Moving onto a mooring and out of the shore powered confines of a marina meant we needed to be more self sufficient for power. To live onboard Millie we need power for lighting, pumping and heating water, refrigeration, heating (diesel fired, but still needs amps to run) not to mention charging phones, laptops and the myriad of other equipment on board. I didn’t want a generator on board so decided to install an inverter, an extra battery (bring the total domestic to 450AH) and a wind generator for charging. With the decision to install the “solar arch” we added solar panels to the list.
Last year I was lucky enough to spot on Gumtree someone selling two 150AH Lucas AGM batteries and a Victron 1600VA Inverter/Charger at a very good price. Although I had no way of checking the batteries, at the price I decided to take the risk, and they have turned out to be good. I’ve now added the third battery, sticking with the same make and model to keep the bank consistent.
Several winter weekends were spent researching and planning, and then drawing up wiring diagrams and doing the maths to calculate current loadings and wire sizes. We were going to need quite a lot of 70mm² tinned copper cable, which cost over £20 a metre at the first place I looked. In the end I found it a little cheaper, but the cables alone cost around £400, not to mention fuses, isolator switches etc.
I decided to run the bow thruster off the engine battery. Many bow thruster installations include an extra battery in the bow so that lighter weight cables can be used from the charging source to the battery. However our main battery compartment is under the starboard saloon seat, which needed less than 4m cable run to the bow thruster. We were also alerted to the risks in siting a lead acid battery in a sleeping compartment (related to production of hydrogen if overcharged or faulty …). Although it can draw up to 400A I figured the bow thruster wouldn’t be running for very long (probably only a few seconds at a time), so the total Amphours drawn would be quite low. However it would likely drop the voltage on the battery bank when switched on and I didn’t want to risk all the instruments going down at a critical time when docking. Because of this I installed a Sterling Pro Splitter split charge controller. This uses ‘intelligent’ logic and MosFETs to control the charging from the alternator to the two battery banks (domestic and engine) and should ensure that the engine battery gets priority charging even if the domestic batteries are fully charged.
The final part of the upgrade was to the AC distribution and shore power connections. The original system had the shore power coming in at the stern and then cabled to an RCD on the main switch panel by the chart table. I was never very comfortable with having mains power on the same panel as all the DC cabling and switching and in any case the RCD should be close to the inlet. This would feed directly to the Victron Inverter/Charger, which automatically switches over when shore power is connected or disconnected. After some research and deep thinking I realised we’d need a second RCD downstream of the inverter, and this would feed into the onboard AC distribution. I also decided to run the water heater off the inverter. It draws 900W so will give the batteries a bit of a beating, but if they are well charged, 30 mins of water heating shouldn’t kill them and it’s preferable to running the engine for that long.
I was a bit concerned about terminating the battery cables until my good friend Malcolm Duckett mentioned in his blog a hydraulic crimper he’d found on ebay for £26. Hard to believe it would be any good, but I bought one anyway and it was brilliant, made easy work of fitting the massive crimps on the cables. Finished with some heat shrink, the end result looked very professional.
I felt the need to draw proper wiring diagrams for two reasons: one, it helped me think through all the issues in detail and aided in putting it all together; and, two, it provided long term documentation, because I know that in a year’s time I’ll be scratching my head trying to remember what connects to what. The professional tool would be something like Autocad Electrical, but the cost of software runs to thousands so had to be ruled out. After some hunting around I found a package called Proficad which has a free version. Although it has some limitations (only three pages in a file and no wiring list facility) it was adequate for what I needed. Here are the drawings for the DC Battery Wiring.
Running the Cables
Probably the most difficult part of the installation was running the cables through the boat. A pair of 70mm² battery cables had to be run from the battery compartment through to the bow thruster, and another pair back to the engine compartment. Because of the movement in a boat all cables should run inside conduit, particularly where they pass through a bulkhead, and secured as far as possible to minimise movement. Also because of the cost of the cable I didn’t want to waste too much so was reluctant to cut them until they were in position. So the process involved routing the plastic flexible conduit through the boat, cutting it to length, then pulling it out and pulling (and pushing) the cables through the conduit. Then the conduit, now full of heavy cable, could be run back through the boat, anchoring it with attachments and cable ties and finally cutting and terminating the cable. This meant that running one pair of cables could easily take half a day’s work and quite a lot of back ache!
Choosing Cable Sizes
Battery cables (and in fact most wiring) is sized by the cross sectional area of the conductor. So a 70mm² cable has a conductor with total cross sectional area of 70mm². Using the formula for the area of a circle (πr² where r is radius, or r=√(a/π) where a is area) tells us that this cable will have a diameter of 9.44mm (plus the insulation, so say around 12mm total). The specification for the cable also tells us that it has a current carrying capacity of 485A and a resistance of 0.000281Ω per metre. Now that doesn’t sound very much, but if you are passing hundreds of amps through the cable it becomes significant, and if the cable is of any length it can translate into a significant voltage drop.
So in order to choose the right cable size, you need to know how much current it’s likely to be carrying and what the acceptable voltage drop is. I used a spreadsheet and started by putting together a table from the specifcations for the battery cables. I used tinned copper Oceanflex cable sourced from Furneaux Riddall or 12 Volt Planet.
I then drew up a list of all the cables I needed and plugged in the parameters to see the voltage drops on various sizes (also in the spreadsheet), calculated by using Ohms Law (in this case Voltage Drop = Current / Resistance). And don’t forget the actual voltage drop is twice the drop on a single cable because the current has to get back to the battery on the ground wire. Another factor is the amount of heat that will be generated, an indication of which is the power consumption (watts) of the cable, calculated by multiplying the voltage drop by the current in Amps. This is not really an issue with short bursts of current, such as used by the starter motor, but could be a problem with an inverter running at full power for a while.
The bible for electrical work for me is Nigel Calder’s “Boatowner’s Mechanical and Electrical Manual”. If you buy a copy make sure it’s the latest edition because there are still a lot of older editions around out there.
After doing most of this work I found the TB Training website which has some excellent notes about electrical installations on boats.