30W cooling fan via inverter

Submitted: Wednesday, Jan 10, 2007 at 22:07
ThreadID: 41086 Views:2040 Replies:3 FollowUps:7
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Trying to get a handle on the Ah draw from a 12v deep cycle marine battery (RC 130)
running an inverter to a 240V 30W cooling fan.
If I was running the 30w appliance direct from the battery, the formula would be:
Ah = RC /2 + 16 = 81. Then divided by 3 = 27 hours.
When converting to 240v is the Ah draw from the battery the same or do different rules apply?.
Secondly, if the fan is run at the lowest setting, is it running at less than 30w?


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Reply By: SimonW - Wednesday, Jan 10, 2007 at 23:45

Wednesday, Jan 10, 2007 at 23:45
Hi T.R,
I don't know the calculations but here are some real world figures from our setup when we tested. We run a 3 speed 240v 50w fan via a 100w can inverter in the CT & the drain on the deep cycle is;
100w inverter no load = .2amp
Fan on via inverter;
Low = 4.5
Med = 4.7
High = 4.9
If you turn on the rotation grid it use's a extra .3amp on all settings.

Cheers Simon.
AnswerID: 214566

Follow Up By: T-Ribby - Thursday, Jan 11, 2007 at 11:02

Thursday, Jan 11, 2007 at 11:02
Thanks Simon, I'll use it as a base guide
FollowupID: 474902

Follow Up By: rolande- Thursday, Jan 11, 2007 at 13:06

Thursday, Jan 11, 2007 at 13:06
G'Day Simon,

Posted your results on another forum as well,

Hope you don't mind


FollowupID: 474940

Follow Up By: SimonW - Thursday, Jan 11, 2007 at 13:53

Thursday, Jan 11, 2007 at 13:53
No problem, happy to help.
The power usage is a little high but worth it to have a good volume air circulation so our 2 little guys can sleep during the daytime.
Our fan is just a "mistral look alike" from bunnings for $18

FollowupID: 474947

Reply By: Member - John and Val W (ACT) - Thursday, Jan 11, 2007 at 07:44

Thursday, Jan 11, 2007 at 07:44

Not too sure about your formula, but it works like this:

First - your battery appears to be rated at 130Ah, so from fully charged it should provide about 130 amps x hours before being totally discharged. That's under ideal conditions, and in practice you won't get that much. Also, battery life is reduced if it is allowed to 100% discharge. (If it is fully discharged it is important to recharge immediately to avoid serious reduction in life.) To be reasonable, assume that your battery can safely provide say 110 amps x hours (that's 1 amp for 110 hours, 2 amps for 55 hours, 3 amps for 36 hours etc.)

The current drain - Regardless of voltage, a 30 watt fan will require about 30 watts, more on starting and running fast than when it is running slowly. Watts = volts x amps so 30 watts at 12 volts will require 30/12 = 2.5 amps. Depending on the size and technology of the inverter, it will be about 80 - 90% efficient, so the current drawn from the battery will in fact be closer to 3 amps. (This is just theoretical stuff - in practice, give or take 25%). On this basis your battery should run the fan for 36 to 48 hours if there is nothing else drawing current from the battery.


J and V
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AnswerID: 214582

Follow Up By: T-Ribby - Thursday, Jan 11, 2007 at 11:02

Thursday, Jan 11, 2007 at 11:02
The battery won't be fully discharged - the RC / 2 formula halves the available current.
I can monitor the voltage as well. I have a new 600w inverter. Around 20hrs running
should be a reasonable margin. Battery is kept charged via my Anderson plug when
travelling - it's when parked I need to know how much juice can be used for the fan.
I have two deep cycle marine batteries - the other one runs the lights which are 10-13w

FollowupID: 474900

Reply By: Member - Arkay (SA) - Thursday, Jan 11, 2007 at 08:42

Thursday, Jan 11, 2007 at 08:42
Consider getting a 12cm (120mm) 12 volt brushless motor computer fan with ball bearing mechanism, square shape about 1 inch deep, and using that. You can balance it on a flat surface or mount it on a base plat etc., but keep fingers well away. You can get a fan blade guard for them for around $7 if you wish. get 2 guards if you want, one for the fron & one for the rear.
These fans run at about 2000 r.p.m. and move quite a bit of air. Try one out in the shop before you buy.
They draw between .4and 1.5 amps. depending on make etc. Some come with a speed control using a little knob which is normally placed in one of the desktop computer tower rear slots & enables the operator to adjust the fan speed from about 300 rpm to max (say 2300rpm or so). Faster = more air & more noise. Those models may also have a temperature control which in normal situation senses the CPU temperature and speeds up the fan depending on how hot the CPU gets. No need to connect that part.
Cost of a basic 12v brushless motor fan would be $20 to $25, or up to $40 for a sophisticated one.
AnswerID: 214591

Follow Up By: T-Ribby - Thursday, Jan 11, 2007 at 11:07

Thursday, Jan 11, 2007 at 11:07
Thanks Arkay - I hadn't thought of that.
FollowupID: 474906

Follow Up By: joc45 - Thursday, Jan 11, 2007 at 12:56

Thursday, Jan 11, 2007 at 12:56
Hi Arkay,
I'd concur with what you're saying; better to use an efficient DC brushless motor.
Now where have you seen these fans? The computer ones are a bit small (tho usually free).
Thinking of one for my tent for when it all gets a bit steamy in there ;-)
FollowupID: 474937

Follow Up By: Member - Arkay (SA) - Friday, Jan 12, 2007 at 09:42

Friday, Jan 12, 2007 at 09:42
Joc45 you say computer fans are a bit small. Some are. What I suggested in my post was the 12cm (120mm, about 4.5 inches in the old language ) fan which is the largest I have seen readily available. Like I said, they move a fair amount of air. Get the shop to demo one and judge for yourself. They are available for Dick Smith, Tandy, Jaycar, or any computer store that sells a reasonable selection of accessories, or does repairs.
FollowupID: 475144

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