Hello all, I am new to this wonderfulforum and to battery projects. I am planning a small Power Wall/Portable pack with about 2kWh capacity for power backup and feasibility testing with thoughts of going bigger if this works out for me.That being said, I have obtained about 50 old laptop battery packs and have started processing the cells. I harvested about 320 18650 cells, separated by their voltage as I found them, and I am now charging and capacity testing them. It seems to be going much like the experiences of others from what I have seen andread so far. I do have a couple of noob type questions that I have not seen discussed in my research so hopefully you guys already know the answers.
I am capacity testing with three OPUS BT3100 chargers and using the default rateof 500mA for testing.
Q1: For charging I have been using two 2-cell chargers that I had laying around already, it turns out that one of them is charging to 4.3V . Will this have an effect on the capacity test? Does the Opus start measuring capacity after it discharges down to 4.2 or does it start measuring capacity at the starting voltage of 4.3? I tested a set of 4 cells as follows...Charged them to 4.3, put them in the Opus for capacity test and subsequent charge to 4.2V, record the result, let them sit for the time it takes to cycle another batch of 4, then re-cycle tested them in the same Opus. Three of the four cells capacity tested about 100mAh higher than previously and one tested about 50mAh lower. Does this fall within normal variation?, does the precharge to 4.3 actually help? does it make any difference at all? should I stop using the charger that goes to 4.3 or not worry about it? Note: I have 3 different DVMs I use for testing and they all significantly agree on the measurements.
Q2: When planning my pack arrangement (cells in P configuration) I see most people mixing capacity ranges to make each pack match the others onaverage and with the same # of cells. I have been thinking that, for example,a 20P pack of 2000mAh cells with 40,000mAh capacity would match nicely with a 25P pack of 1600mAh cells with 40,000mAh capacity. My rationale is that since all ofthe cells in the pack will be matched, they willbehave similarly and no single cellwill get dragged down to a low discharge statewhile it's pack mates are still producing. It seems more important to me to match the pack capacities this way than to match them in cell qty. However, I don't see anyone doing this in their threads orvideos. Then when I look at the pack builder tool on this website there is an option to do it this way so clearly people think aboutit. What are the Pros and Cons of this method vs the "averaging" method? I feel like I am missing something.
Thanks for any help that anyone can provide.
And by the way , I am looking for good sources of used packs in Central FL, not having much luck to date.
Chris
I am capacity testing with three OPUS BT3100 chargers and using the default rateof 500mA for testing.
Q1: For charging I have been using two 2-cell chargers that I had laying around already, it turns out that one of them is charging to 4.3V . Will this have an effect on the capacity test? Does the Opus start measuring capacity after it discharges down to 4.2 or does it start measuring capacity at the starting voltage of 4.3? I tested a set of 4 cells as follows...Charged them to 4.3, put them in the Opus for capacity test and subsequent charge to 4.2V, record the result, let them sit for the time it takes to cycle another batch of 4, then re-cycle tested them in the same Opus. Three of the four cells capacity tested about 100mAh higher than previously and one tested about 50mAh lower. Does this fall within normal variation?, does the precharge to 4.3 actually help? does it make any difference at all? should I stop using the charger that goes to 4.3 or not worry about it? Note: I have 3 different DVMs I use for testing and they all significantly agree on the measurements.
Q2: When planning my pack arrangement (cells in P configuration) I see most people mixing capacity ranges to make each pack match the others onaverage and with the same # of cells. I have been thinking that, for example,a 20P pack of 2000mAh cells with 40,000mAh capacity would match nicely with a 25P pack of 1600mAh cells with 40,000mAh capacity. My rationale is that since all ofthe cells in the pack will be matched, they willbehave similarly and no single cellwill get dragged down to a low discharge statewhile it's pack mates are still producing. It seems more important to me to match the pack capacities this way than to match them in cell qty. However, I don't see anyone doing this in their threads orvideos. Then when I look at the pack builder tool on this website there is an option to do it this way so clearly people think aboutit. What are the Pros and Cons of this method vs the "averaging" method? I feel like I am missing something.
Thanks for any help that anyone can provide.
And by the way , I am looking for good sources of used packs in Central FL, not having much luck to date.
Chris