There is a vast difference in capacities of rechargeable batteries. My hobby besides shooting is radio control airplanes. The radio systems are powered only by rechargeables. We trust planes costing several thousand dollars to these batteries, so we must use the best available. If our batteries go dead, the plane is destroyed. Some days we drain and recharge the batteries several times in one day of flying.
The over-the-counter rechargeable batteries just do not have the capacity of the premium cells that are available. The charge rating of these cells is in milli-amp-hour or mAh. Most of the batteries available at discount stores are in the 200 to 300 mah range. A good Sayno or Kan brand cell will have a rating of 1100 to 1400 mAh for the same size cell. This would mean a 4 to 7 times longer run time. My suggestion would be a 1600 mAh, Sayno, button top, that will drop in your Garmin.
Alkaline will last longer, but tend to get expensive if used a lot. But, if you have a charger that works from a 12 volt system, you can charge in the field and not have to carry extra alkalines.
Here are a couple of sites that sell Sanyo and Kan batteries, plus a site that explaines everything you need to know about rechargeable cells, and then some. Give them a call and explain what you need. They can fix you up with batteries and chargers if needed.
http://www.cheapbatterypacks.comhttp://www.radicalrc.comhttp://www.rcbatteryclinic.com/Nashcat
