Use your ESV to time your battery discharge rate. Many ESV's include instructions on how to do this. Simple way: charge for 24hrs., connect ESV, note time, leave on until meter dips into the red, note time. THIS WILL NOT MEAN YOU CAN FLY THIS LONG! It gives you a baseline to work with. After several flying sessions repeat this and note any duration variance. If there is a large variance, do this several times in a row. This will "Cycle" your batteries (primative way but works). If after cycling the batteries they do not come back up to the original reading, replace them. The ESV will save your plane and the field charger will extend your day.