First things first, what you are buying in any LiPo battery is a controlled explosion in a wrapper. Sounds over the top? well take a look at google video with the search "Lipo Battery Fire" to see what happens if one is overcharged. Now that you are suitably shocked at what can happen (and remember these things are in EVERY mobile phone and laptop, but at much lower charge and usage rates) I'll tell you what to look for when buying and using these amazing devices.
The LiPo battery is indeed an incredible bit of engineering containing anyware from 3 to 5 times the "energy density" of a NiCad or NiMh rechargeable battery, bearing in mind that your average NiMh battery, if shorted, is able to melt through cooker power wire with ease and you begin to see the need to treat a battery with 3 times this power with great respect.
LiPo batteries have a rating measured in C where C is the current rating for the battery, 1200mAh for example, this 1200mAh stands for Milli Ampere Hours and is the standard used to measure the capacity of batteries. A single mAh is 1 Milliamp for 1 hour, you get 1000 Milliamps to 1 Amp. A 1200mAh LiPo may have a continuous discharge rating of 12C meaning 12 times its mAh making our example battery capable of 12 x 1200mAh or 14.4Amps and a peek drain (15 seconds or less usually) of 15C giving a peek drain of 18Amps. One more C figure remains and that is the charging C, this can be upto 5C with specialist cells from hobby stores, usually the rate is 1C or less, less is defiantly more with LiPo battery life
** Update 25th June 2010 **
The newer high recharge rate batteries are starting to filter into the market now, currently (to the best of my knowledge) the new types are 2C to 5C charge rates, this means that you can charge the battery at 3 or 5 times its C rating, e.g. 1200mAh 5C pack can be charged at 6000mAh or 6Amps. This really impressive new technology allows you to "field" charge a LiPo in as little at 15 mins! One word of caution, please do not always charge these cells at 3C or 5C. When you are not in a rush please charge at less than 1C to maximise battery life, also at these high charge rates it is even more important you stay with, and watch your cells while charging.
** Update 29 December 2012 **
10C charge rates are now on the market, again, meaning you can charge them at 10x their mAh rate, so a 1200mAh cell "could" be charged at 12 amps, this is similar to the speed at witch NiMi and NiCad can be charged at.
Do so with care!, more and more the lithium tech from electric car makers is filtering through, but for recreation, great care is needed. 7 Mins to get back in the air is amazing, but again, use sparingly to prolong life. If you do not NEED to get back running in 7 mins do not use the top charge rate, using 1C will give you better battery life.
When buying a used LiPo battery it is rare that it will not have been used at high current drain, this is not a problem so long as the user of the battery did not over drain it. If the battery was over drained beyond its design limits the battery will "Puff Up" as each little pocket of battery expands under too much load, so check the pictures carefully and ask for extra ones to be taken from the side showing the layers of the battery so you can be sure no expansion has taken place. Also of great importance is the integrity of the battery, by this I mean its outer casing (usually a plastic material) UNDER NO CIRCUMSTANCES purchase a LiPo with "slight cracks", "splits", "grazes" or ANY other form of physical damage. I cannot stress this enough, if you buy a LiPo battery with a ruptured outer casing you run the risk of allowing (under high load) the creation of actual lithium and all of its associated toxic and irritant gasses.
When charging your LiPo batteries please do so while you are present, do not walk away and leave them, not a good idea. Also find a suitably fire proof place to charge them on. Ideal places include a concrete paving slab or an old kiln crucible, basically anything non-conductive and fire proof, please do not use metal tin lids as the danger of short circuit is too great. A "FROG" house brick is fine (ones with the carved out middle).
You can buy LiPo fire bags now and the reputable ones are great, but watch out for the cheap rip off's. Irrespective of a fire bag, always charge on a safe non flammable non conductive(i.e. not metal) surface with good ventilation and keep an eye on it.
Always use a computer controlled dedicated LiPo charger, NEVER use an old NiCad or NiMh one. I personally have a 12v LiPo charger with built in computer monitor(not expensive £15-£40). When at home I power it with a large heavy duty regulated 13.8v mains transformer(£17 from a local electronics suppliers though I am sure somthing equivilent could be found on ebay), when out in the field I use a car battery to power it.
All new multi cell LiPo's should now have a second lead attached to them and you may be wondering what this is for, the extra lead ( a 3 or 4 or more, pin connector unlike the 2 pin main connection) is for "Balanced Charging" what this means is that each cell inside the LiPo battery is charged seperatly. This method allows for the correction of possible under/over charging with multi cell LiPo batterys. If you have a 7.4v or perhaps an 11.1v LiPo battery then it is made from either 2 or 3 smaller cells each added together to give the desired total volatge, it is possible for these cells to become charged or used to the point where each cell is at a different voltage, while this difference is small there is no problem, if the difference becomes larger over time it can lead to one or more cells becoming over charged and that is somthing we always want to avoid. If you are new to LiPo I advise you to purchace a "Balancing LiPo Charger" to take advantage of this feature, if you are an existing LiPo owner and you have a battery fitted with this extra plug it is well worth upgrading your charger to take advantage of it.
It is important to check the state of your batteries before charging them, even though most computer controlled chargers do this automatically, do not rely on a machine to perform a task that you should be doing anyway.
Get hold of a multimeter or volt meter (again not expensive £2-£50) and check that the voltage of the LiPo to be charged is not too low. Low voltage for a LiPo battery is measured by taking the cell count (usually 2 to 4 cells) and multiplying by 3v so a 3 cell 1200mAh LiPo would have a minimum charge voltage of 9v if it is below this then something went wrong. Either you let the battery discharge too low by continuing to use the battery after it was already low or the battery has developed a fault.
If you are 100% confident that the low voltage is due only to a slight over use, and I mean SLIGHT, then most chargers have a force charge mode which will put a little current into the battery for a limited time (usually around 30 seconds) and then stop and attempt to charge normally. If the forced charge for 30 seconds does not return the battery to a voltage above 9v for a 3 cell, 6v for a 2 cell and 12v for a 4 cell (cell count x 3v) then the battery has likely suffered damage and must either be disposed of correctly (check with your local council about battery disposal facilities in your area) or returned to the place of purchase for a check-up by a qualified engineer.
Brand new LiPo cells gain performance over the first 5-10 charge cycles, so make sure your flight, buggy or general use duration tests are done after that.
When using your LiPo battery make sure you do not regularly use it at maximum current drain, a LiPo battery used for most of the time at 50% or less of current drain will, and regularly do, provide years of 80%+ capacity service over many hundreds of recharges. 500 recharges is often the point at which most LiPo cells reach the 80% capacity mark, the cells are still perfectly usable but may not last as long as new ones.
After using your LiPo battery check it for temperature, it should never be over 60-65 Celsius (140-149 Fahrenheit). You can use an infra-red remote thermal tester or a contact thermometer to check this. If the battery is extremely hot to the touch (scalding coffee cup), that’s a bad sign and indicates that you need to use more batteries in parallel or reduce the current load on the single battery (changing motor, prop or gear ratio for example). Also check for damage to the external packaging of the LiPo battery and any packs that show signs of splits, cracks, “Puffing Up” or ANY physical damage should be discarded of in the proper manner.
At the start of this guide I said that a LiPo was a controlled explosion in a wrapper and indeed it is, but used correctly it is an explosion that will provide many hours of entertainment and power for months and years to come.
Play it safe and have fun.
Spencer AKA FoxabiloIf you did not find this guide helpfull, drop me a mail and let me know what I can do to improve it.