RC India

What I mean is if I have a lipo battery (http://www.sp4mm3r.com/tattu-uav-battery.html) that is rated for 500-800 charges and I only want it as a backup battery how long will this battery last? That is to say is it 500 charges forever? Like if I take an unused lithium ion battery is 5 years old and start using it will the charge be just as good as if I used it brand new? And will I still get that 500-800 cycles?

Lithium batteries are very powerful densely packed and also very sensitive to temperature changes.
As you have asked yes you can store your Lipo for 5 years but some conditions have to be met like the temperature must be favourable to it and also the storage charge must be close to 50%. Storing in this manner will not do much  damage to your battery. However its efficiency may degrade with time at very less rate as that of new.

If you need help regarding batteries.

I recommend visiting this site. http://www.redsradiocontrol.com/-desire-power--batteries-128-c.asp (http://"http://www.redsradiocontrol.com/-desire-power--batteries-128-c.asp") . Maybe you can ask for details.

Lithium batteries, in general - be it LiIon or LiPo, if treated properly last for a long time. Though their capacity will reduce overtime but its tiny compared to NiMh. Unused battery if stored at a nominal 'storage' voltage won't suffer much. You 'might not' be able to use a 5 year old unused LiIon like brand new but It would still be having a good amount of capacity if it was stored properly at correct voltage.

IIRC its life may reduce it you keep on charging to 4.2v for 3.7v cell , if you reduce the cut off voltage to 4.1V for a 3.7V cell (don't confuse with older 3.6V LiIons which have 4.1V cutoff) its life will(might) increase but at a lesser capacity(since you not fully charging it) and hence you can get more cycles. This is applicable for LiIons - not sure if it applies for LiPos but most probably it must.

Also make a point to use a good charger and if possible calibrate it using a good multimeter(not the cheap chinese ones). Note that Lithium chemistry inside LiIon/LiPol has very less tolerance to overcharging and overdischarging which results in permanent damage.

A side note on primary lithium (non rechargeable Lithium) : Check the page 2 of this - http://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf handbook from energizer which states that : Primary Lithium retains 80% of its capacity upto 15years! Don't know how long can a secondary lithium(rechargeable - LiIons/LiPos) can hold its charge for a given amount of time from any reliable source.

Hope that helps.

No experience of how many cycles, but have some experience of how many years a lipo may last.
Over the last 11 years since I went electric, about a dozen of my lipos died, but most lasted 5-6 years of sporadic use (say 25 flights a year on each).

1. I believe the biggest killer of lipos is storing them fully charged.
2. The second biggest reason is drawing too many amps. I usually never draw over 10c from a 20c lipo.
3. The third is using them for too long in each use. I usually never go below 3.8v per cell.

I guess I'm too conservative, (no helis, no 3d, no quad).
But when stored at 3.85v, I've been able to use them after a gap of 2-3 years!

Lipo may however not be a good choice as a 'backup battery'
Hope you find this of some use
Regards.

Iyer sir has covered most of the things. One other silent killer is the ambient temperature, storing your batteries at high temperature would damage them . Store them at room temperature.