Understanding the Basics of LiPo Batteries

Whether you're brand new to the hobby or if you've been around for a while one thing's for sure about Lipo batteries. There's a lot of information out there, some of it's great, some of it's not so great, so I thought I'd put together a blog to talk a little bit about what I know about Lipo batteries and how I handle them.

This blog is focused on standard Lipo batteries, they are high voltage options out there, but they're outside the scope of this blog.

LiPo are awesome for radio control because they do a great job in maintaining voltage over their discharge curve. This little graph I have from Battery University, shows the starting voltage around 4.2 on the left and the cut off voltage around 3.3 on the right.

You can see during the discharge curve the voltage only fluctuates about 1 volt from start to finish, which makes it really nice because when you're talking about power delivery, volt times amp sequels Watts. So lipos do a great job maintaining voltage over their discharge curve.

LiPos are also very easy to get because they're in such high demand just about every hobby shop carries them. Unfortunately the information on LiPo berried out there I've seen well written documents that are just flat wrong, I've seen information on the forums that's flat wrong, I've seen advice that's dated that can cause problems, I've seen people flat out recommend against parallel charging, which is ridiculous because half these batteries come in parallel formats anyway. In the end you have to decide who to trust.

Personally, I prefer Battery University to get my information because everything they post came from some form of research backed information. And it's normally posted by a company or a university, so they do try and use at least the scientific method when they present data.

When it comes to Lipo batteries there are certain numbers you need to understand.

let's start out with peak voltage. The peak voltage on Lipo batteries is 4.2 volts per cell. So 4.2 volts per cell peak you should never charge above that number.

The next number to understand is minimum voltage. That number is 3 volts per cell. If you go below that number, you're gonna cause permanent damage to the cell.

Safe voltage is considered to be 3.3 volts per sell. Although this is where the arguments will start. The idea is that as long as you land the plane or quadcopter or drone or whatever it is you're flying or driving at 3.3 volts per sell, you'll never go below that damage level of 3 volts per sell. For me my practical loaded voltage minimum is 3.5 volts per sell, what that means, is when I'm flying, I don't like to ever go below 3.5. The main reason for that is because when I start hitting 3.5 volts per cell, I can feel it in the power output for the battery.

Now while you're flying you'll see voltage decrease under load once you take that load away the cell will rebound. so I look for about 3.7 volts per cell unloaded. So when I take an airplane off the flight line and put it on the bench and take the battery out, I expect after a minute or two that my per cell voltage should come back up to about 3.75, which conveniently is the bottom of the range for storage voltage. Storage voltage is another area where I commonly see arguments. I believe that 3.75 to 3.85 is perfectly acceptable. Battery University says there is virtually no self discharge below about 4.0 volts per sell, they go on to say finding the exact 40% to 50 % state of charge level storelion is not that important. So for me storage voltage somewhere between 3.75 and 3.85 volts per sell is perfectly.

One other number you'll hear reference When it comes to Lipo batteries is nominal voltage. The nominal or average voltage for Lipo cells is 3.7 volts, that number is used in the calculation for determining the pack voltage when multiplied by the number of cells per packs. So on a 3s battery for example, 3.7 times 3 is 11.1, so 11.1 is known as the pack voltage, while 12.6 is known as the peak voltage which is 4.2 volts per cell times 3 cells. The rest of this table will show you the nominal pack voltage and peak voltage for 4s and 6s LiPo batteries.

When it comes to peak voltage. I like to point out the study by Chalmers University of technology in Sweden, while we all know 4.2 volts is the normal peak voltage for a Lipo pack. If you lower your peak voltage from 4.2 to 4.15, this study indicated an increase of 50% to 100 % of cycle life for lipo batteries. If you further decrease your peak voltage to 4.1 volts per cell, you'll increase your cycle life by a factor of 2.

let's recap our key numbers

1. Peak voltage is 4.2 volts per cell, however if you lower your voltage peak from 4.2to 4.15 or even better like 10, you can increase the cycle life of your batteries.
2. The minimum voltage before you do damage to a LiPo cell is 3.0 volts.
3. The minimum safe voltage widely recognized out there is 3.3 volts, for me my practical loadeddischarge voltage is 3.5 volts per Your unloaded voltage should be somewhere around 3.75 volts per cell, and your storage voltage can safely be between 3.75 and 3.85 volts per cell. The nominal voltage for Lipo batteries is 3.7 volts per cell and that's used in calculating pack voltage.

Next up is battery markings

It can be really confusing when you first start getting into Lipo batteries, understanding what all these numbers mean. So let's just break it down.

We'll start out with a big number 4, that just means 4 amp hours. You'll also see there's a 4000 over here, that's simply the amp hours times 1,000 so this is a 4000 mill amp hour battery. And what that's a reference to is the size of the gas tank it talks about how much energy is inside this battery or how much energy this battery can hold. So more mah just means it can hold more energy.

The next number I'll talk about is the discharge rate, before I go any further, it's widely recognized these numbers are inflated. Most manufacturers inflate the C rating for the most optimal conditions for their battery. This number is meant to indicate how much current the battery can deliver in a constant state and a burst state. So if we take our amp hours 4 and we multiply that by the continuous current rate of 30, that gives us 120 amps of continuous current, that's what they're trying to tell us with this marking, this battery should be able to deliver 120 amps sustained. The 40 means how many amps it can sustain on a burst basis of no more than about 30 seconds, so 4 times 40 is 160, that means this should be able to sustain 160 amps for about 30 seconds.

The last number to point out is that this is marked as the 5 cell battery. Remember when we talked about nominal voltage earlier being 3.7 volts per cell? If you take 3.7 times 5, you get a nominal pack voltage of 18.5 volts . That's it that covers the markings. you'll see on just about every Lipo battery out on the market.

When it comes to charging Lipo batteries

You should only ever use a balance charger, don't ever use a charger for Lipo batteries, it doesn't include the ability to balance your cells. You can always tell if it's got a balance charger by looking on the side, and you'll see a row of pins, that's meant for this white connector, and it'll also have a main port right here for the main leads on the battery.

Aside from the product description, saying it's a balanced charger. Another way you'll know right

away is by looking at the charging screen, and you'll see in this case I've got a 5 cell battery connected and I'm seeing individual cell voltages for all 5 cells.  So 3.8 /3.8 /3.8 /3.8/ 3.8.

Finally when it comes to care and handling.

This will be another area that gets argumentative, but I'll tell you it just worked for me. So I'm going to stick with it. I always use a battery charger that can give me the internal resistance on these batteries, and for consumer grade devices it's not considered highly accurate or reliable, however what I do know is that when I run internal resistance tests, semi batteries, and I get a value above 74 resistance, that's when I retire the battery. It just worked for me over time so I'll keep doing it, you can do what makes you happy. I'm telling you for me, once I see an internal resistance value of 7 or higher, that's it the battery goes on the shelf.