T-Echo Won’t Run on Battery

[drepamig]

I’m not sure if this is the correct place to ask this question, but hopefully someone can help. I have two T-Echos that were totally stock. I decided I wanted to upgrade the battery from the stock 102535 to a larger 103443. I initially bought the wrong connector size on the replacement batteries (1.5mm instead of 1.25 mm). I thought I might be able to swap the ends and keep the pins, but that failed, so I ended up cutting the connector off of the old battery and soldered it on to the new battery. In doing so, I found that the pins wouldn’t stay seated in the connector (because I tried to swap them earlier) and I ended up plugging in and unplugging the connector about a dozen times trying to get everything seated. During that time, the new battery would power on the T-Echo. I gave up on trying to re-use the connectors from the old battery and bought a 10 pack of new pigtails.

Fast forward a few days where I replaced the connectors and plugged the battery into my echo and nothing happened - no power, no lights, nothing.

• I checked the battery voltage (~4v)
• I checked the voltage from the battery connectors to the circuit boards (also ~4v matching the battery)
• The echos both run fine on USB
• Neither will run on battery, either the originals or the new ones
• The serial debug info on them say: DEBUG | 20:58:33 148 [Power] Battery: usbPower=1, isCharging=1, batMv=4841, batPct=100, so it appears that maybe they're detecting the battery, though batMv=4841 doesn’t match the battery voltage. Is it the USB voltage? I get a flashing blue light on the left and a rapidly flashing amber/red light on the right, so that seems to indicate that the echo isn't detecting the battery.
• I’ve factory reset and re-flashed both devices with no change.

Did I somehow kill the battery circuit on both of these devices? Does anyone have any other suggestions?

Thanks!