Boat Upgrade – Battery Power

living off the grid ?

Our boat has spent most of its life on the Gold Coast, generally small outings and a lot of time in a marina berth plugged into shore power. Therefore the on board power system reflects that in so much as there is limited battery power and power hungry appliances.

Our plan for the boat is totally different. We want to be able to live off the grid for extended periods of time, mainly at anchor so that we can visit remote places and do it economically. While there are places where you must check into a marina, most of the time it is not necessary. The plan is to set up the boats power system so that we never need to access shore power. Indeed, the boat will be based in Sydney Harbour on a mooring so being independent of the grid is absolutely essential. Besides it also contributes towards minimising our carbon footprint.

where to start?

The first step is to analyse what you have and then make appropriate changes. The most economical way to go is to seriously look at what you really need.

Do you really need the microwave oven or the electric kettle and toaster that came with the boat? While it may be nice and convenient they are not essential items. They are very power hungry and to run them on this boat you need to be plugged into shore power or substantially upgrade the 24V dc system and install a 4kw inverter. We have a 4 burner gas stove with oven and grill that is more than sufficient for cooking.

Most of the lights on the boat were incandescent or tungsten halogen. Very inefficient and easily replaced with LED lamps.

So on the power consumption side you need to stick with what you really need and then make sure that what you do have is using the available power efficiently.

On the supply side there needs to be enough battery storage capacity to last a few days and enough charging capacity to top up the batteries after a normal days usage. So an audit is required to find out how many amp hours you will use in a normal day be it sailing or at anchor. And then size the batteries and charging to match.

power usage audit

After doing the sums, we determined that normal usage at anchor would be about 70 amp hours and when sailing with all of the navigation equipment running it would be about 170 amp hours. The large increase when sailing is largely due to having old instrumentation and auto pilot and will in the future reduce significantly when new equipment is required.

The auto pilot and some of the instrumentation is 40 years old but while it still is working well there is no point to replace it. Besides, when sailing we will be running the engines some of the time and we can rune the Honda 2kVA generator if require. So not as bad as it seems.

What Batteries

My battery bank sizing is based on dropping the 85% to 50% in one day under full load and no charging. This means that you need a capacity of about three times the maximum daily usage or about 500 amp hours. Allocated space for the batteries is limited to 110 amp hours for each of 6 batteries based on commercially available lead acid type.

The existing battery set up consisted of four batteries for house power and two starting batteries for the diesel engines. So adopting the existing arrangement would mean the house batteries would be undersized by 25%.

So the battery system needed to be reconfigured. One option is to use a single battery for starting both engines. That leaves potentially 550 amp hours for the house batteries. It is very common practise to separate the starting and house batteries for the simple reason that the batteries can get too low to crank the engines.

However, starting batteries do not do very much. Starting duty is very transient with minimal discharge. So to my mind, provided that you have a reasonable excess of capacity in your house battery and good charging and battery monitoring, the starting batteries are a bit superfluous. While house batteries are deep cycle and not designed for high starting currents, if the starting current is distributed over the 6 batteries, it becomes insignificant. So without finding more space for additional batteries, doing away with the starting batteries means in this case 30% additional storage capacity. That can only be positive for battery life operation.

Charging

The other side of the equation is of course how to keep the batteries charged. These days solar panels are very efficient and super cheap so that is generally the best option. On a catamaran there is normally adequate space on the cockpit roof for installation.

As the majority of time is spent at anchor, you need to be able to comfortably charge the batteries during normal sunny days to adequately replace the daily usage.

Based on solar panel efficiency of 50%, operating 6 hours per day with a battery charging efficiency of 80%, you need around 400W of installed solar panels. This is only an intelligent guess based on the above assumptions.

The four existing solar panels were very old and inefficient. On a bright sunny day the measured output was only 100W. So definitely needs upgrading. Fortunately the with new more efficient solar panels they fit into the existing space.

The other important consideration with solar panels is the charging regulator. The industry standard for many years was simple PWM (Pulse Width Modulation) type of charging. While they work well, they do not optimise the output of the solar panels as the current coming out of the panel at a higher voltage is the same as the current going into the battery at a lower voltage. Modern MPPT (Maximum Power Point Tracking) chargers select the best output voltage from the solar panel and then transform this electronically to maximise charging current into the battery. This typically gives a 20% higher charging current into the batteries which is really significant.

summary

So for an investment of AU$1000 for batteries and AU$500 for solar panels and some labour the 24V dc power supply can be upgraded to a very reliable and adequate system for total independence from the grid.

Of course nothing is as easy as it seems. The brackets for the solar panels had to be changed and new cabling run to handle the higher current output. However the resultant system is a massive improvement.

Having a good battery monitor is absolutely essential. The boat came with one and it is good to know exactly what your batteries are doing.

operational Feedback

When sailing the boat down from Queensland it was necessary to start the generator half way through the night to top up the batteries. In 2019 we spent over 5 months living on the boat and never had to run the generator.

The original calculation on usage at anchor was spot on and most days the solar panels would top up the batteries to 100%. In fact there was only one occasion where the house battery dropped to 80% and that was sailing with a few overcast days. Of course running the motors helps and then there is the generator as a back up. The estimate of solar panel output was good too with typically 12 to 15 amps charging into the batteries on a normal sunny day.

So what we have installed is more than adequate for living off the grid for an extended period of time.

In fact on the topic of lowering our carbon footprint, we have 40 year old diesel engines that are probably not very efficient. But they still work well we use wind power most of the time so no point to change them yet. However they will reach their use by date at some time. Then I will be looking seriously at electric propulsion. And while with current technology the range is limited by battery storage capacity, it is a sailing boat and besides there is less noise, less vibration, less maintenance, less weight and cheaper to install. And as time goes by and technology improves it can only become a more attractive option.