Maximizing LiPo lifespan

So, what tips, tricks and strategies do you have for maximizing LiPo lifespan?

One common complaint I hear on the rcgroups forum is that "LiPo only lasts 2 years anyways, so why care about cycle life?". But I wonder if these types are actually doing what they can to maximize the calendar lifespan.

Some simple facts. Lithium degrades by about 20% in a year when resting at 4.2 volts at room temperature, while if it rests at 40% charge (Which is what, 3.5 volts?), it declines by 4%. At freezing temperature, it declines by about 2% / year if left at 40% charge.

Also, operating temperature affects the lifespan of cells. If you run the cells hot, they degrade much more quickly.

For cycle life, it's also been shown that 4.1 volts significantly improves the cycle life as compared to 4.2 volts - many papers seem to show a doubling in the cycle life (300 as opposed to 600 cycles to 85%). A lot of people prefer 4.15 volts because it's the "sweet spot"(For some) or getting a lot of capacity while increasing cycle life.


So, I'm thinking, a good strategy for longevity is.

If you have a well-defined usage pattern, don't charge until before use. Example, get a timed charger to start charging the batteries at 3:00 A.M if you use them for your 8:00 commute. (Ok, so some LiPo experts will advise against this due to safety; just make sure you charge in the fireplace or something.)

Greater capacity and/or higher c-rates will result in less heating at a given power level.

Tame those charge voltages. 4.1 volts can double your cycle lifespan.

For me, I'm thinking if I used a timed charger, charge to 4.1 volts, and discharge the battery to 40% during the winter time and leave it in a cool place, I should be good. I might also want to use a battery current limit to minimize battery heating, but I'm not really sure if I want to go 20 mph up hills instead of 30 (Well, really, the only time I'll be drawing ~8C will be for a minute while climbing a hill - the rest of the time should only be ~1.5C, so I think I'm probably good there.). With those habits, I think LiPo should last upto 800 cycles or 6 years assuming ~100 cycles per year.

I think the charge to 4.1v------ Don't discharge below 80% dod-------Start out with decent c rate cells plan sounds very sensible. Use really high c rate cells for anything high performance, unless the pack will be really big.

State of charge for storage sounds like a pain to me, forget about it on a dialy use bike. Whatever the gain is isn't worth all that trouble. But on a bike used more recreationally on weekends it would make sense to charge to storage level after the last use. Then top it up friday night. Store it all in the house between rides, or in a place with moderate temps if possible. Winter storage is another issue, so in the warm house might not be so good then if a cooler place is handy.

dogman said:

Don't discharge below 80% dod---

Click to expand...

Yes, that's an important point.

I was just thinking about this; a strategy to keep the resting SOC low without hindering usability. Charging before use, instead of after use, is a logical step, and one I will be using. You can tailor the AH charged to the length of trip. Keeping it around 50% DOD when you don't expect to need it means you still have 20 miles of "emergency" range on a 40 mile pack.

So you're telling me that if I throw my modular battery packs in the refrigerator after use I can keep them fresh for decades?

Very interesting. Now I'm off to ponder active battery thermal management systems (if a huge lifetime jump can be achieved, expect to see these in mass produced vehicles at some point)...now that I think about it the Tesla already does this.

AmpEater said:

I was just thinking about this; a strategy to keep the resting SOC low without hindering usability. Charging before use, instead of after use, is a logical step, and one I will be using. You can tailor the AH charged to the length of trip. Keeping it around 50% DOD when you don't expect to need it means you still have 20 miles of "emergency" range on a 40 mile pack.

Click to expand...

For those with extra eBike battery capacity at the end of the run one could discharge the LiPo to charge a lead acid, like regen, to store for the next charge on the LiPo or to run something else -led lighting? So, you could keep the LiPo at the exact cut-off SOC voltage without having to waste that energy too.

So, I have renewed interest in maximizing lipo's lifespan and I thought I'd add a link to this webpage at http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries and highlight a few details.

According to this place, a 3.92 charge voltage is the theoretical optimum charge voltage to maximize lifespan, but that wouldn't leave much charge, so 4.1 voltage is a good compromise in my opinion. Also, interesting to notice was the DOD versus lifespan table.

100% DoD - 500 cycles

50% DoD - 1500 cycles

25% DoD - 2500 cycles

10% DoD - 4700 cycles

Now, what would be the best DoD for maximizing the total longterm energy that's used from the battery? A lower DoD does increase the cycle life, but the used amount of capacity/energy decreases, causing a decrease in the total used capacity of the battery over its lifespan for a low enough DoD.

So, I calculated the following "energy equivalent in terms of 'full cycles'".

100%*500 = 500
50%*1500 = 750
25%*2500 = 625
10%*4700 = 470

So, it looks like 50% is the optimal DOD in this case. Since I plan on charging to 90% (4.1V) and I do want a little more than 40% of the battery's capacity to use, I'm thinking of discharging down to 40% or 30% before deciding to recharge the battery. Store at 40% SOC overnight, in the fridge, after I'm done using the battery, and recharge the battery to 90% before using it the next and store at 40% SOC in the fridge during longterm storage and I'm thinking this lipo should last about ~1500 cycles. With an average used capacity of 60% during a cycle, that easily beats the 500 cycles that ordinary use of LiPo usually gets (Which is around 90%-100% of the used capacity).

Interesting. In way, that's saying the cycle life golden rule is the same as for lead. 50% dod is best.

Personally, if I get one year out of my lipo I'll be happy. I'd rather romp on em and enjoy than fuss about it too much. My lipo is recreation budget.

Meanwhile my transportation batteries are likely to be the same two pings for several more years.

First, take anything on battery university and forget it.
No greater concentration of wrong/outdated info on batt's exists anywhere on the web.

Second, LG, EIG, A123, Kokam, and a dozen other manufactures make LiPo with >10year shelf life to 80% capacity

NMC is rapidly becomming the dominate lipo chemistry. >10year pack lives with >5000-10000 full 100% DOD cycles is becomming the norm.

If you're only talking about RC lithium cobalt oxide, its shelf life just depends on the quality of electrolyte and contamination. The cheapest stuff fails pretty quick, I've never even had my cheapest most abused heli packs loose 20% in a year, and quality packs (like my HK rhino cells from 3 years ago) still cycle the full label capacity.

The SOC range of the cells as stated above is very important for range. You normally find a window between 20%-90% SOC on most high production EVs and around 30-80% SOC for hybrids that use Li batts. The smaller the window, the more cycles you can get from the battery. However, the smaller the window, the less the energy throughput. There's an optimal point in which the maximum amount of energy that can be put in and taken out is maximized that will give you the greatest $ per Wh.

As mentioned above, temperature is very important as well. Most chemistries have a "happy zone" of around 10-40*C. When the batteries are operated outside of this window their longevity decreases. However, its not just the average temperature of the cell that matters, you also need to consider temperature gradients. A good rule of thumb is no greater than 1*C temperature variation across the surface of a prismatic cell. This sounds a lot easier to do than it actually is. This is often done using a thermal shunt, think flat plate of aluminum, or if the cells are liquid cooled then with some very carefully designed flow plates. GM published their cell cooling design in an SAE paper and is a perfect example of how difficult thermal management can be. The easiest way around this problem is just to oversize your battery and keep your current draw as low as practically possible. Something thats easily doable for Ebikes but no so much for larger EVs.

Another important factor not mentioned yet is compression. Prismatic/pouch/lipoly cells require and adequate amount of compression on each of the cells to prevent delamination. When a battery charges/discharges its electrodes expand/contract which creates mechanical stresses. Over time, especially when charging to a high SOC, the electrodes will begin to separate from the separator and the IR of the cell can increase to the point where it becomes no longer usable. This was the #1 failure mode of my lipoly packs back when I used to fly RC planes.

liveforphysics said:

Second, LG, EIG, A123, Kokam, and a dozen other manufactures make LiPo with >10year shelf life to 80% capacity

NMC is rapidly becomming the dominate lipo chemistry. >10year pack lives with >5000-10000 full 100% DOD cycles is becomming the norm.

Click to expand...

Do you got anything to back this up with? I have yet to see any Li battery technology that can live up to those numbers with the exception of Altair-Nano. Not even LG-CPI, the largest supplier of cells for OEM's, can come close to making those claims.

In terms of longevity I'd think that much of what applies to it's cousin, LiMN, also applies to Lipo. My LiMN Konion cells essentially spend all of their time fully charged except while they are in use. ie I plug in as soon as I pull in the carport. That's over 2.5 years and over 1500 cycles on used cells with an expected life of 500 cycles in the designed use. The only cells I've had with significant degradation was a range booster pack that sat idle for nearly 2 years, starting at full charge. OTOH unprocessed used toolpacks at partial charge sat for the same duration and the cells are fine, so I agree with partial charge for storage, but lower top of charge voltage is far more important than charge timing while in daily use.

I charge to 4.10, run a big pack so low DOD and zero heat stress. I look at voltage as pressure, so just like is typical in many applications, heat and pressure are the enemy.

liveforphysics said:

NMC is rapidly becomming the dominate lipo chemistry. >10year pack lives with >5000-10000 full 100% DOD cycles is becomming the norm.

Click to expand...

Really?? Dang, that sounds like something pulled from my dreams. Any more info on this dreamy tech?

TopherTheME said:

The SOC range of the cells as stated above is very important for range. You normally find a window between 20%-90% SOC on most high production EVs and around 30-80% SOC for hybrids that use Li batts. The smaller the window, the more cycles you can get from the battery. However, the smaller the window, the less the energy throughput. There's an optimal point in which the maximum amount of energy that can be put in and taken out is maximized that will give you the greatest $ per Wh.

Click to expand...

So, what SOC window would maximize the total amount of energy that can be put in and taken out? Since maximizing the usable energy lifespan of LiPo is important to hybrids (Whereas "high production EVs" have to also consider maximizing range which is against the interest of maximizing the total energy throughput over the batt's lifepsan.), I'm guessing it's something like 30%-80% SOC?

I think my happy window of temperature range is much narrower than the batts.

What's the deal with battery university.... owned by cadex.... and they talk up nicd a lot... hrmmmmmmmmmm

auraslip said:

What's the deal with battery university.... owned by cadex.... and they talk up nicd a lot... hrmmmmmmmmmm

Click to expand...

Yes. Battery university has gotta be the biggest source of bad battery info on the web.

liveforphysics said:

auraslip said:

What's the deal with battery university.... owned by cadex.... and they talk up nicd a lot... hrmmmmmmmmmm

Click to expand...

Yes. Battery university has gotta be the biggest source of bad battery info on the web.

Click to expand...

So, what's the implication for me? My projection of 1500 lipo cycles should actually be more like 3000? Excellent, dude!

So, I started scouting out other places for info online, and according to http://www.sciencedirect.com/science/article/pii/S0378775303002222, 60,000 cycles should be expected with 25% DOD!!!

In Low Earth Orbit, rofl.

Yes. Battery university has gotta be the biggest source of bad battery info on the web.

Click to expand...

besides from people like me of course.... which is why I no longer give advice...

I talked to a contractor yesterday about his power tool batteries.... he said that across the board they all only last around three months of daily use... nicd and lithium, cheap stuff and good stuff.
made me realize that a properly sized pack is pretty damn important. More capacity so it doesn't get hammered so much, and don't discharge it to LVC every time!
I'm excited to see how well the lawn tools I've ordered will run on the lipo I've ordered.
It seems like at 1/5th the cost it would be a hell of a deal for contractors. Someone could probably make a lot of money if they made battery packs for contractors that were compatible with their tools and lasted a year of daily use.

Then I talked to a guy tonight who tests and resells electronics on ebay. We talked about batteries. Lots of tool packs with dead cells. It made me wonder, "what's more expensive; the quality control that goes into a pack or the raw materials and manufacturing?"

We're spoiled here with our cheap HK lipo, but obviously we have to do our own QC. I see lots of room in the market for people to buy cheap Chinese lithium, test it, and build high quality packs for LEVs, tools, and electronics. Seems like an emerging market.

/drunk rambling

Which power tool brand? I've seen it range from 2 years daily hard use for makita nicads, to three weeks for other nicads. Getting years out of B&D nicads now, but the use is more like weekly with my job now. Bet that guy overdishcarges a LOT. No matter what you tell a worker, they'll run em completely down of course, then charge for 10 min, and run em down again. Battery murder.

I really believe 90% of the distrust of electric vehicles and batteries in general comes from people committing batterycide by doing what you said. I know I did that when I was a little kid and wanted to play my game boy and with my r/c cars.

Really need to make a battery pack that shuts off well before what the LVC is set at now. Also it won't re-engage until it's been on the charger for long enough. It'd be a boon for the contractors who buy the packs for their employees.

liveforphysics said:

NMC is rapidly becomming the dominate lipo chemistry. >10year pack lives with >5000-10000 full 100% DOD cycles is becomming the norm.

Click to expand...

What do you think, just your opinion is fine, we can expect from the Turnigy Nano packs that are available now? Cycles, lifespan, charge discharge levels?

Also, has anyone got any data or knowledge or experience or an opinion about storing long term (12+ months) without use at 0F LiPo or A123 in a Freezer at zero F vs a Fridg / refrigerator at 32+F.

Will the battery be damaged in any way by storing at 0F ??? I understand the battery would need to be protected from moisture & thaw-out very slowly to room temp before any kind of use, but I'm thinking freezing at 0F should be ok ...unless someone knows otherwise.

Please, anyone, feel free to offer opinions or experience or "hard data". Thanks!