DAVID comes with two batteries, and uninterrupted full-day operation should be possible: while one battery is installed in the device, the other one is charging. Every morning, the batteries are swapped. But this only works if battery capacity is high enough for a full working day (let's say, 8 hours).
We checked battery performance over three weeks, under normal operating conditions. Distance between DAVID and Bluetooth connection box was a few meters. We let DAVID measure continuously: we started a measurement in the morning and let it run until DAVID software reported that battery level was low (DAVID software 1.1 stops the measurement at this point). From the time DAVID software stopped writing to it's log files, we determined the time when connection was lost.
These are the results for battery #1:
| Date | Start measurement | Stop measurement | Time [hh:mm] |
|---|---|---|---|
| 03.11.2010 | 07:00 | 17:45 | 10:45 |
| 05.11.2010 | 07:54 | 17:48 | 09:54 |
| 07.11.2010 | 14:28 | 23:45 | 09:17 |
| 09.11.2010 | 08:37 | 18:47 | 10:10 |
| 11.11.2010 | 07:43 | 16:48 | 09:05 |
| 15.11.2010 | 07:50 | 18:22 | 10:32 |
| 17.11.2010 | 08:07 | 18:41 | 10:34 |
| 19.11.2010 | 09:45 | 18:44 | 08:59 |
| 22.11.2010 | 14:29 | 23:31 | 09:02 |
| 24.11.2010 | 06:58 | 17:54 | 10:56 |
On the average, battery #1 was working for 9 hours 55 minutes, with the minimum around 9 hours.
Now to battery #2:
| Date | Start measurement | Stop measurement | Time [hh:mm] |
|---|---|---|---|
| 02.11.2010 | 14:25 | 23:29 | 09:04 |
| 04.11.2010 | 06:54 | 16:43 | 09:49 |
| 08.11.2010 | 08:42 | 20:10 | 11:28 |
| 10.11.2010 | 11:03 | 00:03 | 13:00 |
| 16.11.2010 | 06:56 | 20:52 | 13:56 |
| 18.11.2010 | 06:53 | 15:49 | 08:56 |
| 23.11.2010 | 08:23 | 17:33 | 09:10 |
Battery #2 lasted even longer, the average time was 10 hours 46 minutes.
Discussion
With the provided battery capacity and treatments on the linac running for 8 to 9 hours a day, the planned workflow with daily battery cycling should definitely be possible. During testing, DAVID was idle (not measuring) only for very brief periods, after Bluetooth connection was lost on a few occations. As soon as this was recognized, a new measurement was started immediately. We do not know whether continuous measurement is draining more power from the battery than more reaslistic measurement cycles with a lot of idle time between the measurements, but if it is, this was some kind of stress test.
The reason for the brief Bluetooth interruptions is not known. The PC running DAVID software was used for other things as well, with a lot of other processor tasks and typical network traffic. Maybe Bluetooth stability is even better if DAVID software runs exclusively on the PC.
In departments with long daily operating times, it is probably a good idea to order more batteries, and a second charger.