Many of us have a fairly good level of knowledge of Lipo (Lithium Polymer) batteries. However Maxamps posted an interesting article about Lipo batteries last year that is well worth a read for anybody interested, or not quite sure what it all means. I have reproduced it here, with a link to the original post at the end.
Ever feel like there is a topic in RC that is just over your head. Like for instance Lipos? How do they work? What is required to use a lipo? What risks are associated with Lipos? These are just some of the questions we get on a daily basis and though you may feel like a complete noob asking them you are on the right track to understanding the electric side of RC. Today I want to cover some of the basics of Lipos.
Trying to understand a lipo by comparing it to other cell types in the same field is not a very successful way to understand this technology. These cells do just about everything differently from the previous technologies that have dominated RC. NiMH and NiCad batteries, which are still used as the stock battery in many setups, fall short in many ways: they are heavier, provide less power, and the power drops off at a much faster rate. Although easier to maintain these power systems are quickly becoming obsolete.
Lipos provide the best energy density on the market. Instead of a constant drain of voltage during discharge (gets slower as you drive) you have a continuous current (constant power) and in most cases will not see a drop in performance until you hit cutoff. Charging in as little as 15 minutes these packs are ready to go as soon as you are. These packs can also discharge at a much higher rate than earlier technologies. They produce more run time and performance and it weighs half as much as its predecessor, so what is the catch?
The catch is unlike those older technologies you need to know how to use, maintain, and operate a lipo otherwise your pack may only last you a single run. With the proper knowledge and setup however your lipo can last you years and provide you with more power than you could dream.
So let’s get started. A lithium Polymer cell or Lipo cell is labeled as 3.7 volts. The cell actually ranges from 3.0V (fully discharged) to 4.2V (fully charged). Going above 4.2V or below 3.0V will cause a pack to lose capacity and performance. This will happen to the point of failure during discharge and can actually cause a fire during a charge.
Lipo fires have become rarer as the technology improves and as the education is spread but having precautions in place like our Lipo Sack will limit the damage in the unlikely event of a fire. The entire point of this however is to help you avoid this ever becoming a possibility. So pay attention and always be there during any process with your charger!
The good news is that the systems developed today revolve around keeping a lipo in the range that will keep it functioning. Most ESCs have a built in low voltage cutoff which will cut power to the motor once the pack has reached cutoff voltage. For most setups this should be about 3.4 volts per cell, well above the point of damage. Smart chargers, once set correctly, will charge and discharge your batteries to the correct voltages while monitoring each individual cell.
Some ESCs do not have a low voltage cutoff built in and in this case you can use a Low Voltage Alarm which will beep when the pack has hit the desired voltage. At that time you would stop driving and go retrieve your RC. This form of alarm is ideal if you do not have a cutoff but it is not as effective as you can run past the point of functionality and not even know it until you go to charge. Better to be safe than sorry so make sure you stop driving once you hear the sound.
Making sure your ESC is setup for running lipos is important. Different brands of systems have ways to indicate whether you are in lipo mode or not. Brands like Castle Creations even allow you to set the cut off voltage. Devices like their Castle Link for cars allow you to setup a range of features including cut off voltage on the fly in the field.
Alongside the Castle Link, having a device in the field to check the status of your battery can be super helpful. The Hyperion Sentry Checker is a great credit card sized device that will allow you to check the voltages of your pack and individual cells on the fly.
Once you are done driving, boating, or flying for the day you should always put your pack into a storage charge. This is 60-70% of the packs capacity; usually between 3.75V and 3.95V per cell. Many of today’s chargers have a storage mode option but you can stop the charge at the right voltage on your own if need be. The reason for this is that at 60-70% capacity your pack has the least internal resistance. Any more or less capacity and the internal resistance rises. Higher internal resistance equals faster cell deterioration. Anytime the packs are not in use make sure to store your pack. If you never use storage mode on your charger you will be lucky to see a year of good use on a high quality lipo. If you store it you can see over 3 years on that same pack.
What do you think is the biggest cause for failure in lipos? Crashes? Over discharge? Although there are a fair number that likely see this unfortunate end what it actually comes down to is lack of use. Lipos are designed to be used. When they sit for long periods they build up internal resistance. After a while the pack will develop dead spots. At this point the pack can puff on the shelf, during the next charge, or on its next use. So how do you avoid this? Cycling (charging and discharging) your packs if they are not being used will keep your cells fresh and your pack working. Again a lot of today’s smart chargers have a cycle option which will do most of the work for you. You should cycle your pack once a month if you do not use it in your RC. If you are not going to play for a while set a reminder to cycle your packs once a month on your smart phone.
The other big killer in our experience is experimentation. Someone reads on a forum somewhere that if they drop the pinion down a couple of teeth the will have more torque. This may be true but there is/was a reason it was geared the way it was and although some modifications work perfectly, others don’t. Heat buildup is the enemy of electronics and an incorrect gearing change can easily lead to this type of failure. If you have an unlimited budget by all means test away. Otherwise, confirm with multiple trusted sources before you make a gearing change that could ruin your ESC, Motor, or battery.
Finally we come to the more exciting killer of lipos. Crash damage! If at your local track you have a 20 foot high jump that you stick on a regular basis you may be damaging your pack without even knowing it. What we have seen over time with packs that have been sent in for warranty is the cells will actually accordion from being smashed into the front of the battery tray over and over and over again. This is usually not a big issue with 1/10th scale short course and similar as the packs are not as tall and the cars themselves cannot jump as high. With 1/8th scale setups however this can be seen more commonly. The cars can accelerate much faster and thus gain more air time. The biggest flaw we have seen with this is the design of the battery tray. These 4s packs are much taller than the tray itself therefore the bottom cells of the pack take the brunt of the impact when the car hits the ground. In response we have created the stainless steel battery tray insert which disperses crash damage and has drastically extended pack life on these larger setups.