Why Fast-Charging Rocks

[Jason McNeil]

We're working with a charger manufacturer on adding a toggle switch that lets users change the voltage from 67v to 6064.8v.
The purpose of this addition is because there's data that suggests that if the battery pack is only topped-up to 60 64.8v they still yield 80% of the capacity (672Wh vs. 850Wh) but reduce cell degradation rates by 5 times!

When investigating this feature a bit further, another previously unrecognized effect is that even when time is allowed to balance the cells to 3.75v 4.05v, because they can still accept a fairly decent amount of current, that last two hours in a standard charge cycle can be condensed into 15 minutes! The net effect of using the 5A charger & a 60 64.8v end-point charge point is massively reduced charging time, from 8 hours with a standard 2A charger to only 2.5 hours with the 840Wh pack.

 

Edit 1: to have a 80% capacity, had increased voltage to 64.8v modifying to a more progress increase in voltage, thanks @Esaj for pointing this out.

ewheels.com, ewheels https://www.ewheels.com/ewheels-fast-chargers-rock/

[mich]

The fast charging voltage graph (brown) looks rather fishy. From 60 to 120 min voltage remains capped at 60V, even though batteries are being charged with 5Amps. How it that possible?

[Jason McNeil]

2 minutes ago, mich said:

How it that possible

Did you read the description? It's fast-charging & capped at 60v by design...

[esaj]

10 minutes ago, mich said:

The fast charging voltage graph (brown) looks rather fishy. From 60 to 120 min voltage remains capped at 60V, even though batteries are being charged with 5Amps. How it that possible?

 

7 minutes ago, Jason McNeil said:

Did you read the description? It's fast-charging & capped at 60v by design...

 

He's referring to the 60V voltage being reached (for the charger), and staying there, yet the amperage-line stays flat at that point (which would mean that the pack voltage isn't rising either, afaik).

If the internal resistance of the circuit formed by the cells in pack(s) + charger output-side would be for example 1ohm (to keep calculations simple), to keep 5A current flowing, the voltage difference between the battery and the charger would have to be

U = R * I   =>  U = 1ohm * 5A = 5V

Meaning that if there's still 5A running in the circuit, and the charger is constantly at a voltage of 60V, the pack would have to be constantly at 55V to keep 5A current running. The current graph would need to start to drop if the pack voltage is getting higher (like it does later on in the graph).

[Keith]

@Jason McNeil, my concern here would be balancing. As I understand it the rather crude balance circuits in most BMS only start to balance once a cell reaches 4.2V, that would mean an 80% charge would result in the pack slowly becoming out of balance. I.e. It is whilst putting that last few percent of charge in that cell balancing is accomplished. If BMS worked like good quality LiPo chargers, balancing to maintain equal voltage in all cells during charge, it wouldn't be a problem.

Now, I'm fairly certain the need to balance lithium cells is somewhat overstated and the more cells you have in parallel the less of an issue it will be as the odds are that any poor cell is in parallel with a good cell. You can certainly get away with doing several 80% charges and an occasional 100% charge to rebalance. The question is how occasional? That, I suspect, needs a bit more research. If every second charge is 100% then very little real improvement in cell degradation will result, if only 1 in 20 charges need to be 100%, it should make a significant difference to cell life.

At the very least it is worth doing for when a quick charge is needed as that last 20% can take as long as the first 80% did.

However, I'm fairly certain the "quick charge" function on most good quality LiPo chargers still charge to 4.2V per cell but discontinue charge as soon as the current drops significantly below maximum I.e. (say) when a 2A charge has dropped to 1A. In practice that is still well below 100% charge. As others have said above, limiting to 60V will still result in a long much reduced current phase before current reduces to cut off condition. 

[Jason McNeil]

For the moment, the graph is just a theoretical, we'll be validating with the real thing when it arrives  

9 hours ago, Keith said:

understand it the rather crude balance circuits in most BMS only start to balance once a cell reaches 4.2V

Maybe I'm missing something, or am wrong, but in a charger where the cutoff current is, say, 400mA, & with BMS distributing between 4.05v  to each cell, as soon as the cell reaches it's max mAh capacity for this voltage, the current will diminish to practically zero & the cells will be balanced right, irrespective of the BMS? Even with a typical (?) BMS doesn't the balancing logic operate not just at 4.2v?

 

The guys at eBikes.ca have quite a bit of experience with partial voltage charging. Months ago I ask the founder, Justin, if he thought the issue of the BMS was of any concern, he didn't seem to think it was.   

http://www.ebikes.ca/tools/charge-simulator.html#benefits-of-partial-charge
"can see drastic improvements in calendar and cycle life when they are not held at the nominal full charge voltage of 4.2 V/cell but are charged to a lower voltage instead. That’s how electric car manufacturers are able to 5-8 year battery warranties on cells that usually only test to ~500 cycles."

[Keith]

@Jason McNeil, whilst there are, I understand, balance circuits available that will maintain all cells at the same voltage at all times  and would work well in this situation ( like the Texas Instruments BQ77PL900 family which I think your diagram comes from) my understanding that a number of them (Gotway for example and possibly many of the Chinese made BMS boards) use the HY2213 or similar. There was a discussion about it below which stated "Once it reaches 4.200V it will turn on the MOSFET and bypass certain amount of current from the cell." I.e. It's balance circuitry is actually the overvoltage protection circuitry as well:

Since that article appeared it looks like the datasheet is now available in English: http://www.hycontek.com/attachments/Battery/DS-HY2213_EN.pdf my reading of that document is that this is exactly how it works: individual cell reaches 4.2V, a MOSFET turns on and shunts current around that cell whilst continuing to charge the others.

Using this, or similar devices, will have no balancing effect at all unless the battery is charged to 100% and, ideally left on for an hour or two after the charge indicator shows complete.

Another argument for better designed BMS boards perhaps?

 

[dmethvin]

So the main reason why you want to balance the cells is to prevent some weak cell from having a voltage that goes too low, right? The BMS would cut out if any monitored cell goes too low, or if you've shunted it the cell will go low and possibly be damaged. If the pack is pretty healthy it seems like a full charge with balancing would only be needed every few charges at most.

 

[checho]

On 4/6/2016 at 1:45 PM, esaj said:

 

 

He's referring to the 60V voltage being reached (for the charger), and staying there, yet the amperage-line stays flat at that point (which would mean that the pack voltage isn't rising either, afaik).

If the internal resistance of the circuit formed by the cells in pack(s) + charger output-side would be for example 1ohm (to keep calculations simple), to keep 5A current flowing, the voltage difference between the battery and the charger would have to be

U = R * I   =>  U = 1ohm * 5A = 5V

Meaning that if there's still 5A running in the circuit, and the charger is constantly at a voltage of 60V, the pack would have to be constantly at 55V to keep 5A current running. The current graph would need to start to drop if the pack voltage is getting higher (like it does later on in the graph).

True for small battery packs, however for very large battery packs it is not true until the last phase of the charge cycle. During the fast charge phase the 5A current it is not because of the difference in voltage potential between the battery and the charger, the 5A is being limited by the charger, but during the last phase of the charge cycle the current is the result of the voltage difference between the battery pack and the charger and as voltage increases the Amps decrease, eventually having 0A and battery pack and charger equal voltage.

[checho]

On 4/6/2016 at 1:11 PM, Jason McNeil said:

We're working with a charger manufacturer on adding a toggle switch that lets users change the voltage from 67v to 6064.8v.
The purpose of this addition is because there's data that suggests that if the battery pack is only topped-up to 60 64.8v they still yield 80% of the capacity (672Wh vs. 850Wh) but reduce cell degradation rates by 5 times!

When investigating this feature a bit further, another previously unrecognized effect is that even when time is allowed to balance the cells to 3.75v 4.05v, because they can still accept a fairly decent amount of current, that last two hours in a standard charge cycle can be condensed into 15 minutes! The net effect of using the 5A charger & a 60 64.8v end-point charge point is massively reduced charging time, from 8 hours with a standard 2A charger to only 2.5 hours with the 840Wh pack.

 

Edit 1: to have a 80% capacity, had increased voltage to 64.8v modifying to a more progress increase in voltage, thanks @Esaj for pointing this out.

ewheels.com, ewheels https://www.ewheels.com/ewheels-fast-chargers-rock/

Such a charger would be great if affordable. There is a charger that is programable that does exactly as you describe, it is a programable charger called satiator 72v it is of high quality, fanless, has a digital display, can be programmed using a laptop, made in Canada, however it is expensive, over $300 once connectors and cable for programing it are added.

[mich]

4 hours ago, checho said:

... it is a programable charger ....

Programmable charger, that's great idea. To minimize battery wear-out, I'm charging battery in evening to about 50%, the rest of charging happens in the morning (to about 90%). It would be very useful, if charger could handle that automatically. Let's say I want to leave at 7:30, the programmable charger would start charging at 5:30 ...

[TwixFix]

Have you guys considered a Charge Doctor V2 from @hobby16?

More info here: http://hobby16.neowp.fr/2016/02/07/charge-doctor-v2-auto-shutdown-function/

It does exactly what you propose for a very reasonable price as well as giving you metrics on each charge session. I've been using it with the 5amp charger that came with my KS14c ordered from @Jason McNeil and it works great. It's programmable, and super easy to toggle the auto shutdown feature on/off so you can occasionally perform a full charge for balancing. I highly recommend it if you're trying to extend your battery life.

[dmethvin]

I have a Charge Doctor V2 as well, really good for avoiding a completely full charge. Its threshold is set by current rather than voltage but I'm not sure there's an advantage to one or the other. 

[Jason McNeil]

Made an interesting discovery yesterday: in the datasheet for the 30Q cell (pp. 14), Samsung actually recommend using a 4.1v charge voltage for eBikes, eWheels, etc..

http://www.nkon.nl/sk/k/30q-specs.pdf

[jbwheel]

4,1V (*16 = 65,6V) is about 86% capacity from http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

Charge level (V/cell)	Discharge cycles	Capacity at full charge	Table 4: Discharge cycles and capacity as a function of charge voltage limit. Every 0.10V drop below 4.20V/cell doubles the cycle but holds less capacity. Raising the voltage above 4.20V/cell would shorten the life. Guideline: Every 70mV drop in charge voltage lowers the usable capacity by 10%.
[4.30] 4.20 4.10 4.00 3.92	[150 – 250] 300 – 500 600 – 1,000 1,200 – 2,000 2,400 – 4,000	~[114%] 100% ~86% ~72% ~58%

 

4,1V is about 84% capacity from charge curve of NCR18650PF (=orbtronic 18650PD) used by some GW / KS batteries 

 

 

 

 

[Jason McNeil]

Justin, the founder & guru behind of the eBike Endless-sphere forum, has responded to a question I asked him about the partial charging & BMSs. He gives a pretty detailed response, so I'll quote it in it's entirety. I'm waiting to hear back from King Song on what chip their BMSs are using. 

This argument is only true if either the pack is made with 

A. shity cells, or 

B. a shitty BMS circuit, 

 

Low quality cells (like hobby LiPo)  can have mismatched self leakage discharge and require relatively frequent balancing, and similarly some poorly designed BMS circuits will themselves drain certain cell groups more than others (a typical case is using the lower 3 cells as a ~12V bus for powering the BMS logic and mosfet gates, rather than using an onboard linear regulator). If you don't have a shitty BMS and you use top tier 18650 cells, then there is almost zero need at all for cell balancing since the relative difference in self discharge between the cells is so small as to be negligible.  And if you have a situation where you do in fact need the cells to rebalance, then you simply run a 100% charge on the pack for a couple days and allow it to trickle into evenness. There is no case where charging to 4.05V the rest of the time is somehow going to be deletrious to pack performance. Quite au contraire. 

 

And finally, actually smart BMS circuits don't sit there and wait until a cell is at 4.2V before they start the bleed balancing process, most programmable BMS's allow you to set a DeltaV threshold between the cells that will trigger the balance bleeding of the high cells regardless of their voltage. 

 

Please quote me directly on these forums if there is any debate still on the topic! 

 

-Justin

[US69]

I also want to mention The LAST Charge Doctor by hobby16!

it is Not on The buy section but You can Order of by Email!

this Charge Doctor Version 2B or 3 Perhaps...has The possibilty To Plug in 2! Chargers of 2 Amp!!! (Or more amp!)

so with a CD V2b You can use 2 conventional cheap 2 Amp Chargers to get a fast charging of 4 Amp! 

And that with all The options to "autocut-Off" on a amprate of your Choice!

so now i have my Standard Charger which come with my Ks14, bought a CD v2B for 30 Euro and an other Standard 2 amp Charger from futurewheel for 29 Euro....and have now a 4 Amp Charger which normally Cost about 150 Euro :-)

look here to See Photos of Version 2b or 3 with Double attach Mode:

 

http://hobby16.neowp.fr/2016/02/29/what-is-the-recommanded-charge-current/

It's fantastic!!!

sorry for my english

[jbwheel]

On Friday, April 08, 2016 at 4:08 PM, dmethvin said:

I have a Charge Doctor V2 as well, really good for avoiding a completely full charge. Its threshold is set by current rather than voltage but I'm not sure there's an advantage to one or the other. 

Very different if you want to cut at 65,6V (4,1V /cell), you would still be charging at max currzent. Personnally I use a mecanical timer (also for safety reason). I juste try not to get over 65,6V unless I need the extra capacity 

[jbwheel]

For who wants to have the best charging parameters :

You need to set voltage and limit max current. Which you can do with CC CV step up (24$, 15A max) using any car battery or computer alimentation or even charger you already have. Program the good voltage for Inmotion, solowheel or your next wheel and amp considering your pack size (1C max !).

Most important you need to cut the charge (fire danger !) when you reach a threshold voltage for example using this item (16$) which can cut using voltage (over voltage protection, MUST be set below alimentation voltage ), time or Wh. I would still use a mechanical timer...

Pictures

 

[jbwheel]

On ‎06‎/‎04‎/‎2016 at 10:11 PM, Jason McNeil said:

We're working with a charger manufacturer on adding a toggle switch that lets users change the voltage from 67v to 6064.8v.

ewheels.com, ewheels https://www.ewheels.com/ewheels-fast-chargers-rock/

It is much faster to keep the voltage setting at 67V (or even above) and then cut the charge when the value 64,8V is reached because you will have a higher current thanks to the bigger voltage difference than to set the voltage of your charger at 64,8V : it will take an infinite amount of time to get there and your curves show that you cut a bit above 64V (72% capacity) in that case.

[dmethvin]

15 hours ago, jbwheel said:

Very different if you want to cut at 65,6V (4,1V /cell), you would still be charging at max currzent. Personnally I use a mecanical timer (also for safety reason). I juste try not to get over 65,6V unless I need the extra capacity 

Looking at the curve that Jason McNeil posted, if the charger is stopped at 1 amp it corresponds to a charge state of a little less than 66 volts. So it's pretty close, right?

[jbwheel]

 

32 minutes ago, dmethvin said:

Looking at the curve that Jason McNeil posted, if the charger is stopped at 1 amp it corresponds to a charge state of a little less than 66 volts. So it's pretty close, right?

I am not sure we are looking at the same curve : voltage is the red one. So for me, you would need to cut at arount 320min to get 65,6V and around 280min to get 64,8V. The earlier you can get with a current threshold is around 360min (>66,2V) when the charge current start to diminish.

[Keith]

What needs to be allowed for if you are looking to both only charge to 4.1V per cell (for longevity) and charge quickly is the internal resistance of the pack. The higher the charge current the more voltage is dropped across the pack. As a result, assuming a given current value as the charge cut off point, then a fast charge will have a much longer constant voltage phase as the max voltage will be hit earlier (due to the internal cell resistance plus higher charge current. I.e. The CV stage will not look like @Jason McNeil's diagram FC Current (A) but will actually be longer than the standard charge current CV curve (indeed, if you think about it, from the point the fast charge current has dropped to 2Amps it HAS TO follow exactly the same curve as the 2Amp charge does.)

The way around this, assuming (say) you wish to charge to 4.1V per cell and quickly is to have the cut off point be an arbitrary but fairly high current (let's say 2 Amps). If we assume internal resistance per cell is 50mOhms then a 16s4p pack will be about 200mOhms which at 2Amps will drop 0.4V therefore the constant voltage phase should be 16x4.1v + 0.4V = 66.0V (assuming no protection diode in the circuit).

However, the bad news is internal cell resistance increases with age. Let us say a year or so later the cells have increased to 150mOhms per cell then internal resistance of the whole 16s4p pack will now be 600mOhms which when the charge has dropped to 2Amps will drop 1.2V that will mean the 66V charge will now have the cells at 64.8volts or 4.05V/cell. I.e. The same 66V charge with 2Amp cut off will now only put around 75% into the pack.

Of course, if your pack was only 16s2p those resistance and voltage drop figures would be doubled, a higher constant voltage would be required (66.4v) to charge to 4.1V/cell and if the cells aged to 150mOhm/cell the voltage drop at 2Amps would be 2.4V dropping the per cell voltage to 4.0V/cell I.e. 72% charge.

[jbwheel]

45 minutes ago, Keith said:

The way around this, assuming (say) you wish to charge to 4.1V per cell and quickly is to have the cut off point be an arbitrary but fairly high current (let's say 2 Amps). If we assume internal resistance per cell is 50mOhms then a 16s4p pack will be about 200mOhms which at 2Amps will drop 0.4V therefore the constant voltage phase should be 16x4.1v + 0.4V = 66.0V (assuming no protection diode in the circuit).

However, the bad news is internal cell resistance increases with age. Let us say a year or so later the cells have increased to 150mOhms per cell then internal resistance of the whole 16s4p pack will now be 600mOhms which when the charge has dropped to 2Amps will drop 1.2V that will mean the 66V charge will now have the cells at 64.8volts or 4.05V/cell. I.e. The same 66V charge with 2Amp cut off will now only put around 75% into the pack.

You are adding much more complexity and I havent see any data allowing to take this into account. From what I see, the aging of battery packs has been studied at constant voltage during the whole life of the batteries (laptops, cars, satellites etc.), not by raising the charging voltage accordingly to the internal resistance. For Battery University, 4,0 or 4,1 / cell during the system life is significantly better than 4,2V/cell if you wish to have more cycles and it has been caracterised like that. If you want to take the original internal resistance into account, then 18650PF internal resistance has been measured at 21 mOhms so it would be 0,16V for 16s.

(http://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die, http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries)

@dmethvin : another reason it is better to have a voltage threshold is that you do not need to know how the charger behave (you can have differences between chargers) and therefore don't need to have the voltage and current curves 

[Keith]

@jbwheel, you are misunderstanding me. You can charge to any voltage below 4.3V per cell but the internal resistance (whatever value it is) will give you the Constant Voltage curve that means the cells are not fully charged (to whatever charge level that voltage gives you) until the charge current has dropped to a very small value - that takes a fair bit of time. If you charge in this way (as all standard charging does) internal resistance is of no concern.

However this thread is about fast charging and not overstressing the cells.

What I have said (and put some numbers against it to demonstrate) is that if you wish to charge quickly AND to a lower voltage to protect the cells then you need to take internal resistance into account. If you stop a (say) 5Amp charge when the voltage reaches 4.1V per cell (65.6v) then, actually you are nowhere near 4.1V per cell or 86% charge at that point.

I looked at actual measurements people had done on real cells and not the quoted figures the manufacturers give for brand new and perfect cells and 50mOhms looks to be a more realistic true figure. You are correct, if the cells IR is better then that the voltage drop will be less, but it will still be there.