Hi guys,
I have a bit of a problem. I want to build up a light battery to use with a Minn Kota Riptide saltwater 55 pound thrust trolling motor which has a digital maximizer control instead of using speed coils (in other words, it pwms the motor instead of dropping voltage in resistance coils like the cheaper 5 speed models). The problem is I want to use some of the many Lithium Manganeese sony US26650VT cells I happen to have (4.1V fully charged, 2600mA, capable of 50A discharge) to build up a pack. The LiMn battery voltage drops from about 4.1V full to about 2.5V empty (don't discharge below this voltage) so if I make a 20 cell ~200Wh using 5p4s pack it will have a fully charged voltage of 16.4V and a fully drained voltage of 10V.
The problem is I cannot get Minn Kota to tell me if the Riptide 55 pwm electronics would be tollerant of 16.4V, and the electronics are all potted (for salt water protection). I assume since the trolling motor is normally connected to a battery which may be charging via an alternator at something around 14.5V that the components are at least rated that high. I could build the pack as 6p3s with a 12.3V full and 7.5V empty, but I don't think the motor would work well below about 10V.
Since the battery would only be at 16.4V for a few minutes until it has drained somewhat, I could run the output through a series string of 3 honkin diodes, dropping the output to under 15V, and then maybe have a circuit to bypass these diodes after the unloaded battery voltage has dropped below 15V? I guess this way I would only have the wasted power (50 amps max times 1.8V = 90 watts) for a few minutes, then have a low Rds on mosfet bypass the three diodes.
Any suggestions for a better way?