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Battery charging question


wstuart

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I have read that charging to 100% can be bad for a battery

I have also read that stopping the charging at 80% is bad (worse) because the cells won't get a chance to balance in the "constant voltage" stage of the cccv charge cycle.

 

Here is my question.  I have a fancy ewheels charger that let's me charge to 80% or 90%.  I just charged my MSP to 80% and when it hit 80% voltage the charger slowly trickled down the current.  - So didn't this charger just balance the cells, even though the cells are not at 100%?  Does this charger allow me to have my cake and eat it too?  - It seems like I'm able to charge my cells to a nicer 90% voltage, bit still allow the cells to balance.  If this is the case, then I don't think I'll ever charge to 100% again.

Let me know if I'm missing something here.... Am I wrong - do cells have to be charged to 100% to truly be balanced?  

 

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2 hours ago, wstuart said:

I have read that charging to 100% can be bad for a battery

I have also read that stopping the charging at 80% is bad (worse) because the cells won't get a chance to balance in the "constant voltage" stage of the cccv charge cycle.

And there's the third opinion to just charge after each ride to 100% and don't hassle around - there are some reports of members owning many wheels for longer times that they have no noticably degradation. Like @Marty Backe.

2 hours ago, wstuart said:

Here is my question.  I have a fancy ewheels charger that let's me charge to 80% or 90%.  I just charged my MSP to 80%

80% according to the charger setting?

2 hours ago, wstuart said:

and when it hit 80% voltage the charger slowly trickled down the current.

What's 80% voltage? Some voltage below the 100.8?

Maybe the charger has some "soft stop"?

Or the 80% charging just stops at the transition between CC and CV - this is exactly when charging current decreases.

2 hours ago, wstuart said:

  - So didn't this charger just balance the cells, even though the cells are not at 100%?

No. Balancing starts once the voltage per cell reaches ~4.2V.

2 hours ago, wstuart said:

  Does this charger allow me to have my cake and eat it too?  - It seems like I'm able to charge my cells to a nicer 90% voltage, bit still allow the cells to balance.

Depends on how the 90% stop condition is realized wirh this charger.

2 hours ago, wstuart said:

  Let me know if I'm missing something here.... Am I wrong - do cells have to be charged to 100% to truly be balanced?  

No. As long as they are balanced, they need no balancing ;)

So there are reports of many people charging to 80% having no problemd.

But as soon as they are out of balance they need 100% charge or the pack degrades rather quickly.

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@wstuartYou might be confusing constant voltage with balancing. The constant voltage phase just means the charger is maintaining a max voltage. Once the battery pack total voltage reaches that setting, current drops but there’s still a little charge the pack can take so the charger doesn’t stop it just keeps enough current to keep topping it up. At some point the current gets so low that the charger notifies you with a green light (on the basic chargers). This is all based on the total pack voltage. I assume your charger can change it’s set point to “80%” ie. 80.3V. 
 

Balancing has to do with the voltages of each cell group in series. You have 24 cells that add up to the total voltage of the pack. If they are all the same, good to go. If any single one is different, various bad things happen. Balancing is only done by the BMS, not the charger. The charger only knows the total pack voltage but the BMS tracks the voltages of each cell group. It would be good to have a BMS that can balance at any state of charge but all EUCs currently only have “passive top balancing” which means they only are able to balance at full state of charge. 
 

This is why the collective wisdom seems to be charge to full (4.2V per cell, ~50mA per parallel final charge current) every time. 

Edited by agranner
Typo, context of reply
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9 minutes ago, agranner said:

You might be confusing constant voltage with balancing.

Just got this comment of yours after reading it a second time - you mean if the charger changes to constant viltage at some lower max voltage for the 80% and 90% option it's just the same charging current decrease as normmally seen?

But still no balancing happening as

10 minutes ago, agranner said:

but all EUCs currently only have “passive top balancing” which means they only are able to balance at full state of charge. 

Yes, balancing happens only by bleeding resistor engaged above around 4.2V cell voltage.

With greater imbalances a little bit before.

And an important part of balancing happens after the charger is unplugged and any cell with the bleeding resistor activated gets discharged to ~4.2V again!

15 minutes ago, agranner said:

This is why the collective wisdom seems to be charge to full (4.2V per cell, ~50mA per parallel final charge current) every time. 

Seems the simplest and safest recommendation for everyone for hassle free battery managment.

Until one has an euc with a bms with cell voltage reporting!

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It's not good to store the packs at 100% for a long period.

It's fine to go 100% as long as you're using the wheel.

If you're nervous, just give it a good charge to 100% and leave it there on the charger for a couple of hours. Open eucworld and write down the voltage readout. 

Now do your 80% trickery or whatever. Then after a couple of months like 3-6 redo the 100% charge and leave it there for a few hours. Compare the voltage readout. Is it the same as before? Good. Is it lower? Try anyother couple of hours on the charger. Better? No? Write in the charging sticky thread.

If anything changes suddenly. Like your charge time went unusually quick or you lost 20% range over a few days. Those are serious warning signs.

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  https://forum.electricunicycle.org/topic/25017-video-on-80-charging-good-or-bad

On 4/14/2022 at 8:06 PM, Funky said:

it's better to charge to 100% each time. Vs 80% charging.. Because some cells can be more empty. And if you don't charge the wheel to 100% each time, but once per 5/10 time.. Those "lower" cells get more empty. At some point balancing them at those 5/10 times don't fill them completely full anymore. And that's how over time the pack dies.. Because the empty cell gets even more empty.

It's safer to charge 100%

 

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