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Author Topic: Swappable "top-off" battery pack?  (Read 752 times)

godot

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Swappable "top-off" battery pack?
« on: August 12, 2021, 10:54:40 PM »

Is it technically feasible to build an e-motorcycle that has a standard-sized built-in battery, but also has a provision for a "top-off" battery in a "glove-box" or similar small compartment?  That way you could:

1.  Have a spare "top-off" battery(s) charging at home while you are using another to ride;

2.  Prolong the life of your built-in battery by avoiding over-exercising the "top-off" last few % of the charge over and over again;

3.  Have a manageable-sized battery to carry into someone's house to charge while traveling;

4.  Have the option to have a charged "top-off" battery brought to you in emergencies.

Thank you.


P.S.  Point of order: 
I just discovered this forum, and this is the first time I have used it.  I looked around, and this seems to be the closest approximation to a category in which to discuss design and architecture issues, so I placed this long-time question I have had for years here.  If I have put a design question in the wrong place, and there is a more fitting category for me to discuss design and architecture issues, please tell me, and I will be happy to move my questions there.  (Thank you.)
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TheRan

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Re: Swappable "top-off" battery pack?
« Reply #1 on: August 13, 2021, 01:56:23 AM »

There are bikes with two batteries like the Zero FX/S modular and some Super Soco and Niu models that can run on just one battery as well. They are however equally sized batteries. Having two differently sized batteries won't work too well as they will have different properties and react differently under load.

Also when a battery's state of charge decreases it's not like using up fuel in a gas bike where you could just split it up among multiple tanks, the charge percentage simply refers to the voltage of the battery. If we take a Zero battery as an example it's 100% at around 116v and 0% closer to 96v. So to achieve the "top off" method that you describe would mean reducing the voltage of the main battery and then having a smaller battery with an even lower voltage and then running those in series to get the 100% voltage (for example you could have a 92v main battery and a 24v "top off" battery). As mentioned above these two vastly different batteries wouldn't work well together, and in the bikes with two batteries they're both full voltage and then run in parallel for double the capacity.
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Crissa

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Re: Swappable "top-off" battery pack?
« Reply #2 on: August 13, 2021, 06:23:46 AM »

Batteries don't like to sit around charged, either.

And if you're thinking you could disconnect one battery and use an auxiliary battery, well, then it would have reduced power because that's also based upon the size of the battery and its ability to deal with voltage slump under high current draw.

-Crissa
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2014 Zero S ZF8.5

godot

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Re: Swappable "top-off" battery pack?
« Reply #3 on: August 13, 2021, 08:44:01 PM »

There are many ways to connect batteries and cells.  One generally uses series connection to achieve increased operating voltage, and parallel-like connections to increase capacity. 

There are many electrical means to interconnect disparate batteries.  I have calculated the power losses which would be due to, for example, diode isolation of series strands, and they are minuscule:  Power = V^2/R, so square-law applies.  E.g. 0.7V^2 is small in comparison to (as in your example) 116V^2.

What I am suggesting is that, due to the way Li-ion batteries work, it is advantageous to avoid charging and discharging the same region of the capacity of the cell repeatedly.  People tend to think the battery does not have a reaction surface, because it changes ("rebuilds itself") from one charge cycle to the next, but the cells do have cathode and anode reaction sites where the ion activity occurs:  Each cell forms its own unique fractal reaction site as charging progresses.  But that portion of the reaction  'forming reaction site which corresponds to a particular charge percentage of the cell'  is formed and reformed from the same material which is located in a particular physical location of the cell.  Hence the materials become fatigued by being repeatedly formed into fractal electrode during charge, and changing back essentially into part of the electrolyte environment as the cell discharges.  A particular "clump" of material can only be formed and reformed a few thousand times before it becomes fatigued.

Hence, it would make financial sense to prevent "topping off" the same region of your large expensive battery over and over again, and, instead, charge and recharge a "throw-away" consumable smaller battery.

However, your point about power being dependent upon the aggregate surface area of the reaction site available ("size of the battery") is well taken. 

In his biography, Musk claims that Lithium batteries are an enabling e-vehicle technology not due to their energy:mass or volume ratio, but due to their ability to deliver huge currents quickly and for prolonged periods.  A battery's internal resistance is the sum of 2 components, one of which is called the "ion resistance."  In other battery technologies, apparently the ion densities in the electrolyte surrounding the electrodes is quickly depleted, and the chemical reactions to replenish the ions produce ions at a slower rate than the ones that had built-up around the electrodes as the battery sat idle while not under load.  (It's very much like capacitance.)  The depletion of the ions in the region near the electrodes then shows up electronically as an increase in the ion-resistance component of the internal resistance of the battery which grows while the battery is under load.  At least that's my understanding.

I was unaware that the the entire aggregate surface area of the batteries' electrodes are always "in play."  I.e. that there was not more current available than required when you max out the throttle.  What I think you are telling me is that the parallelism of the battery (2 dimensions of its size -- effective electrode area, the 3rd dimension being proportional to voltage) all contribute to the "off-the-line" acceleration performance of an e-bike. 

I was unaware that that is the case, and I thank you for your time and effort in explaining that to a newbie like me.

It is interesting to me that the small Zeroes, for example, have the smaller batteries.  I thought that was due to the reduced requirement for energy storage.  But what you are telling me is that the size of battery determines the max "short-circuit" current (power) available, and that, in turn, determines the torque and thrust available.  So the size of the battery determines the mass of the bike in can accelerate at a given performance level, versus NOT BEING the size of the bike determining the size of battery it can carry, and the energy it requires to transport the larger bike for reasonable range, as I thought.  It follows, then, that only a smaller bike can be run at reasonable performance from a smaller battery.  I was under the mistaken impression that a larger (more massive) bike could be run from a smaller battery, but only for a shorter distance and time.

I'll have to think about this and run some numbers.  You have aided my conception of the boundary conditions of the problem immensely.  Thanks, again!
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Crissa

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Re: Swappable "top-off" battery pack?
« Reply #4 on: August 14, 2021, 01:44:51 AM »

There's a reason Tesla and Zero do not repair only half of a vehicle's battery pack.

Those tiny differences in resistance add up badly during sustained discharge.

-Crissa
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2014 Zero S ZF8.5
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