battery maintenance — storing an EUC for 3 months

[RagingGrandpa]

On 10/4/2023 at 2:40 AM, goodsignal said:

1. Is there an Amperage output setting that's too low for a typical BMS to recognize as a charge?

No.
The BMS does not regulate the charging; it only interrupts it when there is a fault.
Even if your charger delivers only 10mA, the cells will receive it. 

On 10/4/2023 at 2:40 AM, goodsignal said:

2. How many volts is 40% and 50% for a 100.8V wheel with Samsung 21700 40T batteries?

For the cells, 0% is 2.5V and 100% is 4.15V. 
Linear approximation gives:
40%: 3.16V/cell, 75.8V for 24s
50%: 3.33V/cell, 79.9V for 24s

On 10/4/2023 at 2:40 AM, goodsignal said:

I'm planning on buying a
DC Power Supply
And setting Volts to whatever voltage my batteries are supposed to be at 40%
Then just leaving the tender on those settings for the entire time

is my plan generally sound?

No.
Because most EUC's have extremely low standby current, and continuously providing "float charging" to Li-Ion chemistries ages them faster than being open-circuit.

Better to store at ~80V and simply check the voltage once every 6 months, recharging if it falls below 2.5V/cell (60V).
I think it would take years to fall that far, for most modern EUCs. 

More ideal-storage data here: 
https://forum.electricunicycle.org/topic/30654-do-you-guys-leave-your-batteries-fully-charged-if-not-riding-for-a-long-time/#comment-443577

On 11/2/2022 at 6:32 PM, RagingGrandpa said:

more evidence for the prevailing wisdom: store at low voltage (less than 3.5V/cell) and cool temperature (~10°C).

...because it minimizes capacity loss:

...and minimizes resistance change:

(Their definition of SoC was this.)

Low is good. Perfect to be between 3.0 and 3.5V.

 

[Chriull]

2 hours ago, RagingGrandpa said:

For the cells, 0% is 2.5V and 100% is 4.15V. 
Linear approximation gives:
40%: 3.16V/cell, 75.8V for 24s
50%: 3.33V/cell, 79.9V for 24s

As between 2.5V and 3V is not really much charge and the 2.5V are more a minimal threshold than a recommended operationial range this linear approximation does imho not fit. This 3.16V to 3.33V are afair the ?30%? maximum allowed voltage for transportation.

For storage afaik some 3.5V-3.7V as "internal ?equilibrium?" voltage of li ion cells is recommended.

But i got this only from some not to thoroughly readings afair. Would need to investigate batteryuniversity.com and some more googling again for some more knowledge.

[alcatraz]

Nominal voltage (50%?) 3.7v.

On some of these phone batteries where they burn the candle on both ends (4.40v) I've seen 3.75v I think.

[0000]

3 hours ago, Chriull said:

Would need to investigate batteryuniversity.com and some more googling again for some more knowledge.

Endless-sphere has a lot of good related battery data buried in various threads - example, this post by docware compares initial vs 700th cycle state of charge differences. Most of the cells tested are the 18650 format, but he did test the Samsung 50E cells which is probably reasonable to assume accuracy (with respect to his footnote disclaimer) for any other 5000 mAh capacity 21700 cell. Most interesting is the voltage at the 10% SoC threshold IMO.

His chart:

Edited October 7, 2023 by Vanturion

[alcatraz]

Hmm, isn't that particular study pointless?

I'm interested in the difference in impedance, heat generation, capacity.

[alcatraz]

Edit: I see that there'a capacity measurements as well in the link provides. Great. That leaves impedance and heat for a (to me) comprehensive test. 

[0000]

38 minutes ago, alcatraz said:

Hmm, isn't that particular study pointless?

You serious? This entire thread has been about what constitutes storage voltage for li-ion batteries. There was also some inaccuracies above about what voltages represented 40% and 30% state of charge/capacity. I'd be hard pressed to link a more relevant dataset to help clear things up. Maybe you can do better.

36 minutes ago, alcatraz said:

Great. That leaves impedance and heat for a (to me) comprehensive test. 

Great, I prefer steak over pizza except on Wednesdays.

[alcatraz]

Didn't mean to piss you off. 

Do you expect the optimal storage voltage to change after a couple hundred cycles?

Storing at 3.75v or 3.70v. Is that it?

[woke rider]

What would be the real danger of just leaving it plugged in the whole time? My Z10 stops charging and the charger is cold after the EUC gets a full charge. My Segway charger gets really hot to the touch while under full charging load. Or, could we give our batteries an 80% charge and then disconnect the battery leads?

My Kingsong 14D held a charge for like a year and went from 100% down to like 70%. If I had a V8, I think I'd just give it a full charge and then disconnect the battery leads. I'd probably do the same thing to my Z10 if I had to leave it unused for 3 months.

Or, let your friend use your wheel and learn to ride. It's probably best to keep your EUC in service if possible. You could sell it and then upgrade to a wheel with suspension when you get back. V8's have a good resell value.

Edited October 7, 2023 by earthtwin
one more thing

[Chriull]

28 minutes ago, earthtwin said:

. I'd probably do the same thing to my Z10 if I had to leave it unused for 3 months.

Be carefull - if you did not mod your Z10 there is some substantial drain even if turned off.

[woke rider]

I do have the @MRN76 but I didn't realize that the mod would prevent vampire drain. If I disconnect the batteries would the vampire drain still be a problem?

[Chriull]

2 hours ago, earthtwin said:

I do have the @MRN76 but I didn't realize that the mod would prevent vampire drain. If I disconnect the batteries would the vampire drain still be a problem?

No, the firmware mod afaik does not help. Here was some hardware mod shown to disconnect rhe communication between bms and mainboard when turned off. Afair.

But disconnecting the batteries should work well, too.

[goatman]

40 T's are more likely to self discharge at full charge than at 3.3v, ive put old salvaged packs in storage for over a year at 3.3v with no self discharge. 40T and 30 Q are known to self discharge above 3.7v to fully charged voltages