I know lead acid batteries need to be tested under load, and should also be run under load before testing as they cam initially measure higher voltage that quickly drops due to "surface charge". Modern rechargeable chemistries tend to produce very flat discharge curves so voltage doesn't vary much except at the start and end of discharge - beyond that you need to know the discharge curve of your particular cells, and also temperature, to take useful readings.
Sometimes you just need to try them - some devices will run better than others at low voltage. You could build a "Joule thief" and run an LED long after a battery is no good for anything else - perhaps handy as a light to keep with your keys, or by the fusebox.
Batteries are a product of chemistry, not physics.
A lot of methods used to characterize them, are not accurate or reproducible. Please do not stare at the maths in the following article. "LEO says GER". Loss of Electrons is Oxidation, Gain of Electrons is Reduction. Chemical equations for half cells, have "e" as part of the equation and represent electrons (for book keeping purposes).
In dilute solutions, the Nernst equation can be expressed directly in the terms of concentrations (since activity coefficients are close to unity).
But at higher concentrations, <=== like in a store bought battery...
the true activities of the ions must be used. This complicates the use of the Nernst equation, since estimation of non-ideal activities of ions generally requires experimental measurements.
The Nernst equation also only applies when there is no net current flow through the electrode. <=== A "no-load" measurement
The activity of ions at the electrode surface changes when there is current flow, and there are additional
overpotential \___ Overpotential happens right after a charging operation resistive loss / Resistive loss is the non-ideal nature of battery impedance
terms which contribute to the measured potential. "
what that means, is single-point measurements, or fancy gizmos ("battery checker") can only give fuzzy concepts like "good" "weak" "bad" indications. The Nernst equation attempted to quantify reduction/oxidation (Redox reactions) for half cells and give precise values.
*******The closest thing to an ideal cell, is the Weston (reference) cell.
The cell has a low temperature coefficient, which means even if the room temperature goes up and down a bit, the voltage remains relatively constant. Unlike lead acid, which has a fairly strong temperature dependence.
You cannot draw any current from a Weston cell. Or rather, you are not supposed to. If you short the leads on a Weston cell, it might take weeks before the voltage value unloaded, returns to 1.018638 V. That's because the chemical concentration of materials is no longer "saturated" along the plate surface. We had one of these cells in a chem lab, against one wall.
*******Notice that the nominal voltage of commercial cells, is not given to a lot of digits.
The battery model for home users, looks like this.
+----- Internal Resistance ---------O <--- Place multimeter here | \ on DC volts range _____ Voltage is a subject \ ___ of usage history, recent Rload _____ discharge or recent charge / ___ / | / +-----------------------------------O <--- Place multimeter here on DC volts rangeIf you remove the test load "Rload", then the measurement gives the internal voltage. If the battery just had a heavy discharge, the voltage is lower than normal. If the battery just received a charging current (for rechargeable types only), the battery will be slightly higher than the real state.
A battery must be allowed to rest. For a lead acid battery, right after charging, it is 13.5V . If you come back 24 hours later, it will be 12.6V or so. And 12.6V is the "full" for the battery in equilibrium state (no float applied). When a lead acid battery is run down, you try not to discharge it too low, and don't draw 100% of the capacity from it. Perhaps you stop at 12.0V equilibrium value.
On my car, with a fully charged battery, the starter motor drew 150 amps. The battery was 12.6V before the test started. At peak current draw, the voltage at the battery terminals was 9.5V. The internal impedance is:
12.6V - 9.5V ----------- = 0.020 ohms 150 ampsAnd that's an example of a determination of the internal resistance.
If I draw 1 ampere from the battery, the seeming battery output voltage will be 12.58V . If I draw 2 amperes, the battery voltage will appear to be 12.56V . This is an estimate of the resistive drop. The chemical dis-equalibrium may make the voltage slightly lower.
*******When you use a battery test meter, with the coloured test bands, it is a single point measurement. The meter does not measure the no-load voltage first. It simply measures the loaded output voltage with a "representative load" and gives a very rough "good-weak-bad" indication. A lot of assumptions go into such a single point measurement. The battery might be perfectly fine for maintaining something which does not have high peak current flows.
A battery test meter is good at qualifying the cells for usage in a flashlight. If the meter says "weak", the cells might not be useful in a flashlight.
Each cell type range uses a different load resistor. It should also be doing something different, for alkaline versus NiMh. And it probably is not doing that. The different terminal voltage (the 1.2V) means the NiMh can never measure better than "Weak" if the meter is expecting 1.5V cells. The load current it is applying, just makes things worse.
The resting battery voltage gives you some idea where in the life cycle the cell is. In that respect, for at least some of the cells, you have a fairly good idea the cell is marginal. You would not use a cell with a low terminal voltage, for your spelunking trip.
Paul
It depends of the capacity of the battery, the nominal voltage, and the load expected.
If you are fitting it to a low power device such as a remote control, the open circuit voltage will be a good guide. Otherwise you should measure it when in situ if possible or with a resistor that takes the same current. I don't see how a "green light" on a battery tester which is just an accessory on a multimeter can tell you much as there are too many assumptions that it makes.
That's your question. The clue you need is that all you need do is charge the cells & stick them in your load. You'll soon know if they work ok. The idea that you need more expensive equipment is just nonsense.
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