Well I finally finished my EXTENSIVE caracterisation of the VTC4 cells I bought from DoctorBass (Makita BL1840 10 cells packs).
I thought posting the results would help people decide by themselves if these cells are suited for their needs and what to expect.
So this post is to give you the results, which quite frankly, are difficult to beat considering the great low price.
So I bought 29 Makita BL1840 Pack from DoctorBass last February 2017.
That is 290 cells. From these, 4 cells were dead (voltage close to 0). The 286 cells lefts were as good as new (see my Excel file below). They were all retrieved at normal operating voltage (Lowest 3.46V ; Highest 4.02 V)... Most in the 3.70V region.
I have actually noticed that the BL1840 packs (Packs of "18"-650 cells ; "4.0" Ah per pack) can have
either one of two kinds of Sony Cells...
I retrieved 218 sony
VTC4 cells (22 Makita packs) and 68 sony
VT4 cells (7 Makita packs).
The VTC4 and VT4 cells seems different, as VTC4 have on average slightly lower DC internal resistance than the VT4 cells (all cells compared at same voltage of 3.65V/50% SOC). Capacity seems almost identical though.
So VTC4 and VT4 are completely different animals with different specs, although I can't seem to find a specsheet for the VT4 cells on the internet.
With all my data, I did a statistics summary :
- BL1840 cells Statisticds.jpg (72.8 KiB) Viewed 1138 times
As you can see I measured DCIR of 22 mOhms for the VTC4 versus 28 mOhms for the VT4.
For the capacity, average was 2190 mAh for VTC4 versus 2119 mAh for VT4.
I did buy them almost 6 months ago, but as of today, these cells are average 2.4 years old for VTC4 and 3.1 years old for VT4 (exact date to the exact day was obtained from date code on each cell).
So now, here are the detailed results. To extensively caracterise each one of the single cells, I did 4 things :
1) Determine the "Salvaged voltage" (anything below 2.5 Volts upon recovery and I would discard these cells).... Note that as soon as I got the packs, I dismantled them to remove the BMS (Battery Murderer Systems that drain down your cells beyond recoverable with long storage times).
2) Measure "Single cell Capacity" (I used 3 x Foxnovo 4S Digital Chargers to measure each cells, 12 cells at a time.... Discharge mAh determined on highest setting (mAh@0.5A discharge/cell.... while charging is done at 1.0A/cell).
3) Measure "DC Internal Resistance" (DCIR) . For that, I used my Imax B6 Mini, which now has the option to determine internal resistance of cells !!! Surprizingly accurate for a non-4-lead measuring device... I found the measures in the 20 mOhm range is around ±5 mOhm margin of error (temperature drifts from time to time).... There are some value drift with temperatures, but repeating measures at different periodes of times help you find the real value (ex : 26 mOhm, 21mOhm, 20 mOhm, 19 mOhm, 22 mOhm... Okay, 26 mOhm is the alien value to eliminate here) ... To compare, I did regression analysis with another method of determination of DCIR (V= -r I + EMF) and getting the slope of graph to get DCIR value, while testing cells at different known loads (Ammeter connected) and reading voltage drop simultanously (voltmeter connected too) for different loads (different 50W resistors used for loads). For exemple, with this method, I measured DCIR of 19.9 mOhm by regression analysis, while for the exact same cell the Imax B6 gives me a value of 22 mOhm. Accurate enough for my needs!!! No cells were of bad DCIR (except the 4 dead ones, reading in the 70 milliohm range when charged before experimenting, then tossed away). I compared with result I got with using different resistors and tracing graphs to get the slope (which gives the DCIR):
viewtopic.php?f=14&t=87173&start=25#p1276187
4) I measured "Ability to Maintain Same Voltage Over Time" (see Excel file below). They did not move significantly over a the course of 3 week test at full 100% SOC : 4.2V. Then, for optimal storage , I discharged all cells to 50% SOC (I did a giant parallel solderless 130P1S and then another 166P1S parallel pack and used a load to discharge the whole thing while monitoring voltage with CellLog 8M to 3.650 V target voltage for storage.
For cells 1-130, Final discharge voltage was 3.651 V. For cells 131-290, Final discharge voltage was 3.657 V.
Three months later, the cells have not moved of a single millivolt. None of them moved (I rechecked every cell's voltage individually, every months). See my Excel file below for data.
So I must say, these cells, I consider of A+++ quality. Just be aware that a few Makita BL1840 pack (about 1 pack out of 4 packs) actually contain VT4 cell in place of VTC4 cells. And these cells I don't know well, but are around 6 milliohms more and about 70 mAh less than the VTC4 on average.
Still, amazing deal if you want my opinion. All in all, I'm glad I bought these cells, as the lower price compared to new cells allowed me to buy valuable tools with that money (Imax-B6 Mini, B168 active balance charger, HP DPS-1200 power supply, Ming He BSW900 boost converter, 3 Foxnovo Digital 4S chargers, A dremel (OMG the best tool ever.... All these tool I will be able to use in the future if I need to). Did a little mod to the foxnovo 4S charger :
viewtopic.php?f=14&t=86556&p=1266049&hi ... o#p1266049
Also, all these cells comes already spotwelded to tabs... And we're talking COPPER TABS here !! (nickel-plated solid copper core), so yes... big amps ! Don't believe me it's copper ? Check pics here :
viewtopic.php?f=14&t=84680&p=1275118&hi ... a#p1274883
Here is the Data I compiled on every single cell I got (extensive caracterisation) :
Thanks again DoctorBass.... I'll be a client again next year when I come back from my training in NYC. Also, when I come back, I intend to build a "real" ebike.... I mean the "10 kW and up" Kind of eBike with massive front fork supension, DOT tires, Hydraulic breaks, Crazy tork arms (maybe the NYX frame), and a suitable battery (Sony cells of course), so I'll need your wisdom...
Cheers,
Matador