Battery Focus : The C rate
If two identical karts are raced side by side, one with a 50 AH lead acid pack onboard and the other with a 50AH lithium battery pack, the lithium kart will win for a number of reasons.
The first thing is that lithium cells are very good at releasing the energy to the kart’s controllers, so the kart’s performance will be better. This can be noticed coming out of tight bends, when the lithium kart picks up faster and the driver feels much more power. When Ross Curnow was test driving the GMS electric kart he said, "It feels much more lively now and is very responsive, it feels super charged with the new lithiums"
The other factor that makes the lithium kart win the race is that the battery pack is much lighter than lead acid cells even though it carries the same energy. The combined effects make the kart seem supercharged! The lithium cells also don't get so hot and they can deliver energy much faster for much longer times. This is because the resistance of the lithium element making up the battery cell is substantially lower than lead. Having a lower resistance means less heating of the battery pack, and less heat means less waste, which in turn means more energy powering the kart. It is worth noting that combustion engines only get hot because they waste huge amounts of energy!
Charging is also important. The lithium phosphate cells can be charged reasonably fast and using a 2 hour charge on all the cells will mean years and years of trouble free use. They can be charged super fast, but it does reduce the life a little. If you are not bothered about lifespan and just want to go as fast as possible then you can ramp up your charger and charge the cells in 20 minutes to 80% of the capacity. To do this takes a serious charger but it's all possible and available now.
All GMS power trains are low voltage and with a 400AH battery system, they can deliver the peak power and performance required by our dual drives comfortably. A 400AH battery running 16 cells can in theory deliver a whopping 12C for 5 milliseconds, that's 4800 Amps @ 50volts! So you can see, the battery is comfortably over-specified for the job. Having this kind of margin or head room, using the lipo4 technology, can deliver years and years of high performance and driving pleasure even at safe low voltages for everyone. Below is the new 70AH 48v battery pack with our project BMS system attached.
Different results under load
The C rate is the measure of a battery’s capacity. C rate is the amount of energy the battery can deliver over 1 hour to 100% DOD, depth of discharge, in other words flat. Under current loads different chemistries of batteries give very different results. The real beauty of lithium battery technology is that it weathers very well under varying loads.
In general the faster you take energy from a battery the lower its capacity. This is very true for lead acid type packs. As you apply a huge load over a lead acid cell, the effective battery capacity falls, so that a 50AH battery under heavy loading may only in the real world deliver 30AH, but it still says 50AH on the side of the battery. When loads from electric motors are high, the actual capacity of a battery is changed by the way you use it.
Lithium cells are much better at delivering energy under high loads.
The actual capacity of a lithium phosphate cell will only reduce a little even under massive loadings and that is why lithium ion phosphate is so popular for use with electric power trains. The other reason is that the added phosphate makes them more stable and also makes the cells much safer than other lithium technologies. You can break a lipo4 cell in half, crash it, snap it and it won't explode or catch fire.
Cells that deliver the true capacity marked on the side of the battery under load are clearly the ones to use. When the cells are tested, scientists give them different C rates so that the customer can understand what they can do under different discharge rates. The cells Green MotorSport offers have a high C rate and are matched to our low voltage, safe drive trains. Almost all the power trains we build use around 150-350A continuous, with surges of current up to 650A as you accelerate away, so we need a cell technology that can offer good range and good life under these high C rate circumstances. Lithium phosphate cells have helped to make GMS power trains viable and cost effective for everyone to drive safe, low voltage electric vehicles.
Example of C rates using a 100AH lithium cell
1C for a 100 AH battery cell is 1 x 100= 100 Amps rated.
2C for 100 AH battery is 200Amps
3C is 300Amp
4C is 400Amps
10C is 1000 Amps
12C is 1200 Amps,
Our new range of high volume, low cost cells is more than capable of delivering 12C for a short burst and 4C continuous.
Charging is also important. The lithium phosphate cells can be charged reasonably fast and using a 2 hour charge on all the cells will mean years and years of trouble free use. They can be charged super fast, but it does reduce the cycle life a little. If you are not bothered about lifespan and just want to go as fast as possible then you can ramp up your charger and charge the cells in 20 minutes to 80% of the capacity. To do this takes a serious charger but it's all possible. Contact us for more info on rapid charging and our super sport cells that can deliver over 40C!
Larger cells are less stressed
The other interesting thing is if you use a larger cell capacity in the same voltage configuration, for example, you use a 200AH lithium cell configuration rather than a 100AH, the battery pack will deliver more power and total energy. There are several reasons for this. The first obvious one is that you have just doubled the battery capacity. The second one isn't so obvious. A larger battery pack using larger format cells suffers much less under load, and as a result the cells will deliver more energy for the application required as the losses are reduced inside the cell chemistry.
The benefits of over sizing a battery pack or adding some head room now start to add up. Using a 400AH cell that has a maximum continuous C rate of 4C can in theory deliver a continuous current of 1600 Amps without having any heating or cycle life issues. If you use 16 of these 400 AH monsters you soon have a battery pack made for a long range, high life, low voltage electric car that has a serious amount of poke!