Monday, 6 April 2020

should I fully charge my scooter or not?

This is an often asked question on forums and results in much discussion, many seem obsessed with preserving their battery on their new scoot. Makes sense, I am interested in not harming mine also.



First, the Short answer:

To me the simple answer is: YES you should.

Details

The devil is always in the details right?
First and foremost you should always examine any answer and check it for veracity. If nothing else I expect anything I write to be a good jumping point for your further research ... if you have doubts.

Now lets examine the proposition here because it relies on people assuming that the makers are stupid (and some unknown twit on the internet is a genius) and then having just enough knowledge to be dangerous. Perhaps there is also some problems here in that some of these self assessed geniuses may be best described in this blog post, but I digress.

Battery and Cell are used interchangably in common language, yet the dictionary is clear:


There in lies an important difference and one which is probably the root of many misunderstandings.

The Cell

The problem (and why people say you shouldn't charge your batteries fully) is based in this often cited Battery University article where discussion centers on cells (that's individual cells and not even restricted to 18650 cells that our scooter packs are currently composed of) were stress tested and a finding was published.  It starts by showing the results of high discharge rates of a particular chemistry (of which there are a few that continue to evolve).

Now, lets look at what they wrote about this:
Figure 1 illustrates the capacity drop of 11 Li-polymer batteries that have been cycled at a Cadex laboratory. The 1,500mAh pouch cells for mobile phones were first charged at a current of 1,500mA (1C) to 4.20V/cell and then allowed to saturate to 0.05C (75mA) as part of the full charge saturation. The batteries were then discharged at 1,500mA to 3.0V/cell, and the cycle was repeated. The expected capacity loss of Li-ion batteries was uniform over the delivered 250 cycles and the batteries performed as expected.

The graph Fig 1 is as below


To get more than 4v out of a cell we arrange cells in strings (called series) to go to higher voltages. The voltage is simply just the addition of how many cells you make in the series. Like this

Now the reality of our battery packs is that they are not strings of single cells because each cell only has about 2 ~ 3 Ah, while we'd often like more like more than ten. So they are in fact composed of strings of bundles of cells:

  1. parallel bundles of cells to increase the available Ah (typically 3, 4 and 6 2,200mAh cells giving 6.6, 8.8 and 13.2Ah)
  2. these parallel bundles are arranged in a string (seen above called Series) to increase the available Voltage, such strings are often 10, 13 and 16 (36V, 48V and 60V) because the "nominal" volatage of a cell is usually given at around 3.7
This is normally given in an abbreviated form like 10S3P or 13S6P or something like that depending on your specific battery (I have 3 scooters with 3 different battery configurations).

Now, to replicate their outcome you'd have to discharge your battery solidly with no break, no let off at the same rate. Meaning if you have a 4P configuration. So you'll have to ride your scooter so that the motor draws 6.6, 8.8 or 13.2Amps unrelentingly till its dead.

I'd argue that's not only not what happens, but probably possible without something else failing.

Next that article goes on to discuss another aspect, depth of discharge and fullness of recharge. They state the following:
In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses ... Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 
Seems compelling,  however then it says:
On the negative side, a lower peak charge voltage reduces the capacity the battery stores
... so you want to have high capacity cells (to make high capacity batteries) but then not actually have high capacity batteries anymore so that they may last longer? This becomes more clear when you look at table 4 and its notes:


and pay attention not just to pick the bigger cycle count, but to read the details, like the "Note:" in the middle over there and to Experiment: making reference to to vehicle (meaning cars) at the bottom, and in among all that TLDR writing.

So to get longer cycle life (defined by what?) you need to reduce the capacity as well as the power of  the battery (recalling power = Volts x Amps).

Personally I love the extra power that my scooter has when 100% charged.

I see many people asking about how to get more power from their scooter, so its pretty clear that the answer can't come from under charging.

Next in reading the details you'll find this:
Discrepancies exist between Table 2 and Figure 6 on cycle count. No clear explanations are available other than assuming differences in battery quality and test methods. Variances between low-cost consumer and durable industrial grades may also play a role. Capacity retention will decline more rapidly at elevated temperatures than at 20ÂșC.

 ... so maybe its not as cut and dried as these studies suggest and then (importantly) will you ever be riding it in summer because temperature plays a bigger margin?

I encourage you to not just skim through that article cherry picking but to really dig in deep if you want the actual answers. If you do you're going to find that this is a very complex subject and has a lot of parameters such as but not limited to:

  • temperatures
  • chemistries
  • cycles of depth 
  • starting charge
  • duration a cell is held at a charge
  • discharge intensity

Are you yet getting the picture that everything is a trade off?

Moving to a battery not just the cell

Now lets consider the actual practical issues in charging our batteries (an array of parallel cells in bundles in a string of bundles) and note that it would be pretty much impossible to charge  all these without a Battery  Management System (BMS) which then distributes the power to each of these. This is the BMS in my battery pack.



Now this is a composite of two images, seen from top and side. The BMS is connected to each bundle so that it can be charged individually. Without this such a system would pretty soon go out of balance and be useless.

Balance?

Yes, each set of cells needs to be not only the exact same chemistry and rating but needs to be at the same voltage when charging is completed, if we just wired these all up and applied 54V across the pack then maybe (maybe) in the beginning it would be able to be completely charged in a uniform way with no cell being forced over the 4.2V and coming to harm (like exploding). This is why its simplistic to just think of that black box that plugs into the scooter as the charger. Its actually not the charger and the BMS is the charger. It makes sure that when any bundle reaches 4.2V its ceases charging, to protect it (and you).

However the reality of this is that because nothing is identical  (and less so with age) not all cell bundles come up at the same time. Were the BMS to just shut down when the first bundle reached 4.2V then pretty quickly the other bundles would fall behind and not have the same capacity. This will result in the pack not being able to deliver its rated output. (disappointing)

Why? Well this is because one of the other functions of the BMS is to protect the cells, you see not only is charging too high a problem but under voltage from "over discharging" is dangerous and can harm a cell. The BMS does this by sensing all the cell bundles Voltage and then shutting down the power output (like an off switch) when any one of the cells falls below its low voltage threshold (often 3V).

Lets examine my pack (which I pulled apart because the BMS was not doing its job, but lets get back to that point soon), when I pulled it down I got the following voltages from each bundle:

You can see that bundles 2, 5 and 11 are significantly lower than the others and bundles 1, 3,4 and 10 are higher.

So this pack was unbalanced and resulting in the following symptoms

  • when BMS shut off charging, scooter display showed 54.4V but unplugging it that fell instantly to 52V (some would cheer saying this was good as it wasn't charging to 100% (52 divided by 13 is 4V which would seem good to the less well informed). lets get to why in a moment.
  • BMS would trigger a pack shutdown before full safe discharge range was reached because the lower cell (bundle 5) would be exhausted and fall below the critical voltage before the other cell bundles. The BMS is then preventing the cells in that bundle from being harmed by excessive discharge.
So why didn't the BMS bring all bundles up to the same Voltage? Well that requires an understanding of how a BMS works, which is a lengthy blog post in itself, but just keep in mind the following things:
  • the BMS can be Active or Passive. A passive BMS can only bleed power from adjacent cells  (so like bundle 2 could get power from bundle 1 or 3 depending which was on its negative side) while the Active type can continue charging individual bundles longer. However all BMS have limits (and trade offs), read this article if you're keen to learn more.
  • many BMS have a limit on the voltage difference which they can cope with for some thats 0.2V ... meaning that if a bundle is greater than 0.2V different to the higher ones then its left behind.
My original factory fitted BMS was a Passive type and I changed it for an Active type, happily the pack quickly recovered giving 54.5V soon (in recharge count) after fitting the new BMS. To nudge it in the right direction however I balance charged it manually first.


which of course took some hours of time ... because I had to recharge each of the 13 bundles.

Why did my pack become unbalanced? Well I'd say there are a number of candidates:
  1. I think my pack behaved like this (dropping to 52V instantly after unplugging) from when I got it. This may have been caused by the dealer I bought it from allowing some people to test ride it (and the tyres showed that to be the case) without fully charging the battery to 100% out of the box. This is a bad thing and should not be done no matter how much of  a worry-wart-fusspot you are. These things need to be done to ENSURE it starts out properly fully balanced on the first "grooming" of the chemistry.
  2. I on a number of occasions was impatient to get going and gave the battery a charge to nearly full and then went on longer rides (like 12 or so km) to go shopping
  3. The BMS may have been faulty or a poor design type of passive BMS
Either way it happened and its fixed.

Conclusion

As you can see there's more to the entire question than the simple "don't fully charge it" as there are many trade offs. Most of the literature focuses on where the "real money" is and that's cars. Cars which like Tesla have over seven thousand (7,000) cells inside, not 30 or 72 or even 128. Given the cost of a 18650 cell at about US$3 that means there more than $21,000 in just 18650 cells in a Tesla and you can buy an entire battery pack for a scooter for about $200.

Ask yourself these questions:

  1. are you fussing over nothing?
  2. are you suffering from new buyer fussing?
  3. might you do more harm to the pack by imbalancing it than potentially preserving the cell life
  4. didn't you want more power anyway?
  5. didn't you want lighter?
  6. in reality how many cycles are you likely to do? Do you run out of charge so often that you need to charge more than 3 times a week?
  7. how can you control temperature in a scooter?
  8. everything has a life span
and a perspective from a well known battery builder:


Lastly to me the low hanging fruit is exactly as said in this video: fully charge it, preferably timed so that its ready just prior to use. Then get the usages out of it and when its at about 30% remaining, charge it. This will minimise the charges, minimise the time at higher voltage and result in the maximisation of cost, benefit, and enjoyment.

So yes, I almost always fully charge mine when I charge it and so far its done over 1000km and based on final charge voltage and range tests (which are a pretty good indicator of battery status) the scoot will still do the same ranges it did from new.

HTH

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