# How build a decent battery tester?

Discussion in 'Electronic Components' started by Alex Coleman, Jul 1, 2007.

1. ### Alex ColemanGuest

How can I build myself a decent battery tester for domestic use? Are
there any circuits? Any info about off-the-shelf equipment? (I am in
the UK).

Of course I don't want to spend more cash than I save in batteries,
so there is a limit budget.

At the moment I use an ordinary digital multimeter which has a
setting to put the cell under a load of approx (a) 10mA or (b) 150
mA.

At present the *main* cells I would like to test are AAA and AA cells
made of NiMH, NiCd, manganese-alkaline or zinc chloride

I would like to have a device which simultaneously shows the current
and voltage of a cell under load. And then it would be useful to
simulate different loads - perhaps using a variable resistor?

If it's not too hard to also get a display of the internal resistance
of the cell that I'm told that can be useful. I gather that internal
resistance of rechargeables can show the battery's state of health.
However int resistance increases as the theoretical max capacity
increases and I guess this means that absolute calibration is not
possible, however perhaps internal resistance varies during charge
and is an indicator of capacity?

Thanks for any info.

--

2. ### Lost in the WastelandGuest

a DVM, note pad, and a resistor you likely already have !

3. ### JeffMGuest

"Testing" rechargable batteries is a waste of time.

To test a disposable battery:
1) Measure the no-load voltage of the cell.
2) Measure the voltage when loaded with a resistor (~10 ohms).
3) Make a chart of what is a "good" reading
(correlated to real-world failure of your battery-powered devices).

4. ### mikeGuest

The whole idea of batteries is that when the thing stops working, you
replace them. For critical items, you use NEW batteries and change them
on a schedule determined empirically.

I use a computer, GPIB programmable power supply and programmable load.
I've found two useful measurements.
Lithium ion rechargeable batteries fail with high internal resistance.
The electrons are still in there, but the device shuts off due to low
voltage and won't let you have them.

I built a tool for sorting lithiums at the surplus dealer.
8 AA nicads in series. A car tail light and switch in series to the
Measure the cell voltage. Under two volts, probably scrap.
Close switch and charge the cell until the dv/dt slows down.
Measure the voltage, let go of the switch, measure the voltage again.
Difference is related to internal resistance. You can do the math and
get a number, sorta, but experience over a few dozen cells compared to
a new cell is your best teacher.

Second useful thing for nicads is to plot the voltage while charging.
The curve shape of a good cell is unmistakable. Ditto for discharge.
mike

5. ### Franc ZabkarGuest

I use Bob Parker's ESR meter.

See the table of battery ESRs at the bottom of this page:
http://www.flippers.com/esrkthnt.html

- Franc Zabkar

6. ### Damit SpockGuest

A \$80 *kit* to test dead batteries.

Capitalism at it's best ...

7. ### Evgenij BarsukovGuest

One thing that rechargeable and alkalines have in common that they
have high impedance when close to discharged state.
So cheap and easy (partially distructive though) test is to use
a multy-meter, switch it to current measurement mode and using a
high-current path (10A), short the cell momentary.
If you see a sustained current (usually 3-4A but it is not
important) than cell is OK. If you see current drop very fast to below
1A, cell is close to dead state.
Yes, you will lose some charge while doing this test, but for that
you don't need any additional equipment, even no resistor. If you happen
to have a 1 Ohm resistor, you can use it instead of the short.
Procedure is the same but current will be much less.

Regards,
Yevgen

8. ### LostgallifreyanGuest

A variant on that that can be revealing is a very brief short-circuit,
about a tenth of a second, then watch how the voltage recovers immediately
afterwards. It might not reveal much, but probably enough to allow less
time at heavy current.