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I am trying to determine the shelf life of a typical Li-Polymer battery. I have Googled but can't find any definitive view on the matter.
There are two questions regarding the battery shown: 1. What is the likely useful shelf life of an unused (from factory) battery 2. Referring to the image, clearly we have + and -, but what might the T terminal be for. The voltage between +/T and also +/- is the same at 3.78v.
Many thanks
Li-Poly batteries have a useful voltage range of 3.0v to 4.2v --under 3.0v they are effectively discharged, and 4.2v they are fully charged. Both the protection circuit in the battery itself and the special L-Poly charger chips limit the high-end voltage (since going above this value can cause the battery to vent and catch fire).
Contrary to popular belief, it is not best to charge the chip to the maximum voltage of 4.2v before putting it in storage; rather a voltage of around 50% of full charge (3.6 to 3.7v) is preferred. This is borne out in the table in Andreas' answer. 3.7v is also the typical voltage the battery is charged to when it leaves the factory, and is considered the nominal working voltage for the battery as well.
In my own work, the company I am contracting at has had over a thousand cells in storage at room temperature for 18 months to over two years without any issues, with only a slight voltage drop. I realize that is anecdotal evidence.
The important thing is before using any Li-Poly cells that have been in storage is to measure their voltage before being hooked up to a charger. If they have dipped below 3v (or show no voltage at all, because the battery's internal discharge protection circuit has kicked in), I would safely get rid of them -- they are damaged goods. (Some would argue that has long as they haven't tripped the battery's low limit, which might be 2.5v, they can still be rescued.) And of course if a battery shows any signs of expansion (starting to look like a little pillow), out it goes.
For LiPo cells you can not strictly speak of a shelf-life, since they basically start aging the moment the batteries are assembled.
Aging depends greatly on the state of charge and storage temperature. The more they are charged, the faster they age. On the other side they are damaged if they are every discharged below a certain point they are unusable and can not be safely charged anymore (consumer device batteries prevent damage in that case by e.g. monitoring the cell temperature)
One table I could find online (at batteryuniversity.com) is sadly for LiIon cells, but LiPo cells should fare similar. It shows capacity loss vs. storage temperature and state of charge:
Storage Temperature | 40% charged | 100% charged
0°C | 2% after 1 year | 6% after 1 year
25°C | 4% after 1 year | 20% after 1 year
40°C | 15% after 1 year | 35% after 1 year
60°C | 25% after 1 year | 40% after 3 months
The thing that makes matters worse is that cells have self-discharge, and normal consumer device batteries have protection circuits embedded in them which consume a small amount of current even if the battery is not being used. So even if you made sure to store all your batteries at e.g. 40% charge, they will not stay at those 40% and slowly drift towards deeply discharged or unusable.
So sadly it's not as easy as to say that such cell have a shelf live of e.g. x years.
Regarding your second question I would refer you to Why are there 3 pins on some batteries?
LiPo batteries, if new and stored at 3.80 volts and at room temperature, can last several years. The measure of battery health is internal resistance, and that resistance increases very, very slowly if the battery has not been cycled and it is stored properly. I have stored numerous new LiPos for more than two years, and they performed as new when charged and used. In use, LiPos can only be cycled about 200-300 times, however, and once oxidation has begun from usage, internal resistance begins to increase and storage/shelf life diminishes.
When lithium ion batteries first came out about 10 years ago, they appeared to loose about half their capacity in three years regardless of use. Considering that most consumer electronics e.g. phones and laptops have had a useful life of about 3 years before becoming too slow computationally to run the latest software in an appealing way, or had too little expandable memory to upgrade, this was somewhat tolerated. As others have already noted, most lithium ion chemistry commonly available now appear to additionally degrade (in terms of capacity) as a result of every full charge/discharge cycle, storage time, temperature and potentially depth of discharge.
Increasing their net storage capacity ie energy extracted on discharge aggragated over all their discharge cycles (or at least until their capacity degrades to a barely useful level,) will likely be achieved by limiting their maximum charge AND discharge voltages and maximum discharge current (beyond the manufacturers recommendations). Some manufacturers may publish data on all of the factors affecting their utility for your specific use or make it available on request. The BatteryUniversity website appears to have some data on these things as noted by others.
Lately I have been looking for lithium ion batteries with considerably longer shelf life - for 'power-wall' like applications. This information is typically not too readily available from inexpensive resellers (e.g. aliexpress) and you may want to avoid them given they could sell manufacturing rejects or 'seconds'/'thirds'. I am also having trouble getting these folk to indicate if the manufacturing date is printed (or better stamped) on their cells. My recommendation if shelf life is of interest to you is to find a manufacturer that clearly marks their cells with a manufacturing date AND gives a shelf life guarantee. Tesla car batteries have longevity/shelf life guarantees of 8-10 years (if memory serves me correctly). Some people sell these on-line used - but unless you know how many cycles they were subjected to (and their depth i.e. minimum discharge voltage,) you may not have a good idea how long they will last you.
I don't recall if lithium ion cells suffer from shallow charge/discharge cycles like earlier chemistries (e.g. nicad cells). This sort of thing would happen if someone only ever drives their Tesla for a few minutes to the store and back and always re-tops the charge on the battery back to 100% between trips. Note that lithium Ion cell manufacturers might not prefer you get the maximum life out of them either, so getting this data might require persistence. Should Tesla have solid guarantees on their battery longevity, it's more likely their battery packs include a sophisticated battery monitoring feature (and charge control/options) that I have yet to search for via google...
My apologies if this appears somewhat poorly edited (noting most electrical engineers are not creative writing enthusiasts.)
