18650 capacity loss test - garage vs basement vs fridge

[Offroader]

Here are the final values.
The values are in amp hours, that is all my charger displays. The time is 1 year difference, from July 2016 to July 2017.

Basement:
2016 --- 2017
2.56 --- 2.56 3.3 volts
2.52 --- 2.53 3.3 volts *2017 slightly higher for some reason.
2.51 --- 2.50 3.3 volts
2.54 --- 2.51 3.7 volts
2.56 --- 2.52 4.1 volts
-------------------------------------------------------------------------------
Garage:
2.53 --- 2.52 3.3 volts
2.53 --- 2.50 3.7 volts
2.54 --- 2.48 3.7 volts *outlier here, will run through charger again, but I did short corner of cell and burned it slightly
2.53 --- 2.49 4.1 volts
--------------------------------------------------------------------------------
Attic:
2.53 ---- 2.46 3.7 volts
----------------------------------------------------------------------------
Freezer
2.54 --- 2.55 3.3 volts
2.54 ---- 2.53 3.7 volts
---------------------------------------------------------------------
Fridge:
2.55 --- 2.55 3.3 volts
2.55 --- 2.54 3.7 volts
----------------------------------------------------------------------
Inside garage 6 months for winter, basement 6 months for summer.
2.53 --- 2.51 3.6 volts

Here are findings:
3.3 storage volts had the lowest capacity loss. Made little difference where cell was stored.
Freezer and Fridge had the best results for 3.7 volts.
The attic had the worst capacity loss, which also had the highest temps by far and biggest temp swings.
The basement had similar results as the garage.This could be because the garage is colder for 6 months out of the year, but hotter in the summer, but not too much hotter in the summer. So it averaged out.
The garage with much higher temp flucuations than the very consistent temps in the basement had little effect on capacity loss. Average temps matter more than stable temps.
The single cell that was placed in the garage during winter and basement during summer, to give cooler stored conditions had best results for 3.6 volt cell not in the refrigerator or freezer.

Average capacity loss per year in percentage. (not freezer or fridge)
4.1 volts = 1.6%
3.7 volts = 1.2%
3.3 volts = 0% to .4%

3.3 volts freezer or fridge = 0%
3.7 volts freezer or fridge = .4%

Best way to store cells:
3.3 volts storage is by far the best way to store 18650 cells.
Freezer or fridge will also reduce the capacity loss, colder is better.
Best to keep bike in basement in the summer and the garage in the winter. Always try to put cells in coldest place.

Thoughts:
The test showed what we all pretty much knew, lower stored voltage and lower stored temps are better. It also showed that you can expect on average about 1% capacity loss per year on your 18650 cells that is not from charge/discharge cycles.
Keeping the bike topped off at 4.1 volts isn't a huge killer, but will cause faster capacity loss.
1% capacity loss is also what my bike pack with 280 of these cells showed in the first year, so most of the loss was from age and not so much from use of the cells.
Most important thing is to discharge your pack to 3.3 volts or as low as you can go when not using the bike. Make sure to disconnect it from controller if stored for long periods of time because it may over discharge your pack if connected to anything.
Moving bike outside to garage in the winter may be better than keeping cells in the warmer basement.
I personally charge my bike right before use and don't leave it stored at a high voltage.
Freezing cells at 0 degrees F / -17 degree C had no ill effects, but didn't show any better results than the fridge at 34 F. This was probably due to my capacity resolution is only .01 amp hours.

 

[justin_le]

Average capacity loss per year in percentage. (not freezer or fridge)
4.1 volts = 1.6%
3.7 volts = 1.2%
3.3 volts = 0% to .4%

This is very useful and encouraging firsthand data, and certainly counters the frequent argument that you really need to discharge the pack to some certain level before storage. Unless people have an accurate amp-hour meter on their ebike most people would never be able to notice a 4-5% capacity decrease let alone 1.6%.

I am really curious to know how much this aging trend continues year after year. Like if you stored the battery for 5 years at 4.1V would the capacity be down about 8% or does the ageing start to accelerate over time? I'm hoping that you'll at least continue this same storage experiment with these same cells for another year or two!

When we first started dealing with lithium ebike batteries in ~2004 they most definitely had a shelf/storage life that was measured in a few years whether they were cycled or not, so it's neat to see the cumulative advancements that have been made since then. ~1% loss per year is nothing.

 

[Offroader]

I am curious if keeping the cells at 3.0 volts instead of 3.3 volts will cause less capacity loss. The best cells that showed the least amount of yearly loss were stored at 3.3 volts or the lowest voltage in my test.
There is 400 MAH capacity between 3.0 and 3.3 volts. Cells have about 3250 MAH between 2.5 and 4.2.

This small yearly loss in cells stored at 3.3 volts could have been the result of that 400MAH capacity. So it only makes sense that cells stored at lower voltages should have less capacity loss, so why not store them at the lowest possible voltage.

I will discharge 6 cells to 3.0 volts. I could go even lower but there is very little capacity between 2.5 and 3.0 volts and not sure if I would risk storing a bike pack below 3.0 volts anyway, but I may discharge 2 cells down to 2.6 volts just to see if those cells show signs of increased capacity loss from storage at such a low voltage.

The rest of my cells will be placed in the refrigerator at 3.3 volts as those showed no capacity loss and will be the control cells to base capacity loss from.

I am looking for the optimal conditions to store a lipo pack when the battery is not in use, as I travel sometimes for many months and keep the bike stored without use. I also can't fit my huge pack in my fridge. It may be best to store the pack, fully disconnected from anything, at 3.0 volts per cell.

 

[justin_le]

I will discharge 6 cells to 3.0 volts. I could go even lower but there is very little capacity between 2.5 and 3.0 volts and not sure if I would risk storing a bike pack below 3.0 volts anyway, but I may discharge 2 cells down to 2.6 volts just to see if those cells show signs of increased capacity loss from storage at such a low voltage.

Yeah, in practice there's not much reserve in the cells to feed the BMS quiescent current draw for very long at that point! And even without a BMS, it's likely the tiny internal leakage currents will result in a much more noticeable drop in cell voltage during storage if you start at 2.6V vs 3.0V.

Anyways, I'll be staying tuned for the results if you do this. There are lots of people who take batteries out of storage which won't charge because of cells being well below cutoff, but still generally above 1.5-2.0V. So data in this realm and its effects on the remaining cell life would be valuable.

 

[Offroader]

Yes you definitely wouldn't want to keep anything connected, even a BMS, if storing at a low voltage. I had my Adaptto Max-e drain my lipo packs below safe limits once and I believe the max-e screen must have come on for some reason.

I'll be checking the cells periodically to see if they drop, but it seems that they really lose no voltage when not being used, a 2.6 volt cell will stay very close to 2.6 volts, assuming it isn't connected to any electronics.

 

[ridethelightning]

Yes you definitely wouldn't want to keep anything connected, even a BMS, if storing at a low voltage. I had my Adaptto Max-e drain my lipo packs below safe limits once and I believe the max-e screen must have come on for some reason.

I'll be checking the cells periodically to see if they drop, but it seems that they really lose no voltage when not being used, a 2.6 volt cell will stay very close to 2.6 volts, assuming it isn't connected to any electronics.

had this happen too. still cant figure out why. i checked the pack voltage, for a few weeks it was like 81v, then i left it for another 2 weeks or so without checking and bam, 16v or something

 

[Offroader]

My ebike which uses 20s14p Sanyo NCR18650GA cells seems to lose about 1% capacity a year.

This test is only based on the accuracy of my MAX-E which seems to be accurate, and also stopping right at 3.3 voltage.
Here are the capacity numbers from 4.10 to 3.3 volts

July 8th 2016: 2,774 watt hours 2862 miles, 170.8 KWH
May 19th 2017: 2,731 watt hours 1.5% capacity drop, (51 KWH of usage) from 2016 3762 miles, 221.9 KWH
June 29th 2018: 2,701 watt hours 1.1% capacity drop. (35 KWH of usage) from 2017 4331 miles, 256.1 KWH

The miles and KWH are from my max-e at those times. I did less usage and had less capacity drop. However, I'm not so sure if the usage played a big part in it as both years the usage was on the low side.

I mostly use my ebike in the summer. The bike is in my garage most of the time which sees cold winters and hot summers. garage is somewhat ventilated and doesn't get extremely hot in summer. 95F highest.

I don't use my ebike all that much. Maybe a couple of times a week from May through September and not at all during the winter. My battery has so much capacity that I usually only charge it to 3.8 volts, and rarely charge it to 4.1 volts.

I always leave my bike in a discharged state when not using it, at around 3.3-3.4 volts a cell, I do this because I only charge my bike enough to allow my bike to be fully discharged when I arrive back home. I am using my bike usually between 3.3 - 3.8 volts.

I'm happy with a 1% loss a year and this bike and pack could easily last me 10+ years and still have an incredible range.

 

[markz]

Wow, at your 48Ah pack of GA's and only losing 1% a year, thats the way to build a pack if one has the money and space on the bike. I was looking for the term shelf life from Justin and came across this as I wanted to see if there was a shelf life on older packs like the used Makita's. I just ripped open my Makita pack and a whole section is toast. Fully charged 15S and P dont matter cuz its not even close, its half capacity if not worse. Full charge would get me 8km if that. Turns out I had a loose connection, got tired of hoofing it that last 1/4 home. Debating going larger on brand new 25R 10Ah. We take care of the good stuff we have I know that, but 48Ah Im gunna build another pack but at 15Ah for the longer summer/spring/fall day rides, like what you do. Fun Times!

.

 

[markz]

Any other updates from a 9 months ago?

 

[Offroader]

Have stopped doing any tests and the batteries just sit in my refrigerator at 3.3 volts.

The capacity loss seems to be consistent each year so I find no reason to run tests on them. I will know my pack capacity by testing it each year and will update with that.

 

[Andrewol]

Offroader, where have you placed so many batteries in your ebike? Could you show how you did this?
I would like to put 273 cells in a triangle of my hardtail, but space is very tight, 1 cm left for case with some buffering.

 

[Offroader]

I just hot glued the pack together by squeezing or compressing it together inside my case. I hot glued it compressed like this to make it as small as possible as gluing it together uncompressed adds space.

Its more difficult to do this because getting the hot glue between the cells is more difficult and I had to plunge it down with something.

 

[Andrewol]

With glued cells it can be difficult to replace some if required. I used honeycomb brackets. Did you spot weld the cells?
Could you please show how did you place it in your ebike? Have you used some kind of buffering, as it should be more than 13kg of weight and bottom cells can be jammed on a rough terrain during a ride?
What case do you use for your pack?

 

[Offroader]

cells can easily be pushed out and replaced in my pack. You may have to heat the bad cell to soften the glue, but no issue.
My cells are all welded together.

Using honeycomb brackets are a waste if you need to fit as many cells as possible. The cells don't heat up all that much for it to make a difference.

You can see in the pictures I used closed cell foam all around the pack. The cells make little indentations in the closed cell foam and it holds it very tight. You will need to add a little foam every 2-3 years as it will slightly shrink but not much.

My side covers hold the battery stable from side to side, I used cut pieces of foam to hold the battery tight.

This is the foam I bought, I then cut it with a table saw.

https://www.amazon.com/gp/product/B00T3KKAP8/ref=oh_aui_search_asin_title?ie=UTF8&psc=1

My 280 cell, 3.2 KW pack.

 

[Andrewol]

You have a special frame made to fit battery pack nicely. As I see you welded the cells in the battery with wires? Or you just soldered wires to the cells?
I bought sheets of soft rubber like silicone to wrap around a battery. To save space I'm thinking to buy or make some triangle bag instead of making hard metal case.
Your side covers are from plastic or metal?

 

[Offroader]

My battery was made with both welding and soldering. I wouldn't recommend that unless you have a round battery and can't weld it properly to pass the current through the cells evenly. I soldered wires to nickel strip, then welded the strip to the battery as you don't want to solder onto a battery directly.

My frame is a Qulbix Q76R, and it is thin frame at 76mm which allows the battery to be tightened between two aluminum covers.

With your bike and also since you want to use over 200 cells, you may want something more sturdy to hold the batteries. I would build a battery enclosure, attach it to the bike, then just try and fit as many cells as you can in the enclosure.

Here is a picture with the cells welded and soldered. Wire is 16 gauge. Each 16 gauge wire connects 2 cells in series. I then used small nickel strips to parallel the cells. My pack was not put in parallel for over 2 years and cells were all perfectly balanced.