Re: Battery voltage as indicator of charge level

pjdd@xxxxxxxxxxxxxx wrote:
> 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.

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.

dan
.

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