Cold weather - almost didn't make it back!

[gljoe]

Simple answer = No!

Low charge = bad for cell health is a residual notion from the lead acid battery technology. Batteries from Yamaha, Brose, Bosch, Shimano have Low Voltage Cutoff point and they prevent any irreversible damage happening. Having said that, ideal would be to store your battery at 40-60%.

Hi Ravi. I'm glad you answered
Because I've been meaning to ask this question, and it would be great to also have a complex answer to help me understand something

So I believe that the problem is:
Electrolyte oxidation products build up on the negative electrode.
The more buildup, the less capacity the cell will have.
The higher the cell voltage, the faster the buildup happens.
The speed of the buildup is also accelerated with higher temperatures
And the total amount of buildup is thus proportional to the voltage, the temperature, and the time the cell spends in this 'elevated' state. Which is why its bad to charge to 100%, and leave it in this state for an extended period. Especially somewhere hot!

And I think (but I'm not an expert here, hence why I'm asking!), that the reason the Oxidisation happens, is that when the cell is charged, the (very active) lithium ions have effectively migrated over to the negative electrode.

So thinking on that, a simple theory would be that at a low charge level (say 5-10%), there would be a minimum amount of ions over on the negative, so the buildup of oxidation products there (on the negative) should be happening at the slowest level. Which suggests that storing at as low a voltage as possible (say 5-10%) would be the preferable level?

But people don't recommend that. They tend to say, as you have, store around the 40-60%. Why?
is it because you then have some leeway then if you DO forget to charge the battery for months, and thus the self discharge isn't going to cause the cells to fall to dangerously low voltage levels (or in some cases stop the BMS itself from functioning) ?
Or is there some other chemical reaction going on? I mean - if the cell is discharged, then it suggests that now, the lithium ions have mostly migrated over to the POSITIVE electrode. Do they cause some similar oxidization process over at that terminal now instead of the negative one? so storing around the 50% mark means the ions are kind of swimming in the middle of the electrolyte, instead of being concentrated at one pole?

I ask because I often come home after a long ride and I'm on the last bar of my battery. I know I'm going to use the bike within a week or two, so there is no chance of it self discharging to a dangerous, unrecoverable level in that time, but because of the above, I don't know if leaving it in the 5-10% charged range is causing the same amount of permanent degradation, as storing it for the same time at 90-95% charge !?!

 

[Ravi Kempaiah]

Electrolyte oxidation products build up on the negative electrode.
The more buildup, the less capacity the cell will have.
The higher the cell voltage, the faster the buildup happens.
The speed of the buildup is also accelerated with higher temperatures
And the total amount of buildup is thus proportional to the voltage, the temperature, and the time the cell spends in this 'elevated' state. Which is why its bad to charge to 100%, and leave it in this state for an extended period. Especially somewhere hot!

I am impressed!
You have far more keen understanding of batteries than 95% of the folks out there.

But people don't recommend that. They tend to say, as you have, store around the 40-60%. Why?
is it because you then have some leeway then if you DO forget to charge the battery for months, and thus the self discharge isn't going to cause the cells to fall to dangerously low voltage levels (or in some cases stop the BMS itself from functioning) ?

That's right, leaving them at 5-10% is perfectly alright if it's only for week or so.
The BMS routinely checks the state of health of cells. So, it's kind of like self diagnosis and this demands some energy from the cells.
So, over a period of month or two, you will start seeing 10% drop or more. So, it would be advisable to give some buffer to the BMS.

Or is there some other chemical reaction going on? I mean - if the cell is discharged, then it suggests that now, the lithium ions have mostly migrated over to the POSITIVE electrode. Do they cause some similar oxidization process over at that terminal now instead of the negative one?

No. They don't cause oxidation at the positive electrode at 5% or 10% state of charge. But, I like the way you think

In electric cars, the BMS is FAR more complicated because they have thermal cooling, autonomous driving radar, sensors of various kinds etc. They all consume power and it is advisable not to store your electric car at 5% for several months. I know people can forget and that's why E-cars have a LOT more buffer than E-bikes. For example, Chevy Volt is a fantastic car and out of 20 Kwhr, it only uses ~14Kwhrs or so. The rest 6Kwhrs is a buffer, a good amount of buffer.

I don't know if leaving it in the 5-10% charged range is causing the same amount of permanent degradation, as storing it for the same time at 90-95% charge !?!

No. you're not. Batteries are degrading anyway. The best thing is to use them. It's called "Calendar aging". Just the nature of the universe (unless you store them in cryogenic temperatures to limit any kind of reaction happening). The degradation at 10$ SOC is FAR lesser than what happens at 90%.

 

[Mr. Coffee]

It is amazing what you can accomplish with a positive mental attitude and tolerance for pain. Take this as an inspiration or a warning:

 

[larry-new]

Get a cheap timer and put in half a charge. Just before the next ride, I charge to 100% and start to use the battery within 30 minutes.

 

[Telkwa]

Ravi, thanks for showing the insides of a typical bike battery pack. Are those 18650's?

For folks following this, 18650 refers to a specific size of Li-Ion cell. Apparently the correct terminology for one 18650 is a cell, not a battery. A bunch of cells packed together is correctly referred to as a battery. 18650's are ubiquitous. Laptop battery packs are made from 18650's. So are Tesla battery packs.

rich, I'm 225 lbs. Patti is less than half that. We usually would clip along at 18 - 19 mph on the return leg in High assist.

We rode yesterday afternoon. Air temp in the 60's. I didn't take the Reflectix blanket off. I rode part of the outbound leg in Eco, then moved up to Std. Returned to the car in Std. The Haibike 1.0 line is pretty basic - the thumbpad shows battery capacity in 10% jumps until you get below 20%. I got back to the car with 60% charge remaining. Coulda been 69% or 61%.

 

[gljoe]

No. you're not. ... The degradation at 10% SOC is FAR lesser than what happens at 90%.

Awesome. Thanks for clarifying that. Saves me being too anal and needing to bump up the battery charge just a little if I know I'm going to fully charge it in a few days anyway before my next ride.
Cheers!

 

[Figs]

I am a vaper, and most use 18650 batteries. I charge to 100% and discharge all the way, every day. They last years. No need to stress about the battery, just charge it and ride!