The 80% charging "rule"

[Mono 3,132]

8 hours ago, mrelwood said:

3) There are numerous threads about EUCs (of all brands) “not charging past xx%”, many of them related to charging only or mainly to 80-90%. The wheels had been used for as little as 1000 miles / 4 months.

OK, then here is three different data points: I have "heavily" used three battery packs, for about 270, 170 and 120 full cycles, respectively (computed based on a 15Wh/km consumption). Two of them I rarely charged to 100% and both seem to be fine even the one with 270 cycles. The pack I charged to 100% more often seemed to show more deterioration, but I can't say for sure and don't have access to it anymore.

A possibly relevant observation: roughly half of my chargers deliver voltages that are significantly below the designated voltage (like 82V instead of 84V). If someone happens to use only such a charger without monitoring the voltage, they may rarely or never get to the balancing state even when always charging "to 100%". At least the InMotion V8 series maps 82.5V already to 100% charge state.

If I were to see, after some time, only 83V maximal voltage in the app, is there a way to determine whether the culprit is the charger or the battery? I mean, without measuring the charger voltage directly or trying a different charger.

Edited February 19 by Mono

[WI_Hedgehog 375]

Thank you @mrelwoodand @Chriull, I stand corrected and learned more about EUC charging. 

Note the voltage while charging should be measured during the constant-voltage phase, otherwise (depending on the charger) the standby voltage can be a bit misleading, at least on the chargers I've used, depending on how smart the charger is and how well the output is filtered.

There's room for advancements in EUC charging...obviously the current flow is going to dictate the charge rate while V is less than Vmax, so a faster charge during the Constant Current phase is dependent on the maximum current. I'm used to seeing the voltage during the Constant Voltage phase at 4.35V instead of 4.25V on fast chargers which increases the current flow into the battery and decreases the charge time, though the balancing circuitry has to support it as @Chriullstated, which thinking back to what @RagingGrandpaposted about EUC BMSs, doesn't support it. 

So @mrelwoodis right, with the small balancing resistors the BMS can't support a higher charge voltage. I've seen 4.35V and 4.30V on fast chargers, 4.25V on normal chargers (now considered "fast chargers" from what @Chriullmentions), what voltage are you reading on the EUC chargers during the Constant Voltage phase? If it's 4.20V I might stroke out...although that would still make sense if the Vmax on the resting cell is meant to be less than 4.20V (like 4.1V).

[Planemo 2,075]

10 hours ago, WI_Hedgehog said:

The V in fast chargers tends to be higher.

I will echo what has already been said - I have never seen this with any EUC chargers either. They all output a flat 58v/84v/100.8v or whatever, irrespective of their max output current.

10 hours ago, WI_Hedgehog said:

Some airplane batteries and high-end motorcycle batteries use this BMS technology, and some high-end R/C chargers do also.

This is true. My R/C charger does indeed do this. You can see the current drop off for a few seconds whilst the charger 'sees' what the cell is doing, then ramps it back up again. It works well, and it really feels like the charger is 'intelligent', constantly looking at the cell health. It makes me feel safe too. Unlike charging my EUC with 144 cells, each string of 3 on one 'sensing' wire and questionable cheap Chinese electronics.

[RagingGrandpa 631]

4 hours ago, WI_Hedgehog said:

the standby voltage can be a bit misleading

what voltage are you reading on the EUC chargers during the Constant Voltage phase?

Just checked. My standard Gotway charger outputs 84.3V in standby and 84.2V during CV. That's 4.21 volts/cell.

"Why do I think my DMM is accurate?"
- it was calibrated during manufacture in 2015 and is in pristine condition
- it matches my charge doctor reported voltage closely (CD is a manually-calibrated device)
- it has been consistent in display of 84.2-84.4 V for brand-new 20s Gotway chargers (correlation)

 

13 hours ago, mrelwood said:

If starting at the top of a hill, one indeed can’t start at a full charge.

How people living on top of hills should take care of balancing is indeed a conundrum.

The "top of a hill" situation really needs clarification: define "hill".

Assuming rider+EUC = 100kg, consistent grade, 32km/h riding (20mph), and no prevailing wind: net energy becomes zero on a 15% grade.
It means: if you are dropping 1500m over a 10km ride distance, you use no energy from the battery pack.
Ride steeper than 15%, and you're charging the pack.

15% is really steep for a roadway.
Public roads in the US are generally limited to 12% grade.
There are some notable exceptions, such as 23% in the city of San Francisco, but they are not sustained for long distances.

So overall, I think it is a very tiny percentage of EUC riders that live on top of a ride route with a 15% grade. Most of us don't need to worry.
Respect the beeps!
Thankfully, there is an audible alert for pack overvoltage. If you hit the alarm, turn around and ride uphill for 1km, then continue back down.

Edited Friday at 04:34 PM by RagingGrandpa

[mrelwood 7,561]

14 hours ago, Mono said:

OK, then here is three different data points:

I never meant to imply that balancing every 10th charge would lead to the same experience I had. Only that it can.

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The pack I charged to 100% more often seemed to show more deterioration, but I can't say for sure

So, even by charging to 100% you can’t be sure if there was or wasn’t deterioration at 120 or 170 full cycles. Doesn’t sound like a situation one should try to avoid very much.

OTOH, a dead pack that makes a 16S into a low power low range wheel, definitely is. And that is the risk that balancing often tries to avoid. Of course the pack can happen to consist of near perfectly matched cells, making balancing much less important. But until we can monitor the individual cell voltages, we are taking a big risk by not balancing at nearly every charge.

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If I were to see, after some time, only 83V maximal voltage in the app, is there a way to determine whether the culprit is the charger or the battery? I mean, without measuring the charger voltage directly or trying a different charger.

Yes, by watching the wheel’s voltage in the app immediately when the charger turns green. If the voltage drops very slightly for half a minute, the BMS has cut off the charging, pointing to an unbalanced pack.

 But if the voltage continues to rise very slowly or stays the same, the charging current is just slowly slowing down enough to turn the charger led green. This point to a low max voltage from the charger.

 

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The "top of a hill" situation really needs clarification: define "hill".

For accurate calculations it needs many more data points. Since unobtainable, it is enough to know that if one starts to ride downhill with a full battery, there is a risk that overvoltage by regen could be an issue.

I sometimes got my 16S to beep at me at the first strong braking even when starting at a flat road. So the “hill” doesn’t have to be anywhere near the actual regen steepness to be a potential issue.

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Thankfully, there is an audible alert for pack overvoltage. If you hit the alarm, turn around and ride uphill for 1km, then continue back down.

Exactly. Although, I would first make sure that the warning works as intended by watching the voltage from an app. And not get into fast speeds downhill until one has carefully tested the situation beforehand. It might not take more than a few strong  accelerations uphill to remedy the situation.

Edited Saturday at 01:43 AM by mrelwood