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Old 26-09-2010, 12:49   #16
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Originally Posted by noelex 77 View Post
The current figure is accurate and the AHr figure is accurate, but not really.
Peukert's Formula shows when you discharge a lead acid battery at these sort of amps its capacity plummets.
The bottom line if don’t want to bother with the math’s is that a hair dryer will draw from a battery much more the just Ohms law would suggest.
If the battery bank is not in good condition it may not cope at all.

Short sun dried hair is good.
OK, a good point and not one to be ignored. Your battery bank has to be sized appropriately for the load.

For a 160A load, what battery capacity would give you (say) a 75% discharge efficiency (in terms of capacity used)?
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Old 26-09-2010, 12:54   #17
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If men can pee standing up into the tiny bowl of a head on a boat that is getting pounded by the waves splattering their mess everywhere, then women should be allowed to have hairdryers that draw more kilowatts than a small city.
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Old 26-09-2010, 13:15   #18
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Also, could you advise - Does this mean that an inverter 'rests' in neutral mode (NO AMPS drawn from battery bank) when no appliance is plugged in, or does the inverter 'steal' energy by being there?
Most line frequency inverters of this size like the Victron you mention will use 15-20 watts of power in idle. High frequency switching inverters in this range like the Prosine will use 50-60 watts of power in idle.

So at 24V, they will consume ~1-3 amps in idle, depending on the type of inverter you have.

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Old 26-09-2010, 13:27   #19
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For a 160A load, what battery capacity would give you (say) a 75% discharge efficiency (in terms of capacity used)?
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.
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Old 26-09-2010, 13:30   #20
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If men can pee standing up into the tiny bowl of a head on a boat that is getting pounded by the waves splattering their mess everywhere, then women should be allowed to have hairdryers that draw more kilowatts than a small city.
Put the invertor next to the head. That will improve the aim
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Old 26-09-2010, 13:46   #21
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Put the invertor next to the head. That will improve the aim
Nothing like a little motivation to ensure accuracy,eh Noelex.
"precision and trajectory are the art of gunnery"
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Old 26-09-2010, 14:16   #22
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A positive side effect of the newly "eco-centric" marketplace is more efficient goodies for sailors.

There are now a couple "ECO" hair dryers out there. It was hard not to spray coffee mid giggle when I googled that one. Hehe. Anyhow the draw seems to be much lower that 1800w, found a couple under 900w that claim to have a more effective heating element as well.
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Old 26-09-2010, 15:00   #23
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If men can pee standing up into the tiny bowl of a head on a boat that is getting pounded by the waves splattering their mess everywhere, then women should be allowed to have hairdryers that draw more kilowatts than a small city.
Nobody pees standing up on my boat!! I tell that to my guests too. Most of the guys (cruisers) I know sit down. No comments about me or my friends please.
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Old 26-09-2010, 23:24   #24
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Originally Posted by noelex 77 View Post
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, etc.

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.
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