Average draw on battery from device equations

[Telgar]

Hello forum, I am new here and really I am only here to ask some you all a question in hopes of possibly helping me out. Here is the question (and yes I briefly skimmed through to find the question, it was not answered in the capacity I was looking for).

I have a device (PSP), and a Battery (It's personal one). I want to find the average draw on the battery form the device in a manner that will allow me to apply this to other batteries (portable battery charger packs) to find out the most efficient battery in terms of not just longevity (obviously that's what the mAh rating is for), but also being able to calculate the average life expectancy of the battery. This is what I have so far:

The battery is 3.6V at 1,800mAh, meaning the battery will provide a current of 90mA at 3.6V per hour for 20 hours in an ideal setting. I have no idea if it is possible to figure out how much watts or how many watts total a battery can pump out at any time.

The device is rated to draw at a maximum, 5V at 1.2A, which means at maximum power consumption (which is never) the device will draw 1.2A per hour at 5V. Due to the equation W = A * V I know the maximum watts this device uses at any given time is 6 Watts.

Being it is a PSP, it obviously does not last for 1 hour, so the AVERAGE draw is not 5V at 1.2A, or my battery would drain faster then 1 hour every use (since a battery draws faster if being used at a higher voltage then rated and at a higher mAh then rated per 20 hours).


This is what I have figured out already. This is what I need to know:

Equation necessary to find out a rough estimate of the devices mAh draw on a battery, as well as its draw when drawn at a higher voltage then rated, or a lower voltage then rated.

How to find a devices average draw per hour having the devices maximum voltage, amps, and watts possible. (would I simply half the voltage and amp's the device is rated for at maximum? or could I use an equation much like the first question asked).


With these questions I should be able to look at any device, any battery, and do the math to figure out how long the device should run at an average setting or maximum power consumption setting with the battery in new condition, and guesstimate its power over its lifetime.

I have not taken battery materials into factor even though I guess I should, If I am correct different types drain at different speeds as well as self-discharge at different speeds, but Li-Ion batteries are pretty much the standard in rechargeable batteries so I would imagine it would be worthless to take that into account.

 

[cdh7]

Hi Telgar,

Unfortunately it's not very possible to calculate a device's average power draw based on its maximum ratings.

If you had a pretty "static" device, say an LED, that uses a constant amount of power, you could simply measure the current being drawn and could calculate that the device always uses X watts (in which case the average power would be equal to the maximum power). But I think in the case of something like a PSP, the power drawn would fluctuate a lot. The backlight needs a lot of energy, and I guess you can probably set the brightness somewhere (I don't have a PSP). So that will change the amount of power you're using. Also I would imagine that even different games require different amounts of power depending on how computationally intensive they are. I suppose you could measure the average power over a period of time under particular conditions, but it sounds like that's not what you wanted to do.

Regards,
Chris

 

[Telgar]

Honestly your post was a lot more helpful then most of the useless ones I have found. now, lets say I do know the power consumption of the device.

PSP - lets say it works at 3.6V (5V 1.2A maximum input)

Battery - 3.6V at 1,800mAh

Knowing the battery information, the psp's maximum input values, and the psp's voltage rating at any given time, can we guesstimate the average amperage that the psp may draw. Is there a way to guess an aprox amps like if maximum V is 5 but devices V is 3.6 and maximum amps is 1.2 then approximate amps is (BLANK). This is what I am trying to figure out if its possible to do.

I know there are many situations where a psp can change its amps used due to load on the device, but lets just say the psp only works at one setting (which many devices do, work at only one amperage). If this was true, would it be possible to make an equation to figure out its approximate drain, and if so how.


Also another thing, is it possible to stick multimeter leads into the charger port (where the AC adapter plugs in) of a device and get a voltage or amperage reading that way, so that I do not have to open my psp up just to figure out its average volt/amp consumption, or put a wire in between the battery and the psp to see how fast the battery is draining.

 

[cdh7]

Hi,

Glad that helped a little.

I'm not sure over which voltage range the PSP works. Have you done any experiments to find out? A Li-Ion battery maintains a pretty constant voltage as it is discharged and then suddenly drops off. As the result, the PSP might not be designed to work with a wide voltage range.

I guess the battery probably stays inside the device and then you plug in an AC adapter to charge it. The adapter will probably have a higher voltage than the battery (hence the 5V rating). The maximum current rating will also be such that the battery can be charged at the same time you're playing a game. So really you'll need a lot less than 1.2A.

Yes, you can estimate the current like you asked. What you need to do is estimate the average power the PSP uses (that's just P = V x I). Say it needs 3W or whatever. Then, if you know the applied voltage, the current will be I = P / V. That assumes 100% efficiency, but I guess the PSP is pretty efficient anyway.

Keep in mind though that you can't just apply any voltage to the battery pins inside the PSP. Even if the case gives a rating of 5V, the power circuit of the PSP might not be able to handle that.

I don't think you can measure anything directly from the charger port when you're running from the battery. What if you take out the battery, plug in the adapter and measure the voltage and current being drawn? Then you can estimate the power being used.

Regards,
Chris

 

[Telgar]

I don't think you can measure anything directly from the charger port when you're running from the battery. What if you take out the battery, plug in the adapter and measure the voltage and current being drawn? Then you can estimate the power being used.
Chris

Thanks for that advice, I did not think of that. Now your talking about plugging in the AC adapter and then checking the battery terminal leads where the device would normally draw from the battery right? If so those are quite easy to access and would be an easy way to test the average consumption. It really does suck though I thought maybe I could take those numbers and figure out an approximate consumption from it but I guess their purpose is more "what adapters I can use" not "this is how much I can use".

By far you have been the most intelligently helpful person without being so intelligent you end up flaming instead. Most of the douches that have answered me have done little more then fight my question then actually attempt to answer it.

 

[Audioguru]

Most of the douches that have answered me have done little more then fight my question then actually attempt to answer it.

We answered you very well on the other website with the same answers as here.
Stop insulting us! We tried to help you!

 

[cdh7]

Now your talking about plugging in the AC adapter and then checking the battery terminal leads where the device would normally draw from the battery right?

Not exactly. The measurements would have to be made at the AC adapter port. You just have to take the battery out because if the battery were being charged, then you'd end up measuring the power the PSP is using plus the power used to charge the battery.

So in order to measure the power consumption, you'll need access to the wires of the AC adapter. I guess you probably don't want to cut open the cable. Is it a standard size plug? Then just get a male and female connector of the same type and wire them together in series with your ammeter. Plug one end into the PSP and the other into the adapter and you can get your measurements.

Most of the douches that have answered me have done little more then fight my question then actually attempt to answer it.

Hm, I guess it was probably just a misunderstanding. Most people do try to be helpful.

Regards,
Chris

 

[Telgar]

Ah. Well I am not willing to open my PSP to do this. I was hoping for something I may be able to use without having to test over wires. Thanks though for the help you have given, I guess at this point at least I just have to look at the mAh and just decide whatever is higher is better, which is a really shitty way to determine whats a better choice for me, but I guess its the only way the market lets you know. Much like how some ink cartridges for printers pretty much say "F you figure out how many pages we print the hard way".

And audioguru, you have been little to no help this whole time, the others have actually went at the question and given decent answers. You just give out information that is obvious. I think even a 6 year old given a psp and charger realizes the charger when plugged in makes the psp work and charges the battery at the same time. I didn't need obvious information, I needed my question worked on.

Thanks again cdh7

 

[Audioguru]

You did not say how much of the operation time the battery is charging so I thought you forgot about it.

We cannot guess at how much current your system uses, and the power supply rating of 1.2A might be much more than it uses because that power supply was available and inexpensive.

 

[hoppy7]

I'm new here too, but I was wondering why don't you measure the time it takes for the psp to go dead. Then you can just divide the number of hours into the batteries amp rating. Example:

It takes 5 hrs. to drain the psp, then 1800mA/hr divieded by 5 so
the average current drawn by your psp is 360 mA/hrs.

I hope this helped!

hoppy7