House Battery Bank: Should we go 48 Volt?

While we wrote Going 100% electric: the “house” after Going 100% electric: the Motor we had in fact made most of the decisions around the house electrical system before we made the decision that we would go straight to an electric motor instead of the diesel.

Now we are thinking about making a change. The things prompting us to consider a change include:

  • The high cost of 48 volt battery chargers. We do need the option of charging our battery bank when in a marina or harbour (or even ashore in the boatyard). We can imagine spending sometime alongside in winter or even popping every so often just to get the batteries fully charged (the expectation of needing to live in colder climates in Winter is influenced by both Covid and Brexit which might limit our options for where we spend our time).
  • We think our house battery bank has ended up a bit small (4 x 120AH) and so are going to be needing to charge it from the Motor bank (4 x 300AH) quite often.
  • Having two battery banks at different voltages ends up creating quite a lot of extra complication.
  • With one exception (the anchor windlass) we have realised that our 12 volt usage is relatively low (LED lighting, boat instruments, water pumps).
  • While we have specified really thick cabling with big busbars and fuses, it is challenging to power 2 x 2,000 watt inverters from a 12 volt battery bank. The current that we need to safely pass is huge and this is where the vast majority of our house consumption will be (induction hobs, microwave, multi-cooker, watermaker, water heater).
  • We didn’t understand enough about how you can power 12 volt systems from a 48 volt battery bank. We thought they were too inefficient but have now realised that we either incur that inefficiency when charging a 12 volt battery bank from the 48 volt bank for all house uses OR when using a 12 volt house appliance (but not a mains powered item from a 48 volt inverter). The total losses are much smaller if we incur them only as we need the 12 volt power rather than to keep a whole batery bank charged.
  • We deliberately chose 4 batteries for the house bank that had enough output so they could be re-wired to be a 48 volt battery bank for the motor if the main bank failed. However, it would take ages to do. So a bigger 48 volt bank with two sets of 4 batteries wired in series and then the sets connected in parallel gives immediate access.

So a little maths about the issue with power over 12v cables.

P = power in watts (W)
V = voltage in volts (V)
I = current in amps (A)

Power = Current x Voltage or P = I x V

Switching it around we have I = P / V
So 4,000 watts from 12 volts = 4,000 / 12 = 333 Amps
Whereas on a 48 volt system we have 83 Amps

More amps = thicker cables and lots of care to avoid melting connections or high losses.

The disadvantages of changing from a 12 volt hour battery bank

Our current thinking

  • As we install them, we will configure all 8 batteries as a single 48 volt battery bank. This is pretty straightforward.
  • We will sell our unused 2 x 2,000 watt Victron Phoenix inverters (get in touch if you are interested).
  • We will use our Victron Orion 48 volt DC to 12 volt DC converter to power all our 12 volt appliances. We can always add extra Orion’s to run together if we need more power (eg for the electric auto-pilot)
  • It would be very expensive to add enough Orion’s to provide all the 1,500 watts at 12 volts for the windlass. So we will add a 12 volt battery close to the windlass. When the windlass isn’t being used we can charge the battery through the standard 12 volt system.
  • We will add 2 x 48 volt 3,000 watt Victon Multi-plus charger/inverters (2 of them to provide redundancy, we can run appliances with some limitations off one of them).

The Multi-plus inverters are smart. They provide mains power to the boat circuit and they automatically take that power from a shore power connection or if that isn’t available from the battery bank. When connected to shore power they automatically charge the battery bank. Two of them can put a total 70 amps into the battery bank.

We will have a 48 Volt battery bank with a total capacity of 1,680 AH (4 x 300 plus 4 x 120). Suppose we arrive at a marina with it fully depleted (ie down to 10% charge). That means we need to put in 90% of 1,680Ah which is 1,512 AH. At 70 Amps charging we are talking about 21 hours to fully recharge the battery bank (realistically we would expect many marinas to be limited to either 16A or 32A supplies so this will be a lot slower). Gradually we would expect marinas to upgrade their electric supply as the number of electric boats increases.

While there are costs to this change it does simplify a number of things, particularly with cabling and charging. All our charging goes into the one battery bank without having to switch solar panels between banks or do inefficient bank to bank charging.

It gives us much simpler use of the battery capacity as we can choose how we allocate the available power between house and motor. For example if we are not going anywhere and expect some sunny days in a while we can use all the capacity for the house. Or if we are motoring up a river to a marina all the house capacity is available for the motor.

In the long term we would expect more boat appliances to be available in 48 volt versions which will gradually reduce the need for DC to DC converters.

We haven’t made a final decision on this yet, but it does look like we are heading this way at the moment.

3 thoughts on “House Battery Bank: Should we go 48 Volt?

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