Current consumption UNO+Eth Shield

My batteries powering my UNO+EthShield+LED+TMP36 did not last as long as anticipated. I am using the LED as a light detector. The batteries (5) are 2300 mAH NiMh loaded into a plastic box that came with my experimenter's kit. The setup lasted 4 hours, which is surprisingly low. My sketch takes analog readings from analog pins 4 and 5 and writes to microSD every 5 minutes.

My thoughts (using data from other threads, thanks): UNO needs ~50 mA Shield needs ~ 150 mA LED+TMP36 need a little more.

With multimeter in series from battery pack I get readings of 220 mA while the logger is running, and 240 mA each time it writes to the microSD. At this rate I should be getting closer to 10 hours, not 4 from the batteries. My suspicion is that my charger is not fully charging the batteries. Are there any other factors I might not be taking into account?

Is the output from those 5 batteries in series going into the Arduino's external power input (barrel connector or GND/Vin pins)? That's the input of a linear voltage regulator that needs at least 6V, preferably a little bit more, to provide good 5V for the Arduino. NiMH batteries only have a nominal voltage of 1.2V, which adds up to 6V for the 5 of them. Granted, they run more like 1.4V when they're freshly charged, but then they drop down to 1.2V, and eventually down to 1.0V each. I wouldn't be surprised if the voltage from the battery pack is going too low to make the regulator happy long before the batteries are actually "dead".

Also: battery capacity figures are always calculated using the best possible conditions, so you should never be surprised if in the real world, mA * h is less than the rated mAh :)

Thanks Hobbified: true, batteries are in series. Voltage when fresh was 6.7, which as you say fell quickly to 6.4 and then to 6.1+ where it seemed to hang for a long time but the logging kept going. Then much later I found no lights on the unit and 3V from the batteries.

They depleted surprisingly rapidly. 4 hours is not long enough for my application, will have to go with another power supply. At least I learn that when presented with low voltage the unit did not go into hysterics, on this occasion.