# AA Battery capacity tester- resistor & clock

Discussion in 'Electronic Basics' started by Jim Elbrecht, Jul 27, 2007.

1. ### Jim ElbrechtGuest

I've got a couple dozen Nimh batteries that are starting to show signs
of age. [some are 7yr old 800mAh, the newest are 2000 mAh & 3-4 yrs
old]

I googled for capacity testers and found a post that said to wire a 1
ohm resistor in series with a aa battery operated clock. Set the
clock to 12, insert battery, wait until clock stops.

I have a box of clock parts so I set up a 4-battery tester and set it
in motion yesterday with some of the 2000 mAh batteries.

Did I wire it wrong, or did I miscalculate how long it should take to
discharge a 2000 mAh battery? At 25 hrs, 45 minutes we're still
running the clocks.

The resistor I used is a Radio Shack- 1.0 Ohm, 10w. - part #271-131.

I connected one end of the resistor to the positive end of the
battery, and the other to the positive terminal in the clock, the
negative end of the battery to the negative terminal on the clock.

Thanks,
Jim

2. ### John FieldsGuest

---
You did nothing wrong but believe that whoever wrote that post knew
what they were talking about. They did not.
---
---
The problem with that method is that the one ohm resistor looks like
such a small impedance compared to that of the clock that the
voltage dropped across it will be largely insignificant and the
clock will continue to run for nearly as long as if the resistor
wasn't in there.

To measure the capacity of the battery you need to know three
things:

1. The specified capacity of the battery.

2. The specified discharge rate.

3. The specified cutoff voltage.

You already know the capacity, 2000mAH, and you can get the
discharge rate and cutoff voltage ratings from the data sheet for
the battery.

What the deal is with the rate of discharge is that if you have a
battery rated for 2000mAH and the rate is 0.1C, then to get the
2000mAH out of it, then you're allowed to take a maximum of 200mA
from the battery for 10 hours before its voltage falls to cutoff.

If you take more than that, then the capacity will fall and the
product of current and time to cutoff will be less than 2000.
Conversely, if you take less the capacity will rise slightly.

So, to do the test you'll need to start with a fully charged battery
and set up some test equipment like this: (View in Courier)

+------+----------+
|+ | |+
| | |
+------+----------+

In order to determine the resistance of the load, first determine
the current that can be taken from the battery by multiplying the
capacity times the rate. In your case, if you have a 2000mAH
battery with a rate of 0.1C, then the current will be 0.2A.

Next, use Ohm's law to determine the resistance:

E 1.2V
R = --- = ------ = 6 ohms,
I 0.2A

where 1.2V is the nominal voltage of your cell (battery). I don't
know if it is, though, consult the data sheet and plug in the
appropriate value.

The power it will need to dissipate will be:

P = I²R = 0.04 * 6 = 0.24 watts,

so you can use two standard 12 ohm +/- 5% 1/4 watt resistors in

In order to start the test, connect the load and the voltmeter to
the cell and note the time and the voltage.

Thereafter, monitor the voltage and when it falls to the cutoff
voltage note the time. If it's less than 10 hours after you started
the test, then the capacity isn't what it's supposed to be.

Keep track of voltage and time and you can plot what the discharge
curve looks like.

3. ### Anthony FremontGuest

Hmm..... I think this is where things actually went wrong. This will never
work.
What you really need to do is to connect the 1R resistor in "parallel" with
the battery, not in series. This will put a hefty load on the battery
(about 1.2A when it is full). The resistor (you said 10W) will still get
warm, so be careful. The problem with discharging like this is that the
load decreases as the battery voltage falls. This makes it a calculus
problem to determine the charge that was in the battery. If you used a
"constant current" load, calculating battery capacity would be as simple as
timing how long it takes.

With that out of the way, I suspect that you can get an approximate capacity
by using the clock as described. Assuming that it stops when the battery
voltage drops to about .8V AND pretending that the voltage curve is linear.
You are averaging a 1A load over time, so if the clock stops at 1:30, you
could sorta say that the battery had 1500mAh charge.....sorta. ;-)

4. ### Phil AllisonGuest

"Jim Elbrecht"

** Not wired in "series" in PARALLEL !!

** Neat idea.

** So four separate clocks ?

However, for testing non rechargeable cells like AA alkalines - use a 10
to 15 ohm resistor.

Shame they will be buggered at the end of the test !

....... Phil

5. ### Jim ElbrechtGuest

Thanks to all- I might set up an excel sheet to plot curves and do it
the way John suggests just for curiosity's sake.

And yes to Phil- 4 clocks- mounted on lexan. It looks so good I was
really disappointed when it didn't work as planned.

On Fri, 27 Jul 2007 13:11:14 -0500, "Anthony Fremont"

-snip-
Aha! I can't find the post now to see if I misread- or he
mis-wrote. At any rate- that's the ticket.

-snip-
For my purposes this will work fine. I've found that usually when a
'set' of batteries goes bad, it really means *one* has bit the dust.
This will locate that one with a minimum of fuss.

Thanks again-
Jim

6. ### Charlie SiegristGuest

Maybe not completely buggered, but indeed, each time a DC load test is
performed on a battery, the lives of the cells are shortened.

OP might want to invest in a pulse tester, which may be less stressful on
the cells. I don't have any experience with these, so perhaps someone
might care to weigh in on the subject? I did find this:

7. ### Phil AllisonGuest

"Charlie Shitegrist"
"Phil Allison"

** They are 100% dead at the end of the test

- you FUCKING IDIOT ! !

** No - YOU need to invest in one of those ....

Just to see if you are DEAD or not.

IMBECILE !!

......... Phil

9. ### Charlie SiegristGuest

Well, I'm not the idiot who posted a recommendation to do a load test on
non-rechargeable batteries, that would have been, ah, you. I'm only the
idiot who overlooked the fact that some imbecile, in reference to a
to load-test alkaline batteries. In an objective idiot comparison, I think
you'd win.

So, there's your cue. You set a trap in the hopes that you would be given
the chance to spew some puerile venom. Good work! However, in doing so,
you've only shown yourself to be the imbecile. Now, start a thread
entitled "Charlie Siegrist = PANKING MORON" or some such other puerile
nonsense, and thus try to shield yourself from the fact that you've once
again pathetically out-idioted yourself.

10. ### Phil AllisonGuest

"Charlie Shitegrist"

= FUCKING IDIOT ! !

** No - YOU need to invest in one of those ....

Just to see if you are DEAD or not.

IMBECILE !!

11. ### Anthony FremontGuest

Why are you bringing this up? The thread is about rechargeable batteries.
Did you figure that out only after trying it yourself?

12. ### Phil AllisonGuest

** Cos 1 ohm load is not suitable for testing all kinds of AA cells -
alkalines for example.

** The thread is about a neat technique for testing cell capacity with a
clock.

Try fucking reading it - you autism fucked pile of

STEAMING DOG SHIT !!!!!!!!

....... Phil

13. ### Anthony FremontGuest

Again, you have completely missed the point of my question. sigh.....

But I'll go ahead and bite anyway. How can you make that assumption?
Perhaps someone wanted to test some non-rechargeable alkalines and they were
destined for a high current application (like say powering a video
transmitter on a model rocket/plane). IOW, why do you assume a ~20 hour
discharge as the only viable test?
s/a neat/debugging an obvious/
s/cell/rechargeable cell/

NiCd chargers have been doing basically the same thing for DECADES.
Yes I do, it looks arguably like you are threatening my life. At any rate,
had you been able to grasp the context of the thread, you would have noticed
that the OP was only testing rechargeables. I think you purposely threw in
that tripe about alkalines to bait Charlie.

BTW there are rechargeable alkalines, so technically you weren't even
"buggered".

This is SEB Phil, why can't you at least try to restrain yourself here. You
get to act an ass in plenty of other newsgroups, why can't you treat this on
like it's holy ground or something?

HAND

14. ### Phil AllisonGuest

** Cos 1 ohm load is not suitable for testing all kinds of AA cells -
alkalines for example.

** The thread is about a neat technique for testing cell capacity with a
clock.

See the OP's bloody heading ??

Try fucking reading it - you autism fucked pile of

STEAMING DOG SHIT !!!!!!!!

....... Phil

15. ### Phil AllisonGuest

** Why doesn't someone kill this vile public menace & autistic mental
retard ???

....... Phil

16. ### neon

1,325
0
Oct 21, 2006
i am not an idiot but you two are not all battery are created equal. thanks god. here is a new concept battery have memory too. they like to perform the same function as opposed to ramdom function. some are designed for long life no drain some are designed for short burst of energy. the manufactures allways recomend a charge and discharge rate. meaning for you to control heat that is the real killer for a battery.