Am I empowered?
As touched on in my previous post about gears, the Newbie Corner is intended to be an information draw from a newcomer to the hobby to share my experiences (the newbie) with others so that you can hopefully learn from my mistakes! And this second post is about batteries and not-so-coincidentally, my biggest mistake to date.
I relied on others with experience to kit me up with my first car (now about 6 months old) including the motor, ESC and batteries. I didn’t really understand the figures listed on the batteries but had confidence in those who chose for me. What I didn’t understand, and was never really explained to me, was that these batteries need to retain a certain amount of charge in them or they will be severely impacted in future. I’ve already made that mistake and the 2 batteries I have purchased upfront in my initial foray in to the hobby, are now both depleted in their ability to perform optimally. I’m not going to replace them yet as they still function and I’m still learning in any case, so the impact on me is still negligible.
But I did want to understand where I went wrong and understand why letting the batteries fully discharge causes damage to them. Hopefully this post gives you some insights and a better understanding of what all these numbers on battery packs mean!
Lithium Polymer. These are the types of batteries used in our RC car racing. But there appear to be a ridiculous number of variants by way of measurements / ratings on these suckers. My initial purchase was 2 x Team Zombie 5500 mAh 75C 7.4v batteries. Let’s start with what all of these numbers actually mean. And I’m going to start with the easiest one first.
I say easiest as it is going to be consistent among all batteries we use in the on-road touring class cars. 7.4v. The batteries we use are commonly known as 2S which represents 2 cells in series format, meaning you combine the voltage of each cell together for the total voltage. Each cell, unsurprisingly is nominally 3.7v so the total nominal voltage rating is 2 x 3.7 or 7.4v as you will see on any 2S LiPo battery we use.
The nominal value is essentially the “resting” value that is used to benchmark and compare against different battery types but it isn’t the maximum capacity. Each cell will charge to 4.2v and conversely, the minimum safe state that you should not go under (as I did; refer opening remarks!) is 3.0v/cell. So the nominal measurement of 3.7v/cell is essentially the middle mark between the 4.2 (max) and 3.0 (min safe) values.
So what does that voltage value actually mean?
It is essentially the measurement of how fast your car will go. In the Spec class series, we are using a Hobbywing 3650 21.5T brushless motor. The 3650 motor is rated at 1750kv or, the ability to turn 1750 RPM for every volt powering it. This motor therefore, powered by a 2S battery providing 7.4v input, can provide 7.4 x 1750 (12,950 RPM) or, at maximum capacity 14,350 RPM (8.4 x 1750). All of our on-road racing classes dictate a 2S battery with 7.4v (common stock racing spec) so that the power provided to the cars are all equal.
The 5500 mAh reference refers to the capacity of the battery, or, how much power the battery can hold. It’s pretty easy to draw a conclusion that the larger the number, the better the battery but this isn’t really the case in our racing environment. The unit of measure here is milliamp hours (mAh), or, how much drain can be put on the battery to discharge it in one hour.
Our races only go for 6 minutes. A fully charged battery pack at 8.4v will provide sufficient full power for this race duration so a battery pack of size 5500 mAh will provide sufficient power for a race the same as a battery 8200 mAh. The 8200 mAh battery would power a car longer than a 5500 mAh battery, but in the context of a 6 minute race, this is not necessarily a primary consideration. The higher the mAh rating too, the more weight in the battery and this also can be worth noting when considering your car race weight.
The C Rating
My Team Zombie 5500 mAh battery has a C rating of 75. So what’s with this? The C rating on a battery refers to the “Capacity” rating or more succinctly, the rate that it can be discharged safely without harming it. My battery is powering a Hobbywing ESC with a continuous power draw of 45A and a burst draw of 260A as well as the Justock motor draw of 35A.
With my 5500 mAh battery, I can determine the following:
75C = 75 x 5.5 (capacity) = 412.5A. Any more draw than this would at best, reduce the lifespan of the battery or at worst, burst in to flames. I’m OK here with my 75C rating as the ESC and motor, at burst capacity totals 305A, well under the 412.5A capability of the battery.
Most batteries these days have 2 distinct C ratings; a continual rating and a burst rating (10 seconds). By understanding the ratings on these and the continual and burst specifications on the devices the battery is powering, you should be able to determine whether the C rating on your battery is sufficient.
From what I’ve read in my research here, the C rating is a bit of a real debatable topic in terms of performance benefits. Some swear that a larger C rating is better but the more measured response here is that as part of the manufacturing process, the higher C rating usually (but not always) results in lower resistance and this is indisputably tied to the performance of the battery.
Internal Resistance (IR) isn’t something that you will find as a rating on a battery as it is variable and will alter over time. The only real valid way to measure resistance is through a battery charger as part of the charging process. So what is it?
Resistance has to do with your battery’s health. As a LiPo battery is used, a build up forms on the inside terminals of the battery and the resistance value increases. Effectively, this is a measure of the battery’s efficiency. After many, many uses, the battery will simply wear out and be unable to hold on to any energy you put in during charging – most of it will be lost as heat. If you’ve ever seen a supposed fully charged battery discharge almost instantly, a high IR is probably to blame.
I won’t (can’t really) go all physic mode here and adequately explain the correlation between Ohms Law and how resistance works; there are far more cleaver people (and blogs) that can run through this. The relevant take-out here is that a battery that is displaying a high IR will have an impact on its efficiency. I got a good gauge for how to start looking at your battery health from a website I used in researching a lot of this information at https://rogershobbycenter.com/lipoguide/ and I encourage you to read their blog in full !
To use their guide on battery health though, you can consider the following resistance measurements when read from your charger as a gauge for when to consider replacing your batteries:
* Between 7 and 12 mΩ is reasonable per cell.
* Between 12 to 20 mΩ is where you start to see the signs of ageing on a battery
* Beyond 20mΩ per cell, you’ll want to start thinking about retiring the battery pack
Also note that higher internal resistance equals higher operating temperature. The higher the operating temperature, the greater the risk of the battery catching fire!
Battery chargers come in many shapes and sizes. I am now using an EV-Peak AR1 which can charge all the way up to 25A compared to the original charger I had that was restricted to 5A.
As for the charging process itself, the charger will accommodate programming based on the mAh rating and number of cells. If you are reading this before you’ve purchased a charger, make sure you get one with capabilities to charge the cells in balanced mode. Balancing is a term used to describe the act of equalising the voltage of each cell in a battery pack. We balance LiPo batteries to ensure each cell discharges the same amount. This helps with the performance of the battery and is also crucial for safety reasons.
Charges are also used to discharge the batteries as well and there is a process for this too. Most chargers run a storage mode and LiPo batteries shouldn’t be stored at full charge nor should they ever be discharged below 3.0v/cell. The optimal storage should be 3.8v/cell.
This is by no means a definitive guide on batteries but I hope, if you’ve managed to read this far, that you got a few takeaways from this. I’ve learned a lot in how to interpret batteries and how to charge, discharge (store) and evaluate their health. So hopefully this helps you too!
One final shoutout yet again, is to the website I gleaned the majority of my research from; Rogers Hobby Centre. Thank you for the comprehensive guide. I encourage anyone who wants to read deeper than what I have provided here, to check out their guide here: https://rogershobbycenter.com/lipoguide/
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