EddySPalm wrote:Now, are you guys saying they can't really handle 10A continuous and still keep close to their rated capacity? Are you even saying they will get hot and/or damaged?
I did check them on the graph on lygteinfo.dk, but what am I looking for? All I can see from the chart is that (for example) for a 7A continuous draw their capacity will drop to 3.1Ah instead of 3.5Ah. I can live with that...
The problem is with how much of the pack's energy is turning into heat. Lithium does not like heat and will degrade at 160F and beyond.
Even at 1/2 the continuous maximum.. you are going to get a hell of a lot of heat.
Here's the thing. These cells are rated for their maximum continuous discharge based on the result of a single cell in a laboratory at room temperature. When you finish a pack, you have a bunch of cells heating each other. Enclose the pack in a box or with shrinkwrap and you retain even more heat.
Look at the 10A graph. The cell goes from 3.7v nominal to 3.3v nominal. ( get nominal voltage by looking at the voltage in the center of the graph )
If we take the nominal voltage x the capacity, we get: 8.58 watt hours.
But at 1 amp, we get 3.6v nominal x 3.2ah = 11.52 watt hours.
The difference between 11.52 watt hours and 8.85 watt hours? you're losing 25.6% of your cell's battery energy to internal resistance. Therefore, 25.6% of the battery output will be heat.
So let's say you've built a pack and are running the full 10A at 4000W. The cells will be producing 1000W of heat in an enclosed space. That will go kaboom in short order without liquid cooling.. and it'll be extremely inefficient. suddenly your wonderful 250whrs/kg pack is delivering an effective 187.5whrs/kg to your controller.
Ah, here's the other problem with that. You built a 17S pack thinking that 3.6v nominal x 17 = 61.2v nominal, right? but when the cells sag to 3.3v nominal, you get 56.1v nominal and take a hit in performance and speed. You have a major resistance problem here. It is like running 40 amps through a 12 gauge wire.
Now let me make this simpler for you so you don't have to sit around with a calculator.
The thing you want to look for when figuring out how a cell with perform at a certain amp level is the voltage sag. The voltage sag is an indicator of resistance going on ( just like in a wire ), which indicates a loss in efficiency and an increase in heat.
Compare the 0.2A and 5A discharge graphs to see what the voltage drop is relative to baseline performance at a super wimpy discharge..
Notice that we've got a difference of 3.45v to 3.7v, right in the middle of the discharge? well, a drop more than 0.2v is quite big. We've got a 0.25v loss though.
Let's do the math on the 5A scenario, using the voltages in the middle of the graph as the nominal voltages.
3.2AH x 3.7v = 11.84watt hours at a 0.2A discharge.
3.125 ah x 3.44v = 10.75 watt hours at 5A discharge.
We've got a 9.3% loss of energy to heat, so at 4000W, we're making 372W of heat, which is still pretty bad.
You might say.. 'yeah, i can figure out how to disperse 372W of heat in this pack'.. and you could be right, when the pack is brand new.. but as it ages, it's discharge rating is going to go down as you put more cycles and calendar life on it.
I have 7 year old 20C turnigy packs here and their internal resistance has gone up 5X. They're effectively 4C rated packs by now. But i've always used them at 2C maximum, so i've always had lots of headroom. The amount of headroom you have is zero here. But your pack is going to need some form of cooling right out the gate, whereas my massive headroom has allowed me to just put my packs in a bag and forget about the thermal considerations.
I would design your pack with more cells in parallel or start thinking about a cell with lower internal resistance. Samsung 25Rs are the champion in this regard.. but there are a few cells that are closer to a middle ground. Maybe the Samsung 30Q is better? look for no more than 0.2v drop when you are comparing discharges at various C / amp in order to find the sweet cell for your needs.