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Topic: How to load a battery / wall-wart for a voltage test? (Read 1 time) previous topic - next topic

JimboZA

MaJiG suggested this in another thread:

Quote
Be sure the wall wart's voltage is measured with a reasonable load on it.


So my question is, what is a reasonable load to test batteries, wall warts and other power supplies? Are there some rules-of-thumb please, for different nominal voltages and (perhaps) types of batteries especially?

TIA,

Jim

Roy from ITCrowd: Have you tried turning it off an on again?
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fungus

For a wall wart/power supply ... 50% of rated amperage?

Batteries are harder, it depends on their size but ... 250mA for AA/AAA seems about right (they'd normally last for a few hours at that load).
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JimboZA

Quote
250mA for AA/AAA seems about right


So if my arithmetic is correct I want a resistor of 1.5V / 0.25A = 6 ohms.

As for the wattage of that resistor well 1.5 x 0.25 = .375 so I want 1/2W ones I guess....
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JimboZA

#3
Feb 02, 2013, 12:54 pm Last Edit: Feb 02, 2013, 01:11 pm by JimboZA Reason: 1
I only have 1/4 Watt resistors in my Sparkfun kit, so....

I have 10 ohm ones, so if I put two in parallel I'll get 5 ohms and a slightly higher nominal current than suggested, viz 300mA.

The power will be 1.5V x .3A = 450mW.... and two 1/4W resistors in parallel will each be able to handle their share of that.

(EDIT: Alternatively a 10 ohm by itself would draw a current of 1.5 / 10 = 150mA for less of a test, but the power's just on the money for a 1/4 ohm resistor at 1.5 x .15 = 225mW)
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JimboZA

Ok, here's an interesting twist on this- I happened to find an old (1997) RS catalog today while looking for something else. Seems they used to sell a battery checker (I can't find it online in their current (haha) offering, but it had the SKU 596-034), and according to the catalog this tests a 1.5V cell under a load at 35mA.

That's an order of magnitude different from fungus' suggestion of 250mA.... anybody got any thoughts?

35mA would require a resistance of 1.5 / .035 which is 43 ohms. The current would be 1.5 x .035 = 0.05 and so a 47ohm, 1/4W resistor would be good...

Supplementary question: how do I type the Ohm "omega" symbol please?
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