question for you electrical engineers out there

Mark · Post by **Mark** » Fri Jun 19, 2009 8:12 pm

say i use a 9 volt battery to power a small 12volt fan that draws 0.10amps, is there any way to tell about how long the battery would last, i know nothing about Ohms law or whatever so i thought i would ask.

J-high · Post by **J-high** » Fri Jun 19, 2009 8:37 pm

you would get about 5 hours maybe with a 9v battery with a .10a load.

Rainbow · Post by **Rainbow** » Fri Jun 19, 2009 8:40 pm

Depends on other factors, what kind of 9 volt battery? Regular , alkaline, Nicad etc.
A typical 9 volt battery is 400 ma/hrs. Without being exact @ 9 volt your motor will draw a little more so a typical battery will last a little over 3 hours maybe as much as 4

Faust · Post by **Faust** » Fri Jun 19, 2009 8:41 pm

im not even close to being an electrical engineer, but if im not mistaken, it depends on the VA rating on the battery, yes?

Mark · Post by **Mark** » Fri Jun 19, 2009 8:42 pm

it is a regular alkaline battery, i am curious how you guys figured that out , can to share with a lay person

CableDude · Post by **CableDude** » Fri Jun 19, 2009 8:44 pm

Mark · Post by **Mark** » Fri Jun 19, 2009 8:44 pm

here is what i could find

Mark · Post by **Mark** » Fri Jun 19, 2009 8:45 pm

CableDude wrote:[m]

done that dude

CableDude · Post by **CableDude** » Fri Jun 19, 2009 8:47 pm

Mark · Post by **Mark** » Fri Jun 19, 2009 8:47 pm

CableDude wrote:[h]

yes LOL

Rainbow · Post by **Rainbow** » Fri Jun 19, 2009 8:47 pm

Yes an Alkaline battery is about 575 ma/hrs so that would give you about 5 and a half hours.
The 400 ma/hrs was for a standard carbon battery.

J-high · Post by **J-high** » Fri Jun 19, 2009 8:47 pm

a typical 9v battery has a .55Ah rating. This is the total power the battery contains and can deliver continuously.

.10a load continuous on a .55Ah battery = 5 to 5.5 hours

Faust · Post by **Faust** » Fri Jun 19, 2009 8:48 pm

ah, thats right, forgot the "hours" part of it. essentially the measure of the length of time a battery can supply a given current. you see it a lot on UPS batteries and cordless phone batteries.

Mark · Post by **Mark** » Fri Jun 19, 2009 8:49 pm

wow, thanks guys, you have made my work easy

Rainbow · Post by **Rainbow** » Fri Jun 19, 2009 8:49 pm

J-high wrote:a typical 9v battery has a .55Ah rating. This is the total power the battery contains and can deliver continuously.

.10a load continuous on a .55Ah battery = 5 to 5.5 hours

Not a typical one, by typical I believe you mean standard carbon zinc that is 400

CableDude · Post by **CableDude** » Fri Jun 19, 2009 8:49 pm

4chan?

Mark · Post by **Mark** » Fri Jun 19, 2009 8:51 pm

CableDude wrote:4chan?

ewwww, gross

CableDude · Post by **CableDude** » Fri Jun 19, 2009 8:51 pm

Mark wrote:ewwww, gross

J-high · Post by **J-high** » Fri Jun 19, 2009 8:54 pm

typical as in alkaline (duracell) or (energizer).

RoundEye · Post by **RoundEye** » Fri Jun 19, 2009 10:17 pm

Here’s another thing that may throw a wrench into the figures. Say the fan draws .9 watts, as the battery goes down in voltage it will draw more current in order to try and maintain that wattage.

Ohms law

Fan wattage at 9 volts

E x I = W Voltage times current equals wattage 9 x .10 = .9 watts

Current at lower voltages

W / E = I Wattage divided by voltage equals current .9 / 9 = .1

W / E = I Wattage divided by voltage equals current .9 / 7 = .13

W / E = I Wattage divided by voltage equals current .9 / 5 = .18

W / E = I Wattage divided by voltage equals current .9 / 3 = .3

As you can see it’s really small numbers with a battery but as the voltage goes down the current goes up. The fan will try to maintain the wattage it’s designed for. As the battery drops in voltage it will discharge even faster. Wattage is the amount of work done and the load (fan) will always try to maintain that load.

nightowl · Post by **nightowl** » Sat Jun 20, 2009 10:20 pm

That battery is also going to be very hot due to the 12 volt fan drawing more engergy then the battery is rated for!