Because of my job, my rc plane hobby was put on hold for about 5 years. Now, I found that I am able to rejuvinate my Ni-Cd batteries but the nickel metal hydride batteries seemed beyond repair. I invested quite a bit in these Ni-H batteries and now they appear useless.
What is your experience with these batteries? How do the new LiPO batteries hold up during long periods of inactivity?
Nimh last at best a couple of years. Nicads can be revivd. LIPOS generally wont self discharge in much less than a few years. Certainly with no special storage they are better than 95% charged after an idle winter..
LiPo batteries have limited shelf life. They will go bad by themsleves, even if you are not using them, and they will go bad faster than NiCds or NiMHs. There are some recommendations that are supposed to prolong the shelf life of LiPo batteries, like storing them half-charged and storing them in cool place, but the bottom line is that once you get yourself a LiPo battery, you have to fly it.
Some more information of typical Lithium accumulators:
"... When compared to the lithium-ion battery, Li-poly has a greater life cycle degradation rate. However, in recent years, manufacturers have been declaring upwards of 500 charge-discharge cycles before the capacity drops to 80%. ..."
How to prolong lithium-based batteries:
"... A lithium-ion battery provides 300-500 discharge/charge cycles. ... Aging of lithium-ion is an issue that is often *ignored*. A lithium-ion battery in use typically lasts between 2-3 years. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the cell resistance reaches a point where the pack can no longer deliver the stored energy although the battery may still have ample charge. For this reason, an aged battery can be kept longer in applications that draw low current as opposed to a function that demands heavy loads. ... Figure 1: Permanent capacity loss of lithium-ion as a function of temperature and charge level. ..."
They are much lighter than the NiMh or NiCad for the same output, nearly half the weight. The advent of LiPos has really let electric flying hit the mainstream. The biggest downsides are cost ( but they have gotten much more reasonable lately) and safety. Damaged or mishandled LiPos can and will burn. That said, I've been an all electric and all LiPo flyer for 5+ years now with no regrets. I'm also very careful in how I handle/charge/discharge them.
LiPo AKA LiPoly stands for Lithium Polymer. This is the main battery type used in electric RC planes.
Once again, to reiterate the _correct_ information I already provided before:
For extended term storage manufacturers recommend
- storing LiPo packs at 50% charge.
- storing LiPo packs in cool place (one of the recommendations I saw suggested 32 F = 0 C)
LiPo batteries degrade with time even if not used. The speed of degradation depends on the specific cell chemistry (and, of course, storage conditions). High C-rating chemistry usually degrades much faster than low C-rating chemistry. For example, Thunder Power Extreme V2 series batteries (now being outphased) had very good C-rating, but rather short by modern standards shelf life.
Note, that by "shelf life" I don't mean the ability of the battery to retain the charge (self-discharge rate) or the cycle count in active use. (Self-discharge rate of LiPo is excellent, much better than NiMH and even NiCd. Cycle count is also very good). By "shelf life" I mean the general degradation of the battery when not in use. And this still remains a weak point of LiPo, even though it is constantly improving.
Thanks for all your responses. I was confusing the Li polymer batteries with the Li Iron Phosphate ones. After some research it appears these are geared more for electric motor scooters and motor vehicles. I'm guessing the light weight of the Li Polymers makes them more suited for rc planes. The general opinion seems to be that LiPO are a durable time tested battery with a long shelf life as long as you don't exceed recommended charge/discharge rates. Perhaps Andrey had a marginal battery and was just relating his experience with it.
Lipo's need to be charged before storage, leave it discharged and store it and it probably won't be happy when you get it back. They self discharge less but below a critical voltage per cell (2.4V I think) they will become unrecoverable. So fully charging them and leaving them and they'll be fine, using them till they are flat then storing them is going to give you trouble..
Actually there were some reports on RCGroups that seem to demonstrate that combination of full charge and room (or above room) temperatures are pretty hard of LiPoly batteries, i.e they do promote faster degradation. While there are RC products out there that recommend storing LiPoly in charged state (= fully charged) in their manuals (Hirobo helis, for example), the manufacturers stated repeatedly that this is not a good practice. TP datasheet can be found here
and it explicitly says that the batteries are better stored at 50% charge. As it has been said here, modern LiPoly chargers support "50% charge" mode specifically for that purpose.
LiPo's require a bit more care in their care and feeding, but they sure are nice. Just remember that they've been known to burst into flame while charging, and charge and store them accordingly. You need to take an attitude toward a LiPo that you would toward a similarly-sized flask of gasoline, rather than you would toward a similarly-sized pile of flashlight batteries.
If it's for an electric powered plane then you just need a charger and ESC that are made to 'understand' LiPo's and are set up for them, and follow the due precautions mentioned above.
If it's for a receiver pack then you have a choice between 3.7 volts (which is too little for all but teeny planes) or 7.4 volts (which is too much for just about anything without a regulator). Apparently the jet folks do use LiPo packs for their receivers (I've been approached professionally to design a regulator), but I have no clue about where to get a regulator, how much to expect to spend, or who makes good reliable ones.