Batteries And Capacities
Truth or Lies - Part 2
If you have not read Part 1, It can be found here.
10/9/21
Lithium Polymer (LiPo) Batteries
Hello my friends and fellow RC enthusiasts from around the world.
In this post I am following up on the investigation into stated battery capacity. As I said in Part 1, batteries are at the heart of our beloved RC hobbies and can be quite expensive. As with any item, the more you pay, the better quality you can expect to receive. The same thing is true of batteries. The bigger the capacity of a battery is reflected in the price. Obviously there a lot more factors involved around pricing, but today I want to talk about "stated" capacity in Ampere hours. In part 1, I talked about Nickel Metal Hydride (NiMH) batteries and in part 2, I am going to be talking about the capacities of Lithium Polymer (LiPo) batteries.
2S (2 cell) LiPo Battery
The 2S LiPo battery consists of 2 cells. These cells are not the normal, cylindrical battery we are used to seeing. A LiPo cell (1) has a flat rectangular shape and comes in various sizes. They are sometimes covered in a hard protective shell to protect the chemical cells inside from damage. This is because of volatile chemicals/gases inside, that if exposed to air, will spontaneously ignite and will be very difficult to extinguish. These batteries are so dangerous it is advised to never leave a battery fully charged. It is also highly recommended to use a fire proof bag for the battery when charging.
Another difference a LiPo battery has is a lot more wires. For example a 2S battery will have 5 wires coming out of it. The 2 big thick wires are used in a conventional way and connect to the electronic speed controller (esc). The other 3 smaller wires are only used when charging. A LiPo charger uses the 3 smaller cables to monitor each cell and keep them charge balanced. This is where it will keep each cell as identical (in voltage and capacity) as possible. The balance leads are not used when discharging (running the battery), however it is critical that the LiPo is NEVER fully discharged as this will damage the battery. Some esc's have a battery cut off, some don't. After market, low voltage audio alarms are also widely available.
The arrow in the picture above shows the voltage selection button to set off the audio alarm. It is very loud and audible from quite a distance. The alarm can be set from 2.5 volts, up to 3.8 volts per cell. Once any cell reaches the minimum set voltage, the alarm will sound. It's ear piercing to be honest, what with 2 sounders, one on each side of the button. The alarm will not switch off or disconnect the battery, it is simply an audial warning system. The alarms themselves are quite inexpensive and cost around £3.50. They are also normally covered in clear heat shrink/tubing but I removed mine due to other ongoing projects.
With the chemistry lesson over, I can now talk about the stated capacity of my (or any) 2S LiPo battery. The 2S pack I am talking about today is my own Turnigy 2S "Shorty" 3000mAh battery pictured below.
After a 3 month design, build and test extravaganza I could finally charge and discharge (cycle) a battery in a reasonable time. We are still talking 60 minutes for either a 2S & 3S (3000mAh) battery pack, but it's a lot faster than it was. So, after 10 cycles of charge and discharge, I found the following results. Please excuse my messy writing.
The above results were observed and recorded on the 19/1/2021. This was the first test of this 2S LiPo battery, with a charge/nominal/start voltage of 7.92volts. After 17 minutes of discharging at 2.77Ohms, the battery was found to have only 740 of the alleged 3000mAh. I have worked with rechargeable batteries throughout my career, so I wasn't surprised by this result. It can take 5 to 10 cycles to reach it's full potential. With that in mind, I did another 9 cycles with the results below of the tenth discharge.
This was the tenth and last test result. As you can hopefully read in the above results, they were recorded yesterday. The battery has been used daily during the summer months, so it should be by now up to it's stated potential of 3000mAh. However as you can see in the results above, it is still a whopping 480mAh short. That equates to 16% of capacity. It is also down on power by approximately 4Wh. Even if my test equipment is 10% out of calibration (which it isn't), the battery is still 6% short. As you can see, the battery has never been abused and only ever operated in the correct manner, so what is happening with the advertised or stated capacity? To be fair, I don't know. Are we being taken for a ride or lied to. Again, I can't answer that question. Another solution would be to discharged the battery another 0.1 volt per cell, but even that (IMHO) would not claw back the 480mAh that is missing.
The red arrow in the above picture is pointing to the total capacity of the battery. The yellow arrow point to the total power of the battery. As you can see they are far below what they are stated to be. This battery is 18.2% down on power. Again, I don't think that another 0.1volt per cell discharge would replace the lost wattage. You can also see in the above picture, the discharge time was 59 minutes 52 seconds at an average discharge of 2.5Ah.
These results for me speak volumes of how the manufacturers mislead buyers like you and me into spending extra money on extra capacity that simply doesn't exist. Annoying, isn't it?
3S (3 cell) LiPo Battery
Let's see if another reputable manufacturer can do any better.
This time I will be cycling a 3S, 11.1Volt, 3600mAh, LiPo and Voltz branded pack. Again this pack was brand new in the box prior to the 10 cycle test. Unfortunately I forgot to take any pictures of the 3S pack under test.
Above is the recorded results of the first charge & discharge cycle. As you can see after the first test the pack had a capacity of only 830mAh, only 23% of potential capacity.
The above picture is the tenth discharge cycle. After 40 minutes and 39 seconds the battery had a capacity of 2710. Yes it's a lot better but still a massive 890mAh short of the stated 3600mAh. In percentage terms it equates to 24%, almost a quarter. I do not know how much the battery cost as it belongs to a friend. I can't imagine it was cheap so I didn't tell him the awful truth and upset the man. But Me, I was very annoyed. Yes I have only tested 2 LiPo batteries and more types of battery would give a clearer picture but I know where this is heading.
Lithium-Ion (Li-Ion) Batteries.
To further study battery types, I have taken a Li-ion battery pack from a cordless tool. The battery pack has been well used but it's own battery indicator light shows it fully charged. I have no information on the chemical makeup of this type of cell, other than they were designed to be used in the Aeronautical industry as they are light weight. I know these batteries are well established and used in anything from a laptop computer to an electric car. They are a very robust, reliable and widely available cell. They are also very cheap to buy, depending on capacity.
Above is the Li-ion under scrutiny. As you can see it is a 12volt, 1.3Ah with 15.6Watts of power. It's own built in battery indicator displays a full charge. So let's discharge it and look at the results.
With the battery under discharge, I waited patiently for the results. I did not write them down as I thought the battery would not have a lot of capacity. I was wrong, the battery was surprisingly strong.
The red arrow is pointing to the capacity which was 1.01Ah with a power of 9.98Watts. It retains, after 1 cycle a capacity of 77.7% and 64% power. I know this battery has been heavily used and yet still retains (in % terms) 77.7% capacity, which is better than a brand new 3S LiPo pack. The battery was drained in 17:32 minutes but discharged at a heavy rate of 3.6Amps, so it is about right. If I cycled this battery, I doubt it would increase the capacity or the power of the battery. However, I have been wrong in the past and maybe quite surprised if I did. I have low expectations of this battery but, it will be employed in another project that does not require heavy discharge. Waste not, want not!
Whilst I was researching for this post, I came across some highly interesting facts about battery capacities. The rule of thumb (and this information is from a major battery manufacturer), that if the capacity of any rechargeable battery is less than 2500mAh, then the pack can be cycled approximately 1000 times. However, if the capacity of the rechargeable battery is greater than 2500mAh, the cycle frequency falls to 500 cycles approximately. Why, I know not, but I will say this, from now on I will buy two, 2500mAh packs instead of one, 5000mAh pack. It also makes sense, as if one battery goes off, at least you have a spare to keep you going.
Again if you have not read Part 1, it can be found here.
There will be a Part 3 in the future where I will be testing further types of battery and sharing their results with you. I think I will also test some single cells (ie. AA & AAA, 18650, NiMH and Li-ion) to see if it is the same case with those.
I hope you have found this useful in any way, shape or form.
Until my next post, take care my friends.
CATXLS
10/9/21
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