Kokam Lithium Polymer 5S 4000mAh 30C battery (WHL #30)
Yawn. Another battery pack. But also new hardware for the electronic load, so testing conditions have improved 
Today’s item is a Kokam SLPB 400030-0501K(9543140H5) battery pack. Kokam is one of the big players in lithium polymer batteries, and given the battery was purchased at the same time as the last item, I guess this one wasn’t a cheapie as well. No Wan Hung Lo this time, but after all, it’s not an item that I bought… 
It is clearly aimed at RC stuff, having 4000mAh at 30C discharge rate, 18.5V nominal / 21.0V max voltage, 8000mA charging current. The front also indicates 120A/200A discharge current, 120A being the 30C nominal figure, 200A hinting at some label on the back of the pack…but there’s no note about this 50C rate and when and how this applies. Still, 120 amps at 18 volts is more than 2kW, which is quite a thing for a 298g battery pack…
Front, once again: (the slight Kokain (cocaine) mis-labelling was kindly fixed by my old friend and former work colleague Dave…)
Backside:
There’s no other labelling on the pack, and there’s also not much to be seen without ripping apart the envelope, unfortunately. It does have some PCB inside, but for said 2kW figure, I better not test if there’s an short-circuit prevention integrated 
The front label is a bit squishy for some reason. It might be gassing cells, but I found the weakest cell is actually #3. So unless they have a very weird wiring order, having the #1 cell gassing a lot and the others not doing a thing is a tad strange. I also doubt this battery has seen more discharge current than in my tests – and that’s only 8 amps or 2C, which is nothing compared to the 30C rating. When I picked it up from the lab, it wasn’t over-discharged, despite sitting around for a long time. So basically it should be alright. But is it?
Turns out: It only has about 60% of capacity left. Bummer!
It also looks like the pack is dying cycle over cycle. First run I took was at 4A, then 2A, then the revised 4A (I’ll elaborate on that in a bit). Capacity decreases each run – 2535mAh, 2425mAh and 2315mAh respectively (the exact 110mAh steps might be a coincidence). I never made it to stop the tests before the hard 3.0V per cell cutoff that my electronic load does by default, so the weakest cell went below 3V each time. Once again – capacity gains when squeezing out the last drips are negligible, but it’s probably hurting the cells.
I cannot find a datasheet for the pack, but there’s a really old test at elektromodellflug of similar cells. They say, these cells have some improvements in intrinsic safety, so they don’t explode or catch on fire when mistreated. The cells also show very low internal resistance, which is more than an order of magnitude lower than mine. So I’d say we keep them for tests that may kill them, but they’ll never find their way into a prototype that is tested by ordinary workers or without supervision in our plant. Given the delicate structure of the pouch cells, I’m also reluctant to just remove the weak cell and put the rest back together…but before throwing the pack into the bin (yes, the “old battery for recycling” bin), I might have a try.
As for the improvements in test gear: Have a look at the blue curves in the graph above, where the battery was tested with the upgraded version of the electronic load. The amp reading has much better resolution due to the doubling in shunt resistance (20mΩ instead of 9-10mΩ RDSon), which has some downsides as well – there’s now a fixed 30mΩ resistance in series instead of only 10mΩ from the switching MOSFET. 20mΩ of shunt resistance also means a much higher power dissipation, which is where the temperature coefficient comes into play. Instead of the semiconductor with pretty shitty temp co, there’s now a 50W, 50ppm/K power resistor installed. Only 500ppm or 0.05% of resistance change over a typical 10K temperature rise during tests is a big improvement!
Using data from the last WHL post, I can do a comparison at 4A load on the 12V ATX power supply. Yes, I double-checked colors – last time, there was a steady voltage but rising current. This time, current is pretty steady, but the voltage looks like it is jumping around a lot. Don’t be fooled by the two straight lines on top – one shows voltage from the old test, one shows current from the new one!
But have a closer look at the numbers: In the former test, current went up from 3.9A to almost 4.1A (5%), while voltage was regulated within less than 50mV (less than 0.5%). This time, current was stable within 20mA (0.5%), and voltage regulated between 10.95V and 11.10V – just shy of 1.5%. So overall stability increased by a factor of 3!
Unfortunately, this doesn’t seem to be the full story. I ran another test at 8 amps, which is shown in the graph above. I divided currents by 4 and 8 so that we can see some detail. Voltage wasn’t changed (but axis range had to be extended, due to the voltage drop in the leads). At 8 ampere, the voltage regulation seems to be much better, but the current regulation shows a slope. It’s ramping down from 8133mA to 8058mA (only short of 1% – axis range can be deceptive…). Voltage stayed within 10471mV to 10520mV, which is another 0.5% of variation. So for higher currents where contact resistance and also wire resistance plays a role, current regulation seems to become a bit less stable, but overall performance stays consistent.
Sure, this isn’t exactly BK Precision 8600 accuracy levels (0.05% of reading + 0.05% of full scale) – but it’s also far away from the 1000++ USD price sticker as well
As for the Kokam battery: It’s dead, Jim. Well, almost. Let’s keep it in storage and dispose of it when we completely murdered it.
