Twice this summer I've needed to make a really early morning departure and didn't want to go through the rigamarole of folding down the TM on the departure morning. Also, my TM takes up my entire driveway when setup, which necessitates my tow vehicle being parked on the street...and I'm not thrilled about doing that on the night before a big trip. So I decided to investigate whether I could fully pack-up the TM on the day before the trip...including fully loading the refer...and use a battery charger to keep the refer running over night while maintaining the TM's battery at full charge. That would also let me put the TM back in my garage and bring my tow vehicle off the street into my much safer driveway.
Since the TM refer's DC heating strip is about 100 watts, I calculated I would need a charger capable of supplying about 8 amps for that plus at least 3 or 4 more amps for residual battery charging and running the refer fan. So I went out and bought a 15/2/100 battery charger at Wally World (also available from Harbor Freight
http://www.harborfreight.com/cpi/cta...emnumber=47449). The going price for these 15 amp chargers seems to be around $69. Since my TM (2720SL) has the battery in a back compartment, I decided the easiest approach would be to use the TM's existing charging circuit so I also got a 7-pin pollack receptacle ("car end"
from an RV dealer and wired the appropriate connectors with 10 guage red and black wires. I stripped some insulation from the ends of both wires so I could clip the charger's alligator clips to them.
Before doing any testing, I set the TM up and plugged it into 120 AC for about 6 hours to cool down the refer (running on AC) and fully charge up its battery.
Once the refer was cooled to the desired temp (see my other tip on refer gadgets), I switched it to off and unplugged the TM from 120 AC. I then
very carefully connected the pollack connector to the TM's umbilical cord, the charger to the wires coming out of the pollack connector, and finally the charger to 120 AC. Note: Because the red wire has 12 volts on it as soon as the umbilical and pollack connector are mated, great care has to be used to make sure it doesn't touch the black wire or the TM's frame. If you do (even momentarily), your next step will be to replace the TM's battery fuse! Judicious use of some electrical tape is recommended.
I found that with no load on the TM's battery, the charger's ammeter would show about 4 amps residual charging current. Turning on the TM's refer fan upped that to about 4.5 amps. Switching the TM's refer to DC further upped the output from the charger to between 11 and 12 amps...about what was expected and well within the charger's sustained output capability.
After disconnecting the charger and pollack connector, I finished loading the TM (including putting a bag of ice cubes and a carton of ice cream into the refer's freezer compartment), set the refer to DC and turned on the refer fan. I then folded down the TM, put it in my garage, and hooked the charger back up for the night. The next morning, I hooked up and did a 5 hour drive to the campsite. When I arrived, I found the battery was fully charged and that neither the ice cubes nor the ice cream showed any sign of melting/refreezing. Success!
A big caution: Do NOT do this if your TM's battery is not already fully charged (and capable of holding a full charge). First, the charger has very little spare capacity to charge the battery
and power the refer. Second, a battery that's being charged does tend to outgas hydrogen...and one that can't be fully charged really outgasses hydrogen.
On the 2720 SL, any battery outgassing with the trailer closed goes into the interior of the rear top shell at about the rear window and will flow upward from there. Although my battery was not noticeably outgassing (about 1 bubble from any of the cells every couple of minutes), I released the hold-downs on the front top shell after I got the TM into my garage so that it's aft end rose about a foot above the rear shell. In so doing, I ensured there would be no hydrogen trapped within the trailer. Slightly opening the rear window before closing up the TM would also probably provide sufficient ventilation for any battery outgassing from a healthy, fully charged battery.