The Vbattery reading gives us a clue as to what the problem is.. Thats your batteries voltage. It should be putting out a mostly solid and steady 8.36 volts.. Some variance there isnt bad, if its .5 volt + or Minus. But dropping all t he way to zero?
That tells us one of three things.
1. The battery is defective.
2. The battery isnt connected properly to the mainboard. ( you could open it and check the clip, make sure its fastened snugly
3. The voltage regulator on the mainboard has burnt out. The main board will need to be replaced.
It should be easy enough to eliminate 1 or 2 as the cause. if you still have the old battery, open the device. remove the battery connector and reconnect it and then check the Vbattery setting again. If it stilll fluctuates to zero. Try the older battery. If the older battery doesnt fluctuate to zero t hen the newer battery is faulty and needs to be replaced. If it also fluctuates, and you double check the battery connection.
You may want to consider replacing the main board, which is extremely difficult to say the least. Probably the largest challenge you would face with replacing the main / logic board. Is finding it for sale!. Used 296 units, even non working used 296 units still sell for upwards of 200$. To replace the unit with an exact model would cost around 600 http://www.ebay.com/itm/Garmin-GPSMAP-296-Bundle-with-ALL-CURRENT-Aviation-Terrain-Obstacle-Databases-/171906929358?hash=item28067392ce
You may be better served with investing in a somewhat newer model ( If it comes down to having to replace the logic board.)
For example, this is a newer model ( compared to the 296 ) and a solid choice. http://amzn.to/1MTVMnL
If you do decide to go for a newer model. Here is what I suggest. List your current not fully functional device on ebay for 400$.. Show that it can boot up, explain that it is for parts, and that it wont run off the battery. If you look at this page and scroll down, you will see that broken units that can still boot up, sell for upwards of 500$ still!
If you take a few pictures showing that the device can boot, you should be able to get 400$ out of it rather easily.
That would cut the cost down on the Aera 500 or purchasing a new Aviation GPS , significantly.
I do hope that youre able to repair t he unit. Maybe its a faulty battery, or the battery connection wasnt solid. If you have further questions or concerns, let me know.
I strive to provide great service, and greatly appreciate a rating of excellent ( 5 stars ) selected and submitted below this answer or at the top right of the screen. I always answer follow up questions.
Senior Technician and Network Switching Systems Operator / Maintainer.