Pros and cons of 21700 batteries

[Tawpie]

44 minutes ago, mike_bike_kite said:

create 50% more power

They store more power per cell, how quickly they deliver the stored power and how warm they get in the process of converting and delivering that power is an independent  characteristic.

[Planemo]

Agreed. 21700's don't provide 50% more current than a 18650. And the point regarding heating is simply comparing surface area for a given amount of potential Wh. 18650's will generally ventilate better simply because there will be more airspace around them for a given size Wh pack. But no, we should not be reaching critical temps for either type of battery either during riding or charging.

As has been said many times before, there are no benefits to 21700 other than cost and reduced number of interconnects.

[mike_bike_kite]

23 minutes ago, Planemo said:

Agreed. 21700's don't provide 50% more current than a 18650. And the point regarding heating is simply comparing surface area for a given amount of potential Wh. 18650's will generally ventilate better simply because there will be more airspace around them for a given size Wh pack. But no, we should not be reaching critical temps for either type of battery either during riding or charging.

As has been said many times before, there are no benefits to 21700 other than cost and reduced number of interconnects.

I still can't see how they ventilate better if the batteries are all wrapped in shrink wrapped plastic. Should we be asking manufacturers to put cooling tubes through these packs or do they really not heat up that much?

One difference for me was the charge time, the 21700 1800Wh Nikola charges in 6 hours while the previous 18650 1600Wh Nikola charges in 13.5 hours. That means I don't have to leave it charging overnight.

[Tawpie]

The pack itself shouldn't heat up much, but I wouldn't expect that any manufacturer monitors pack temperature directly anyway. The cost/benefit of monitoring doesn't work out if the design and construction of the pack's thermal management has enough margin to ensure acceptable temperatures in the expected operating environments.

When charging, if any single Li ion cell goes up 10°C (I think that's the number) it's a bad sign and you should investigate. In use, rapid discharge will cause the cells to heat more quickly than they do while charging and here we have to put our faith in the pack designers to have properly accounted for the self heating. They have many (infinite really) options to deal with heat; cell-to-cell spacing, airflow, external heat sinking, cell arrangement, cell size, cell shape, heat pipes, refrigeration, current limiting, pack voltage, the list goes on and on. For the most part they'll get it right enough, but they will try to get as close to the limits as they dare because over design costs money and may make the product so expensive nobody will buy it. Some like to live right on the verge, others prefer to stand well back.

 

Edited January 14, 2021 by Tawpie

[Planemo]

36 minutes ago, mike_bike_kite said:

I still can't see how they ventilate better if the batteries are all wrapped in shrink wrapped plastic. Should we be asking manufacturers to put cooling tubes through these packs or do they really not heat up that much?

You're correct to a point. Having the whole lot encapsulated doesn't make much odds really, but a load of 18650's will still dissipate heat better than a load of 21700's simply because theres more airspace around 18650's and therefore less 'shoulder rubbing' with other cells. It's a moot point anyway as our packs shouldn't be getting that hot to matter either way.

36 minutes ago, mike_bike_kite said:

One difference for me was the charge time, the 21700 1800Wh Nikola charges in 6 hours while the previous 18650 1600Wh Nikola charges in 13.5 hours. That means I don't have to leave it charging overnight.

I suspect that's everything to do with the charger spec, BMS or the capabilities of the wiring rather than the actual choice of cells used. I charge my MSX (18650) 1860Wh at 6A which takes about 4 hours.