18650 or similar for EV conversions?

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OzSpider
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18650 or similar for EV conversions?

Post by OzSpider »

Hi, as a noob, I'm a bit surprised this community doesn't use the 18650 / 21700 form factors for car conversions more often. Noting @jonescg is building something special as I type this....

What's the main driver for selecting LiFePO or similar over cylindrical cells?

Cost? Energy Density? Safety? Or just extra work?
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jonescg
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Re: 18650 or similar for EV conversions?

Post by jonescg »

You listed most of them I think 🙂. The cells are actually well priced on a per kWh basis, but the main drawback is complexity and difficulty ensuring a reliable contact between the cell end and the busbar. Spot welds are OK but not the greatest.

Energy density is often as good as it gets. 250 Wh/kg at the cell level and 220 Wh/kg at the module level is outstanding. But they are high impedance cells so they get hot, and heat is the enemy. So a detailed cooling option needs to be used.

Otherwise they can be customised to suit almost any space, if you have the patience. I'll keep you all posted.
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brendon_m
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Re: 18650 or similar for EV conversions?

Post by brendon_m »

Extra work is what drove me away from them. Same reason I avoided pouch cells.
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Re: 18650 or similar for EV conversions?

Post by jonescg »

The other problem with pouch cells is they are very fragile. They need to be held snugly in position without any opportunity to fret or rub against another surface. Also, the cut edges of the foils are prone to corrosion, and serve as a time-delay failure if they aren't well protected from moisture and air. I'm prettymuch of the the mind now that pouch cells have their place in high power applications. While high energy pouch cells can be had with respectable C-rates, the same can be achieved with cylindrical or even prismatic cells for much less hassle. Cylindrical cells are the best for high energy density and low power - which is EVs for the most part.

I'm actually thinking of building the CRX conversion as a high energy cylindrical pack, but with a portable high discharge LiPo pack as a 'booster' for silly shenanigans.
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OzSpider
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Re: 18650 or similar for EV conversions?

Post by OzSpider »

Does anyone make an 18650 module kit for liquid cooled modules? Smaller bricks than a full Tesla module? More like the LG Chem size for better options to tuck cells into small spaces.
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Re: 18650 or similar for EV conversions?

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Re: 18650 or similar for EV conversions?

Post by coulomb »

I've never understood the attraction of thousands of used 18650s for power walls. The chances of avoiding a fire, or at least disappointing performance, seems negligible to me.

However, arrays of new, known-good quality cells is a different situation. Still much more work than I'm personally willing to put into a conversion, but I think it could work in some situations.
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OzSpider
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Re: 18650 or similar for EV conversions?

Post by OzSpider »

Yeah I did see those HBPowerwall carnage clips. Very hard to work out what's gone wrong with the limited info he has. 2nd hand unknown cells could be part of the answer. Agree not a good option to use odd cells for an EV.

Guess the chemistry carries an inherent higher risk of disaster. Does active cooling do enough to negate that risk?
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jonescg
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Re: 18650 or similar for EV conversions?

Post by jonescg »

It's a combination of things really. Starting with uniform cells (same capacity, same DCIR) is the main thing, and maintaining a good state of balance throughout the life of the battery too. Heat generation from the cells themselves is about half the damage, and heat from the environment is the other half. You can't really do much about the car parked on the street at 40°C but you can keep it cool when in operation and feasibly while charging.

Poor interconnects is another source of failure modes, so trying to ensure all the connections are solid is a good move. As Peter said in his videos, the packs were well made so it was probably a dud cell or three. Individual fusing isn't going to prevent a cell with a high DCIR and/or lower than average capacity from getting hot. The risk of an internal short is very, very low.

But generally, high temperature, high states of charge and long periods of time in either, or both of these states, is the best way to ensure a short cell life, and potentially a spectacular end.
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Re: 18650 or similar for EV conversions?

Post by T1 Terry »

The LTO 66160 Linlong cylindrical cells look promising. The power to weight isn't that great but the ability to high rate charge and discharge yet retain a long life and the mechanical screw tight contact each end look promising for enclosing them in Lexon with a seal on each stud and liquid cooling them ..... just need the leaves to grow back on the money tree ....

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