Batteries, there is a lot to know about them, but the basics are not difficult to understand when explained without the technical jargon. At the present time, Lithium Iron Phosphate, (LiFeP04, or simply LFP) is the cell chemistry best suited for motorsport applications. These cells are widely available and work well for the high discharge and charge rates we require in our sport. The basic cell characteristics are described by an industry standard rating system which I will try to explain.
● I’ll start with voltage: LFP cells are rated at 3.2 to 3.3 Volts with a standardized test load on them. In fact, the standard charge voltage is 3.65V. As soon as charging is cut off, the voltage begins to “settle”. After a few days (or weeks) the cell will usually measure around 3.35-3.45V. After months, they will measure around 3.33 volts and still retain most of their charge. Of course, we need more than 3.3V for our race cars and Karts. In fact, for a “Senior” kart we want 66V. That requires 20 cell packs (3.3V x 20 = 66V). To accomplish this, we use two 33V batteries connected in series. Each 33V battery has 10 cell packs in series. This called a “10S” battery.
● LFP cells are available as “energy cells” and “power cells”. Energy cells have a higher storage capacity, but are designed for relatively low discharge and charge rates. This is because they have more internal resistance. There are more square inches of thin cell plate material inside energy cells. Power cells, conversely, have less surface area of thicker plate material inside, so they have less internal resistance and can flow higher currents without heating up as much. But because there’s less surface area of cell plate material, they have less energy capacity. For heavy discharge applications, power cells are best. For maximum power storage applications, energy cells are best.
●The discharge rating of a cell is called it’s “C rate”. Low C rate cells are energy cells designed for slow discharge (and recharge), while high C rate cells are power cells designed for fast discharge (and recharge). “C” refers to the fraction of an hour that it takes to discharge a cell fully without exceeding it’s design parameters. It’s really very simple: a 1C cell is designed to be discharged fully in one hour. A 3C cell is designed to be discharged fully in 1/3 of an hour (20 minutes). A 10C cell is designed to be discharged fully in 1/10 of an hour (6 minutes). A 20C cell… 3 min! Generally cells under 10C are considered energy cells and cells over 10C are considered power cells.
Consider the following example: A Kart print race is about 8 minutes long, which is more than 1/10 of an hour, so you may think that 10C cells would be ideal. But actually, it’s not the length of time of the race, but the cumulative time we are at full throttle that is important. In fact, we are only at full throttle (maximum discharge) for (maybe) 2/3 of the time we are racing, or ~ 5 minutes of an 8 minute race. So to make sure our batteries can handle the discharge rate we need without overheating, we actually want 15C or 20C cells, which can be safely discharged in 5 minutes or less.
● Cells also have a “pulse discharge” C rate which is usually around twice what the steady state C rate is. A “pulse” usually means 5 or 10 seconds.
● These ratings are established using industry standard testing procedures. They don’t account for 100 degree days in Utah, or lack of ventilation in your battery case. They also do not account for heat created during off throttle power regeneration…. throwing a charge back into the battery when we lift at the end of a straightaway. When you pulse discharge a battery
at C rates higher than the continuous rating, it causes extra heat in the cell. Unless the cell has time to cool between pulses, it can easily overheat, which will shorten the lifespan or in extreme cases even cause failure.
●The upshot is this: if you want a battery to have a long service life and also have the ability to discharge at the highest rate for fast acceleration, a high C rate cell is what you need. My recommendation is: 20C cells will give the longest service life for race Kart batteries.
● Battery manufacturers are (for the most part) located in China. Like all suppliers, some are good and some not so much. Many of the claims made by resellers on eBay and other sites cannot be trusted, so beware. If it sounds too good to be true, it probably is. I have found that dealing directly with the manufacturer is the best way to go, and I’ve developed relationships with several trusted suppliers and import the cells directly from them. Additionally, we’ve built a “battery dyno” that enables me to test the cells to verify they will perform in our battery pack at the advertised C rate.
Charging: Battery charging has to be managed. It can be done manually, but that is time consuming and easy to get wrong. It’s best handled by a “balancing charger”.This is different from a so called “smart charger”. Smart chargers are better than dumb chargers, but they really just look at the total voltage across your battery while charging. Once they sense the voltage adds up to 3.65V per pack (or cell), they shut off. The problem is, a battery is made up of multiple cells or cell packs. Some of these packs may be low, while others are high. If one pack is at 3.75V and another is at 3.55V, the average of the two voltages is 3.65V, yet one pack has not reached full charge and the other is overcharged. In order for a smart charger to work effectively, the battery must have separate passive balancing circuits attached to each cell pack to prevent the cell packs that reach 3.65V first from becoming overcharged. These passive balancing circuits turn on at 3.65V and shunt the excess current through resistors while the lower voltage packs have time to fully charge. At least that’s how it’s supposed to work.
● A “balancing charger” is different. This type of charger requires a “charging harness” connected to every cell (or parallel cell pack) in the battery. Each cell pack is charged separately through the harness while the charger senses individual pack voltages. Once a pack reaches the desired state of charge, the current to it is cut off. They won’t all finish at precisely the same time, but when they do finish, all will be charged to the same electrical potential; in other words, they will be “balanced”. This is important to maximize battery life and performance, because a battery is only as good as the weakest pack in it. A good balancing charger will pay for itself many times over.
● We’ve made our own balancing charger. Our CuicPower Perfect Balance Charger will charge every cell pack in a 10S battery to a precision of .005 Volt. This is at least 10 X more accurate than industry standards and 50X more accurate than any “smart charger” without benefit of a BMS or individual balancing circuits. Our Perfect Balance LFP Fast Charger™ will take a 33 volt 45 amp hour kart battery from discharged to full charge in about 1.25 hours.
● Of course, the choice of battery chargers is up to the individual. But understand that once a LFP cell is charged to full capacity, trying to put ever more voltage into it provides no benefit, it just destroys the cell.