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The 80% charging "rule"


Tawpie

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Various internet sources and battery gurus say that you can extend the usable cycle count of a Li Ion battery by only charging to 80%. Being able to have something last longer is always tempting even if it's silly**.

But, charge to 80% of what? a) 80% of the terminal charge voltage? b) 80% of the difference between terminal charge and 'walk' voltage? c) 80% of the difference between terminal charge and cell shutdown voltage? Does anybody know?

For my 84V KS wheel these would be:

a) 84 * .8 = 67.6

b) ((84 - 63) * .8) +63 = 79.8

c) ((84 - 50) * .8) +50 = 77.2

"a" seems like I wouldn't ride anywhere for long but both b and c are over 90% of the terminal voltage so that seems like more than 80% state of charge.

**it's a silly question because assuming I get 100km from a full charge on my wheel, and full-charging every time (max life reduction) gives me 300 full cycles (it's probably more like 500 but this is an example). That's 30k kilometers before the pack is permanently degraded to 80 km/charge. I personally don't have the kind of time it takes to ride the wheel 30k km much less some multiple of that.

Edited by Tawpie
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Personally, I think the answer is 67.6V is the mythical 80% mark, which is not useful in this application.

If this is true and we're already over the 80% SOC mark just to be useful, that begs a second question: is it better to go 100% with balancing every time than it is to stop in the 90% range?

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15 minutes ago, meepmeepmayer said:

Store

Tiny parenthesis here: battery forums seem to indicate storage at nominal voltage (3,6V - 3,7V) (not including vampire drain or very long term drop)

Edited by null
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14 minutes ago, /Dev/Null said:

Agreed with null.   I charge to 100%, use within 12-24 hours & get it back down to 45-55% and then they stay there (store) until I charge them again.  Seems to work well.

This is absolutely the best thing to do with li-on cells.

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Kinda what I thought. A person like me that rides for entertainment would be best served by:

  • ride
  • on return, don't plug the wheel in (to allow the batteries to come back to room temperature)
  • next morning if it looks like a day for riding, plug it in (be mindful that if yesterday was a long hard ride this could take a while)
  • unplug and power cycle when KS gives me 7 slow beeps (KS16X firmware 2.0.2)

Seems like a good task for IFTTT or Alexa.

Edited by Tawpie
forgot I have a slow charger!
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16 minutes ago, Tawpie said:

Kinda what I thought. A person like me that rides for entertainment would be best served by:

  • ride
  • on return, don't plug the wheel in (to allow the batteries to come back to room temperature)
  • next morning if it looks like a day for riding, plug it in (be mindful that if yesterday was a long hard ride this could take a while)
  • unplug and power cycle when KS gives me 7 slow beeps (KS16X firmware 2.0.2)

Seems like a good task for IFTTT or Alexa.

Sounds like good advice to me!  I purchased a fast charger from Ewheels but only use the "Fast" part when I have a back to back ride opportunity [only once have I done a 5 amp charge].  In Seattle you have to take advantage of good weather when you have it!

Mostly though, I use the "Fast" charger to actually charge slow.  I charge at the slowest speed [lowest amps] for the required result to be ultra safe.

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2 minutes ago, Rawnei said:

don't overthink it

you are 100% correct, but I do ride in the cold and don't want to be charging a wheel until it's finished developing dew on the inside and has dried out any dew that did form, so there is that to overthink. Besides, making IFTTT do the work would be a worthless-to-the-good-of-humankind but entertaining diversion!

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I treat my battery the same way.  Just charge it up before I ride.  If I get back and have a low battery, I charge up to just 70% and let it sit.  I never let my wheel sit around that long at 100%.  With the newer 16X firmware, I turn down my eWheels chager to 4A so I can avoid most of the annoying beeps!

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Just enjoy riding and don't worry so much ;). Unless there is something inherently wrong with your battery pack, you will have other critical problems (mechanical, motherboard, etc.) loooooong before you would notice loss of range due to battery life. Even if you recharge to 100% every night.

I'm not saying people are wrong when they say what to do and what not to, but don't sweat it.

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As your pack ages you might want to increase the time spent at 100% to give the balancing more time.

If only we could monitor individual voltages so we wouldn't have to rely on guesswork.

80% or 90% refers to the charge level. 90% of a 4.2v cell is usually around 4.1v. 80% is a bit extreme for euc use as the packs don't cost a fortune, but on cars 80% is likely very real.

Charging to 80-90% increases the amount of cycles drastically. If 100% offers 1000 cycles before the effective capacity declines to 80% of new, charging to 80/90% increases that tenfold.  The jump from 100 to 90 is big, the jump from 90 to 80 is smaller. I saw a paper on the subject. The numbers were like 1000 - 8000 - 20000 (cycles) or something of the sort.

Another interesting discovery was that even with 100% charging, after the cell being tested (NCR18650B) reached 80% of original capacity, it basically stopped deteriorating and stayed over 75% capacity for thousands of cycles. So they come to 80% rather quickly but after that they stay there for a loooong time. The deterioration tapers off and flattens out.

The problem is that the bms essentially kills the cells as the imbalances become too great to handle. It's not the fault of the cell. With stronger balancing your battery pack could outlive you and still have 75% of original capacity.

Edited by alcatraz
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3 hours ago, Seba said:

To get benefit from extended battery life by partial charging you need a smart BMS equipped with enhanced balancer that works also when cells are partially charged. Not the one built into KS batteries.

Do you know of any EUC manufacturer using smart BMS?  Or heard rumors of brand adoption?

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Excellent battery discussion guys.  Just to be clear and spell it out for me, the balancing we are talking about is when the battery reaches 100% and the charger still shows red [under load], or is it the period after the charger goes green [no load] that the balancing occurs?

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4 minutes ago, mrelwood said:

 Yet I wasn’t able to convince eWheels to fix the disastrous 80% charging recommendations from their shop’s fast charger product descriptions.

Thanks @mrelwood, yeah that is how they sold me a fast charger! hehe.  But after some extended research into the topic I quickly learned about balancing and how it may not be the best choice to not charge to 100%.

Ultimately I'm not salty about the purchase of the charger as I still have a fast charger, and I can also take one to work and one at home.

I feel this info should be stickied somewhere in a maintenance section!

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For my KS on 16XS v2.0.2 firmware, it beeps slowly 7 6 times long after (hours after) the charger has gone green and the reported voltage is well full... it is believed (I possess no facts to back this up) that the 7 6 slow beeps means the wheel has decided that it has finished balancing. I have a slow charger, but during balancing that doesn't matter as the current drawn during balancing is very tiny.

Edited by Tawpie
only beeps six times, not seven
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1 minute ago, Tawpie said:

For my KS on 16XS v2.0.2 firmware, it beeps slowly 7 times long after (hours after) the charger has gone green and the reported voltage is well full... it is believed (I possess no facts to back this up) that the 7 slow beeps means the wheel has decided that it has finished balancing. I have a slow charger, but during balancing that doesn't matter as the current drawn during balancing is very tiny.

I'm going to perk my ears up on my next charge cycle and see if I hear some cool sounds when my V11 is done charging.  It makes kind of a Jaws [Da Duh] kind of sound when it starts charging.

Problem is my fast charger is kind of Loud to have it in my home office, I wish the fan on this thing was thermal triggered and not always max fan speed.  Need to pull in quieter charger inside next time.

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11 hours ago, Rich Sam said:

I'm going to perk my ears up on my next charge cycle and see if I hear some cool sounds when my V11 is done charging.  It makes kind of a Jaws [Da Duh] kind of sound when it starts charging.

Problem is my fast charger is kind of Loud to have it in my home office, I wish the fan on this thing was thermal triggered and not always max fan speed.  Need to pull in quieter charger inside next time.

Same here, on my Gotway I have a fast charger (5/7/9, always charge at 5A) where the fan is always full on and kinda loud.

On the V11 I use dual stock chargers which will also give 5A combined and those are totally quiet which are super nice.

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12 hours ago, Rich Sam said:

Problem is my fast charger is kind of Loud to have it in my home office, I wish the fan on this thing was thermal triggered and not always max fan speed.  Need to pull in quieter charger inside next time.

It's very easy to replace fan to a silent one:

 

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