Wednesday, 10 February 2016

Fledgling 12v electrical system... job #5

Time to start thinking about job #5, installing some electrics.. been two years now and that's long enough - I feel the need to be able to press switches and see lights come on....

So basic requirements at this stage is nothing more than:
  1. battery
  2. isolator switch
  3. bus bar
  4. switch panel
 ...working towards the following....

Click to "embiggen" - this diagram shamelessly ripped off from the Sailing Around blog [clicky] (worth visiting by the way)
 1. Battery

So my good mate Giblets gifted me a battery when we exchanged the spondoolics for the new outboard the summer before last, and up until now it has been sat in my garage.

I have put it on an occasional charge, and it seems to hold that charge so I think it's good to go, and if push comes to shove I can always replace it...

Some basics - the battery dimensions are 13cm by 19cm by 24cm (W/L/H), and it is 40 amph's - so not huge, but suitable to my purposes at this stage..  most importantly it's a sealed unit so I can turn it over, even upside down, with impunity while getting it into position

It will be in a battery box, which will then be strapped to a sheet of epoxied marine ply, which will be either:
  • bolted between the sides of the quarter berths, just behind the cockpit drains and under the cockpit sole....  or..
  • bolted to the lip of the opening for the same space
...this shows the spot, and the opening... option one is widthways roughly where that white rag is , the lip of the opening for option 2 (lengthways) is in the foreground..

 The opening I've taken the picture through is 22cm high so everything will fit through in one dimension or the other...  I also have limited access to that space from above via the quarter berths...

2. Isolator switch

Any number of these are available on eBay and the like, I am an electrical novice, but I had one of these on Pap and if it was good enough there, it's good enough for Sparrow..  this is the master switch to the whole system, if this is off, there's no power to any of the devices in the boat...

The studs underneath are M10 - one is connected via cable to the positive on the battery, the other connects via cable to the switch panel..

For cables I've bought a set of car jump leads - yes, I know tinned cable would be better, but they are 4 or more times the cost..  From what I can tell these are 16mm2 (they are 100Amp capacity so plenty of oomph) , so 8mm outside diameter and 2mtrs long. The planned cable run will be short, no more than a metre, maybe a metre and a half.

For connectivity at the battery I already have quick release cable clamps (you can get bolt on one's, but I had quick release on Pap so I'm falling back on duplicating what I know and am comfortable with.
The other end of the positive cable will have a copper tube terminal (16mm2 with 10mm opening) to attach to the stud on the isolator switch. I will take a length off - length TBD once I see where the switch physically is - to attach to the other side of the switch, and connect to the next element in the puzzle...

3. Bus bar

So.... thus far we have battery, in box, bolted to boat (via battery box/ply board), with 16mm2 copper cables attached to the battery using quick release clamps - one positive/one negative.

On the positive cable we have the isolator switch wired in, but on the negative side currently just the bare cable length. This (negative) cable I will connect to a 12v bus bar so I have a common negative for anything I want to attach to the switch panel...  you could use a power distribution post (like a big bolt) but a bus bar allows you to work on each device without having to take everything off the post to get at the cable you want (probably the one at the bottom!)

You can buy a pukka bus bar, but they are 6 times the cost of the following which was recommended to me by the guys on the  Practical Boat Owner web forum - this is a stainless steel 8 Way Earth Terminal Block...

...and on to the next step in the puzzle..

4. Switch panel

So.... thus far we have battery, in box, bolted to boat (via battery box/ply board), with 16mm2 copper cables attached to the battery using quick release clamps - one positive/one negative. Positive cable (red) has an isolator switch wired in, the negative (black) runs direct from the battery to the bus bar/earthing block. Now I need the thing to allow me to turn individual devices off and on - the switch panel....

These switch panels are available in a bewildering number of styles & prices, and are a bit of a God-send as they come pre-wired, and with fuses...  I've chosen a 6 switch (6 gang) panel as my requirements will always be pretty simple, and I plan to combine some devices on a single switch anyway (nav lights for example) - I've also chose one where the light comes on when the switch is turned on (at least you know there's power there when you see the light )

The wiring basics are as follows:

..and I'll:
  • attach the positive cable (from the battery via the isolator switch) to the switch panel
  • the switch panel is a device in it's own right (as the switches light up when they are on), so it also needs a negative - the board comes with one wired in, and that needs to be connected to the bus bar - at this point if everything is connected up, and the isolator switch is off (ie. power is on), then the lights in the switches will work when you click them...  hopefully....
...seemples....   he also said hopefully.....

All the subsequent things I'm going to connect to the switches on the panel (VHF/nav lights etc.) will then have their negatives screwed into the bus bar and their positives to the required switch on the panel.. depending on the panel I get, the fuses will be a variety, so I will choose switch based on fuse...  but that's next stage....

Just in case anyone is interested - here's the costs so far...
  1. battery - free
  2. battery box - £15.99
  3. jump leads for battery cables - £4.90
  4. isolator switch - £4.99
  5. bus bar - stainless steel 8 Way Earth Terminal Block - £3.18
  6. switch panel - £18.95


  1. Ooh, excellent. Good to have some leccy on board. If you want solar panel, I have a spare brand new one. That would keep it trickle charged while on the mooring.
    I originally had my battery under the cockpit like that but it was a pain if you wanted to get to it.. which sods laws dictates you will only need to when it's blowing a hooly. Mine is now in thee cavernous looker under the quarter berth. Very easy to access and leaves the middle free for long things like paddles etc.

    1. Watcha Phil -apologies for not replying sooner - your comment went into a moderation folder and I only just found it! Thought about quarter berths, and cockpit lockers, but it's nice and neat down there - out of the way as well... and on Sparrow the quarter berth panels lift out completely so there's a lot of access to the area under the cockpit...