Trailer Vent Fan

I just checked two trailer vent fans and both were dead. With a little checking I found one had a dead battery and the other had a bad motor. They are both made by NICR and are day/night fans which employ a battery that charges all day, so it can run all night. Why did the battery go dead? I suspect it died because that trailer has been stored inside where the battery soon went to zero volts and there was no sunlight available to recharge it. This condition probably lasted all Winter long. There is an on/off switch, but I'm sure I would put the ship away and forget to turn the fan off or take it out and forget to turn it back on. Better for me to replace the fan with a day only unit made by SUNVENT which is a lot cheaper too......$60 vs $160. The other trailer will probably be left out most of the time and a working day/ night fan is required. Anyone know where to get a Ni Cd 2500 mAh 1.2v?

JJ