Perry's Blog

Background
In a previous review we tested the SPOT Messenger on the Mittagong to Katoomba walk in preparation of the much longer walk of the Bibbulmun Track (See the 54 On-Track Review of the SPOT2 here: SPOT2 GPS Messenger Review on the Bibbulmun). One of the things I wanted to determine was an accurate estimation of what the battery usage was going to be, as we were going to be using the Voltaic Fuse Solar Charger to charge our batteries for the 2 month duration of the Bibbulmun (I wanted to make sure that the Fuse was going to cope with all our charging requirements).

tely I swapped the batteries out when they came up with their first warning (100 to go), because I got a little too excited about charging them on the FUSE .

The manufacturers recommend, and all their specifications are based on, AAA lithium batteries; which they claim should return 350-700 Check-In Messages. We'd already made the decision to use the Voltaic Solar charger, so I purchased some new AAA rechargables from ProTog: Powerex Imedion 950mAh and set about seeing how they stack up against the Lithiums.

The Test
The purpose of this test is to determine the performance for my Powerex Imedion 950mAh batteries. It should also provide a better picture of how the SPOT performs in various situations over a longer period.

Basically I took the SPOT everywhere and sent Check-In Messages whenever I could. I recorded the following times:

• Started the Check-In
• GPS Fix
• Message Cycle Started
• Message Cycle Finished

The full message cycle comprises 3 attempts by the SPOT to transmit the message back to their server via a commercial satellite. As they say in their FAQs, environmental obstacles may get in the way and block the transmittion. This is the reason 3 messages are sent. By evaluating the time the message was received, it was possible to fairly accurately guess which message attempt succeeded.

The Results
The test ended up running over a period 46 days, from April 22nd to June 7th when the SPOT would not pass its Start-up Test (due to low batteries, which tested: 1.09, 1.14, 1.13 volts).

The low battery indicator started flashing at Message #16 and continued to flash right through to Message #211.

Raw results:

Note: The graphic at right does not include the 2 attempts that could not get a GPS fix.

Comments
Many of the message attempts were sent in the same area. The "walk to the train", "walk to work", "walk home"... all featured quite a few times. Interestingly the SPOT often found it difficult on the road to my local train station, the GPS location being in the area, but nowhere near accurate.

The results also contain numerous attempts that were cut short, generally because I couldn't be bothered waiting outside a building for the message cycle to complete, or my train came (and wouldn't wait ). When the failures that were likely due to a truncated message cycle are removed from the results (which is probably more accurate to a bushwalking situation anyway), the results look more like this:

Why the Failures?
Generally there were obstacles above somewhere. Canopy was only an issue if there was a lot of wood (trunk and branches). Open Eucalypt forest (like we got on the Mittagong to Katoomba) walk is generally fine, but the three messages I tried in the depths of a hoop pine forest all failed (actually the GPS was pretty flakey too, so I wasn't surprised not to see the messages in my inbox when I got home). The message from the outdoor cafe at Circular Quay (but under a glass awning) failed.

The other reason is low batteries! I was surprised to see the number of failures when the batteries were just about due to give up: 4 of the 8 messages failed on the last day! This 50% failure rate is in stark contrast to the overall 5 failures from the previous 177 (i.e. 2.8%). What's difficult is that the device gave no warning (apart from its low battery warning that it had been giving for the previous 185 messages -- which probably should have been enough?). I don't really want to swap the batteries out after only 16 messages, so I'll probably keep track of the number I'm up to and swap the batteries out around 150 messages.

Some interesting statistics

Conclusion
The important point to take from these statistics is how essential it is to let the device progress through the complete message cycle! 26% of messages fail on their first attempt, so if you turn the SPOT off too quickly your message success rate will be lower than 75%!

I'm pretty happy with 200 messages on a set of batteries. It's certainly less that the lithiums, but some of the attempts were in less than ideal situations (which uses more battery); the three attempts in the hoop pine forest took over 15 minutes trying to acquire a GPS fix. The FUSE takes a short 2.5 hours to fully charge two of the batteries, so there will be no problems keeping the SPOT powered during the Bibbulmun Track walk.

I'm also extremely happy with the message success rate. 95% success rate is great considering I wasn't paying any attention to where I was attempting the send, but when you discount the last day's low batteries the success rate is over 97%.

It will be interesting to see what the statistics are for the Bibbulmun Track! Edit: The results were interesting – See the 54 On-Track Review of the SPOT2 here: SPOT2 GPS Messenger Review.