by TassiePete on Wed Jul 05, 2017 5:23 am
Charging a 30V battery with from a 12V supply is nothing special, so called DC-to-DC Step-up Converters have been around for a long time. I have a couple of concerns about charging the motor battery from your house battery. The regular 110/240V charging unit charges the battery (about 300Wh) with 3A (or 4A) and it takes 8+ hours to do so. A trickle charger cable, as the name implies, will take a lot longer. Even if it were to charge at the same rate of 3A, you probably will need a solar panel rated at 80-100W, which is rather large, and it will need a charge controller. A 100W solar panel on the boat will rarely be oriented optimally, will be shaded by the sail at times, the sun will not always shine, etc. So the real output of a 100W panel is probably about 40W on average over time (and not necessarily over 8+ hours), which is what you need to have 300Wh available (12V x 3A x 9 hours -> 324Wh). Remember that you will need the 40W+ to supply 3A at 12V (36W). The difference are losses in the charger (chargers usually get a bit warm). On a rainy day, the solar output is zilch, so you will drain your house battery quite a bit.
Either way, sunny or not, if you are ever in trouble and run your motor battery down, you have no options left. To get 15 minutes of motoring time back, you will need to charge for 2 hours, or a lot more with the trickle charger cable ... wind and waves might carry you a long way during that time and in a direction you may not want to travel.