Hi everyone! Electronics hobbyist here, but new to 18650s (and Li-ion in general).
I'm planning to build an 18650 ebike battery to replace the crappy 3s 12V SLAs I'm currently using (used bike, bought cheaply). I have measured the current draw from the current batteries to 13A at stall (brushed motor, so I'm guessing the controller is unsophisticated and stall current is max current).
I have 120 salvaged 18650s with reasonable capacity, no overheating, and low leakage. They consist of 6 different types. I want to build a 10s12p pack out of these, because I have enough space in the casing and because it will still be lighter than the SLAs.
The capacities range from ~1450 to 2500 mAh. The reason I want to include the low mAh cells is that they are from a power tool battery (original capacity 1500 mAh/cell) and reasonably new.
In order to get a reasonable balance in internal resistances, I arranged all cell groups so that they got an equal number of each type of cell, as closely as possible. Every group got three red cells, three green, two blue, one power tool, one light blue, and two mixed (I've checked that battery type and color corresponds).
In order to get every group as close as possible in capacity, I made a spreadsheet of every cells capacity, that listed cell types vertically and cell groups horizontally. I then calculated the average capacity of all groups, and made a matrix showing which cells of the same type to swap between high and low groups in order to get them as close to the average as possible. Using this method, I got the groups within 12 mAh of one another, which is less than a tenth of a percent from the average of 24.4 Ah.
I have not made any absolute measurements of the internal resistances of the batteries, but I've ran them all through a Liitokala lii-500 for capacity testing. The resistance testing on this charger is not reliable, but it gave ballpark figures, and no cell that worked fine showed unusual values when it came to internal resistance.
So my question is, how do you think this battery will fare?
I made an assumption that cells of the same type would have their internal resistance rise proportional to the wear, and also have their capacity drop in the same proportional way, so that in assuring that all groups have about the same numbers of each type, and the same total capacity, they will have reasonably matched inner resistances. Is that a safe assumption?
I also assumed that the voltage drop due to internal resistance will be low to negligible when loading the cells well below 1C. Is that a safe assumption to make, without having measured the actual internal resistance? Or to reformulate, is it safe to assume that it is unproblematic to load a used, tested 18650 below 1C?
Can I further assume that cells that differ in both capacity and internal resistance will work well in parallel at this load level? I'm thinking that the power tool cells may be drained a little quicker, since they have both lower internal resistance and lower capacity, but unless I plan on draining the pack very low, I think that won't be a big issue.
I plan on using a BMS and/or other battery supervision circuit, in order to keep the groups in balance and see if any of them start to develop issues. What electronics do you recommend? I read about a bluetooth BMS somewhere here, where you could get lots of info to your phone; is that thing accurate, useful, cost effective and shippable to the EU? If not, what would you recommend?
I'm planning to build an 18650 ebike battery to replace the crappy 3s 12V SLAs I'm currently using (used bike, bought cheaply). I have measured the current draw from the current batteries to 13A at stall (brushed motor, so I'm guessing the controller is unsophisticated and stall current is max current).
I have 120 salvaged 18650s with reasonable capacity, no overheating, and low leakage. They consist of 6 different types. I want to build a 10s12p pack out of these, because I have enough space in the casing and because it will still be lighter than the SLAs.
The capacities range from ~1450 to 2500 mAh. The reason I want to include the low mAh cells is that they are from a power tool battery (original capacity 1500 mAh/cell) and reasonably new.
In order to get a reasonable balance in internal resistances, I arranged all cell groups so that they got an equal number of each type of cell, as closely as possible. Every group got three red cells, three green, two blue, one power tool, one light blue, and two mixed (I've checked that battery type and color corresponds).
In order to get every group as close as possible in capacity, I made a spreadsheet of every cells capacity, that listed cell types vertically and cell groups horizontally. I then calculated the average capacity of all groups, and made a matrix showing which cells of the same type to swap between high and low groups in order to get them as close to the average as possible. Using this method, I got the groups within 12 mAh of one another, which is less than a tenth of a percent from the average of 24.4 Ah.
I have not made any absolute measurements of the internal resistances of the batteries, but I've ran them all through a Liitokala lii-500 for capacity testing. The resistance testing on this charger is not reliable, but it gave ballpark figures, and no cell that worked fine showed unusual values when it came to internal resistance.
So my question is, how do you think this battery will fare?
I made an assumption that cells of the same type would have their internal resistance rise proportional to the wear, and also have their capacity drop in the same proportional way, so that in assuring that all groups have about the same numbers of each type, and the same total capacity, they will have reasonably matched inner resistances. Is that a safe assumption?
I also assumed that the voltage drop due to internal resistance will be low to negligible when loading the cells well below 1C. Is that a safe assumption to make, without having measured the actual internal resistance? Or to reformulate, is it safe to assume that it is unproblematic to load a used, tested 18650 below 1C?
Can I further assume that cells that differ in both capacity and internal resistance will work well in parallel at this load level? I'm thinking that the power tool cells may be drained a little quicker, since they have both lower internal resistance and lower capacity, but unless I plan on draining the pack very low, I think that won't be a big issue.
I plan on using a BMS and/or other battery supervision circuit, in order to keep the groups in balance and see if any of them start to develop issues. What electronics do you recommend? I read about a bluetooth BMS somewhere here, where you could get lots of info to your phone; is that thing accurate, useful, cost effective and shippable to the EU? If not, what would you recommend?