Energy Unlimited by Victron

8.5. Thinking different

8.5.1. Daily energy needed

Both for the DC concept and the battery assisted AC concept the first question to ask is not “ what is the maximum AC power to be expected? ” and then size inverters and the generator to that power. Instead the first question should be “ what is the daily electric energy need? ”

It is the daily energy need that determines the rating of the source of electric power.

The daily run-time needed to produce the required energy is calculated with the following formula: run-time (hours) = daily energy need (kWh) / output of the source(s) of electric power (kW)

Alternatively, if the requirement is to limit generator run-time to a certain amount of hours, the formula is: output of the source(s) of electric power = daily energy need / run-time Some examples: Source: alternator on the main engine supplying 100 A into a 12 V system, i. e. 100 A x 12 V = 1.2 kW Daily run-time needed: 4 kWh / 1.2 kW = 3.3 h (In practice the run-time will be somewhat longer due to losses in the system and possibly a reduced current absorption capacity of the battery at the end of the charge cycle, but for a first approximation the calculation is ok) 8.5.1.1 Daily energy needed: 4 kWh (see chapter 9)

8.5.1.2 Daily energy needed: 14 kWh (see chapter 10)

Source: diesel generator, but should not run more than 4 hours per day Minimum rating of the generator: 14 kWh / 4 h = 3.5 kW

8.5.2. Battery capacity

When power generation is limited to a few hours per day (alternator on the main engine or generator with generator free period), the size of the battery is determined by the amount of energy that the battery has to supply during the periods that the main engine or generator are off: the generator free period. In practice, due to the short recharge periods, the battery will be recharged to not more than 80 % (20 % DoD). The battery also should not be discharged to more than 70 % (70 % DoD). This would mean a usable battery capacity of at most 70 % - 20 % = 50 %. We should include a safety margin: when a battery has been discharged to 70 % there is no margin left if anything unexpected happens. There is no general rule for the amount of margin, but let’s take 10 %. This leaves us with 40 % usable capacity and a DoD of 60 %. Then we have to build-in a factor of 0.8 to account for 20 % capacity loss when the battery gets older: 40 % x 0.8 = 32 %. And finally, if we discharge a battery faster, or slower, than rated (the rated discharge time is in general 20 hours, see sect. 2.5.3) another correction factor will have to be applied. In most cases the time between recharges of the house battery is 8 to 12 h, and 32 % discharge in 8 hours is equivalent to 32 x 24 / 8 = 96 % discharge in 20 h. Very close to the rated discharge time, so no additional correction needed for batteries rated at 20 h, and a positive correction for tubular plate traction batteries, for Exide / Sonnenschein A600 cells (see sect.2.5.3.). (I imagine a breath of relief here: the usable capacity would have gone to nearly zero if even more corrections had to be applied)