Originally Posted by noelex 77
Not sure I understand the question, but to put some numbers on Peukert's Formula my batteries are Sonnenschein solar
block batteries. The manufacturer says my nominal 185Ahr batteries only have a capacity of 103 AHrs if discharged at 185A and 150 AHrs if discharged at 37A..
For a nominal 400AHr battery bank when discharged at 176 A would be Reduced by something like 1/3.
Thus the real “cost” of the hairdryer is not 14.66 AHrs,but around 20AHrs. (note this will be less is your battery bank is larger, or you run the hairdryer while some charge is put back into the battery) These are manufacturers figures based on brand new batteries older batteries can perform much worse, even when they appear healthy.
Tell your wife you love short hair.
Thanks for the numbers. I have a fairly big battery bank (1000AH @12V, AGM) and when giving advice I sometimes forget that not all boats are similarly equipped. I couldn't find high-amp discharge data for my Lifelines
, but did find curves for some Rolls AGMs, and your example seems reasonable. It might even be optimistic, as they allow the terminal voltage to go lower at high loads when calculating battery capacity.
Of course the Ohm's Law formula will still correctly give the current
drawn from the battery, if you allow for the reduced battery terminal voltage. What Peukert's Equation shows is that the battery will not deliver it's rated capacity at the high loads, and at the end of the day this is pretty important. Current drain is still good to know when you are sizing conductors, fuses
Back to the original question, assuming a healthy 12V 400AH battery bank, running the 1800W hair drier for five minutes will effectively drain about 23AH of capacity from the battery (15AH / 0.66). This is about 12% of the useful capacity of the battery bank -- probably not the end of the world.