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Battery balancing


Ruslan

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Hi all!

I was trying to find threads with the "battery balancing" topic, but for some reason couldn't find any.

My everyday commute is 9.6 miles. So I charge my ks16s once a day to 80% using EWheels Dual mode Charger. When I am back, the charge level is at 45%. 

I have heard from someone that every so often I need to discharge batteries to 0% and then back to 100% to balance them.

What would be your suggestion on battery balancing for a unicycle? 

How often is the battery balancing necessary?

Thanks!

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1 hour ago, Ruslan said:

Hi all!

I was trying to find threads with the "battery balancing" topic, but for some reason couldn't find any.

My everyday commute is 9.6 miles. So I charge my ks16s once a day to 80% using EWheels Dual mode Charger. When I am back, the charge level is at 45%. 

I have heard from someone that every so often I need to discharge batteries to 0% and then back to 100% to balance them.

What would be your suggestion on battery balancing for a unicycle? 

How often is the battery balancing necessary?

Thanks!

just charge it to 100% and commute for 2 days then recharge again ? 

i just used to charge my electric bikes to full always and never noticed much battery depletion over 3 years anyway 

 

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4 minutes ago, Ruslan said:

Thank you! 

Does it also mean that I don't really have to deplete batteries to 0 every now and again?

No. Just leaving the battery on the charger "at the end of" and after full charging to 100% does the balancing.

@zlymex imho once posted a "device" (?100 mA constant current source?) which can be used just for balancing, too?

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15 hours ago, Ruslan said:

Thank you! 

Does it also mean that I don't really have to deplete batteries to 0 every now and again?

 

15 hours ago, Chriull said:

No. Just leaving the battery on the charger "at the end of" and after full charging to 100% does the balancing.

@zlymex imho once posted a "device" (?100 mA constant current source?) which can be used just for balancing, too?


I agree… If you think that the Charge Doctor will have a long term benefit then keep using it. But every 4-5 charges use the standard charger and charge until the LED is green (ie. 100%) and that should keep your wheel running well.

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  • 1 year later...

I unearth this for a question I can't seem to find answer to:

How long to leave the charger on after green so that the balancing gets done?
I suppose it depends of how of balance the batteries are, what would be a reasonable time for a 1000+Wh wheel?

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Thanks for the indications. It would be nice to have an idea rather than navigating in the dark.
It wears on the batteries to leave them charging, so we might as well balance as best while at it.
Someone on the French forum mentioned leaving it overnight (had some math I think, trickle current was very low), and that was just a 320wh.

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If your cells are degrading at the same speed the tiny balancing currents will be enough to keep the pack balanced.

If your battery has bad cells in it, no amount of bms balancing is going to be enough. (Only by external balancing through a charger can you keep running such a battery safely, but it needs regular monitoring.)

If you have a voltage readout for the pack and you can reach full voltage (or 100%) consistently then that's likely a good pack. Most bms prevent cells to go over 4.25V.

If you can't get to 100% with a voltage output checked charger and after charging a long time that's one warning flag. It could be other things but it could be the battery.

Also if the battery has some corrosion then you'll see single cells discharging faster than others and possibly deplete completely at 0V. Such batteries can not be rescued. You'd need to replace those cells which is a tricky job. If you do recharge a 0V cell you risk blowing up your machine (unless you run a safe chemistry like lithium iron phosphate).

Edited by alcatraz
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8 hours ago, null said:

I unearth this for a question I can't seem to find answer to:

How long to leave the charger on after green so that the balancing gets done?
I suppose it depends of how of balance the batteries are, what would be a reasonable time for a 1000+Wh wheel?

2-4 hours seems to be a common recommendation, but exactly as you said, depends on how unbalanced the cells are. I see no real problems with even a lot longer times, as long as you don’t leave the batteries full for several weeks.

The Charge Doctor is handy for balancing as well, since I can see exactly how much current is going in. Once it reaches 15mA or so, I personally consider the balancing done.

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11 hours ago, null said:

I unearth this for a question I can't seem to find answer to:

How long to leave the charger on after green so that the balancing gets done?
I suppose it depends of how of balance the batteries are, what would be a reasonable time for a 1000+Wh wheel?

Let's start with the charging stages for LiIon (from batteryuniversity.com):

ion1.jpg

Stage 1 - constant current(CC):

Here shown for a 1C charge. This can quite easily be estimated by the battery capacity:

CC phase duration in hours = (capacity in Wh) / (3.7V times cells in series) / charging current

So for a KS18XL with 1554Wh and 84V (capacity in Wh) / (3.7V times cells in series) = 1554Wh/(3.7*20)=21Ah - so 21A would be a 1C charge, 2A would be a 2A/21Ah~0.1C charge.

For a 67.2V 840Wh KS16S one has 840/(3.7*16)=14.2Ah

With a 2A charger this leads to ~21Ah/2~10h for the KS18XL and ~14.2Ah/2~7h for the KS16S.

At this stage (as good as) no balancing happens.

Stage 2 - constant Voltage (CV):

The duration for this stage is harder to estimate - very (very) roughly it should imho take about this 3 hours as shown in the diagram no matter which battery capacity or charging current is used...

At this stage cell balancing takes place!

At the end of this stage (as shown in the diagram at about current < 3% C) the charger should end (cut off) charging.

I never really looked at red/green light vs. charging state- but imho the only thing that would make sense (too me) would be that the light turns green once both stages are finished and the charger has cut off!

So if this is true there is no sense in letting the wheel on the charger after the led turned green ... please correct me if this works different!

With KS wheels and wheellog/euc world one has additionally the possibility to log/watch the battery voltage while charging!:

ZXOVQRy.png

The red line is the smoothened battery voltage - as the measured values (blue line) was very noisy...

The first stage (CC) was very short - i use an 8A charger. Just took a couple of minutes up to ~66-66.5V, then comes the transition from CC to CV and then the CV phase. The CV phase is in this example not really constant. The slowly rising voltage could be the balancing happening or just some "charger artefact".

Then at ~18:43 the charging ended. Either the charger cut off or i unplugged it - don't remember :ph34r: (since it was about 2h could be the build in charger cutoff?) But should be easily to be seen, even with low charge current cut off, once the batteries are really full.

11 hours ago, null said:

...It wears on the batteries to leave them charging, so we might as well balance as best while at it.

This is why a LiIon charger always (should) turns off once the current reaches some threshold (~3% C according to batteryuniversity.com). How the cheapo chinese charges delivered with the EUCs are adjusted - no one knows? But from many reports here is shown that they have 3 potentiometers to adjust  the max voltage, the max charging current and the cut off threshold current.

Quote


Someone on the French forum mentioned leaving it overnight (had some math I think, trickle current was very low), and that was just a 320wh.

Yes. Some 10h+3h=13h (KS18XL) or 7h+3h=10h (KS16S) should be about the time for a full charge for an absolute empty battery.

11 hours ago, meepmeepmayer said:

Someone did the math on this once (balancing currents vs. voltage differences or something like this). It was longer than expected. Maybe @Chriull was that guy?

Could be.Did some posts regarding this topic - maybe with some other focus - don't really remember...

Edited by Chriull
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52 minutes ago, Chriull said:

At the end of this stage (as shown in the diagram at about current < 3% C) the charger should end (cut off) charging.

None of my EUC chargers have ever stopped applying voltage or current by itself. These include several cheapo 67.2V and one 84V brick, as well as two fanned plastic 4A cheapos.

52 minutes ago, Chriull said:

I never really looked at red/green light vs. charging state- but imho the only thing that would make sense (too me) would be that the light turns green once both stages are finished and the charger has cut off!

It definitely would, but it doesn’t seem to be the case. I checked a few chargers, and the led turned green once the charge current got below something like 0.5 - 1A.

I have forgotten to unplug the charger a few times, and even after more than 24 hours, Charge Doctor still shows < 10mA flowing. Most of it turning to heat I assume.

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32 minutes ago, mrelwood said:

None of my EUC chargers have ever stopped applying voltage or current by itself. These include several cheapo 67.2V and one 84V brick, as well as two fanned plastic 4A cheapos.

That's bad...
Maybe the one adjustment potentiometer for the threshold is misadjusted - or just the charger has not cut off functionality... :(

Quote

It definitely would, but it doesn’t seem to be the case. I checked a few chargers, and the led turned green once the charge current got below something like 0.5 - 1A.

Then it seems to change color sometimes after the change from CC to CV, which is quite "senseless" ;(

Edit: After some seconds of thinking about this the green light could also show the "cut-off current threshold" as this values could be about what i wrote below, but the charger just does not cut off...

... and the 1A is a bit too high, so just something misadjusted?

Quote

I have forgotten to unplug the charger a few times, and even after more than 24 hours, Charge Doctor still shows < 10mA flowing. Most of it turning to heat I assume.

Something about 100-300mA should be a nice cut off point.

Regarding to batteryuniversity.com the 3% C would be about ~600 mA for the KS18XL and ~400mA for the KS16S as cut off threshold.... (Seems to be quite about 100mA per parallel 3500mAh cell...)

Edited by Chriull
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Thanks a lot everyone, I'm not technical enough to give an interesting reply to everyone, but I'm reading through a few times.

I'm sure it will be useful for others too, I couldn't find much on the subject.

18XL been charging maybe +6h from green as I hadn't balanced it since 1500km. (live on a hill, not practical for that)
The volage reads to be 84.50V, I've use the stock charger. Too bad there is such discrepancy between measuring equipment, my voltmeter measures 83.x exiting the same charger.

 

I'll go drain it a bit on the small flat area there is.

Edited by null
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1 hour ago, mrelwood said:

...

I have forgotten to unplug the charger a few times, and even after more than 24 hours, Charge Doctor still shows < 10mA flowing. Most of it turning to heat I assume.

Just an idea/speculation (as i have no idea how much power the charge doctor consumes) - maybe the charge doctor pevents the charger from cutting off by draining too much current by himself?

But afaik one could set the charge doctor to cut off once some 100 mA are reached?!

13 minutes ago, null said:

I've been charging maybe +6h from green as I hadn't balanced since 1500km. (live on a hill, not practical for that)

My personal assumption/believe is that the best way to balance is to just discharge the battery a little (from 100% downto 90-85%) and then charge it through the ~3h CV stage.the 10-15% used could need some 1-2h during the CC stage (depending on the charger and battery capacity) - so around 5h in total.

The balancing imho occurs better by repetition of this process than by leaving it for long durations on the charger with low currents...

13 minutes ago, null said:


The volage reads to be 84.50V, I've use the stock charger. Too bad there is such discrepancy between measuring equipment, my voltmeter measures 83.x exiting the same charger.

Yes. Unfortionately +/- 0.5% (for an already good DMM) + some (least significant) digits  give +/- 0.5V at 84V...

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1 minute ago, Chriull said:

My personal assumption/believe is that the best way to balance is to just discharge the battery a little (from 100% downto 90-85%) and then charge it through the ~3h CV stage.the 10-15% used could need some 1-2h during the CC stage (depending on the charger and battery capacity) - so around 5h in total.

The balancing imho occurs better by repetition of this process than by leaving it for long durations on the charger with low currents...

Yes it would be less wear from charging at max, but isn't the max needed to stop the fuller cells from charging further, so that the less full cells can catch up? Or maybe that happens in CC stage anyway..

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1 hour ago, Chriull said:

Just an idea/speculation (as i have no idea how much power the charge doctor consumes) - maybe the charge doctor pevents the charger from cutting off by draining too much current by himself?

When not connected to a wheel, CD shows 0.000A. And the charger voltage measures exactly the same to a multimeter with or without CD connected.

Of course CD only shows it’s output current, but I’d imagine CD consumption being in the 10mA range. In any case much less than 100mA.

I can’t see how the charger could recognise the CD and only not cut off when CD is connected. But this is now a case of interest, so I might have to investigate a bit!

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