Battery Capacity

‘The most unreliable part of the boat is its electric systems.’

–Tony Brooks

At the Boat Show we managed to attend 2 (free!) mini lectures, the second of which was on ‘Electrics for Boaters’ with Tony Brooks. 

Tony is an instructor offering boaters’ courses and technical help. His site with some good technical notes on it can be found here.

He only had 20 minutes to speak and focused on calculating how much battery capacity you actually need to have on your boat.

Here are the highlights…

First do an energy audit

1. Turn all electrical items off on your boat. Then one by one, turn each item on and off to determine how many amps each item on your boat draws (eg lamp uses 3.4 amps, pump uses 5 amps).
2. Estimate how many hours you use each item for per day (eg lamp used 2 hours, pump used 2 hours).
3. Multiply each item’s amps by the # hours/day used – this will give you amphours per day per item (3.4×2=6.8 and 5×2=10).
4. Add up all the amphours of all the items together (6.8+10=16.8 amphours).
5. This tells you the amount of usuable battery capacity needed.

BUT

• You can’t use 100% of any battery all in one day! 
• In fact, you should only use up 50% of the charge in your batteries each day (otherwise you significantly shorten the battery’s life).
• AND you can only realistically charge the battery to 80% of its so-called total rated capacity.
• Therefore really only 30% of the battery’s claimed capacity is available to you (80%-50%=30%).

Then calculate battery capacity

1. 16.8 amphours per day is only 30% of rated battery capacity needed. So how many amphours of rated capacity do you actually need?
2. Solve for x: 
• 16.8/x = 30/100
• x = 56 amphours needed per day

So 1×110 ah batteries should be enough. 

On the other hand, if you only charge your battery every third day, then you’ll need 168 amphours and should do fine with 2×110 ah batteries.
  
Listening to this lecture also showed us… we don’t know a thing about engines yet either. Hmm.

So the Can Opener will be trolling the electrics section in our Don Casey’s Sailboat Maintenance Manual as well. And the RYA marine Electrics course has also been bumped up the list…

Have you ever done an energy audit? 
Find any surprises?

–Captain Cat

(transcribed by the Can Opener)


Cruising Lectures
Next: Chuck Paine Speaks
Previously: Diesel Bug
 

Don Casey’s Sailboat Maintenance Manual – Arrived!

links below at bottom of post

I just got Don Casey’s Complete Illustrated Sailboat Maintenance Manual in the mail to add to the library. ‘Six books in one’ is anyone’s idea of good value. 

I’ve assigned it to my First mate by day for some light reading (ha!). He needs to cover this giant reference book cover to cover. In particular, I want him to focus on its overview of how to survey boats. The book weighs in at about 3 lbs, so it’ll take him a while.

By night, however, the book is mine! A few good snoozes stretched over this tome should allow all the info contained therein to leech upwards into my sizable and osmotically inclined brain. I’ll be done absorbing its info long before the Can Opener. 

Then I can move on to selecting the right shade of organic catnip slip covers for the berths in the saloon. 

A captain’s work is never done.

 

–Captain Cat

(transcribed by the Can Opener)

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Electrical Systems – One More Thing for the Training List

 

Can Opener: I’m sure there is a good reason you have added ‘a thorough knowledge of complete boat electrical system’ to the ever-growing list of things you think I am about to learn.

Captain Cat: Best practices says we should both be knowledgeable about everything on our boat. Everything. 

We need a working knowledge of all systems and should be able to do basic repairs. And then some.

Know how to maintain marine wiring? How to calculate our electrical usage and needs? How to rig solar panels and make sure we’ve got a non-integrated system (so that if one thing busts we don’t lose all our electrics at once…)? 

Can Opener: Hm…

Captain Cat: I have spent years snoozing my way on the very best books to electrical savoir faire. You, on the other paw, have some studying to do.

Can Opener: So exactly what would happen if you didn’t get your way all the time, every time? 

Captain Cat: No one knows…

–transcribed by the Can Opener

Reference: The Training List