Difference in current draw between adapter and battery?

Thread Starter

jellytot

Joined May 20, 2014
72
A few days ago, I measured the current draw of a load, powered by an 1.5V AA alkaline battery. It was around 100ma (and today, a retest with a fresh battery, the load draws 150ma).
Today, I measured the same load, but this time it's powered by a 1.5V (1A max) wall adapter. This time, the current draw is 450ma.

Why the difference? It's my understanding the load will draw as much current as it needs, so if it really needs 450ma, why didn't it draw that much from the battery (which I believe can output a maximum of around 2A)? Is there an error in my understanding on how current works?

It's actually better for me if the load uses less current (it's OK if the load doesn't "work" as well). I was going to switch the load from battery power to a 1.5V regulator, but there aren't a lot of 1.5V regulators that can handle 0.5A, plus 0.5A would create a lot of heat in the regulator. Anyway, can anyone shed some light on whats going on?

Last edited:

AlbertHall

Joined Jun 4, 2014
11,505
What is the load?
ETA You can use an adjustable regulator to get 1.5V e.g. LM317T can handle 1.5A. How hot it gets depends on what voltage you supply it with.

Thread Starter

jellytot

Joined May 20, 2014
72
What is the load?
ETA You can use an adjustable regulator to get 1.5V e.g. LM317T can handle 1.5A. How hot it gets depends on what voltage you supply it with.
The load is a TEC (Thermoelectric Cooler) max rating 15.4V, 3A.
I do have a 1.5V (out) linear regulator, that can handle 0.5A. Input voltage will be 5V. I worry about the amount of heat that will be generated, but I haven't tried it yet. Would first like to know the reason for the difference in current draw.

Something else odd that I noticed, at 12V the load draws 950ma from my power supply (1A max). Shouldn't it draw... more, if the TEC can draw 3A at 15.V volts?
And I don't think this makes a difference, but my 1A max wall power supply has select-able voltages (1.5V min to 12V max).

AlbertHall

Joined Jun 4, 2014
11,505
Your such a long way from the usual operating voltage for the TEC I have no idea how it would perform. Maybe it is going to vary with temperature - perhaps particularly difference in temperature between the two sides.

Thread Starter

jellytot

Joined May 20, 2014
72
The TEC is hooked up to heatsinks. Temperature should not be a factor, as I tried to replicate the test as exactly as I could (except switching the battery/power adapter).
For argument's sake, let's pretend the load was something more mundane, like a motor or light. Should the current draw should be the same, regardless if I use battery or wall power?

AlbertHall

Joined Jun 4, 2014
11,505
The current will depend on the voltage - the AA voltage is likely to drop more than the wall wart when you draw >100mA.
At the same voltage the light would draw the same current. The current drawn by small motors generally depends quite a lot on what speed they are running at, so small changes of voltage may make large changes in current.
ETA At these low voltages and relatively high current a very little contact resistance could make a lot of difference. Be sure to get good solid connections.

Alec_t

Joined Sep 17, 2013
12,208
Would first like to know the reason for the difference in current draw.
The internal resistance of the battery is higher than that of the adapter and so reduces the current.

WBahn

Joined Mar 31, 2012
26,398
It would be a LOT more helpful if, in addition to the current draw figures, you could also furnish the ACTUAL voltage values under load. Ideally both value would be measured at the same time, but it will be almost as good if you measure one and then the other so that you can use a single VOM meter.

KeepItSimpleStupid

Joined Mar 4, 2014
5,084
At that low of a voltage you have to be careful of two things:

1) The voltage burden of the ammeter. I'd check the voltage across the TEC at the same time your measuring current.
2) The internal resistance of the battery and battery discharge curve.

dannyf

Joined Sep 13, 2015
2,197
Why the difference?
The battery has much higher internal resistance.

Thread Starter

jellytot

Joined May 20, 2014
72
Thanks for the assistance, everyone!

Looks like the problem is solved. The select-able voltage power supply is faulty.

When the power supply is set to 1.5V, it measures 2.9V with the multimeter. It draws ~400ma-450ma.
The 1.5V battery measures ~1.4V (slowly drops when in use). It draws ~200-230ma. These results are in-line witht he results above. Not sure why it drew ~150ma earlier today though.

Anyway, thanks again!

WBahn

Joined Mar 31, 2012
26,398
Is that 2.9 V under load or unloaded? Many adaptors are unregulated without a load and tend to be at a much higher voltage.

Thread Starter

jellytot

Joined May 20, 2014
72
Is that 2.9 V under load or unloaded? Many adaptors are unregulated without a load and tend to be at a much higher voltage.
That was loaded.