achieve this with cells in series? Do you kill an entire row and put it on "maintenance voltage"? I know how balance charging RC batteries work but they're smaller scale and "single row".

Also in RC car world it's generally preferred to have one cell per voltage step, I've had way more dual-cell(per series) packs fail than single ones. Though my experience is only 6s/~22v but it's "the same shit on a bigger scale" as far as I can comprehend.

No EV that I'm aware of has the ability to bypass a single cell in a series string.

I have often wondered if it would be worth designing an EV battery that can permanently short out a bad cell in a string, perhaps by deliberately disabling balancing, letting the bad cells voltage fall to zero, and then perhaps having a single use 'bypass' that latches on.

It wouldn't be a seamless user experience, because if you discharge the cell to say 0.5 volts but then the user tries to charge their car, you can't let them, since you cannot safely charge a lithium cell which has fallen below the minimum voltage, but you also cannot bypass it till the voltage falls to zero. Could be done automatically at 3am though like system updates.

I guess you can put a relais/switch in for each cell in a series but then you need to account for voltage differences when taking them out. Either by over provisioning within the series and rotating in different cells. Or by have other strings take up the slack.

Either way you need some form of overbooking / compensating capacity.

A relay would allow you to switch it out and then back in again. Which you don't need. Just a fusible link that can be blown to permanently disconnect the battery from the string might be simpler and more ideal for the application.

Nearly all EV's today would run just fine on 3.7 volts fewer.

My car's high voltage circuitry seems to work down to about half of the nominal voltage.

Is the failure mode of whatever chemistry is in an EV that they just conduct electricity? Li-Po usually fail in more spectacular ways than so.

There's NCA (drones), NMC (laptops, phones, many electric cars), LFP (stationary/grid), and LFMP (many new electric cars; slightly more expensive but higher current variant of LFP).

Yes, this happens sometimes.

Search this page for "PTC": https://www.electricbike.com/inside-18650-cell/

The PTC protects the rest of the battery if a single cell internally fails short.

Why can you not bypass until the voltage falls to zero? I'm with you aside from that.

If you just close the bypass switch, a large current will flow out of the (dying) cell, making a lot of heat.

You could have a two-way bypass, disconnecting the original cell, but that would cost more. Remember the bypass switch is duplicated for every series cell group (hundreds) and must carry the whole battery current.

Or you could have some kind of slow drain resistor - but then you're back to the time issue.

Okay, I understand. I was definitely imagining some sort of latching switch, that would connect to the bypass instead of the cell. Makes sense that would be more expensive.

I don’t know to be honest, I just know that you can find second hand car batteries that have been used for 15/20 years that still have 40% charge and I don’t know of any single phone battery that’d be the same. I could be talking out of my ass but that’s what I thought.

> How do you achieve this with cells in series?

You don't, if one cell fail you shut off the whole array of cells that is in series. But a pack has several arrays of cells.