LiPo's - How long can you float them for?

[rc_sjo]

To all the LiPo experts....

Is it possible to indefinately float LiPo cells at 4.2v? I know most chargers have a cut-off current (50mA or so I believe - please advise me if different) to identify 'fully charged', but i'm wondering what harm could come of the cells if you kept them at 4.2v for extended periods of time (at currents below the typical cut-off above)..

I know keeping LiPos fully charged can/will reduce their lifespan but i'm wondering more about if it's dangerous to do so, eg increase the risk of fire.

Thanks in advance! :cool:

[Julez]

As LiPos practically have zero self discharge, you can simply put them in the drawer, once they are fully charged. No need to float.

However, storing them fully charged makes them age faster.
The longer you store them fully charged, the more they age.

[rc_sjo]

Julez- Thanks! Yes I am aware of this, but that wasn't the nature of my enquiry. My question is not relating to keeping the cells at 100% indefinately, but really aimed more around general charging strategy.

For example, to charge a single lipo cell, is it safe to put it on a precision 4.20v power supply for extended periods of time?

[Julez]

Yes, this is perfectly ok, as long as you limit the charge current.

Do you want to use it as a backup or something?

In this case, I would advise 4-4.1V. You will hardly lose any capacity, but the cell will last longer.

[rc_sjo]

Julez- it's actually for a solar powered balancer i'm designing (idea based on another thread in this forum).. so i'm wanting to ensure it can be left out for the day whilst flying or whatever without risk of damage.. I don't really want to have to introduce a low current cut-off if I don't have to!

[Julez]

Be assured, there is no need for a low current cutoff.
Most chargers terminate the charge at one point or another because

a) There is no point in waiting one hour to get the last 50mAh in at a 50mA charge rate.
b) Chargers cannot sufficiently control very low currents.

[AndyKunz]

In my last job (until July of 2008) I worked on a commercial battery charger system. Most of our systems were Pb- or Ni-based chemistries, but we were experimenting with (and they've now released) Li-based (A123 and LiPo) systems. My job was the hardware and software for controlling the charge.

Basically, we put a shunt between the cells and the power supplies, and limited current to very close to 0A by adjusting output voltage, with temperature compensation. It's quite a package, but runs nicely.

We essentially floated the Li cells at 0A with 4.20V/c @ 25C, and adjusted from there. These are systems that back up computer systems (we have several large-name accounts), banks (big names there, too), lots of cell and land line sites. They have to stay alive for 10 years with virtually no maintenance.

Andy

[rc_sjo]

AndyKunz- that's very interesting, thankyou.

Would I be right in thinking the basic strategy would be to drop the current below what is necessary to float at 4.20v when the cell temp hits the threshold (25C)? If so, does this also cause the voltage to drop? I am curious as to how this additional parameter comes into the typical cccv charging strategy, beyond acting simply as a safety device.

[AndyKunz]

Batteries change their voltage based on temperature. This is called the temperature coefficient (tempco) and is provided by the manufacturer. It takes into account many parameters of the cell composition and manufacturing process. You can measure it yourself, but we just used mfg specs so as to stay within their warranty. The tempco is in the units Volts per cell per degree offset from a reference temperature.

The process is intellectual property, but basically the concept is to keep the current at 0 when the voltage on the bus is X and the target voltage (cell count * temp coefficient * temperature offset from 25C) is also X. If not, adjust output voltage accordingly.

There are also safety and alarm conditions for excessive hot or cold cells, unbalanced cells, etc. It's just that the packs are a LOT bigger.

In other words, as long as the current is 0 (neither charging nor discharging for LiPo, slightly charging for Pb, cycling for Ni) the cells are supposed to last a rated life, in our case we used 10-year and 20-year cells.

Andy

[rich smith]

Lipos kept at 4.2v for extended time do not age or lose capacity but IR goes up. This means cell will not be able to supply max current which may or may not be a problem.

[Tomapowa]

Here's a few articles worth reading re: Lithium batteries and internal resistance of various battery chemistries... below are a few excerpt:

"Li-ion offers internal resistance characteristics that are between those of NiMH and NiCd. Usage does not contribute much to the increase in resistance, but aging does. The typical life span of a Li-ion battery is two to three years, whether it is used or not. Cool storage and keeping the battery in a partially charged state when not in use retard the aging process.

The internal resistance of the Li-ion batteries cannot be improved with cycling. The cell oxidation, which causes high resistance, is non-reversible. The ultimate cause of failure is high internal resistance. Energy may still be present in the battery, but it can no longer be delivered due to poor conductivity."

and

"The Li-ion does not like prolonged storage. Irreversible capacity loss occurs after 6 to 12 months, especially if the battery is stored at full charge and at warm temperatures. It is often necessary to keep a battery fully charged as in the case of emergency response, public safety and defense. Running a laptop (or other portable device) continuously on an external power source with the battery engaged will have the same effect."


http://www.buchmann.ca/Chap6-page3.asp

http://www.buchmann.ca/chap9-page1.asp

http://batteryuniversity.com/partone-12.htm

The known life-time issue with LiPos is what talked me into reverting to A123, Lithium Nano-Phosphate cell technologies.

[rich smith]

There's no significant difference between nano and lipo in terms of shelf life. The ones I have in storage (some full, some half) all check out to 90-95% capacity after 4+ yrs.. They differ significantly in IR though. Cycling is primarily what wears out secondary cells of all types.

Many of the info sources some swear by are out of date or simply incorrect (NET=Not Everything's True). So don't go by what I say either, find out for yourself.

[Tomapowa]

There's no significant difference between nano and lipo in terms of shelf life. The ones I have in storage (some full, some half) all check out to 90-95% capacity after 4+ yrs.. They differ significantly in IR though. Cycling is primarily what wears out secondary cells of all types.

Many of the info sources some swear by are out of date or simply incorrect (NET=Not Everything's True). So don't go by what I say either, find out for yourself.
Impedance growth is one of the top failure modes of high power batteries and a major cause of power fade over the life of most Lipos. A123 cells are engineered (patented) so their internal resistance will decrease with their use. This is the opposite effect to most Li Ion cells which experience a growth in their internal resistance as they are cycled at high rates and/or temperatures. This is a significant benefit in applications requiring long calendar life such as hybrid electric vehicles or in devices that simply must work such as medical devices or mission critical systems, hence why you see big corporations like GE and Chrysler investing in the nano-phosphate technology.
http://www.a123systems.com/technology/power

I have used A123 cells for the last 4 flying seasons, and I've also used LiPos before most manufacturers were selling packs. In the last 3-4 years of A123 and LiPo use, I have seen 1/2 of my LiPo cells (4000+maH) go south in around two years of normal use (maybe 120-150 cycles). There are also good LiPos and bad LiPos... so beware what you buy (YGWYPF). There are many factors that affect LiPo life such as excessive current draw, overheating, and excessive charge rate to name a few. I balance charge all my packs (LiPo and A123) and usually store them at 50-60% cap. So far using A123 Nano-Phosphate cells, I've had only one cell go south on me, that's it, and I have just over 200 cycles on twelve 6s packs I use regularly. I have in fact found out for myself that I get better performance and useage out of A123s vice LiPos. Sometimes I like to provide valuable and knowledgeable links/info so some can learn. Rather than say some info is wrong or incorrect to your belief, mention what you think is incorrect and why, and we all can then discuss and learn together.

[vintage1]

Li-ion != LIPO.

Beware of out of date information applying to older technology cells.

LIPO technology is new every year, and what applied to last years cells may not apply to this years cells.

I've left LIPOS fully charged for considerable periods. Whilst I agree that when they go, its internal resistance (and capacity drop) that kills them, this has always been in my case because they went flat ..very flat. Usually left in a model, or attached to something for an extended period.

[Tomapowa]

Again, be specific on what info is "out of date" and we can discuss. My main objection is that A123 technology is totally different than either of the other lithium battery technologies out there.

[rc_sjo]

Some interesting comments above, but remember folks it was quite a simple and specific question so no need to turn this thread into a lipo war ;)


Is it safe to measure internal resistance of LiPos? Excessive current can cause them to fail from what I understand, but I suppose it's relative to how long the load is applied for? Does anyone have any experience of how long you need to 'zap' a LiPo for to get a stable measurement?

I remember 20 years ago with matched ni-cad packs they'd have the IR detailed on the sticker on each cell - and as far as I know this was measured during the standard 30A discharge cycle for matching..

[rich smith]

Actually all lipos=lion but not all lion=lipo.

Everything you say is correct though. There is huge difference between 2 yrs ago and now. And also true that most failures occur in use and not sitting on the shelf.

There is no doubt nano hase major advantages over lipo but shelf life is not one of them. In general usage they outlast lipos significantly mostly because resistant to physical and electrical abuse rather than idle shelf life. Lipos treated with kid gloves are surprisingly resilient. This is rare in the real world so many are much better off with nano.

Note that you don't need to supply damaging current to find IR. High current is used to test load capacity with some battery types though but not lipos.

Li-ion != LIPO.

Beware of out of date information applying to older technology cells.

LIPO technology is new every year, and what applied to last years cells may not apply to this years cells.

I've left LIPOS fully charged for considerable periods. Whilst I agree that when they go, its internal resistance (and capacity drop) that kills them, this has always been in my case because they went flat ..very flat. Usually left in a model, or attached to something for an extended period.

[rich smith]

Impedance growth is one of the top failure modes of high power batteries and a major cause of power fade over the life of most Lipos.

Impedance (AC) is not of general interest with batteries. Resistance (DC) far more useful. But you are correct that a123 cells live much longer than lipos in general use. Sometimes YGWYPF is true.

[Tomapowa]

Some interesting comments above, but remember folks it was quite a simple and specific question so no need to turn this thread into a lipo war ;)


Is it safe to measure internal resistance of LiPos? Excessive current can cause them to fail from what I understand, but I suppose it's relative to how long the load is applied for? Does anyone have any experience of how long you need to 'zap' a LiPo for to get a stable measurement?

I remember 20 years ago with matched ni-cad packs they'd have the IR detailed on the sticker on each cell - and as far as I know this was measured during the standard 30A discharge cycle for matching..

No war started (not intentionally at least :) ) I happily use both...

You don't normally "zap" LiPos like you do with NiCads and/or NiMH, as they don't inhibit that "memory" effect you read about. Normal cycling and balancing will do the cells well. I found out the key with buying LiPos is to buy the best (always new stock too!.... Ask!) and maintain them (i.e. 40-50% storage in cool area, charge the night before you fly). Excessive current draw near max C and heat are the biggest culprits I've seen which cause damage to LiPos, besides improperly charging & balancing the cells.

Post Note: Some chargers also allow you to see pack IR... such as the Hyperion Duos I use... easy to keep track of pack performace also when using their software.

And lastly... don't forget "LiPo Safety" !