# Battery testing?

Discussion in 'General Electronics Chat' started by Mathematics!, Dec 28, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I am curious how you can test if a batteries good or bad?
For instance I have used DMM to test the voltage of the battery but this doesn't really tell me if the battery is good....

Like I have measured 9volt batteries and they read 8.5 and work fine and then I have measured them at 8.6 and they don't work.... so the DMM is not a good way of judging

I am curious how these battery tester devices work to indicate if the battery is good or bad.... some of them even have the ability to tell you if it is at 60% , ....90% ,...etc of its full capacity...

Curious if their is a simply battery tester circuit that I could make ....

First though I need to know how the test is done is it by current or voltage ,,......

Thanks for any help

2. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
A load will knock that 8.5 volts down in a second,try It. You can make load tester
a diagram should not be hard to find.

I think most bench guys have them,along with a simple curcuit tracer
that never go's out of style.

A short tester with a 100 watt bulb to save fuses. Some one will jump on line
and set you up.Give the post a day.

and answer a question for you.Take an extension cord and split the wire so when
you use your ac clamp on current meter,you can wrap a couple of turns around
the clamp so you can pick up a more accurate reading of ac current. Then you won't
have to split your good cord.

Just In fun trying give every one a Happy New Years,And separate my quotes.
How does this look guys.

Last edited: Dec 29, 2009
3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
You can make a simple battery tester using just a 360 Ohm resistor in series with your DMM set to read on the 200mA scale.

A good NiCD or NiMH will read around 3.8mA.

It won't work for a lead-acid battery; not enough of a load.

Do NOT try to test mains current this way.

4. ### Mike33 AAC Fanatic!

Feb 4, 2005
349
25
While on this subject, does anyone have a good (inexpensive) means of testing a 12V car battery this way?
I mean, I understand that to get an idea of its health, you'd probably have to load it up and get it to give a couple of hundred of those "Cold-Cranking Amps", so maybe there isn't a 'quick and dirty' test...maybe a piece of copper pipe as a resistor, LOL. Just curious...

5. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
Kel equipment make an excellent car battery checker,It put a 10-15
second load on the car battery.

N.A.P.A auto parts sell them for around \$90.OO, It has a chrome
plated cover. If the Indicator needle go's south you need to change
battery.A good battery the Indicator will stay steady for 10-15
seconds.

It will also check your alternator,It has 14 volt scale.
Maybe a car repair person can varifiy this Information.

6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Thanks but how did you come up with using that resistor value?
And How do you know what current is great , good , bad, terrible bad with the particular battery.... Like how do you know when the battery will work or not by the amount of current read by the DMM ???

What I was think is if I knew the max current each battery could put out then I could figure out the percentage based on the DMM test..... Maybe that is the way to get how used a battery is.... (but to figure out the max current for a particular battery I would need to know th batteries voltage and internal resistance unless standard batteries give a max current rating on them or if their is some place to look this info up?)

For the lead-acid batteries what resistor could I use to test these?

........Anybody?

Last edited: Dec 30, 2009