Perhaps this experiment has been done before, but I couldn't really find a good answer on bottom vs. top balancing, BMS systems with balancing, etc., lots of conjecture and anectode and not much hard data. So I built a basic test setup to measure the cell voltages of each cell in an 8s1p pack and run it through charge/discharge cycles. The pack in question is made of Samsung 18650-22P cells from the Hoverboard packs on eBay, I picked my lowest capacity set of 8 cells for this since I'm using them for a project already. I started by bottom balancing all the cells to 3.15V and starting the cycles from there. Charging is done at 2A (~1C) and discharging at 1A (~0.5C) which makes the capacity math easy (hours = amp-hours).
So far the capacity hasn't changed much over the 6 cycles, but the cell voltage difference is slowly creeping up, especially at high load(charging at 2A in this case) and termination. So it seems to me it's still necessary to keep the pack consistently balanced via BMS, or at least be able to monitor cell voltages to make sure none drop below/above your safety limits. I'll let this continue to run and post data at 10 or 20 cycles if there's interest.
The project I will be putting these cells into is a range extender pack for a Nissan Leaf, basically following the procedure of Leaf Xpack on Youtube. So far I've only bought 200 cells (~1.4kWH) which is enough for a 96s2p pack to add hopefully around 5 miles of range.
And for the nerds out there, here's the data in real-time (no offset/gain correction, channel 2 of the ADC is wonky so it needs to be corrected by 0.75): https://thingspeak.com/channels/308182
So far the capacity hasn't changed much over the 6 cycles, but the cell voltage difference is slowly creeping up, especially at high load(charging at 2A in this case) and termination. So it seems to me it's still necessary to keep the pack consistently balanced via BMS, or at least be able to monitor cell voltages to make sure none drop below/above your safety limits. I'll let this continue to run and post data at 10 or 20 cycles if there's interest.
The project I will be putting these cells into is a range extender pack for a Nissan Leaf, basically following the procedure of Leaf Xpack on Youtube. So far I've only bought 200 cells (~1.4kWH) which is enough for a 96s2p pack to add hopefully around 5 miles of range.
And for the nerds out there, here's the data in real-time (no offset/gain correction, channel 2 of the ADC is wonky so it needs to be corrected by 0.75): https://thingspeak.com/channels/308182