Many brands of portable charges, outputs, are badly controlled and do more damage than good to batteries. They get you going when needed..but many boil the batteries/heat the plates if you leave them hooked up overnight. If you have a GOOD onboard charger for your trolling batteries...there is no reason you can't plug it in every night, no matter how little you've run the batteries down, when you come back in off the water....as they shut off automatically after taper charging for the last few % of capacity.