LiPo Battery Guide

LiPo cells are no longer brand new technology and while NiMH packs are still kicking around, it’s undeniable that LiPos are the prevailing RC battery chemistry. As a result, there are a lot of options out there. One of the top companies associated with high performance voltage is SMC Racing (Superior Matching Concepts). SMC packs have been helping racers win long before anyone even dropped a LiPo in a car. They’ve been at it for decades, and at the head of SMC is Danny Sullivan. And while, again, LiPos aren’t new, a lot of people are still making costly, even dangerous, mistakes or are intimidated by the technology and not trying it. So, we turned to Danny and his wealth of knowledge to help us guide the RC community in the proper use of LiPo–and maybe debunk some hype too. If you want to make sure you’re using LiPos correctly or want to make sure you really know what you’re doing before you switch to LiPos, check out the expert advice below directly from the experts at SMC.

What is C-rating, why is a higher rating better, how is it determined and how comparable are each brand’s ratings?
C-rate is supposed to be the amount of amps a cell can provide. In theory, this is very good way to rate a cell’s performance, but in reality it isn’t. The reason why it isn’t is there is no official standard C-rate test. This means factory A can get the rating using a method that is different than factory B. I’m fortunate enough to be able to get info from factories and when I ask about C-rate testing some have told me they use 90% capacity retention and others have told me 80%. As you load up a cell at higher amps, the mAh it puts out drops. A cell’s mAh is rated at 1C discharge and to get what the 1C equals to in amps you must multiply the cell’s mAh by 1 so on a 5000 mAh cell this would equal to 5000 mAh. To bring this to amps you divide by 1000, which is 5 amps. The 5000 mAh cell will give its full mAh when discharged at 5 amps which is 1C. If a cell is rated at 20C this means you must discharge it at 100 amps. For a 5000 mAh cell discharged at 20C/100amps it must put out 4500 mAh to be considered 20C—if you use 90% capacity retention. If you use 80%, the cell must give out 4000 mAh. I’ve personally done C-Rate tests and I noticed that when a cell has a true rating that it will be able to do the C-Rate it’s rated at, but this typically damages the cell and it tends to form gas due to high heat generated under the higher amp load. The factories know it’s very hard for the consumer to test for C-rate so most of them will inflate the rate to generate more sales. Most factories that make RC style cells will sell them to agents/resellers in China who then sell to companies in the USA or other countries. These agents/resellers will inflate the C rates some more, so in the end, C-rates have pretty much become useless, in my opinion. This is why I created a True Amp rating system a few years ago but have now decided to move away from it as it was a bit confusing for our customers. I recently updated to a Power Factor rating for our packs.  This Power Factor rating shows the customer which pack has more power as the higher the number the more powerful the pack will be. The test is simple as it consists of discharging a pack at 100 amps for 5 seconds and looking at the voltage of the pack at 5 seconds. I then take the voltage per cell to get the rating. If a 2S pack is at 7.80V at 5 seconds under 100 amp discharge this would equal to 3.90V per cell.  In this case I would rate it as a 390 pack. By knowing the cell voltage if you were to buy a 6S pack with the same cells rated at 3.90V per cell then this pack would be at 23.4V at 100 amps at 5 seconds.  In conclusion, you can’t rely on C-rates to determine the performance of the cells/packs as no one uses real C rates or the same standard to determine C rates.  Typically, when buying from the same company you can use the C-rates they provide as a way to know which packs have better performance as I would have to assume that a company who sells 50C and 75C packs would rate them in a way that the 75C is more powerful than the 50C.

“This cell tester can charge and discharge cells and is programmable via the PC. It can do up to 40 amps for charge and discharge. I use this to test cells and do cycle life testing on the cells to see how they react over the cycle life. My cycle life testing is harsh on cells as I only let 10 minutes of cool down between cycles. This shows me how different formulations hold up or degrade faster.”

We all get mAh is a measurement of capacity, size of the gas tank, and the higher the number the more runtime is provided, but how does a higher mAh impact performance otherwise, if at all?
Many think the higher the mAh the longer the voltage will stay up under load. This is partly true but doesn’t give you the full picture. When it comes to RC race packs for sanctioned racing you have a size limit on the packs. Since the size of the cell can’t be increased to achieve higher mAh you must change the cells real C-rate to increase the mAh. This typically increases the internal resistance of the cells which means the voltage under load will be lower than a cell with lower internal resistance. If the mAh could be increased by making the cells bigger we would see an increase in mAh and the internal resistance would get even lower. At SMC, we use to offer a 6200 mAh-1S under our Extreme Graphene series. This pack had very low internal resistance. We offer a 8000 mAh-1S under this same series, but the internal resistance of the cells is higher due to the higher mAh. Under a 35 amp discharge, which is what many oval racers use for testing, the racers look at how long the cell will hold 4V under the 35 amp discharge. The 6200 would take 10-15 second more before it dropped under 4V compared to the 8000. The racers also look at the voltage at 240 seconds at 35 amp discharge and the 8000 would be around 0.01-0.02 higher in voltage which makes sense as the mAh is longer. On the race track, it seems like most racers preferred the 6200 as they claimed the upfront lap times were a bit better and the pack finished at a similar pace than the 8000.  We are talking a small difference overall but when racing at a high level every bit counts. In conclusion, the internal resistance of the cells in the pack has more of an impact than the mAh. With more and more low center of gravity packs being offered this means the packs typically have higher internal resistance and lower mAh as the cells are thinner/smaller. One thing to consider is the weight of the pack and lower center of gravity will help handling on high bite tracks. Handling can’t be neglected as it typically helps with faster lap times and more consistent lap times. Speed is only one part of what makes for a fast lap. If you can use a LCG pack and not have to add weight, the lower weight of this pack can compensate for the higher internal resistance and lower mAh. If you need to add weight to make minimum weight then you will be better off with a bigger pack assuming it doesn’t hurt handling too much.  Using the SMC Power Factor number you can see which ones of our packs give the best power as voltage drop under load is directly related to the internal resistance of the cells. The lower the internal resistance the higher the voltage will be under load.

“This is how I test packs for mAh , average voltage and my Power Factor test. This setup can do up to 1700 watts so this means I could discharge a 2S pack up to 202 amps assuming the wires and connectors could handle it. The tester itself is 40 amps 200 watts and the 3 extra load you see are 500 watts.”

In addition to C-rating and mAh, but do amps and Wh ratings mean and why are they important?
If the amp rating of a cell would be true and every factory, agent/reseller and company selling the packs would use the same method to calculate the amp rating then it would mean something, but unfortunately this isn’t the case. Wh is just the cells rated voltage multiplied by the mAh. A pack rated at 7.4V and 5000 mAh will have a 37Wh rating, but this is just a static calculation and doesn’t reflect the true performance of the pack.

What is the ideal low-voltage each cell should be allowed to reach?
I don’t recommend going under 3.60V per cell and when the pack/cells have cooled down if they are in the low 3.70V range, this is good. Best way to test the voltage is to test it as soon as the speed control cut-off or LiPo alarm goes off and make sure the cells are not under 3.60V. As they cool down they will go back up a bit. When using the SMC Precision LiPo Alarm, I recommend you start with a cut-off of 3.70V.  

 

What does over discharging do?
Over discharging increases the chance of cell damage and will shorten the cycle life. It also increases cell expansion. All cells have natural expansion under charge and discharge, but over discharge creates extra heat in the cells and can make the cells expand more. By using a proper voltage cut-off, the cells will remain cooler and have a higher cycle life.

What is balance charging and how often should you use this charge method?
Balance charging makes sure that all the cells reach the same termination voltage at the end of the charge. In a perfect world cells would be identical and perfectly matched, but this is not easy to accomplish. Due to this, balance charging makes sure each cell gets charged equally. It should always be used when charging LiPos. If you don’t use balance mode you are creating a safety risk because if a cell has an issue and over charges it can ignite. By using balance mode you are making sure the charger is checking the voltage of each cell. I have some good info on this on the SMC site. (Editor’s note: you can access this information here)

What has happened when a battery swells or puffs and is there any saving it?
There is a difference between a cell or pack that has some expansion and one that puffs up like an airbag. If a cell or pack has some expansion it’s still fine to use the pack as long as the cells are charging properly and in balance. Every Lithium pouch cell will have some natural expansion under charge and discharge.

We should leave clearance in the cases to compensate for minor expansion, but we don’t because manufacturers is trying to put as much mAh or performance in the cells so the bigger the cell the better it will be.

In my testing of cells, I’ve noticed around 0.2 to 0.3 mm of expansion under normal cycling of the cells. If you take an 8000-2S2P hard case pack this means there are four cells in this pack so you can get as much as 1.2 mm of natural expansion which will make the case rounded as the cells will push up on the plastic. There are a few things that increase the expansion of the cells like discharging them down too low under load, leaving them stored with too much charge or running them under higher amp loads where the cells will get very hot. If a pack has a cell that puffed up like an airbag then that pack is no longer safe and usable.  

Speaking of unusable packs, how do you properly dispose of a LiPo?
To dispose of a LiPo pack you must make it safe by discharging it fully. Best way to do this is to submerge it in salted water. You take a plastic container that will allow the pack to submerge in it and put water with a few table spoons of salt. Leave the pack submerged for two to three days. I recommend you put the container in a safe place where kids and pets can’t access it as the water will get pretty nasty with chemicals leaking out of the cells.  When you want to remove the pack from the water I recommend you empty the container and rinse the pack before handling it. It’s then safe to dispose of and you should check in your area for a recycling center.

Why are some racers heating LiPos before a race and why is this being banned at more events and tracks?
The internal resistance of a cell will be lower when a cell is warmer. This is why racers try to heat up the packs. One thing to note is Lithium cells cool down very fast, so I personally think heating of packs isn’t as big of an advantage as some think as it’s very hard to time so the cells are just charged before the race. Normally there is tech and delays so in my opinion this is not as beneficial as some think. Racers who claim this makes a difference tend to test this in practice where you can run a pack on the track as soon as it’s charged/heated up. As to why this is being banned at tracks, I assume it’s partly for safety and to keep things more simple. There is already enough things to work on to have a competitive race car that I believe having to worry about heating up packs and timing it as close to your race as possible makes no sense.

Unlike older cell chemistries, LiPos are said to have no memory, so you can discharge it 3/4 of the way and then recharge it without completely discharging it, but doing can impact performance. Why might racers want to always discharge their LiPos before recharging them?
There is no memory effect in a LiPo, but racers discharge to build up heat and to have the charge take a bit longer to build up heat. Once again, I think this is overrated as cells will cool down very fast.

What is the best charge rate to use for LiPos? How about discharge rate?
When it comes to charge rates, it depends on the cell formulation. Certain cells can be charged faster than others. That being said, I find 2C to be the best charge rate based on time of charge and mAh put into the pack. So, a 5000 mAh pack charged at 2C would be charged at 10 amps. Charging at higher rates puts less mAh into the pack as the constant voltage part of Lithium charging will stop when the amps reach a percentage of the amps set for charging. On most chargers, this is 3%. If you charge at 5 amps this means the charger will stop the charge when the cell holds 4.20V until the amps drop to 0.15 amps. If you charge at 30 amps then it will stop when the amps reached 0.90 amps so this tops off the cells a bit less. For discharge rate, there is really no best discharge rate. Typically discharge is used to put a pack in storage mode so if your charge can do 20 amps you can discharge at 20 amps on normal RC car size packs. I personally test packs for performance under a 35 amp discharge and the cells have no issues being discharged at this rate

What are some common LiPo mistakes?
The biggest mistakes are discharging the pack down too low under load and leaving it stored with too much charge. Both of these can increase the cell expansion and will shorten the cycle life of the pack. Using too small of a pack for the power system and weight of your vehicle. If you are going to run a bigger 4WD truck you shouldn’t run a small pack designed to be run in a smaller 1/10-scale vehicle. For example, SMC offers a 5000-3S pack that is the same size as a 5000-2S pack. This pack is for Traxxas Rustlers and other kits of a similar size. If you try to run these packs in a Traxxas E-Revo this can lead to possible issues as the cells are small to fit in the smaller case which means they can’t dissipate heat as well as a bigger pack. The hotter cells get the more they can expand and the lower the cycle life will be.

LiPos aren’t new anymore, but what are some of the things people still don’t know or get about this technology?
LiPo is the best technology when it comes to energy density, so you get the most mAh per size with this technology. That being said, there are a few things you must do properly to get the best cycle life out of this technology. You shouldn’t discharge the cells down too low under load or leave them stored with too much voltage. Most chargers have a storage mode where the pack will be charged or discharged to a set voltage per cell. This is normally 3.80 to 3.85V per cell. If the customer takes proper care of the packs and use packs that are well suited for their applications you will get good cycle life out of a quality pack.

What does oil in LiPo cells?
When it comes to LiPo cells there are many different formulations based on what you want to get out of the cells. One way to improve the internal resistance of a cell is to have oil in the formulation instead of water. This lowers the resistance so the cell will put out more voltage under load. With a cell that uses water instead of oil you get higher resistance. The trade off in using oil is that oil cells tend to expand easier then cells with water as they don’t handle the heat as well. SMC has always been known to offer high performance packs and all of our cells used oil, but this past year we released the new DV series packs with no oil in the cells. We believe this is better for the basher market as the cells are less prone to expand under normal use. As mentioned above, a cell that expands can still be used but many don’t like to see this and feel the pack is defective.

LIPO MUST-KNOW FACTS

  • Recommended charge rate is 2C or twice the capacity. For example, a 4000 mAh pack should be charged at 8 amps (or less)
  • Balance charge mode should always be used
  • Always charge with the battery in a LiPo sack or similar protective device
  • Never leave a charging pack unattended
  • Do not discharge cells below 3.60 volts
  • Do not store LiPo packs in hot spaces such as car trunks
  • Store LiPo packs partially charged

Links
SMC Racing

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Filed Under: FeaturedGeneral RCTech

Matt Higgins About the Author: Matt has over 30 years of experience in RC and has worked professionally in media for two decades. Matt enjoys everything from racing to rock crawling to bashing, and he believes RC should be all about having fun. Matt is as at home covering a world championship in an exotic country as he is showing a new hobbyist how to set gear mesh. His desire to share the hobby with as many people as possible inspired him to create RC Truck Stop and RCTruckStop.com.

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