Watts and Amps and Volts -- Oh my!

Posted By: John Turpin, P-15 "Tetra", Edmond, OK
Date: 12/11/08 6:34a.m.

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I need your collective brain power and electronical "know how". This year, I'm going to install a Raymarine ST1000+ autotiller unit on Tetra. I've decided to sail her to the South Pacific and I'd like to nap while she steers herself. (Okay, that South Pacific part isn't completely true.) Here's my dilemma. Tetra is not electrified. All this time, I've used jump start batteries to power any electric needs: masthead tricolor/anchor light, depth sounder, accessory charging, etc.. If possible, I'd like to NOT install a permanent battery. Portable batteries have worked for me and give me flexibility that I like. Messing around with a post-factory battery installation sounds like a real hassle and, quite frankly, I just don't use that much power. When I decide to put in A/C and a freezer, that might change. Here's my question. Could I reasonably power my new autotiller from a portable battery? I am an electricity idiot and don't see that changing any time soon. I have two jump start batteries. One is a small battery that I've used for years and it has lost all labels that identify its specifications. I use it for accessory stuff and it works fine, so forget about that one. The other is a big honkin' battery that has 600 starting amps (and like a gazillion peak amps). It's big in size, weighs a ton, and has lots of lights and gizmos on it. It frequently draws admiring stares and nods from manly men. I'm wondering if I could run my new autotiller from this battery for week-long trips. So, here's my assumption. In an eight-hour sailing day, I would never use "Ray" for longer than four hours--most days just a fraction of that. But, let's assume a total of twenty hours of autotiller use in a five-day trip. Assume that I would not be able to find a 110 outlet to charge him up during that time. Here's what Raymarine says about current consumption for this unit: Standby: 40 mA (90 mA with full lighting) Auto: 0.5 A to 1.5 A depending on boat trim, helm load and sailing conditions Assume the higher end of that scale because when Ray's steering, I'll be scooting around messing with stuff and he'll be working hard to keep her on course as the ballast (me) constantly shifts. As for the battery, it was obviously meant to start cars and is certainly not optimized for deep cycleish duty. So, it's already at a disadvantage. I envision a 6 foot (+ -) run of battery cable from the battery's location in the cabin to the unit's location toward the stern. Assume that I'd use this battery solely for Ray's power. If/when I find out that the battery might not support 20 hours of autotiller power, I would plan to mount a solar panel somewhere to provide some extra juice. If possible, I'd have the panel trickling power into one cigarette lighter outlet at the same time that Ray is sucking power out of a second cigarette lighter outlet. So, here's my question. Can anyone estimate how many hours of autotiller use this battery might get me? Then, what size solar panel (in watts) would be needed to keep up with the current outflow? There are small solar panels that provide 5-6 Watts (per hour?) and then the next range tends to jump up to about 8-10 watts. Anything beyond 10 watts would be cost prohibitive and I'd probably be forced into a real marine battery purchase. My tired arms and tiller-calloused hands thank you in advance. Sincerely, Electrically Confused in Oklahoma