Is it possible to test batteries accurately with a multimeter to see if they're still good?
Is the reading accurate if you connect the meter across the battery terminals or do you need a parallel resistance (and what would be appropriate)?
How much oomph does a battery need to be any use? One site said that
20-30% below its stated voltage, the battery was US, another that batteries are good down to 20-30% of stated voltage.Thanks for any advice.
I used to "test" D-size cells using the 1A range of a multimeter, which put a nice hefty load on the cell. A bit of practice with new and older cells will give you an idea of the sort of readings to go for. It's not recommended to try this with NiCads though. Or lead-acids.
Yes.
No and yes.
It depends on the battery and the usage. there's no good universal rule of thumb, other than does it work.
We used to test zinc-carbon batteries 30 odd years ago by measuring teh current drawable by an avo on the 10A range for a couple of seconds. Value depended on the size of battery. I wouldn't try it on more modern types, they could explode.
Sort of.
Charge a NiCd, leave it alone for a couple of days, measure the remaining open circuit voltage. It won't tell you the remaining capacity (that's hard), but it detects the usual failure mode of the development of internal shorts that makes them require either discharge cycling, or throwing away.
For lead acids you care more about the current capacity on heavy load (i.e. their internal resistance). For that you need a dummy load (coil of heater element / few headlamp bulbs) and a heavy-duty ammeter. If you can estimate the load impedance when hot, then a voltmeter might suffice. The easiest way to get this is with a commercially-made prong tester.
By "batteries" do you mean something like a car battery (or other multi- cell type) or single cells?
The off-load voltage is meaningless so the answer to your question is no. The idea of using the 1A range on a meter is interesting, but risky to the meter and not really meaningful apart from when used for comparing similar cells. Don't consider doing this on anything that has a reasonable current capability!
You need to use a dummy load resistor. The value and current rating for this is chosen depending on the battery voltage and anticipated internal resistance of it (e.g. the load resistor for a D cell will be lower than that used for a AA cell and lead-acid cells need lower values than standard alkaline). The idea is to draw a current that is sufficient to cause the terminal voltage to fall noticeably when there is a known % of charge remaining. Once calibrated this has pretty good repeat accuracy. For a rough test on single cells use a 4.7 ohms 1W resistor. This will draw about 320mA (about the same as a torch bulb) from a 1.5V cell.
There is no known *accurate* way of testing NiCd and NiMH batteries for "goodness" other than using them. Their very low internal resistance causes the terminal voltage to drop very quickly at end of life so the dummy load test can't judge a % with any accuracy.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.