Is no-load voltage a reliable indicator of charge condition ? Are there universal voltage levels for a particular type of cell (e.g. NiCd, NiMH, Li-ion, lead-acid, etc.) that can be taken to indicate full-charge and full-discharge conditions ?
There are gadzillion articles that discuss the general characteristics of various types of rechargeable cells, but I have not found any that specifically says that an X-type cell reaches Y Volts at full charge, and should not be discharged below Z Volts.
Is it at all possible to specify voltage levels this way ? Temperature and internal resistance (and therefore charge and discharge currents) must play a part, but standard voltage levels under no-load, no-charging at standard room temperature would be very helpful.
Lead Acid battery noload terminal voltage can be used as an indicator of charge level (See other comment above). Other cells are not quite as easy.
Li+. Fully charged : 4.2V per cell. Discharged 3.0V per cell (nominal). Nominal cell voltage: 3.7V. There are charts available, and maybe I'll dig out the links tomorrow at work. Fairly linear, but as with NiMH somewhat susceptible to ambient conditions.
NiMH. 1.2V per cell. More difficult, but 1.1V is nominally discharged,
1.3V (roughly) is fully charged. Fairly linear betwen these especially for the 20-8% range. Varies somewhat, see the manufacturers data sheets.
NiCad. The noload voltage for a particular cell has zero bearing on the charge state of a NiCad. A discharged battery might read anywhere between 1.1V and 1.3V. The loaded voltage likewise has little bearing. I spent many years with these, and the only method I know that actually works is measuring the recovery time of the cell postload.
Details: Measure the noload voltage. Load the cell at some arbitrary current (but well below the max load rating) for sufficient time for the loaded voltage to stabilise (perhaps 100millisec). Release the load. Measure the recovery curve (which is very close to a RC charge curve). The shorter the curve, the higher the charge. For the mathematical details, you'll have to wait until I dig out some books from 15 years ago (when I did those experiments and filed patent disclosure).
So the general answer is --No--, there is no universal voltage for most cells. Indeed, we used to determine the charge of a lead acid battery by electrolyte specific gravity (that was quite a while ago).
Not really very helpful - an indicator, nothing more. For anything approaching reasonable accuracy, noload voltage is not much use for most solid battery types.
It depends what you are doing. Simply measuring no-load voltage is most likely not reliable on most batteries. However, you can probably get a crude state of charge by looking at the voltage. All battery chemistries will have their own unique voltage versus state of charge per cell. But again, in most cases the best you can hope for is just an estimate and if the battery isn't functioning properly the estimate could be *WAY* off.
That said, if you are trying to charge a battery and want to know when to stop the correct algorithm varies wildly with battery chemistry, and you should never attempt to charge any battery without understanding what the correct behavior is for that algorithm. In particular, NiMH and Lithium based batteries require careful attention to proper charging or it can have... unfortunate... consequences. NiCd and lead acid are generally safer from that perspective.
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