21s for next battery design

I have already started designing the layout of a "next generation" battery for my eZuma project. Currently my bike is using 20s17p, which is just adequate but it fit without any issues in the existing battery box. I'm aiming for 21s26p which will require some mods to my center stand and supports to fit the larger size pack. I plan on limiting charging the 21s pack to only 84V which is 4V max per cell. The goal being to never have to worry about overcharging the pack and presumably extending the life of the pack as well. Another goal is to maintain all my existing "72V" accessories as is. I "believe" there are smart BMSs out there that will balance a 21s pack properly @ 84V if you program it to max out at 84V (e.g. via a Xiaoxiang app).

One example appears to be: https://www.aliexpress.com/item/32827394534.html

Just looking for confirmation.

Remember just spec'ing a stop-charge voltage on its own means next to nothing about the resulting SoC / range, much less longevity or safety.

It is easy to overcharge at 4.0V, just keep holding CV / Absorb stage until amps accepted drops to zero.

And C-rate is a critical factor, if not holding Absorb at all (CC-only, "charge to xV and stop") then going to a much higher voltage at a high rate, is safer than stopping at a lower voltage when current is very low.

In fact at low enough currents, you can't even determine a safe terminate-charge point, need to resort to using CEF and mAh counting to determine when to stop.

But in reality, charging at normal healthy rates like 0.2-4C, using CC-only stop points, 4.00Vpc is giving up too much range for no benefit.

4.05V is very safe already, and really 4.10V is fine especially at the higher amps range. Above 0.5C rates going to 4.15Vpc is fine, long as no CV/Absorb is held by the charge source.

Of course, the above is just talking about the cells, ignoring the other components.

If you are relying on a balancing method / device with a low balance rate, that only starts balancing near the top of your voltage range,

as is the case for most "balancing BMS",

that may force you to lengthen AHT in order to get to your "balancing finished" spec,

requiring adjustments to the voltage and/or current parts of the charge profile.

john61ct said:

... 4.00Vpc is giving up too much range for no benefit...

Click to expand...

I'm not understanding the above statement. I currently charge my 20s pack at 4.1V. And the plan is to charge a 21s pack at 4.0V. That gives 82V and 84V respectively. Not seeing how I would be "losing range" if those are the two choices to compare against each other (all other things being equal)?

Additionally, I would be using a "72V" charger for both. Doesn't the CC/CV curve derive from the charger and not the BMS? In which case the curve will be ideal for both. Or am I missing something?

M