Newer units are much improved in this respect over older models, and at the same time, batteries themselves have improved. According to Garmins own specs, the run-time king is the Etrex Legend or Venture C or Cx series (NB the Legend and Venture are exactly the same hardware, apart from the colour of the case). They quote 'up to 36 hours' battery life for the Legend C, which uses 2x AA cells, and 'up to 32 hours' for the Cx.
Other Garmins either use smaller cells (Geko) or are rechargable-only (Edge) or have onboard compass and barometer (Vista) or use older less efficient circuitry (Etrex mono models) or use a more sensitive receiver (Etrex H series, or the 60 or 76 or Colorado models) or a bigger or brighter screen (60/76/Colorado). All these things make for shorter runtime, though its not always a serious hit - I wouldn't let runtime alone put me off buying an H or a 60 or a Vista. The new Colorado series do look a bit problematic though, for runtime.
(Scroll down to see some current drain figures from user tests.)
A good pair of NiMH AA cells, freshly charged, will easily run a Legend/Venture Cx for 24 hours non-stop, including using screen backlight and warning tones. That seems to me to be good enough, really. To get round a 600km, you could take one of several options:
* ony turn the GPS on when you need it. (OK, that's a bit extreme!)
* change the battery at around half time, buying new alkalines from a garage or carrying a spare set with you.
* run the GPS in 'battery save' mode.
* fit a pair of Energizer Lithiums and run the GPS in 'alkaline' battery mode.
Lithiums is probably the best option. Sheila did this for PBP and she was halfway back to Paris before the first pair of lithiums ran down - a runtime of maybe 50 hours. Lithiums have another virtue - they are very lightweight, so help to reduce the possibility of 'Etrex Jitter', of which see more below.
Battery Save Mode works pretty well - I've used it a lot on the Geko because run time is a problem with that unit and battery changing by the roadside a bit of a pain. Battery Save reduces the frequency at which the satellites are polled - from about 1 second intervals down to about 5 seconds I believe. I don't really notice any difference in performance, in real-world riding situations. You might notice it if you tested for it, by riding round a tight roundabout or something with your eyes glued to the screen - but in practice its completely usable. On the Geko it seems to extend battery life by about 20-30% - just enough to make a good pair of NiMH AAAs last all day.
Incidentally, enabling WAAS would also be a slight battery hit - and the increased accuracy (from EGNOS, in Europe) hardly seems necessary for cyclists' purposes.
If carrying spare cells for GPS (or lights) in your luggage or pocket - get hold of one of these plastic containers to keep things neat and safe and free from short-circuits. They come in 2x, 4x or bigger, and various sizes to suit AA, AAA or CR123.
If carrying spare cells, Energizer Lithiums are ideal - they have a very long storage life, weigh less than other cells, and pack lots of punch. That tiny pair on the left could stay in your pack for years, you won't know they're there, and they'll still work when you need them.
Etrex Jitter is when a bit of vibration causes the GPS to turn itself off. Its caused by a momentary loss of power to the circuit board, apparently there is a dodgy contact between the battery connections and the board itself. One cure is just to find a more compliant mounting option - Sheila got a bad dose of this problem when she started riding her new bike, which has oversized (and so stiffer) handlebars, with much less space for rubber wrapped under the GPS bike mount. Use of NiMH batteries, which are 30% heavier than alkalines and twice the weight of lithiums, probably doesn't help, because of their increased 'battering ram' effect.
With normal diameter bars, try using the oversized Garmin bike mount, with extra rubber under it. Another trick is to stuff a fragment of rubber behind the terminals in the battery box, to make the cells a tighter fit. If you remove the cells and pull upwards on the lower pair of terminals, you'll find they are actually a single strip of metal that slides right out. You can now insert a bit of something into the fold of the metal - don't overdo it - I used about an inch of sleeve stripped from some wire flex - and just slide the terminals back into place. This definitely seems to improve matters, though I don't know why, if the problem is actually a connection elsewhere.
The ultimate fix is to add a small electrolytic capacitor across the power rail which will smooth any short interruptions in power supply. 22uF should be enough. I couldn't find one physically small enough to fit in the battery box, and in any case to be most effective it needs to be done on the circuit board itself. This means opening the case - probably the GPS will never be waterproof again, afterwards. But if you want to try it, the instructions are here
Francis Cooke
Appendix: These battery drain figures are from www.gpsinformation.net/main/bat-5.txt
GARMIN UNITS WITH TWO AA BATTERIES
Internally Powered with Fresh Alkaline Batteries
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(Test Mode) Antenna Connected
Model Version Sim Searching GA27C or GA26C
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eMap 2.04 60ma 120ma 130ma NA
Legend 2.30 50ma 120ma @ @
Vista 2.16 50ma 120ma* @ @
*165ma with compass ON
Vista C 2.10 52ma 62ma* @ @
*100ma with compass ON
Vista Cx 2.20 50ma 65ma* @ @
*75ma with compass ON
GPSMap 60CS 3.50 38ma 71ma# Gilsson: 82ma
#Backlight low: add 23ma
#Backlight med: add 44ma
#Backlight max: add 100ma
#Compass ON: add 40ma
GPSmap 76 2.04 62ma 120ma 130ma NA
GPSmap 76S 1.06 100ma 125ma* 185ma NA
*Compass ON: add 50ma
GPSmap 76C 3.43 52ma 62ma* 70ma NA
*With backlight full: add 100ma
GPSmap 76CS 55ma 62ma*
*With backlight: 20% add 25ma, 30% 33ma, 50% 53ma, 100% 103ma
*Compass ON: add 35ma, Tone: add 70ma (Approx)
GPSmap 76Cx 2.40 40ma 100ma*
*With backlight full: add 100ma
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* Searching
° On Garmin models, the lower the battery voltage, the higher the current drain.
Living with a Garmin: The Waypoints LimitationLiving with a Garmin: Track, Route or Autoroute
Living with a Garmin: Struggling with GPX
Living with a Garmin: Battery Runtime and Etrex Jitter
Living with a Garmin: Living with Metroguide Maps
Living with a Garmin: Declutter the Page Sequence
Living with a Garmin: Screens you don't see every day
Francis Cooke's Garmin Menu Map (pdf)