Pack: 16s15p comprised by (240) 2.2ah lithium 18650s from all-battery.
Controller: 35amp 72 volt crystalyte
Problem: The ammeter on my handlebars, verified by my sense of normal torque, says when just starting out the controller is getting/pulling a max 32 amps instead of the usual 35. After riding for 15 minutes or so, when I crank the throttle, amps go back up to 34-35.
By later in the ride, when the cells are down to ~50% capacity at 3.85 volts, max amps drops to 30 under load. During all these times, the voltage sag remains in it's normal range of 4-5 volts on a fresh charge to ~7 volts at 50% charge.
Lately it's been about 40F colder than in the summer when I first put the pack together. Could cold weather cause a dramatic increase in impedance in these lithium cells? If so, why doesn't the voltage sag under load also increase? Can I expect the cells to return to full power with increasing temperature?
About 6 months ago, I ruptured and killed two of these cells by running them too low under full load . The heat killed two dozen more nearby without rupturing them. I replaced these cells with new ones. Could I have also damaged other cells still in my pack, causing the problem I'm only seeing now, 6 months later? If so, what's the easiest way to find/isolate the damaged cells? I have only a DVM to test with. I could pull the 15-cell parallel-wired subpacks one-by-one. If the culprit is in one of these packs, can I expect full amps again when I take it out of the circuit (albeit at less voltage)?
If I continue to ride the bike this way, do I risk causing more damage?
Thank you for your time and help.
Here's pics of one of the two ruptured cells I replaced -- the heat killed 7 others in the top two rows I also replaced. I'm wondering if the heat damaged the four in the bottom row I'm still using such that it's causing a low-current problem for the entire pack.
Here's a pic of the pack charging after I resoldered it 15s15p, replacing the dead cells (this is before I added the 16th subpack):