do devices pull more current at lower voltage?

[akikhia]

ive studied a bit about the chemistry and electricity that goes into batteries and i wanted to know whether devices such as cell phones or laptops pull more current out of a lithium ion battery at lower voltages vs. higher voltage?
reason i imagine this is true is because i would expect these devices to pull a constant wattage so voltage and ampere would be inverseley proportional.

any input would be highly appreciated. thanks. im new to the site but ive had good experience with other forums so i thought ill see if you guys can help.

 

[DerStrom8]

Quick answer: No. Consider Ohm's Law--V=IR, where V is voltage, I is current, and R is resistance. The resistance of the circuit remains the same. So let's assume a resistance of 10 ohms for this example, and we'll start with 12 volts. I=12/10 = 1.2 amps. Now let's lower the voltage to 9 volts: I=9/10 = 0.9 amps. A lower voltage means a lower current, provided the circuit's resistance remains the same. Does this help?

 

[cowana]

While DerStrom8's answer holds for a very basic device like a resistor, an embedded system such as a cell phone is much more complex.

Most battery operated devices use switching regulators to generate the required voltages - in these cases the current will rise as the voltage falls. Other devices using linear regulators will draw a constant current regardless of the batter voltage (provided it is above the required threshold) - any excess is dissipated as heat.

Andrew

 

[DerStrom8]

While DerStrom8's answer holds for a very basic device like a resistor, an embedded system such as a cell phone is much more complex.

Most battery operated devices use switching regulators to generate the required voltages - in these cases the current will rise as the voltage falls. Other devices using linear regulators will draw a constant current regardless of the batter voltage (provided it is above the required threshold) - any excess is dissipated as heat.

Andrew

Thanks for that. My answer was very basic and over-simplified. You're absolutely right--the relationship between voltage and current will differ from circuit to circuit, depending on what it's made up of.

 

[akikhia]

so this means the devices dont run at a constant wattage? i thought it would be a bit more complicated than assuming it has a constant resistance. but i guess your right.
well why would it seem that the batteries drain faster when at a lower voltage?
im guna go buy myself a new voltmeter and do some testing today

 

[hexreader]

Quick answer: No. Consider Ohm's Law

You are usually right, so I am a little nervous to be asking for clarification...

...but

Won't complicated devices like phones and laptops draw more current as battery voltage drops?

I would have thought that many of these devices would use boost circuitry, which would have to work harder (and thus draw more current) to get a low voltage to the correct level? I am no expert and will be happy to be corrected.

For simple devices, I of course agree with you fully.

EDIT: please ignore - you answered in the time it took me to write my post (can't find an embarrassed emoticon)

 

[DerStrom8]

so this means the devices dont run at a constant wattage? i thought it would be a bit more complicated than assuming it has a constant resistance. but i guess your right.
well why would it seem that the batteries drain faster when at a lower voltage?
im guna go buy myself a new voltmeter and do some testing today

When the battery is at a lower voltage, it means the chemicals inside have already deteriorated and are almost "gone". Its "drain" has nothing to do with the voltage--only the chemicals that are decaying.

If you can find a multimeter, I'd highly recommend that. Digital is by far the best. That way, you can measure resistance, voltage, current, and others.

 

[DerStrom8]

You are usually right, so I am a little nervous to be asking for clarification...

...but

Won't complicated devices like phones and laptops draw more current as battery voltage drops?

I would have thought that many of these devices would use boost circuitry, which would have to work harder (and thus draw more current) to get a low voltage to the correct level? I am no expert and will be happy to be corrected.

For simple devices, I of course agree with you fully.

Yup, you're right. See my reply above

 

[akikhia]

While DerStrom8's answer holds for a very basic device like a resistor, an embedded system such as a cell phone is much more complex.

Most battery operated devices use switching regulators to generate the required voltages - in these cases the current will rise as the voltage falls. Other devices using linear regulators will draw a constant current regardless of the batter voltage (provided it is above the required threshold) - any excess is dissipated as heat.

Andrew

ah didnt see your post thanks cowana i thought that was what happened inside the device.
so in that case i was thinking that if i were to double a battery's capacity (specifically a lithium ion battery) it would provide more than twice as much usage time considering if we draw out the graph, the lithium battery would spend more time at higher voltages.
would it by the way have a greater than double Watt-hour? im judging based on what i recall studying about lithium ion battery discharging patterns

 

[akikhia]

You are usually right, so I am a little nervous to be asking for clarification...

...but

Won't complicated devices like phones and laptops draw more current as battery voltage drops?

I would have thought that many of these devices would use boost circuitry, which would have to work harder (and thus draw more current) to get a low voltage to the correct level? I am no expert and will be happy to be corrected.

For simple devices, I of course agree with you fully.

EDIT: please ignore - you answered in the time it took me to write my post (can't find an embarrassed emoticon)

haha feel the same here

 

[akikhia]

When the battery is at a lower voltage, it means the chemicals inside have already deteriorated and are almost "gone". Its "drain" has nothing to do with the voltage--only the chemicals that are decaying.

If you can find a multimeter, I'd highly recommend that. Digital is by far the best. That way, you can measure resistance, voltage, current, and others.

yes will definitely get a digital multimeter. reason im referring to lower voltage is because i tested this out once and my results showed that there seems to be more Ampere draw at lower voltage of course there was error but it seemed to be a recurring pattern. and as was mentioned above its due to voltage adjustments within the device.

 

[audioguru]

Why not look at the datasheet of the battery you use on a battery manufacturer's website like Energizer?
They show graphs of the voltage dropping with a constant current or with a resistor.
They have a graph with constant power.

 

[crutschow]

Switching power supplies do generally take a constant power from the battery for a constant power output. The input thus actually exhibits a negative resistance characteristic (input current goes down as voltage goes up).

 

[KeepItSimpleStupid]

Derstrom:
In post #2. you answered three questions with a single yes which is very confusing.

Crutschow's post #13 makes the most sense although efficiency of the switching regulator would vary slightly. Resistive losses go up as the current goes up.

 

[DerStrom8]

Derstrom:
In post #2. you answered three questions with a single yes which is very confusing.

Yeah, I realized that, which is why I tried to elaborate a bit