I'm currently doing some research into how firefly hybrid generators can save fuel emissions. I'm currently trying to calculate how exactly they charge using some simple calculations. The technical data sheet ((http://www.fireflycleanenergy.co.uk/wp-content/uploads/2015/03/Firefly_Cygnus_Hybrid_Power_Generator_Rental_Spec_ Brochure.pdf) pages 10,11 ) says it has a charge time of 7.5 hours, however it states this for units of various sizes with varying accompanying generator sizes, therefore I don't understand how this can be true.

In essence I'm building a rough idea of an expected load throughout the day a generator would be put under, and I'm trying to calculate when required how the firefly would charge when the generator is charging the firefly and also supporting the load upon it.

Does anybody know how to do this?

The limiting factor in charge time of a battery is the heat created by the charging current - you can't go too high or you can destroy the battery. As a rule of thumb, the charging current should be no more than 20% of the rated Amp-Hr capacity of the battery. The specs doesn't tell us the Amp-Hr capacity of the batteries, but for illustration: for a 24 A-Hr battery it should be charged at no more than 24 x 0.2=4.8 amps, and a 48 A-Hr battery should be charged at no more than 9.6A, and both batteries would have a 5-hour recharging time. I suspect that the charging circuit on these generators uses lower current levels than the above calculation would suggest, perhaps to extend battery life or to reduce the charging power load.

To determine the load that charging puts onto the generator - take the energy storage figures for the batteries and divide by 7.5 hours to get the power required in watts. In practice then divide that by 2, assuming that the batteries discharge only half way. For example the 48 kW-Hr batteries would require 48/7.5 = 6.4 kW to fully recharge in 7.5 hours, or more practically speaking half of that = 3.2 kW.

Thanks ebaines

Because the generator and battery back in question is 3-phase does this change in anyway

Also in the tech specs it states that the depth of discharge is at 60%, does this mean that the 20% you quoted is 40% on this unit?

Because the generator and battery back in question is 3-phase does this change in anyway

No.


Also in the tech specs it states that the depth of discharge is at 60%, does this mean that the 20% you quoted is 40% on this unit?

No, it means the "discharge only half way" that I mentioned should be "discharge only 60% of the way."