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

Bajdi

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

retrolefty


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?

JimboZA

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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retrolefty


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

fungus


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".

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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