Conversion from mW to mAh

[(unknown)]

Hello everyone,

I'm new to this, so can anybody please answer this: I have a small sensor (transceiver) , which says it consumes 14,4 mW (op voltage 4V). I want to connect this on a 4Volts 200mAh battery. Can anybody calculate the operation time ? Say, in seconds?

Thanx ALOT in advance, this is extremely important for me

Nikos.

[johnlm]

Theoretically you can get 55.5 hours, but in practical application you will get maybe  90% of that or 50 hours, if the battery is starting out at a full charge.

John

[(unknown)]

John, many thanks!

Can you tell me what you divide in order to calculate this?

Thanx again mate

[johnlm]

14. 4 milli Watts/ 4 Volts = 3.6 mA as the amount of current being drawn.
    
200. milli Amp Hrs / 3.6 milli Amps = 55.55 hours.
     
     

See the posting by IFred on a reply to a post below about some guy testing his generator to light up a light and then getting 70 Amps short ckt current.  Fred give the ohms law and power law equations to calculate this.

regards

john

[ghurd]

Nikos,

'P' is watts in iFred's post.

G-

[(unknown)]

Fully understood (nice old physics )

John, u rule, if you come to Greece I'll buy you as much ouzo as you want

Thanx alot!

Nikos

[johnlm]

Niko,

Your welcome.  I doubt Ill make it to Greece anytime soon.  

Come to Colorado USA and we'll have Rocky Mountain Oysters.

Glad to help.

John

[thunderhead]

The simplest answer is that current times voltage is power, so a battery that delivers 30Ah at 12v will supply 2A to provide 48W, and supply it for 15 hours.

The practice is that for lead-acid cells, the actual power delivered is less than this.  The normal model used for this is something called Peukert's Equation, which involves raising your current to the power of the mysterious Peukert number before calculating the battery life.  So if our battery has a Peukert number of 1.1, then the 2A becomes 2.144A, and it only lasts 14 hours (30Ah divided by 2.144A equals 14h).

But if the battery had a Peukert number of 1.25, then the 2A becomes 2.378A, and the battery only lasts 12 hours and 37 mins.

Peukert number varies for types of cells - the best traction batteries might be as low as 1.03, but UPS backup batteries might easily have numbers of 1.25 or more.

[ghurd]

I also believe the faster the current is drained, the shorter (less than Amp-Hour rating) it lasts.

Loose example... A 100ah battery is tested to deliver 1a for 100h.  At 10a, it should last 10h, but only lasts 9.5h.  At 20a it should last 5h, but only lasts 4h.

It gets worse as the current goes up.

Most people with some experience seem to know this intuitively, and don't expect a 100ah battery to deliver 100a for a few minutes, let alone an hour.

I haven't seen the Peukert Number mentioned in a long time!

G-

[johnlm]

I think your math is incorrect.  How is 2 amps at 12 V equal 48 watts.  The rest of your info is informative, but I doubt this guy is using a lead acid battery if its only rated at 200 ma hour.  He's got something much smaller than a AA alkaline as they are more than 200maH,   I think he just wanted to know how long a primary cell of some kind would power his small electronic device.

John