At what point does the advantage of having more battery packs in parallel significantly diminish

[Heyzeus]

I'm not an electrical engineer or hobbyist, so those more in the know, please correct me if I get any of this wrong. My understanding from reading posts on these boards is that having more battery packs in parallel reduces the voltage sag when under load, heavy acceleration, or hitting bumps or other things that suddenly spike the amount of current needed by the motor and thus reduces the risk of a cut out.

So my interpretation of what I've read is that the more battery packs in parallel you have, the safer it is (all else the same and assuming reliability in other parts), 3p is better than 2p, 4p better than 3p, 6p better than 4p, etc...

I've seen some people comment that the new king song s18 battery capacity is low(1100wh) and is unfortunately only 3P.  This makes me wonder, given that it's going to be a lower range wheel than others, would people rather just consider it to be a low range commuter/fun wheel and sacrifice more battery capacity in exchange for more packs in parallel.

Ie: if they could make the s18 6p and 800wh instead of 3p and 1100wh would people consider that a worthwhile trade off.

Or with the veteran Sherman, how many people are truly going to want 128 miles of range(I'm sure some hair will love it, but in general), I don't know how many parallel packs it has but let's say it has 6P, if they could make it 10p but it reduced the theoretical range to 90 miles, would people find that is a worthwhile trade off.

So basically:

At which point do you stop facing serious gains by increasing the battery packs in parallel, ie: at what point does increasing the number of parallel packs stop significantly decreasing voltage sag when under load.

And

Would people be willing to highly reduce range or potentially increased safety

 

If I've got all this wrong and this is a dumb post let me know but it's something I've been wondering based off my information (misinformation?)

 

[mrelwood]

Since the 1600Wh MSX with 18650 cells is already 6p, Sherman with 18650 cells must be something like 10p.

 The way I see it, the answer to your question depends completely on personal riding style. For example, today we did a long trip with a local meet group with a top speed of somewhere between 25 and 30 km/h. Despite all of us having 45km/h or faster wheels. For such peaceful cruising the voltage drop even of a 2p wheel just wouldn’t be a relevant concern by any measure.

 If we have only fast riders doing the trip, we ride much more aggressively and peak at around 50km/h. That’s when the voltage sag becomes relevant. The members of this group would probably exchange a little range for less sag.

 Personally I tend to ride semi aggressively until about 50% battery, after which I start to tone it down. If voltage drop wasn’t a concern, I would probably keep going harder further down the battery. For me there must be a reasonable balance between sag and range.

Edited June 14, 2020 by mrelwood

[Chriull]

1 hour ago, Heyzeus said:

would people rather just consider it to be a low range commuter/fun wheel and sacrifice more battery capacity in exchange for more packs in parallel.

Ie: if they could make the s18 6p and 800wh instead of 3p and 1100wh would people consider that a worthwhile trade off

(Most probably) No. The used 21700 cells have much lower internal resistance as the 18650. ?about half? So 6p 18650 versus 3p 21700 won't really change anything - at least according to the specs from the datasheet.

But internal resistance of li ion cells is afaik quite "dynamic".

21700 are quite new and not too much is known on their "behaviour" in EUCs by now...

Driving at 50 km/h needs much power - so the 1100 Wh will be eaten up relatively fast. Imho noone would really prefer less capacity....

1 hour ago, Heyzeus said:

So basically:

At which point do you stop facing serious gains by increasing the battery packs in parallel, ie: at what point does increasing the number of parallel packs stop significantly decreasing voltage sag when under load.

Its a parallel circuit of the internal resistance - doubling the paralleled cells halves the resistance. So going from 2p to 4p is a huge step. From 4p to 6 or 8p a big step. And the further one goes the less are the gains.

In series with the batteries internal resistance is the motor coils ohmic resistance roughly in the range of some 0.1-0.3Ohms - so this relativizes gains of paralleling cells even more.

1 hour ago, Heyzeus said:

And

Would people be willing to highly reduce range or potentially increased safety

As with your above example one cannot increase one on the cost of another.

If one compares 18650 vs 21700 with same capacity they have about the same characteristics. Now used lg 21700 m50t have a bit less capacity per volume and a bit less internal resistance (according to the datasheet). But they have notable less continous current delivery capability.

One will know more (or something definitly at all) once more comparisons/reports of 21700 wheels are available.

[Ben Kim]

8 hours ago, Heyzeus said:

I'm not an electrical engineer or hobbyist, so those more in the know, please correct me if I get any of this wrong. My understanding from reading posts on these boards is that having more battery packs in parallel reduces the voltage sag when under load, heavy acceleration, or hitting bumps or other things that suddenly spike the amount of current needed by the motor and thus reduces the risk of a cut out.

So my interpretation of what I've read is that the more battery packs in parallel you have, the safer it is (all else the same and assuming reliability in other parts), 3p is better than 2p, 4p better than 3p, 6p better than 4p, etc...

I've seen some people comment that the new king song s18 battery capacity is low(1100wh) and is unfortunately only 3P.  This makes me wonder, given that it's going to be a lower range wheel than others, would people rather just consider it to be a low range commuter/fun wheel and sacrifice more battery capacity in exchange for more packs in parallel.

Ie: if they could make the s18 6p and 800wh instead of 3p and 1100wh would people consider that a worthwhile trade off.

Or with the veteran Sherman, how many people are truly going to want 128 miles of range(I'm sure some hair will love it, but in general), I don't know how many parallel packs it has but let's say it has 6P, if they could make it 10p but it reduced the theoretical range to 90 miles, would people find that is a worthwhile trade off.

So basically:

At which point do you stop facing serious gains by increasing the battery packs in parallel, ie: at what point does increasing the number of parallel packs stop significantly decreasing voltage sag when under load.

And

Would people be willing to highly reduce range or potentially increased safety

 

If I've got all this wrong and this is a dumb post let me know but it's something I've been wondering based off my information (misinformation?)

 

There are multiple benefits to having more cells in parallel:

- Higher total discharge rate (4p = 40A max semi-sustained discharge rate, 10p = 100A)
- Lower overall resistance within the battery packs e.g. lower voltage sag
- More range (obviously)
- Due to overall lower draw per cell, they will run cooler and have marginally higher overall discharge capacity than a lower number of cells in parallel).

And you missed one important point,

It's not all about total range when it comes to having so many batteries in the wheel.  It's also about how long you can sustain higher speeds deeper into the battery cycle. 

There is never much harm (aside from additional weight) to adding more parallel to a battery setup.  

Edited June 15, 2020 by Ben Kim

[tudordewolf]

A larger battery also means fewer charge cycles over time, and/or lower depth of discharge between charges, both of which will increase the long-term performance of the battery.

The only time I'd consider too much battery a con is portability; on an ultralight wheel like the i5, being able to walk around with it like a briefcase without significant fatigue lets it fill use-case scenarios that an Msuper simply couldn't; but for wheels with walking handles, which is every flagship by now, the point is moot.

There's one more advantage to oversize battery packs, although it's relatively niche - the ability to use faster chargers. I've caught myself on low battery needing to go somewhere, and the option to top up 20 minutes of ride time with a 20 minute charge only applies to wheels that can absorb 4+ amp charge rates, for which you'd probably want at least a 4p battery.

 

 

Edited June 15, 2020 by tudordewolf

[Ben Kim]

1 hour ago, tudordewolf said:

A larger battery also means fewer charge cycles over time, and/or lower depth of discharge between charges, both of which will increase the long-term performance of the battery.

The only time I'd consider too much battery a con is portability; on an ultralight wheel like the i5, being able to walk around with it like a briefcase without significant fatigue lets it fill use-case scenarios that an Msuper simply couldn't; but for wheels with walking handles, which is every flagship by now, the point is moot.

There's one more advantage to oversize battery packs, although it's relatively niche - the ability to use faster chargers. I've caught myself on low battery needing to go somewhere, and the option to top up 20 minutes of ride time with a 20 minute charge only applies to wheels that can absorb 4+ amp charge rates, for which you'd probably want at least a 4p battery.

 

 

Figure out the total Ah of the battery, cut it in half, voila “fast” charger. I use a 3A charger to charge my Mten3 when I’m out; my larger wheels typically have enough range where I don’t need to carry a charger.