We were very fortunate to have a lot of help at the weekend from our son who is halfway through an HND in electrical engineering. So we now have a wiring diagram and all the technical details worked out for the solar charging, batteries, through to the inverters and to the 12v circuit breakers.
The plan is to keep the engine starting and alternator charging almost entirely separate from the Lithium house battery bank. The only connection will be a battery to battery charger that can be manually turned on to top up the house battery bank if we are using the engine and it’s starter battery is fully charged. We do not want to plan to run the engine just to charge the batteries. Short term that means using less diesel and it means we don’t spend money upgrading the alternator (which would be pushed far too hard by the demands of directly charging the big lithium house battery bank). Long term that is because we want to end up with an electric motor anyway so money spent improving the engine charging ability is wasted.
This simplifies things. However, due to my decision to keep a 12v battery bank, for wide availability of appliances and to make it cheaper to expand battery bank capacity (you can add individual extra 12v batteries rather than needing multiples), makes the wiring more critical for safety and to avoid lots of inefficiency with the high current needed for big appliances (particularly the inverters [producing 240v AC] and the windlass).
So our batteries will be in two layers using the original battery box.
The engine starter battery will be at the bottom left of the picture. House battery 1 will run lengthwise to fill the rest of the bottom of the battery box. The other two house batteries will sit side by side on top of the first two (on a shelf, not directly on top). They will be set slightly aft of the lower battery so that it’s little lcd screen for it’s BMS (Battery Management System) will be visible (fortunately for us these big batteries have the terminals and LCD screen all at one end). Should we ever need it we could potentially add a 3rd layer of batteries (for a potential massive bank of 1,500AH), although this would reduce access to the rest of the cockpit locker).
One of the challenges will be lifting 38kg batteries down into the cockpit locker. So I’m going to sort out a “crane”, basically a block and tackle fastened to the wheelhouse above the locker. However, I’ll also need one to get the batteries from the ground onto the boat in the first place. I’m going to need to put my thinking cap on for that as the “normal” solution would be to use the main boom but the mast is currently down.
The space reclaimed from the paraffin fuel tank will be used for a +ve and a -ve busbar (following advice from Nigel Calder‘s book) for the house batteries. This allows the most even balancing of both load and charge for the batteries. We are over-sizing this busbar by using Stainless steel 30mm by 5mm bar and M10 bolts, all mounted into a wood frame with a transparent acrylic top to prevent shorting by tools etc falling on it, plus it keeps water off. After the -ve busbar everything on the load side goes through a “shunt”, this comes as part of a Victron battery and power monitoring system. It will give us a display of the battery condition as well as things such as total current power use, total power use etc.
We carefully priced all the big cables between the batteries and inverters (and windlass). It was 25% cheaper to buy the cable in reels, with connectors and tools than to buy them ready made. These are massive and are going to be a huge pain to bend when we are installing them. However, they are sized so that we have no worries about overloading them, even if we were to run both inverters at full power (that is 4,000 watts!) plus have the windlass running at the same time (12v 1,500 watts).
We are going to re-purpose the “wet locker”, the hanging space between the engine and the steps as an electrical supply cabinet. So on the load side two big cables for the -ve and two for the +ve will come to a big 600A on/off switch mounted under the steps down to the cabin so it is easily accessible. From the switch power goes to another pair of busbars to which all the big electrical stuff is connected: 2 x inverters (each with a circuit breaker); windlass circuit breaker; and two 12 volt distribution boards with circuit breakers.
That means we are going to be pretty much replacing all the 12v wiring in the boat. Given it is all over 40 years old we feel that isn’t surprising. It does mean we can move the switch board from over the galley which will create more space there. We were going to need new lighting cable anyway as there are so few lights in the cabins at the moment. It also means we can sort out the problems with the lack of a continuous earth at the same time.
The solar charge controllers (initially 3 Victron MPPT controllers) will also go in the “wet locker” and be directly connected to the main battery bank busbars again with overspecced wire for efficiency and safety. We can monitor and control these using bluetooth from our phones.
Whilst the wet locker is “round the corner” from the companionway so should be very protected from waves it does have an open front at the moment so we will make a canvas “curtain” to stop any water from ever getting in. It is the best place we can find for a combination of accessibility, weight distribution and closeness to the batteries. It should mean a total cable length between battery and inverter of under 5m. That would be acceptable with 70mm2 wire but we are using 95mm2 so we are well oversized.
As there won’t be an official wet locker anymore (not that it has been used for that in the recent past) we expect to use the forward heads compartment for our wet waterproofs where they can drain into the shower sump. If we have lots of guests it will just have to be only in dry weather 😉
Renewing the electrics is one of the bigger jobs we have to do so we are really pleased that we now have concrete plans and lots of the parts on order.