The primary reason for this article is to explain the importance of using a balancer for LiPo battery charging every single time. Balancing will greatly reduce the risk of your batteries going bad prematurely.
The above diagram shows what happens if we charge or discharge a Lithium cell at a constant rate (a very bad idea) instead of slowing and stopping the way it should be done.
Cell Capacity Illustration
It is
important that we understand the role that individual cell capacity plays in
the balancing process. Lets start with an illustration for those who may be
electronically challenged:
• Imagine two 5 gallon buckets. One has been used for mixing paint and has several layers of it coating the inside.
• The bucket with reduced capacity (painter’s bucket) will fill faster and will empty faster if the flow rate for each is the same.
•
Two buckets just
purchased from Walmart may not be the same size.
• Normal aging and cell damage are like adding layers of paint. The cell with less capacity will charge or discharge faster than the other cells in the pack.
• Cell balancing is like drilling a hole in the bottom of the painter’s bucket so it will fill no faster than the clean new bucket. We can put our finger over the hole as necessary to keep the two buckets in balance as we fill them.
• The drip hole in the bottom of the painter’s bucket may not be big enough if too much paint has built up.
• Using a balancer does not scrape the paint out of the painter’s bucket or add layers of paint to the clean bucket.
Lithium
Polymer (LiPo) chemistry accumulates a charge over a fairly narrow voltage
range with rapidly diminishing capacity exhibited above and below this range.
E.g. the water level will rise much faster in the neck of the jug at right. This
explains why the voltage rises or falls more rapidly above and below this
chemistry range. Operating outside this range of voltages will at best
accelerate the aging process and can result in serious cell damage and even
smoke and flame. A battery that could have lasted three years might fade away
in less than a week if one cell has a significantly reduced cell capacity
relative to it’s mates.
Can I use a NiCAD Trickle Charger for my LiPo?
NiCAD and NiMH
cells self limit at full charge voltage at which point they start getting hot
which is why it is very important that NiCAD and NiMH chargers detect full
charge and switch to a trickle charge rate. LiPo cell voltage is not self
limiting which is why you should never use a NiCAD style trickle charger. The
following diagram normalizes the three charging curves so that their respective
full charge voltages appear to be the same.
For all three chemistries, continuing the full charge rate introduces a very high risk of fire. Slowing to a trickle charge rate works for NiCAD and NiMH batteries, but not for LiPo cells.
One of the leading reasons why LiPo batteries have precipitated car and house fires is that the charger thought a three Cell LiPo had four cells and the charge process did not slow down and stop at the 3 cell voltage. When cell voltage gets moderately too high or low it accelerates the aging process, i.e. cell capacity is reduced and internal cell resistance increases. Let the cell voltage get below 2 volts and you have essentially destroyed it. Same if cell voltage gets too high. High enough and the cell becomes a fire hazard.
Can I use my LiPo after a crash?
The first thing to
worry about after a crash is that there could be an internal short circuit that
has not quite made contact. The larger LiPo packs especially can put out 100s
of Amps with violent results. Think fire hazard for the next 24 hours or so
(you don’t want the next big pot hole on your way home to ignite an arc welder
in the back of your car).
A crash that impacts the shape of the LiPo pack will reduce the cell capacity of one or more cells. The resulting imbalance can be serious. I experimented with a three cell pack that had one cell significantly out of balance. When the pack was charged, the cell voltage of the problem cell soon climbed to 7 volts and I disconnected the charger. Next I connected a car head light to load the battery. I was surprised at how quickly the good cells had driven the bad cell all the way from +7 down to -7 volts. I quickly disconnected the load. I was able to get the battery to puff up and produce some stinky smoke, but almost no flame. These batteries are much more robust than they were a couple of years ago. You can still purchase LiPo packs that use a high risk internal connection scheme and are much more inclined to burst into flame without much provocation. Better to pay a bit more for a properly designed LiPo pack.
Flying a damaged battery in an airplane stands the chance of catching the battery and the airplane on fire. As you start down the runway for takeoff you will notice that the battery is performing very poorly because the bad cell is furnishing very little power. When ever you notice that one of your batteries is not performing as expected, become suspicious that one or more cells are damaged. Chances are your charging system can no longer keep the pack in balance or that the pack is safe to fly in your airplane. Time to properly retire the battery (don’t just toss it in the trash if any of the cells carry a charge).
After a crash, even
if there is no apparent damage, conduct all the necessary tests to ensure that
it is safe to use again. Get a second opinion from someone with LiPo battery
experience. As a minimum, be especially paranoid about the battery until it has
been through a couple of charge and discharge cycles (e.g. use a LiPo Sack,
metal or ceramic container).
Do I need to purchase an expensive LiPo charger?
You
may be a newcomer to radio control flying of indoor or park flier airplanes on
a very limited budget. This article need not scare you out of the hobby or into
purchasing an expensive charging system. The smaller simpler LiPo chargers do
not charge batteries all the way to the top, allowing some margin for a cell
being out of balance. At least do the following if you don’t use a balancer for
a small light weight system:
• Make sure the charger is charging to an adequately conservative voltage that is less than 4.2 times the cell count.
• Check and double check the cell count each time you use the charger.
• Once in awhile use a volt meter to confirm that none of the cells are being charged to more than 4.2 volts. If a trend is developing in that direction then its time purchase a balancer. Such a trend can develop rather quickly.
If the above is stretching it a bit, then
you should at least add an external balancer (e.g. Blinky balancer). Understand
that a Blinky balancer may be adequate today and not so in a few weeks. Once in
awhile be sure and check cell voltages as full charge is reached to make sure
that balancer is adequately doing its job for that battery. Small voltage
testers are available that connect to the balance leads to conveniently read
out all the cell voltages. In flight balance lead loggers are also available.
Consider the following limitations of an external balancer:
• May not be aggressive enough, especially for larger batteries or any battery with cells more seriously out of balance (Blinky balancing cost me an expensive battery).
• An integrated balancer can easily produce an alarm if you dial in the wrong cell count. An external balancer may or may not give you the warning you need.
• A charger with an integrated balancer will slow down or pause when ever the balancer is not keeping up.
If
your flying practice sounds anything like the following, then you should (must)
use a charger with an integrated balancing system:
• High battery stress style of flying (e.g. lots of full throttle, hot weather, flying until the battery fades, outdoor helicopters).
• Cell counts greater than three.
• Cell capacity greater than 2100 mAh for which you should at least use a Blinky external balancer.
Some chargers have an external balancer that communicates with the charger over a cable link. This can be as effective as having the balancer built into the charger.