Battery life is always going to be a problem with wrist top GPS units. The Ambit has one of the best rated batteries and with careful use, it could be used for an extended multi day trip into the mountains. I spent 6 days in the Southern Alps of NZ in May 2012 and had temperatures rarely above 0°C. Due to good weather and hence easy navigation, I left the mountains with 57% battery remaining. I recorded 46.5Hrs of ABC data but no HR or GPS tracks. The only GPS use was to confirm a few waypoints etc. It would have been nice to GPS log the route but I was concerned about battery depletion.
I decided I needed a back up charging device and my requirements was that it was to be:
- relatively inexpensive.
The two (of many) contenders I chose:
1. Standard two AA emergency iPhone charger solution. Unit cost $3.50, Battery cost (Energizer Ultimate Lithium) $10. Unit weight inc. batteries 51g. Total weight 172g
2. Lightweight solar charge 1000mAh Li battery solution. Unit cost $10. Unit weight 35g. Total weight 158g
To compare weight and battery life, my existing GPS device (which the Ambit will replace if in field charging solution is reliable) is a Garmin eTrex which weighs in at 152g with Ultimate Li batteries and has a specified battery life of 22Hrs (not field tested). I usually carry a spare pair of batteries because it does go through them, thus add another 29g. Total weight 181g.
To compare two potential devices in the field I simulated a typical 7 day walk in the NZ mountains. If I was to log my routes using the GPS, I would need to use the 60 sec GPS update and 10 sec ABC logging to maximise memory capacity. BUT I will assume I need to use navigation mode heavily, perhaps 4-5 hours per day e.g. navigating a ridge in poor weather, descending a valley and requiring to identify correct side branch etc. I will assume that in difficult GPS acquisition terrain, using in watch navigation, I will deplete the battery every two days. In all reality I would keep the Ambit topped up by charging it each night.
I substituted the iPhone as the recipient of the joules so I didn't have to hammer the Ambit for no good reason. I did a rough "calibration" test for the iPhone vs Ambit: Using new AA alkaline batteries I did a recharge of Ambit from 30% -> 100% using the emergency charger. I then measured the battery remaining by using a universal digital battery tester both immediately after charging then after a 12 Hr "resting" period (39% remaining initially, on resting 64%) and I cross-referenced the battery tester by slipping the batteries into my iMac's Track Pad and asking what my iMac thought was left (initially 62%, resting 85%). Using two new AA batteries, I then did a charge calibration using the iPhone starting at 30% iPhone battery capacity and tested the AA batteries at 15 then 5 minute intervals until they reached an approximately similar used capacity (34% initially, 56% after resting). Cross referenced with iMac (initially 54%, resting 79%). This correlated to 55% iPhone battery level or a 25% iPhone battery charge.
Now, there are problems with this method I know, mainly because iPhones will draw up to 1000mA, if available, whereas the Ambit is unknown but is probably closer to 500mA as per USB specifications. This difference in current draw changes the "capacity" of alkaline batteries - the slower the current draw the more energy you can extract. However Li have far less problem with this (and hence Energizer's outrageous claim the Li Ultimates will last 9 times longer for the same rated battery )
I am assuming a 25% iPhone charge can be representative of an Ambit 70% charge for the purposes of this review. Also because I am comparing two different devices for their charging ability, the recipient of the charges is not necessarily critical to the outcome. This test would probably favour the solar rechargeable solution as the Energizers should last longer with a low current draw device i.e. the Ambit.
Next I thought about environmental conditions in the field. Typically the charger will be used in the evening in cold damp conditions. Huts or tent depending on trip.
Night time temperatures are often well below 0°C and daytime temperatures not much better.
Direct sunlight hours will vary from zero (raining/snowing all day)………
to brilliant clear skies where 8 hrs of direct solar gain could be achieved…….
I will test two scenarios:
1. poor weather (and hence need Navigation mode on the Ambit often) and;
2. good weather.
The Emergency Charger would be brought out each evening, batteries inserted and charge the Ambit then remove batteries and pack away. The Solar Charger would be clipped on the pack during the day if terrain/vegetation allowed me to safely do so. Otherwise it would be taken out at breaks and hopefully not forgotten on the side of the track. This is a potential drawback of the solar solution. "Rest days" on the track (if any), tend to be poor weather days so unlikely to get much solar gain then either. Of course Ambit wouldn't be using much battery on the rest days either.
Test 1 Emergency Charger with 2 x Energizer Ultimate Li AA
- New batteries at start
- Left batteries in fridge at 3°C each night
- Stored in cool place during day
- 25% iPhone charge in fridge in evening.
Over four evenings the Li AA achieved a 25% iPhone charge successfully. On the fifth evening the iPhone would not accept the charger (due to low current output). I used ambit for the final drain but only achieved an increase from 75% to 80% before the Ambit stopped charging as well.
- Four effective 70% charges of the Ambit with one pair of AA batteries.
- Likely duration of Ambit use in the field: at least 7 Days.
- Add another 29g of battery and you have significant charging capacity for total of 80g.
Test 2 Solar Rechargeable 1000mAh Li Battery, "forest/poor weather"
- Initially charged battery to full capacity via USB
- Left battery in fridge at 3°C each night
- Exposed solar panel to direct sun between 7am and 9am (hanging vertically on fence facing east) then in semi-shaded area for the rest of the day.
- Attempted a 25% iPhone charge in fridge in evening.
Straight away it was evident the 1000mAh internal battery had little storage capacity. It only achieved a 10% iPhone top up even after a full USB charge on the iMac. I did this USB recharge 3 times just to confirm it wasn't an erroneous result. So back to the methodology which will now not be able to be completed. I did a full USB charge and measured the Ambit recharge ability. Starting from 38% the Ambit increased to 84% before it ceased to charge further. So from this test a 45% ambit charge is approx. a 10% iPhone charge which is somewhat different to my original assumptions.
After the initial discharge, the unit did not effectively recharge at all with the above exposure. After the day solar charging in semi shade, it managed to increase the iPhone from 29% to 30%.
Thus I had to convert to the "good weather scenario" having given up on the poor weather/forest scenario.
Test 3 Solar Rechargeable 1000mAh Li Battery, "good weather"
- Exposed solar panel to direct sun between 7am and 12MD (lying tilted to face sun directly) then in semi shade for the rest of the day
- Ambit charge in the evening
Even after a prolonged recharge in full sun (23° South latitude mid summer!) it only managed to charge the ambit from 43% to 56%. This unit also specifically says it is not in any way water-resistant so if there was any chance of precipitin it would not be coming out to charge>
Based on my uncontrolled testing I believe the AA battery solution would be a reliable means to recharge the Ambit in the field. The solar charging device has severe limitations which would make it unreliable on a field trip. It may perform in optimal conditions and outlast the battery solution but in areas where I venture I would not rely upon it.