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Author Topic: How to load a battery / wall-wart for a voltage test?  (Read 1125 times)
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MaJiG suggested this in another thread:

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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

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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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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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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)
« Last Edit: February 02, 2013, 07:11:18 am by JimboZA » Logged

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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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I bought a bunch of low resistance (5ohm & 10ohm) 10W cement resistors on Ebay.
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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?

It depends on the current capacity of the cell being used. The testing load current should not be the same for say a AAA cell Vs a C or D cell? A good load current testing value would/should be based on a constant percentage of current capacity I would think? At least that is how I would design a 'battery tester', with a users choice of current draw to use.
Lefty


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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Yes Lefty that makes sense, but that RS one does say it measures AA, AAA, C and D on the same load of 35mA. PP3 9Vs it does at 20mA; and it says not to test Nicad batteries at all.

But as a matter of principal, and to test my understanding, it is presumably batter to test a cell under at least "some load" whatever that might be? And presumably, the closer to the load characteristics encountered in real life, the better?
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Yes Lefty that makes sense, but that RS one does say it measures AA, AAA, C and D on the same load of 35mA. PP3 9Vs it does at 20mA; and it says not to test Nicad batteries at all.

But as a matter of principal, and to test my understanding, it is presumably batter to test a cell under at least "some load" whatever that might be? And presumably, the closer to the load characteristics encountered in real life, the better?

Certainly measuring a batteries terminal voltage without any load is the least desirable method, and a small fixed load is better then no load, but a load proportional to the the cells normal capacity would be even better I would think.

I know the R/C folks into using high performance (and expensive) battery packs for their aircraft often purchase battery 'cycle testers' that will measure the true mAH capacity of a specific pack by placing a user selectable load resistance on the pack and measuring the voltage drop over time to then come up with the true pack discharge capacity, which does slowly decrease over time and with total number of pack charge/discharge cycles it has accumulated. With some R/C aircraft having some thousands of dollars invested some take proactive battery pack testing very seriously.

Lefty
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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?

I invented my 250 mA number up based on the mAH capacity of a battery. I figured that a load that took several hours to discharge a battery would be about right. An AA should last about 8 hours at that load, an AAA about 4.

35mA seems tiny. An AAA battery with 2000 mAH capacity would take 2.4 days to discharge at that load.

Yes, a C or D cell would need a higher load to give a useful value. That's what I meant when I said "...depends on their size".

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This is what my battery-tester multimeter (claims) to do:



In principle, what would a good load be? Assuming, say, that you have a 1000 mAH battery, draining 200 mA would be 1/5 of its capacity, or should take 5 hours, anyway.
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There to kinds of test one to see how long the battery last and two to see if the battery is good.

To see if a battery is good doesn't need a big load to see how long a battery last and is still good the user load would be a better choice.

Testing a  Nicad with a 6 ohm resistor 1/2 watt would be like short not last not very long because the Nicad can dump out amps when shorted needs a 3 watt resistor.
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My cheap HF multimeter battery check has 4ma for a good 1.5v cell and 25ma for a good 9v battery. As to dummy loads, I use various types of light bulbs. 12v auto bulbs have lower resistance than the 120vac types. An old car headlight comes in handy for a bigger load. One might also get a space heater/hair dryer type device with heating coils to use for a variable load. Connect to the coils at varying distances apart with alagator clips to have an adjustable resistance.
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Testing a  Nicad with a 6 ohm resistor 1/2 watt would be like short not last not very long because the Nicad can dump out amps when shorted needs a 3 watt resistor.
A battery 'short' isn't really a short. The current limiting under conditions where a nominally 0R is placed across the terminals will be governed by the internal resistance of the cells.

A 6R means you are limiting the current to a known value. The true value will be slightly less than you calculate as there will be the cell internal resistance in series with your load.

The battery can't 'dump amps' just based on it's chemistry. It can only obey Ohms law in providing current into a fixed load resistance, based on it's terminal voltage, state of charge and internal resistance.

If you short the terminals then different battery types will be able to deliver a different 'short circuit' current, although it's not really a short as any voltage would drive an infinite current into a zero Ohm load.
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