There is a lot to know about batteries, but the basics are not difficult to understand when explained without the technical jargon. Lithium Iron Phosphate, (LiFeP04, or simply LFP) is the cell chemistry best suited for the high discharge rates and regeneration requirements of motorsport use. The cells are inherently rugged and are lower in cost compared to the higher energy density cells such as lithium cobalt which are fragile and tend to fail dramatically when over-stressed. Cells are rated multiple ways, which I will try to explain. First is simply voltage: LFP cells are rated at 3.2V to 3.3V with an industry standard test load on them. In fact, we charge them to 3.65V, but as soon as charging is cut off, the voltage begins to “settle”. After a few days (or weeks) a cell will usually measure around 3.4 to 3.45V with no electrical load on it. Many months later, they will still be around 3.33 volts and retain 95% of their charge. When there is an electrical load on a cell (as it’s being discharged) the voltage will measure lower than when the cell is not loaded. The voltage drop will be greater at a higher current draw. Fully charged LFP cells are fairly “stiff” when electrically loaded, but as they approach the end of their discharge cycle the voltage will drop sharply while under load.
LFP cells are available as “energy cells” and “power cells”. Energy cells have a higher storage capacity, but are designed for relatively low discharge rates. This is because they have more internal electrical resistance. There are more square inches of thin cell plate material and electrolyte in energy cells which maximizes their ability to store energy. Power cells, on the other hand, have less surface area of thicker plate material inside and less room for electrolyte, so they have less internal electrical resistance and thus can flow higher currents. Because of this, they have somewhat less total energy available, but even though there is a bit less total energy in power cells, their ability to discharge (and charge) at higher currents makes them better suited to our needs in most cases.
When electrical current flows in a conductor, heat is generated. Consequently, the internal conductors in a cell heat up in proportion to amount of current flow. If we ask a cell to flow current greater than what it’s been designed for, there will be a temperature rise that exceeds it’s design parameter. When a cell gets hot outside of it’s design spec it degrades the electrolyte which shortens lifespan.
The important specification in regards to discharge rate is called “C”, which is a numerical value equivalent to the fraction of an hour that the cell is designed to discharge continuously at. It’s actually simple: a 1C cell is designed to be discharged fully in one hour. A 3C cell can be discharged fully in 1/3 of an hour (20 minutes). A 10C cell can be discharged fully in 1/10 of an hour (6 minutes). A 20C cell… 3 min! Generally cells rated less than 10C are considered energy cells and cells rated 10C or higher are considered power cells.
In fact, LFP cells also have “pulse discharge” C ratings. Some manufacturers specify a “long pulse” and a “short pulse”. This can be up to 60 seconds for a long pulse and as little at 3 seconds for a short pulse, depending on the state of charge the cell is at. These specs can be double or more than what the steady state C rate is. The take-away is this: if we “pulse” them within their rating, and allow them time to cool between these high current pulses, then our cells will likely provide a long service life.
Another factor we need to consider in some motorsport applications is “off throttle power regeneration”… throwing a charge back into the battery when we lift at the end of a straightaway. Power controls are sophisticated electronic devices. Most have a function called “dynamic braking” or “regeneration”. When dynamic braking is enabled, the EV motor becomes a generator and the controller puts energy back into the batteries. This helps us extend driving range because some of the kinetic energy of the moving vehicle that is usually wasted when we apply the brakes can now be recovered and used to recharge the battery. While most batteries are not designed to be charged at the same rate as they can be discharged, whenever electrical current is flowing, heat is created.
Bear in mind that these ratings are established using industry standard testing procedures that do not account for 100 degree ambient temperatures or lack of ventilation in your battery case. Please see my discussion of “DUTY CYCLE” below.
In most racing applications we want to carry only as many cells as it takes to complete the race because otherwise, we’ll be carrying extra mass which will slow us down and we want to have used at least 90% of the available charge by race end.
The below example of a battery for eKart racing will illustrate how we can utilize the battery spec to determine what type of cell we should use for a given application.
A sprint Kart heat race is about 8 minutes long, but when specifying a cell, the important parameter is the cumulative time we are at full throttle, not the length of the race. Suppose we are only at full throttle for (maybe) 2/3 of the time we are racing, which would be about 5 minutes. 60/5=12 which informs that we need at least 12C to not overheat our batteries. A 15C battery would likely serve well, but we are also going to use a fairly heavy regeneration setting in our control to help extend range, so, to manage that extra heat and as a safety factor to protect the batteries, we will choose 20C cells, which can be discharged in 5 minutes or less without exceeding their heat spec.
Here is the takeaway: if you want to maximize the lifespan of your expensive batteries and discharge them at the highest rate for fast acceleration while also using the dynamic braking function to extend range, then 20C power cells are always a good choice.
15C cells may be a good choice too, but unless we limit the current or use more of them in parallel, do not expect the same service life as 20C cells in traction applications.
In the case of a road going EV where range may be more important than performance, a properly designed larger battery comprised of lower C-rate (but higher capacity) energy cells can provide the necessary current to achieve good performance and also extended range. We just need to be sure there are enough cells wired in parallel to deliver the current and remain within the C-rate.
DUTY CYCLE
One last factor to consider is “duty cycle”. Electrical devices always have a duty cycle associated with their operation which accounts for ambient temperature, overloads and other factors that contribute to heat rise and cooling rate.
Manufacture’s published cell specs are always for a single cell in open air at a controlled temperature of 25 deg. Celsius (77 deg. Fahrenheit). In the real world, cells are usually tightly packed in small spaces with little ventilation. This is where duty cycle becomes important. If we discharge or charge our cells at a rate that heats them close to their maximum rated operating temperature, then we need to allow time for them to cool adequately between power cycles. For instance, if after 30 minutes of run time the cells are at their maximum recommended temperature and after sitting idle for 30 minutes they are back at ambient temperature, then we would say the duty cycle for that application is 50%. But for example, if the cells reach their maximum rated operating temperature in 15 minutes, but it still takes 30 minutes for them to cool to ambient, then that would be a duty cycle of 33%. For safety purposes and in order to maximize the life of our expensive cells, a battery temperature monitor is an important part of any battery management system.
Ron Coffin update 3/8/25