That sounds a little short for battery time. I haven't done through testing, but I was getting about 65 hours. I wonder if the MicroSD card is drawing more power than it should. Are you using the 256 MB that it came with or another one? If you're using the 256 MB card, I suggest trying a high capacity card from a known manufacturer. Did you have the backlight off?
I've opened a github issues about the battery drain https://github.com/PaxInstruments/t400-firmware/issues/203. Probably leakage current through the diodes. In the next device I will do better about protecting against completely draining the battery. I'll look into this further.
The T400 can be powered via the MicroUSB port. Not all external USB battery packs will work through. USB battery packs typically have circuitry that will cut off power if the current draw is too low. While charging the T400 the battery pack will power the device. However, when the T400 is fully charged the current draw will be below the battery pack's threshold.
To test an external battery pack I suggest removing the battery from the T400 and seeing if it can be powered by the battery pack alone. If it doesn't turn off, you should be good. A USB cable to a computer or wall power source would work nicely.