Leaving ground connected should not present any problem.

It would be best to disconnect and charge to the battery directly, of course.

If you cannot do that and want to have things permanently wired, go ahead and leave the ground connected to the ECU, battery and charge port in a "Y" connection. Then get a small single pole double throw switch (from radio shack) for the "+" or hot side wiring. To hook this up, connect the center contact (com) of the switch to the battery, one end (L1) to the ECU and the other end (L2) to the charge terminal. Then the charger port cannot possibly damage the ECU.

See a SPDT switch diagram here: http://en.wikipedia.org/wiki/File:SPDT-Switch.svg

In the first position of the switch, it connects battery to ECU (this would be the "run" position), in the second position, it would connect the charger to the battery and leave the ECU floating. This would be the "charge" position.

I tried to draw it with ascii graphics but could not get it to survive posting. Looked good with the editor though :-)

The reason you are cautioned not to leave the battery connected while charging is that the charger may take the battery to a rather high voltage at the end of the charge (depends on the internal resistance of the pack, wire sizes, etc). It could easily be much higher than the fully charged battery voltage (e.g. some 5-cell NiCd packs, when charged aggressively, can get over 8V at the charger terminals).

Caution, I have not tried this, so while I believe this is correct, you need to be careful, especially if you use the same field battery and/or charger to simultaneously charge other batteries in the plane, which can in some cases cause odd problems if the charger uses the - side of the battery as the charging lead and uses the + side as common.

Dave McQueeney