18650 Cycle Life vs Discharge/Charge Rate

[chaka]

After having seen countless discussions, graphs and lectures and my own testing I have come to the conclusion that these cells, no matter what their rating, will degrade by as much as 40% after only 200 charge/discharge cycles when used in a high discharge environment. It has been shown that you can increase cycle life by 400% or more simply by discharging at 1C or less and charging at 0.5C. Some cells can go 2000 cycles at this rate and still retain 80% of their original capacity.

What this means in regards to cost and efficiency. Cost is an easy one to figure, not only are high discharge cells double the cost they also will have to be replaced at a much higher rate when discharged heavily. Usable capacity is another factor of cost, when a cell is discharged at 1C or lower it has more usable capacity due to a lower voltage sag giving you a higher return on your investment.

Discharging at 1C or less also flattens the discharge curve giving you a higher average voltage and minimal initial voltage sag.

Charging at 0.5C or less may seem tedious until you realize this means charging at 350watts in a 700wh pack. Not so tedious now is it? This shows that we can charge our packs safely at high wattage with the use of a high capacity pack.

The only industry I have seen that is paying close attention to these characteristics are auto manufactures, it wouldn't be very profitable for them if they had to replace packs under warranty after only 1 or 2 years of service. On the other hand, if you are in the business of selling batteries, it is more profitable to have consumers replace batteries frequently if there is no long term warranty to worry about.

One reoccurring discussion is the use of easily swappable batteries to gain more range. It works but why not use all the batteries at once and have them work together? This will extend the overall range and cycle life of your investment.

The initial investment in a large capacity pack may seem a bit high at first but in the end it will cost you nearly 10x less than a high discharge low capacity pack. This factor is further increased by the fact that you now have more lower cost options to choose from since you are no longer dependent on expensive high discharge cells.

 

[torqueboards]

Makes sense... 200 cycles is bad :? 2000 cycles is a big difference

 

[elkick]

Just assuming: as a conclusion this would mean to carry a 10-15kg battery attached to my 5kg Board?

 

[torqueboards]

Just assuming: as a conclusion this would mean to carry a 10-15kg battery attached to my 5kg Board?

Yeah, that's the unfortunate part

 

[Vanarian]

This or you simply change chemistry and you're good to go.

If you want more cycles at high discharge rates you but LifePo4 from a123 systems, downside is you need to counter lower voltage with more cells. But you're good for a thousand cycle at least.

I think the main factor in battery choice is the price of initial pack. If you can afford more powerful and more durable batteries then you can get a quality setup for all your needs. If you can't you have to make compromises.

 

[chaka]

Just assuming: as a conclusion this would mean to carry a 10-15kg battery attached to my 5kg Board?

More like 4 to 5kg, a complete board with over 700wh comes in around 10kg/23lbs giving you at least a 2 hour run time at 30kph/20mph.

Surprisingly, a board of this weight is much more stable. The added weight seems to dampen the ride giving the rider more confidence at speed. Plus you could extend the cycle life further by limiting full charges to times when it is actually needed.

You could go with a 350wh pack and drop the total weight down to around 7.5kg/16lbs but you will induce some deterioration with burst currents while accelerating or climbing steep hills. This deterioration could be halted with the use of a cooling system like Tesla Motors used in their roadster.

 

[tomjasz]

Satiator.

 

[onloop]

I have come to the conclusion that these cells, no matter what their rating, will degrade by as much as 40% after only 200 charge/discharge cycles when used in a high discharge environment.

I'm not sure what your agenda is but until you provide some references or test data for how you have come to this conclusion I think your "opinion" is misleading.

Maybe you are battery chemistry engineer? That would give weight to your opinion....

When you say "these cells" what are you actually talking about? Generic references such as this make it very hard for you to structure a compelling argument. 18650 is a form factor only, It has nothing to do with performance rating or discharge specification.

"High discharge environment" what is this exactly?

I can only assume you are referring to observations from your own setup that has a gearing ratio of 1.6:1 (20:32)

You said your system pulls a constant 2000W

So you are constantly pulling over 60A? from these 8S packs you are using.

So please tell us specifically;
- What cells are you using?
- What configuration is the pack?
- What is the BMS rated for? Do you have a BMS?

not only are high discharge cells double the cost they also will have to be replaced at a much higher rate when discharged heavily.

This means nothing without data, Please show us the calculations used, what are you comparing?

What does replaced at "higher rate" & "discharged heavily" mean in actual numbers? What are your sources of test data?

The initial investment in a large capacity pack may seem a bit high at first but in the end it will cost you nearly 10x less than a high discharge low capacity pack.

HOW?.... how can this magical battery cost 10× less?



ALSO..why have you not mentioned anything about SOC and duty cycle.... if someone is interested in pack longevity & cycle life, as I assume you are, it is worth discussion. There is compelling evidence that by charging the pack 20% to 10% less than total capacity can have a dramatic result in duty cycles.

"Most lithium batteries are capable of vastly improved cycle life if they are not fully charged to saturation all the time, but at the expense of reduced range".

A battery is simply one element of a propulsion system. It must not be designed without first considering the other elements of the system....

Of course in any propulsion system a massive battery (eg.12S10P) will have lower amp draw /discharge rate per cell then a smaller pack (eg. 10s3p) but if the smaller pack is designed and built to work safely within its own specifications in an effecient system that has a drive train that also is running within the same system limitations the only advantage of a bigger pack is more Wh.


Don't design a battery design a system.


If you are concerned with battery life cycle focus on DOD & SOC. Buy this: http://www.ebikes.ca/product-info/cycle-satiator.html

 

[onloop]

Benefits of Partial Charge
one of the key benefits of the Cycle Satiator is its ability to let you easily control the charge level of your battery. It is now well known that most lithium chemistries (with the exception of LiFePO4) can see drastic improvements in calendar and cycle life when they are not held at the nominal full charge voltage of 4.2 V/cell but are charged to a lower voltage instead. That’s how electric car manufacturers are able to 5-8 year battery warranties on cells that usually only test to ~500 cycles.
With most ebike chargers, you have no ability to set the full charge voltage and have to accept topping it up to 4.2 V/cell. This gives the most range on a charge, but if you don’t require the full capacity of your battery on most of your trips then you are unnecessarily reducing the battery life every time you charge it. In many cases that means replacing your ~$1000 lithium battery pack every 1-2 years, when with proper management it could be lasting more like 4-5 years. In fact the further from full charge you go, the more pronounced the life cycle improvements.
With the Charge Simulator we’ve made it really easy to produce profiles that will charge a battery to a given percentage of its full capacity, so you can easily create say 70%, 80%, 90%, and 100% charge curves for your pack, and the Charge Simulator will figure out the required full charge voltage for each. If you have a 20Ah battery, and typical trips only require 12Ah or less, then the 70% charge (to 14Ah) would be fitting most of the time. If you knew you needed just over 16Ah for a longer journey, then you would choose the 90% profile instead, and when you want to get full range from the battery or let the BMS balance the cells, then that is your only occasion to use the 100% profile.
Used in this manner, the Cycle Satiator will pay for itself many times over just by extending the useful life or your expensive lithium battery packs. Nevermind all the other benefits of having a compact, programmable, sealed, high power battery charger with a graphical display screen.
Partial Charge and Cell Balancing
One of the only downsides to partial charging is that many inexpensive battery management system (BMS) circuits will only do active bleed balancing of the cells when they are at or near the full charge voltage of 4.2 V/cell. This means that with partial charge profiles that don’t reach that voltage, the BMS circuit will never be able to rebalance cells if they are drifting apart. Over time you may have less available capacity from the pack as certain cells will hit the low voltage cutoff on discharge well before others.
If this is an issue it can be easily remedied by occasionally (like once every month or two) leaving the pack connected to a 100% charge cycle overnight.
Good quality programmable BMS circuits will usually attempt to balance the cells whenever they see more than a certain voltage spread between the highest and lowest cell in the group, and in that case there is no problem with partial charges. Similarly, good quality cells rarely drift out of balance in a series string, and can easily handle 100 or more cycles and maintain a perfect voltage matching even if the BMS circuit doesn’t do any active balancing. But if you aren’t sure of the makeup of your battery pack, then the protocol of occasionally giving a 100% top-up is a good bet to ensure both a long cycle life and evenly matched cell voltages.

 

[tomjasz]

Satiator.

Satiator. I charge my 24V, 36V, 48V, Lithium, SLA, and more all at varying charge rates and charge levels.
I've now sold a few and none come back. Everyone is happy. Best $300 ever. If the information is even close to accurate the $300 is paid back in maintaining just one of my Cell_man batts. BTW he sells them too! My shipping is cheaper.

 

[chaka]

Here is a link to an interesting teardown recently posted by a guy building a battery bank using a salvaged Tesla Model S battery pack. http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack

This pack has 85kWh @ 400v and gives the Model S a rated range of 270 miles (434 km) at 70 mph (112 km/h), a discharge rate close to C/4. The smaller 60kWh pack has a range of 240 miles (386 km) at a discharge rate close to C/3.

I am assuming Tesla is doing their best to balance battery cost and longevity when designing their packs not to mention the need for a constant voltage with little sag. I supposes it is the difference between designing a toy or designing a long lasting commuter vehicle.

Another interesting spec on the Model S pack is the 4.15 charge voltage, meaning we are only seeing numbers on 95% of the overall capacity of these batteries.

A board could easily be made to match these numbers and remain under 30 lbs (13 kg), a little more if dual motors are used.

Lithium being a finite resource, we should do everything we can to prolong the overall service life of our cells and dollar.

 

[maxchilton]

After having seen countless discussions, graphs and lectures and my own testing I have come to the conclusion that these cells, no matter what their rating, will degrade by as much as 40% after only 200 charge/discharge cycles when used in a high discharge environment. it has been shown that you can increase cycle life by 400% or more simply by discharging at 1C or less and charging at 0.5C. Some cells can go 2000 cycles at this rate and still retain 80% of their original capacity.

proof???

 

[maxchilton]

op, you should read this if you haven't already.

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

 

[chaka]

op, you should read this if you haven't already.

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

Thanks, yes I've seen that. Pretty much all reasons to go as big as you can with your pack.

Everything points to heat being the major culprit behind the low cycle life of high discharge cells.

Lets pick a high discharge cell at random. How about the LG HE2, lets look at the specs. http://www.powerstream.com/p/LG-ICR18650HE2-REV0.pdf

From the factory spec sheet we can see after 300 cycles @ 15 amps the cell only has 60% it's original capacity. It has been shown that a high amp pulsed discharge results in even fewer usable cycles. So far I have only seen anecdotal evidence from user testing on various vaping forums to the latter. Once a cell has reached this point it will deteriorate rapidly if you continued to pull high amps due to the high internal resistance and heat generated.

It is interesting how this information seems to be so "matter of fact" to groups who have been using these cells for a period of time vs. new converts.

The ability for these cells to output such large amperages is truly amazing but is better suited for items with space limitations such as power tools, the suggest application for the HE2.

 

[torqueboards]

I think it's safe to say.. If you can afford it you might as well go bigger and add more P's to your setup and/or if you don't need the voltage (S) then exchange it for adding it to the (P) capacity.

Adding more cells in parallel wouldn't do no harm besides added weight and longer battery life which aren't bad things.

 

[Vanarian]

The issue is pretty simple : the closer you are from 1C discharge rate of your pack (plus nominal voltage and not maximum voltage), the longest calendar cycle you will achieve.

If we had motorcycles to drive we could try to get say ultra heavy packs made of 200,000 mAh at 150V nominal. The continuous juice pumped out of these batteries at 1C is 30Kw. And yes the dragster could run for thousands cycles before getting deteriorated.

But we don't, we try to pack more possible batteries in the smallest and s lightest form factor. So you must accept compromises because no matter how hard you try you won't achieve virtually immortal batteries if you seek power. Not with actual chemistries.

 

[agraham]

I think it's safe to say.. If you can afford it you might as well go bigger and add more P's to your setup and/or if you don't need the voltage (S) then exchange it for adding it to the (P) capacity.

Adding more cells in parallel wouldn't do no harm besides added weight and longer battery life which aren't bad things.

You are confused. Adding cells no matter whether you add them in series or parallel will increase your capacity.

But wait you say. The amp-hours only go up if if I add them in parallel

Yes but the watt-hours goes up no matter which way you add the cells and its the watt hours that counts. If the voltage is higher then you will use less amps to achieve the same output power so your capacity is increased either way.

Can we please stop with the "parallel is the way to achieve more capacity" misinformation.

 

[torqueboards]

I think it's safe to say.. If you can afford it you might as well go bigger and add more P's to your setup and/or if you don't need the voltage (S) then exchange it for adding it to the (P) capacity.

Adding more cells in parallel wouldn't do no harm besides added weight and longer battery life which aren't bad things.

You are confused. Adding cells no matter whether you add them in series or parallel will increase your capacity.

But wait you say. The amp-hours only go up if if I add them in parallel

Yes but the watt-hours goes up no matter which way you add the cells and its the watt hours that counts. If the voltage is higher then you will use less amps to achieve the same output power so your capacity is increased either way.

Can we please stop with the "parallel is the way to achieve more capacity" misinformation.

The point I'm trying to make is that by adding parallel you decrease the discharge rate per cell versus running in series which most people wouldn't use.

Which would increase the life cycle of the pack.

Parallel is a way to increase capacity (watt hours) without increasing power.

 

[chaka]

Can we please stop with the "parallel is the way to achieve more capacity" misinformation.

Try to take into account we are limited to 12s on most motors and speed controllers.

I am assuming everyone here knows their watt hour from their amp, so yes, once we have settled on a voltage adding in parallel is how it's done.

The auto makers are really doing their homework on this, we should look at their numbers a little if we are going to pretend we are concerned with economy.

 

[chuttney1]

I can confirm from basic rules of electricity, adding batteries in parallel increases total capacity. Amperage draw is independent of capacity and dependent on the system.

 

[onloop]

It's really simple.... build more effecient drive trains.

Do you really? think using examples of tesla batteries and how "big" they are actually means anything if their drive train was ineffecient? this generic reference doesn't support your false augment.

OP. Please report back when you have built a more efficient drive train.

Otherwise: This thread should be deleted as it is full of S H ! ♧

List of questions not answered by OP
1. What batteries are you using (brand)
2. What configuration
3. Proof of concept. Shows us some real life data of your setup.
4. Explain why you are using 1.6:1 reduction
5. What is the peak, avg, and minimum amp draw in your system .

 

[torqueboards]

Just because the OP has a crazy reduction doesn't mean that the point wouldn't be valid.

It's common sense that more P's would lower the discharge rate across all cells which would mean less stress on each individual cell.

Do you need 6P? No, not really? What it be better to have 6P vs 2P, 3P setup? Well, yeah..

6P wouldn't hurt the setup besides added weight.

I still wouldn't sell a 12S6P battery pack, because I don't think many people would want to spend $400 bucks for a battery pack. Perhaps, an 8S6P or 6S6P setup might be better.

Either way it's just good for food for thought.

 

[chaka]

Maybe we should look into a more modular system of designing batteries around 1s5p and 1s10p packs with protection circuits and 1s3p high discharge for people who don't mind the reduced cycle life and want a lightweight build.

It would allow easy reconfiguration into other voltages plus it would allow for unregulated shipping by keeping each module under 100wh's. Justin over at Grin Technologies is working on this idea for e-bikes, I think it is just as suited over here with us e-boarders. Its just a matter of time before more people start getting caught breaking the shipping regs on shipping over 100wh without paying all the hazmat fees.

Not all boarders have the same power requirements so a modular system would be great if someone found their pack lacking after a build was completed. They could just add to it by adding more S's or another round of P's. Overhead would also be reduced since only a few different sized modules would handle every variation imaginable.

 

[torqueboards]

If they are modular, how do you plan on connecting them easily? With added BMS and all?

Max for USA UPS Ground shipping is 300Wh. Air is another story 100Wh max usually.

The funny thing is HobbyKing ships these RC Lipo packs via USPS and I don't think they label them either lol. Although, ground in USA but they should still label it on the outside that way someone doesn't chuck it onto my porch.

 

[chaka]

They would have protection circuits individually so they would be good there, You can then hook them in series just as you would with hobby grade lipo's. Maybe have the connections at opposite ends to make this easier. A separate balance board could then be used for balance function when charging. Completely modular.

Edit: Yeah ground is a totally different story, shipping by air seems to be the hurdle.