[electricboats] Solar charging

OK friends, here's the deal.  I have a 36v.  trolling motor as aux. propulsion on my HA18, that's a catboat.  I power it with a 36v lit ion rechargeable bike battery. Works good.  I'm trying to be able to put a little solar juice in it while underway.  What's the simplest way to do that?  

thanks,

Glenn/NC