Battery Capacity Calculation

Maringret is an electrically complex boat and requires an efficient electrical supply based on an appropriately sized battery system.

Through the Maxi 95 we had gained experience with wind power, solar power, LED lighting, and power management systems. In undertaking the refurbishment of our HR 41 we took the lessons learned from our first boat, added the experiences of the HR 41 (which came to us with some advanced electrical circuitry on it and Gel Cell batteries), mixed in what we had gathered from our research and came up with a design and installation plan.

Energy systems are a good example of something that benefits greatly from planning and review. Knowing what is desired, what priority it has, what budget is available and what complexity is tolerable all enter into a rational plan. Once a “shopping list” of desired items (e.g. lighting, stereo, pressurized water) is put together then a list of the typical consumption for them should be assembled:

Rating Usage Consumption

Appliance
(amps) (hours per
24 hour day)
(amp-hours per
24 hour day)
lighting 1 amp 4 4
stereo 2 amps 2 4
water pump 4 amps 0.5 2
Total: 10 amp-hours

The example above is very simple and only for demonstration, a better and thorough description is available in Nigel Calder’s Boatowner’s Mechanical and Electrical Manual. A working spreadsheet for these calculations is available on the Von Wentzel website.

In our example above, using the figure of 10 amp-hours usage per 24 hour day, the batteries can be sized accordingly. There is no purpose to re-detailing all the calculations that are covered in the book and website, suffice it to say that a battery capacity of 10 amp-hours will cover 1 day of usage before being totally depleted, a 30 amp-hour battery will cover 3 days and so forth.

One nasty aspect of batteries is that they hate to be discharged and have a physical memory of being discharged – the more they are discharged, the less resiliency they have and the shorter their lifespan. Discharging batteries fully is much worse than discharging them 50% or 20% or 10%. The less they are discharged before re-charging the more discharges they can provide and the longer they will last. A typical set of figures for battery discharges is:

ie41_victronBatteryCycles

Discharging a battery to 30% gives 4 times as many discharges (i.e. 4 times the useful life) as completely discharging until the battery is flat. Another way to phrase this is that constantly discharging a battery completely cuts the lifespan of the battery by three quarters.

The configuration of Maringret’s electrical system is:

Quantity Battery Size Capacity
5 Victron 150 Ah gel cell 750 Ah domestic battery
1 Victron 125 Ah gel cell 125 Ah engine battery
Activity Consumption % of Domestic Battery
Daily consumption
(summer in harbour)
20 Ah 3%
Daily consumption
(winter in harbour)
25 Ah 3.5%
Daily consumption
under way
35 Ah 5%

Soft energy sources are dependent on the weather (i.e. wind & sun) and so it is hard to determine what contribution they will make on a “typical” day. Also there are sunny days that are still and windy days that are heavily overcast. Given the capacity of the wind and solar on Maringret, perhaps the most sensible thing is to list how long it would take each to replace the daily consumption (assuming each device is working at half capacity for an 8 hour period in each 24 hour day).

Source Total
Capacity
Half
Capacity
Half Capacity
over 8 Hours
Solar panels (2 @ 50W & 2 @ 100W) 300W or 24 A 12 A 96 amp-hours
Wind generator (16 A @ 32 knots of wind) 208W or 16 A 8 A 64 amp-hours
160 amp-hours
(a windy & sunny day)

Note: the figure of 160 amp-hours in the above table is a bit misleading. Yes that amount is what could be generated by wind and sun together, but not what the batteries could necessarily accept.

Now to return to the daily consumption figures:

Activity Consumption Deficit
or Surplus from Solar Charging
Deficit
or Surplus from Wind Charging
Deficit
or Surplus from Solar & Wind
Charging
Daily consumption
(summer in harbour)
20 Ah 76 Ah surplus 48 Ah surplus 140 Ah surplus
Daily consumption
(winter in harbour)
25 Ah 71 Ah surplus 43 Ah surplus 135 Ah surplus
Daily consumption
under way
35 Ah 61 Ah surplus 33 Ah surplus 125 Ah surplus

Note: the figures in the above table assume an unlimited ability to accept charge on the part of the batteries, this is misleading. Generally the chemical activity within batteries limits their ability accept charge – and this limiting increases with heat. Which quickly leads to the paradox of a warmer (maybe hot) summer day with a lot of sun but then the batteries become warmer and are thereby less able to accept the charge which is in abundance.

The above figures confirm that the energy generation on Maringret more than doubly covers the expected consumption. Of course none of this means anything if the wind is still and cloud cover thick. In that event the battery bank of 750 Ah would cover approximately 15 days of typical consumption, at the end the battery bank would be depleted 50%.

  • A power manager is invaluable and also irreplaceable for managing a battery bank of any size, see our page here.

After the first year the domestic batteries have not been discharged lower than 10% (and that was only once). The engine battery is easily able to turn over the diesel engine – even in the lower temperatures of winter. The bow thruster batteries are able to drive the bow thruster as needed – they are charged via a 12 to 24 step up charger or shore power.


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