As mentioned in the HOBO RX3000 and U30 documentation, if the battery voltage is allowed to drop too low, it may not be charged within the logger. In these cases, the battery must be charged with another source.
You will need to obtain a DC power source that can be set to 4.5 volts and limited to a max of 2 amps. Something like this will do just fine:
To recharge a battery from a U30 or RX3000, perform the following steps:
1. Disconnect the AC adapter or solar panel.
2. Disconnect the battery.
3. Unplug all sensors, the USB cable, relay, and grounding wire.
4. Unscrew the antenna cable with a 5/16-inch wrench. (U30-GSM only)
5. Use a Phillips-head screwdriver to remove the four screws at the corners of the clear inner case.
For the RX3003, the GSM antenna wire cannot be removed; therefore you must be very careful while removing the clear case. It may be best to have a helper holding the clear case to the side when removing the battery
6. Remove the clear inner case.
7. Use a Phillips-head screwdriver to remove the two screws that secure the battery.
8. Note the orientation of the battery and then remove it.
9. Set DC power source to 4.5 VDC, limited to a Max of 2 amps. Connect the battery to the power supply, observing polarity. Allow several hours to fully charge. (If the battery starts to get hot to the touch, remove the battery from the charger and seek a new battery) Warm is okay. Check with volt meter after charging is complete
10. Install the charged battery with the same orientation and reverse the steps to reassemble.