200 KHZ will give you the best definition. It'll give you minute details of underwater structure including fish. 50 - 90 KHZ give you much greater depth penetration but less definition.
I still like the Compres. I have 4 as it is and as soon as they're phased out later this year, I'll probably pick up another 4 just to have because. Nowadays my goto rods are all Spiel rods.
I'm using this one for my two TM batteries...the main starter battery looks after itself. http://www.cabelas.com/product/Boating/AutoBoat-Batteries-Chargers/On-Board-Chargers-Inverters|/pc/104794380/c/104698080/sc/104465880/Guest-Charge-Pro-Waterproof-On-Board-Battery-Chargers/737490.uts?destination=%2Fcatalog%2Fbrowse.cmd%3FN%3D1100565%26WTz_l%3DSBC%253BMMcat104794380%253Bcat104698080&WTz_l=SBC%3BMMcat104794380%3Bcat104698080%3Bcat104465880
The C-Tek is more a maintainer than a charger. It's a great little gizmo but I'd say your boat would have to be plugged in all the time it's not on the water.
Sorry folks...extremely busy here. I will turn it up a notch or two. I guess some of the newer members have not read the rules. It's a pretty bad state of affairs when you need rules to keep people civil.