Join Date: Sep 2007
Location: Pasadena Md - Magothy side
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Re: Basic marine deep cycle battery questions
Others have already addressed the 'what's watt' issue of amp hours and usage.
There is no such thing as a standard battery - you need to look up yours and see what it it rated at, then because you have no idea what the current state of it is (age, condition) go with a safety factor that only you can determine (i.e. 80%).
Voltage can be used as a simple indicator of state of charge, but it's not totally accurate. There are many threads here discussing that (search 'voltage state of charge').
For extra 'hang time' on the hook there are lots of options, solar, wind, low amp anchor lights etc..the idea is either put the amp's back, or reduce the use.
Be realistic on what a anchor light draws - the old ones are 1.5 or more amps per hour - for 30 bucks you can get a perfectly legal LED base hook on the back stay type and it will draw .1 amps.
Solar - calculate your gain to a charged battery based on a simple rule of thumb - you'll get 25% of the watt rating in amp hours (e.g. 100w panel, 25 amp hours) per day.
A 20-30w panel is a fine trickle charge over a week on a mooring, it will not significantly extend anchor hang time.
If budget is a concern you can get a 900w gas generator at Harbor Freight for under a 100 bucks. They are loud, smelly and use gas - but - they are cheap, almost disposable and great for that time when you do in fact kill the battery.
Lessons learned are opportunities earned.