The Electric plan: Storage

Our Electric cooking plan and the need to charge our Electric Outboard (plus other projects) has made it obvious that we will need a big house battery bank. On boats there are typically separate battery banks for engine starting and domestic/house purposes. The intention being that even if you leave all the lights on and flatten the house battery you will still be able to start the engine. That was especially important when the engine alternator was the only way of charging your batteries.

The house bank on the boat as we bought it was made up of 3 lead acid 12V batteries that had been left alone for more than 12 months. They will hold enough charge for using a interior light for a short while but no more than that.

There is now a ton of evidence that it is most cost effective on a boat to buy Lithium-ion Phosphate batteries. They have numerous advantages over older technologies including:

  • Easier recycling of the Lithium
  • They can last orders of magnitude more discharge/recharge cycles than any other battery technology
  • They can be discharged to around 90% without damage, unlike lead acid which can can only be discharged to 50% before needing to be recharged.
  • They can be discharged faster for high power applications
  • They can be charged much faster
  • They are lighter
  • You can add new batteries into a bank of old batteries (can’t do this with lead acid etc)

So looking around there are many people building battery banks from a variety of non-marine specific batteries (mainly due to the price premium charged for “marine” batteries). Also many people are building battery banks from lots of smaller batteries eg 6volt golf cart buggies, for similar reasons.

When building a battery bank one of the absolutely key decisions to make is what voltage you are going to configure the bank as. For example two 12 volt batteries can be connected in series to appear as a 24volt bank or in parallel to appear as a larger 12volt battery. You get the same total capacity but if the voltage is higher you get the same power from lower currents.

For example if I want 2,000 watts for an induction hob then from a 12volt battery I will need to draw 166 amps of current (ignore the conversion from DC to AC at the moment). That is a lot of current and will require a very thick cable if you are not going to melt it or have very low efficiency. With a 24volt battery the same 2,000 watts will only need half the current: 83 amps, so the cable can be thinner and cheaper and easier to install.

From this logic the temptation is to go for the highest voltage bank of batteries that you can. For electric motors, used instead of diesel engines to move boats (the most power hungry of all devices), the battery banks are often 48volt or even 72volt. However, there are disadvantages too. 48volts is into a new level of dangerous if you get it flowing through you. From here “In industry, 30 volts is generally considered to be a conservative threshold value for dangerous voltage. The cautious person should regard any voltage above 30 volts as threatening, not relying on normal body resistance for protection against shock.”

Then there is the matter of how you configure your batteries to create a bank. To get a 48volt bank from 12 volt batteries you need to connect 4 batteries in series. That means you can only extend your battery bank by adding 4 extra batteries at a time. A 72 volt bank needs batteries adding in sets of 6. A 12volt bank allows you to add one 12volt battery at a time.

Also we need to consider what the battery bank is powering and where it is situated. In our case most DC devices for boats are 12volt (many are now also available in 24 volt but not stocked as widely and more expensive). For us the big power devices are going to be AC at 240 volts. The AC power can run through much cheaper power cables. That just leaves very high levels of current running to the inverter before they get switched to high voltage AC. We can minimise this problem by keeping the inverter close to the batteries and using big cables, also by using multiple inverters so that the current gets split between them.

So we have gone for a huge 12 volt battery bank with three big 300 ampere hour batteries (traditionally 12 volt batteries have been about 100 ampere hours). This is slightly cheaper and significantly simpler to install than 9x100AH batteries. I’ll be using very thick, oversized cables between the batteries and to/from the inverters to minimise the losses. We have gone for a pretty low profile brand at about 2/3 the cost of better known ones as a calculated risk.

Will let you know how we get on when they arrive.

3 thoughts on “The Electric plan: Storage

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