Lithium polymer batteries, like most battery types, are complicated chemical and mechanical devices. They are awesome power sources for rc use but they are also fragile and prone to problems. Below I will attempt to cover, and offer advise on, many of the common problems that arise when buying, using and abusing lipos.
New lipo problems
Just because a lipo is new does not mean it is perfect. Many brand new lipos have problems and this has to do with everything from the quality of the cells to the workmanship of the assembly and even the thoroughness of the packaging for shipping. Lipos are delicate devices that require great care when manufacturing, assembling and handling. As you can imagine, this is where the brand of the lipo may have a large impact. Just like anything, the more you spend on a lipo, the better chance you have of getting a quality product. A high quality lipo, read expensive, usually goes through a much more careful testing, assembly and packaging process than a lower quality lipo. Thus a high-end lipo should have far less problems out of the box. Below are a few of the problems people might see when receiving a new lipo.
Any product is susceptible to damage during transit and lipos are no different. Lipos seem fragile, and in many ways they are, but they can take certain abuse in stride. For example lipos can handle shock loads quite well but they can not handle being crushed. If you receive a damaged package containing a lipo, open it and inspect everything. Do not reject the package and return to sender! A damaged lipo could cause a fire on the return trip or even after it arrives at the business that sold it. It is your responsibility to make the lipo safe once it is in your possession. Remove and carefully inspect each lipo. If a lipo shows any physical damage or swelling, place the lipo outside in a safe place and contact the business you purchased it from. The business should replace it and should not ask for you to return it, though they may ask for photos or a description of the problem. If each lipo appears to be in perfect physical condition, assume it is fine and treat it like any other new lipo.
Low voltage cell(s)
This is a problem I am seeing on the forum more and more. People are receiving lipos with one or more low voltage cells. Lipo cells are never charged before they arrive at your door. The energy in them is created when the lipo is manufactured. That energy gives the lipo a voltage that should be approx. 3.85V. The exact voltage does not usually matter, the difference between the cell voltages is what matters. The battery you receive should have all cells within say .03V of each other. A 3s pack should have cell voltages like 3.85V, 3.84V, 3.85V. If it instead has cell voltages like 3.85V, 3.84V, 3.74V, cell 3 has a flaw in it. Tiny shorts between the plates cause energy to bleed off and that lowers the voltage. If you receive a pack with a slightly low cell, it will probably never be as good as a well matched pack but it will likely still be useful. If you receive a pack with a very low cell, like 1.5V, then it may never useful. In any case, balance charging these packs may give you some use, even good solid use, but don't expect them to last as long or perform as well as a properly matched pack. As a note: cycling the packs will never "fix" manufacturing flaws. A low cell is an indicator of a flaw and the pack will always suffer from it.
A high quality lipo assembler will test the voltage of each cell and match them together to create matched packs. They will not use any cell that shows a problem. As such, high quality packs seldom ship with cell voltage problems. If you do receive one, contact the seller and request a replacement.
Low quality packs often do not undergo this process and are prone to having bad cells. Unfortunately in addition to lower quality cells and assembly, you usually give up support when you save money on lower quality packs. That means getting bad packs replaced is hard to impossible.
Bad cell / 0V cell
There are normally 2 reasons a pack will have a zero voltage cell, a bad cell or a bad solder/wire connection. If the pack voltage is 3.85V * # of cells (a 3s pack should read approx. 11.55V), than it is likely a bad solder joint, broken wire, bad connector, etc. on the balance lead. If the pack voltage is one cell low (3s pack voltage reads approx 7.7V) than there is likely a bad cell.
If either of these problems occurs on a high quality pack, contact the seller and request a replacement. Like low cells, these problems are rare on higher quality packs.
If you get a low quality pack with one of these problems, you will likely have to deal with the problem yourself. In the case of a bad cell, there is nothing you can do but dispose of the pack. In the case of a balance lead problem, it can usually be fixed fairly simply. Do your best to locate the break and repair it. This may be as simple as adjusting the pins in the balance connector housing or as complicated as removing the shrink wrap around the pack, and resoldering the broken lead to the cell tab. How much you do is up to you. If you are scared of the pack, dispose of it.
Cell IR problems
Most chargers these days have the ability to measure individual cell internal resistances or IR. If you have one of these chargers, you can use the IR measurement to further judge the health of a new lipo. If you have a cell that shows an IR far higher than the other cells, you may have a weak cell. Caution: There are many things that greatly influence the IR readings given by chargers. They are only to be taken as rough estimates unless you have a very solid understanding of what impacts the readings.
Charging related problemsWith all the chargers, adapter boards, battery connectors, user selectable settings, etc., time to time there are bound to be lipo problems tied to charging. It is not possible for me to cover every possible problem the average hobbyist may encounter but below are a few.
With there being at least 18 electrical connections needed to charge a 6s pack on most chargers (7 for the balance adapter, 7 for the battery balance connector, 2 for the main leans at the charger and 2 more on the battery main leads), there are bound to be a bad connection from time to time. This can be something as simple as not connecting the battery fully, to something more show stopping like a bad connector or loose wire. Here is how I handle this sort of problem.
Lipo balancing issues
There can be many problems related to balancing but they usually fall into two categories, low voltage cells and long balance times.
Most times a low cell is due to a problem cell, like a damaged cell or a bad one from the factory, but there are other times when it is charger related. If you find that you have several packs with the same low cell, it could indicate a problem with the charger or wiring. Try using different balance leads/boards and see if that helps. Try a different charger and see if it helps. If you decide your charger has a balancing problem, you may be able to re-calibrate it or it may need to be replaced to fix the problem.
If a lipo is balanced, meaning all the cells are within say .01-.02V of each other, but the cells are not at or near 4.20V, then you have a undercharged lipo. Don't freak out though, as this can be quiet normal, the reasons for it vary from "this is just how it is" to the lipo being old and weak.
When you charge a lipo, see How lipo chargers work for more info, there are several factors that decide how close to 4.20V each cell is when the charge finishes and most of it has to do with the charger itself and the setting chosen by the user.
Not all low voltage issues are related to the charger. Sometimes battery is to blame. For example high resistance cells will end up at a lower voltage at the end of a charge cycle. This is often why a battery may have balanced cells while charging but when allowed to rest for a time, will have cell voltages all over. This happens to older, weak or low quality batteries quite often.
If a lipo is balanced, meaning all the cells are within say .01-.02V of each other, and some or all of the cells are over 4.20V, then you have a overcharged lipo. It is not common to have an overcharged lipo but there are several ways it can happen. The most likely way is to overcharge it, either on purpose or by accident. Lets assume you never want to purposely overcharge a lipo, that leaves accidental overcharging.
The most common way to overcharge a lipo is to use an improperly set-up charger. For example say you have a 3s lipo and you tell the charger it is a 4s lipo. The charger will charge it to 4s voltage without concern and you will end up with a severely overcharged pack. These days nearly all the chargers have an integrated balancer, making these sort mistakes very uncommon, but it is still possible to use these chargers without the balancer attached and that is where users can get in trouble. If for example you want to charge your buddies lipo but don't have the correct balance lead for it, you can charge it without a balancer.
Now let me cover what to do when you end up with an overcharged lipo, but first lets talk about severity.
Use related problems
Coming later ...
Age/cycle related problems
Coming later ...