this post was submitted on 06 Oct 2023
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The problem with replacing individual cells is you'll end up with cells with different wear levels which means different discharge rates and capacities (max voltage). The battery management system can mitigate some of the problems that arise but can only do that while chargering. While the battery is in use or sits for an extented period of time the cells will try to level out the voltage difference causing loss in capacity and a potential fire risk if the cells are too out of balanced.
Replacing the battery with new better batteries isn't really an option either as any significant increase in power density would be from a change in battery chemistry and that would also require changing out the on board charging circuit and related systems. There's also the issue that the majority of charge points providers state in their TOS that the use of any aftermarket parts or modifications to your battery or charger is forbidden so you are essential blacklisted from using any DC fast charger.
That's why I mentioned pairing cells from multiple used battery packs... This isn't magic, it's numbers... You pair like with like, and by removing the underperforming cells, you get rid of the dead cells bringing down the whole bank because they can't hold a charge. The fire risk is also greatly overstated... Batteries and management circuits have come a long way. You could swap in new cells and it would be fine... It would just be a waste, the new cells would degrade faster than their spec sheet implies.
And as far as matching like with like, you can build a lithium cell tester at home to profile each cell for a few dozen dollars... Literal hobbiest level stuff
And you most definitely can upgrade with newer, better batteries. There's three numbers - energy density, voltage, and discharge rate. You can upgrade the density to the moon, so long as the other two match it's a drop in replacement... And these three things are a tradeoff when you build the battery, so a better battery bank with the same output and voltage is just going to make you car run further, easy as that.
Yeah, TOS might stand in your way, but that's economic pressures, not an engineering issue. We could entirely solve that problem through force of law. On the topic, let me take this opportunity to promote right to repair - companies are going to feed us a lot of bullshit reasons why we need to throw it all away and replace everything fresh... They have every incentive to make us believe that. It's less work for them to say "that's not an approved use", they make far more money if they convince us they are the only ones that can fix it... it's an economic alignment problem, the engineering solution is well understood