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SPOT 2 GPS Messenger: Battery Test
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 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 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.
|No GPS fix||2|
Note: The graphic at right does not include the 2 attempts that could not get a GPS fix.
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
|Average Time to Receive Message||5:24||Ranged from 20:57 down to 0:40|
|Average Time to Acquire GPS Fix||2:30||15:55 was the maximum, and it failed twice|
|Average Message Cycle||15:21||The message cycle is a random schedule up to 20 minutes, though my SPOT recorded a 35 minute cycle once!|
|Average Delay after First Message Failure||7:14||The messages are sent out on a random schedule.|
|Average Delay after Second Message Failure||6:56|
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.
Hope your research goes well and your happy with the outcome (love to know what it is BTW) :)
I'm still having good results with my Spot 1.
I'm not sure about the discharge curve of Lithiums, but NiMH is very different to standard alkaline batteries: they stay high for most of their life then drop off at the end very quickly.
The whole "battery level" thing seems to be a bit of a black art, and the different battery types doesn't make it any easier for the manufacturers (I only got 16 messages off before the low battery indicator started!). This is why I wanted to test my devices with the batteries I was going to use them with: which seems simplistic, but it's essential.
I was 'tracking' and had successful GPS signal lights and successful message lights for the entire day, including a couple of 'check in' messages but NOT ONE of those messages got through, this resulted in an overdue hiker report to law enforcement because I was late getting out, and unnecessary worry and embarrassment. All the while I thought the signals were getting out because the message light was blinking green through the whole time.
And I imagine in such a "low powered" device, that there is not a lot of room to play with. Therefore: it is essential to test your device with the batteries you plan to use. As lithiums, NiMH, etc all have different discharge curves. I'm not sure at what point the SPOT doesn't send with enough grunt to get it to the satellite: but it is significant that the success rate on the last day's messages was 50% (nowhere near the overall 95%!!)
Also note that this test was only Check-in messages, not tracking: I have not thought too much what difference that would make??